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Near-Earth Asteroid

Near-Earth asteroid

Near-Earth asteroids (NEAs) are asteroids whose orbit intersects Earth's orbit and which may therefore pose a collision danger, as well as being most easily accessible for spacecraft from Earth. In fact, some near-Earth asteroids can be reached with much less delta-v than it takes to reach the Moon. The most famous near-Earth asteroid is 433 Eros that was visited by NASA's Near Earth Asteroid Rendezvous probe. A few hundred such near-Earth asteroids are known, ranging in size up to four kilometres. Tens of thousands probably exist, with estimates placing the number of NEAs larger than one kilometre in diameter at up to 2,000. Astronomers believe that NEAs only survive in their orbits for 10 million to 100 million years. They are eventually eliminated either by collisions with the inner planets, or by being ejected from the solar system by near misses with the planets. Such processes should have eliminated them all long ago, but it appears they are resupplied on a regular basis.

NEA classification

Some of the NEAs with highly eccentric orbits appear to actually be extinct "short period" comets that have lost all their volatiles, and in fact a few NEAs still show faint comet-like tails. These NEAs were likely derived from the Kuiper belt, a repository of comets residing beyond the orbit of Neptune. The rest of the NEAs appear to be true asteroids, driven out of the asteroid belt by gravitational interactions with Jupiter. There are three families of NEAs:
- The Atens, which have average orbital diameters closer than one astronomical unit (AU, the distance from the Earth to the Sun) and aphelia of greater than Earth's perihelion, placing them usually inside the orbit of Earth.
- The Apollos, which have average orbital diameters greater than that of the Earth and perihelia less than Earth's aphelion.
- The Amors, which have average orbital diameters in between the orbits of Earth and Mars and perihelia slightly outside Earth's orbit (1.017 - 1.3 AU). Amors often cross the orbit of Mars, but they do not cross the orbit of Earth. The two moons of Mars, Deimos and Phobos, appear to be Amor asteroids that were captured by the Red Planet. Notice that all Atens and Apollos have eccentric orbits that cross the orbit of the Earth, making them potential threats to our planet while Amors do not cross Earth's orbit but some may come very close. Also sometimes used is the Arjuna asteroid classification for asteroids with extremely Earth-like orbits. Near-Earth asteroid is a more restrictive term than near-Earth object.

The NEA threat

The general acceptance of the Alvarez hypothesis, explaining the Cretaceous-Tertiary extinction event as the result of a large asteroid or comet impact event, has raised the awareness of the possibility of future Earth impacts with asteroids that cross the Earth's orbit. The threat of an Earth impact was emphasized by the collision of the comet Shoemaker-Levy 9 with Jupiter on July 16, 1994, resulting in explosive impacts that would have been catastrophic on Earth. To be sure, Jupiter is far larger and more massive than the Earth and so undergoes far more impacts, but the event still illustrates that such things do happen and can be unimaginably destructive. On March 23, 1989 the 300 metre (1,000-foot) diameter Apollo asteroid 4581 Asclepius (1989 FC) missed the Earth by 700,000 kilometres (400,000 miles) passing through the exact position where the earth was only 6 hours before. If the asteroid had impacted it would have created the largest explosion in recorded history. Asteroids with a 1 kilometre diameter hit the Earth a few times in each million year interval. Large collisions with 5 kilometre objects happen every ten million years. Small collisions (but still potentially dangerous ones) occur a few times each month. Although there have been a few false alarms, a number of asteroids are definitely known to be threats to the Earth. Asteroid (29075) 1950 DA was lost after its discovery in 1950 since not enough observations were made to allow plotting its orbit, and then rediscovered on December 31, 2000. Proper calculation of its orbit then demonstrated that it has a 1 in 300 chance of hitting the Earth on March 16, 2880. This probability is a thousand times greater than any other known asteroid threat, and 50% greater than all other known asteroid threats combined. (29075) 1950 DA has a diameter of a kilometre. On March 18, 2004, LINEAR announced a 30 metre asteroid 2004 FH which would pass the Earth that day at only 42,600 km (26,500 miles), about one-tenth the distance to the moon, and the closest miss ever noticed. They estimated that similar sized asteroids come as close about every two years. It is difficult to determine the chances of its impact better than that. The uncertainty is due to minor irregularities in the Sun's shape, and so its gravitational field; weakening of the Sun's gravity through mass loss from the solar wind of particles that streams out from its atmosphere; uncertainties in the masses and so the gravitational pull of the planets; variations in the tidal pull of the surrounding galaxy; the subtle pressure of sunlight; and, in particular, a phenomenon known as the "Yarkovsky effect". This effect was discovered by a Russian engineer named I. O. Yarkovsky a century ago. It is a subtle process: the heating of the asteroid's surface causes it to emit thermal radiation, which creates a slight amount of thrust. It is somewhat unpredictable, since an asteroid's ability to soak up heat from the Sun depends on its terrain, and the effect is also influenced by the asteroid's spin orientation and rotation rate.

Projects to ameliorate the threat

Astronomers have been conducting surveys to locate the NEAs. One of the best-known is the LINEAR which began in 1996. By 2004 LINEAR was discovering tens of thousands of objects each year and accounting for 70% of all asteroid detections. LINEAR uses two one-metre telescopes and one half-metre one based in New Mexico. Spacewatch, which uses an old 90 centimetre telescope sited at the Kitt Peak Observatory in Arizona, updated with automatic pointing, imaging, and analysis gear to search the skies for intruders. The project was set up in 1980 by Tom Gehrels and Dr. Robert S. McMillan of the Lunar and Planetary Laboratory of the University of Arizona in Tucson, and is now being operated by Dr. McMillan. The Spacewatch project has acquired a 1.8 metre telescope, also at Kitt Peak, to hunt for NEAs, and has provided the old 90 centimetre telescope with an improved electronic imaging system with much greater resolution, improving its search capability. These new resources promise to increase the rate of NEA discoveries by Spacewatch from 20 to 30 a year to 200 or more. Other near-earth asteroid tracking programs include Near-Earth Asteroid Tracking (NEAT), Lowell Observatory Near-Earth-Object Search (LONEOS), Catalina Sky Survey, Campo Imperatore Near-Earth Objects Survey (CINEOS), Japanese Spaceguard Association, and Asiago-DLR Asteroid Survey. "Spaceguard" is the name for these loosely affiliated programs, some of which receive NASA funding to meet a U.S. Congressional requirement to detect 90% of near-earth asteroids over 1 km diameter by 2008. A 2003 NASA study of a follow-on program suggests spending US$250-450 million to detect 90% of all near-earth asteroids 140 metres and larger by 2028. Nonetheless, the fact that an impact of an NEA a kilometre or more in size would be a catastrophe unparalleled in human history has kept the idea of a defensive network alive, as well as led to speculations on how to divert objects that might be a threat. Detonating an explosive nuclear device above the surface of an NEA would be one option, with the blast vaporizing part of the surface of the object and nudging it off course with the reaction. This is a form of nuclear pulse propulsion. However, it is becoming increasingly obvious that many asteroids are "flying rubble piles" that are loosely glued together, and a nuclear detonation might just break up the object without adjusting its course. In some ways, being struck with a loose cloud of smaller asteroids is worse than being struck with just one big one. This has led to a variety of other ideas for dealing with the threat:
- Setting up "mass drivers" on the object to scoop up dusty material and shoot it away, giving the object a slow, steady nudge.
- Flying a big sheet of reflective Mylar to wrap itself around the asteroid, acting as a "solar sail" to use the pressure of sunlight to shift the object's orbit.
- Dusting the object with powdered chalk or soot to perform a similar adjustment, utilising the Yarkovsky effect. Thinking on the matter continues - see Asteroid deflection strategies - and if there is no prospect of immediate action, the issue isn't going away, either. As our space technology and space infrastructure advance, our choices improve. For example, it might become possible to bring asteroids (that are not flying rubble piles) to orbits near Earth, then maneuver them so that their small gravitational influence can move the NEAs to orbits that do not threaten us.

An example of a recent asteroid impact

On June 6, 2002 an object with an estimated diameter of 10 metres collided with Earth. The collision occurred over the Mediterranean Sea, at approximately 34°N 21°E and the object detonated in mid-air. The energy released was estimated (from infrasound measurements) to be equivalent to 26 kilotons of TNT, comparable to a medium-size nuclear weapon [http://www.astro.uwo.ca/~pbrown/documents/flux-final.pdf]. At that time India and Pakistan were at a heightened state of alert, ready to initiate a nuclear war with each other. If this asteroid impact had hit in this area the results might have been catastrophic.

See also


- Near-Earth object
- List of NEAs by distance from Sun
- Sentry monitoring system

External link


- [http://neat.jpl.nasa.gov/ JPL Near Earth Asteroid Tracking program (NEAT)] There is an excellent article in the October 2003 issue of Scientific American regarding NEA's and long term strategies for protecting Earth from them.
- ja:地球近傍小惑星

Asteroid

:This page is about the astronomical body Asteroid. For the arcade game, see Asteroids. An asteroid is a small, solid object in our Solar System, orbiting the Sun. An asteroid is an example of a minor planet (or planetoid), which are much smaller than planets. Most asteroids are believed to be remnants of the protoplanetary disc which were not incorporated into planets during the system's formation. Some asteroids have moons. The vast majority of the asteroids are within the main asteroid belt, with elliptical orbits between those of Mars and Jupiter. Jupiter

Definition

The term "asteroid", meaning star-like (from the Greek asteroeides, aster "star" + -eidos "form, shape"), was coined in 1802 by Sir William Herschel shortly after Olbers discovered the second one, 2 Pallas, in late March of the same year, to describe their star-like appearance; the other then-known planets all show discs, by comparison. He also applied that term to the small moons of the giant planets. The first [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1840AN.....17...81E&db_key=AST&high=41e14f475d05983 scientific paper] to use the word in its title was published in 1840 by Erman. The exact definition of an asteroid is unsettled. The term "Minor planet" (or "planetoid") carries no strong suggestion about the composition of the object or its general location in the solar system, and some argue that not every minor planet should be called an "asteroid". One way to classify asteroids is in terms of size. A working definition is that asteroids are larger than 50 m in diameter, distinguishing them from meteoroids, which are typically boulder-sized or smaller. The distinction is made because asteroids are large enough to survive passage through Earth's atmosphere and strike Earth largely intact while the smaller meteoroids generally break up high in Earth's atmosphere. Thus, it would be safest to use the term "asteroid" for Solar System objects that are bigger than meteoroids, smaller than planets, and made out of rock, not ice. See Solar System for a complete taxonomy of objects in our system, and minor planet for a taxonomy of the subplanetary objects that include asteroids. The term artificial asteroid is sometimes used to designate man-made objects which have ended up in solar orbits, such as the Mariner IV probe.

Asteroids in the solar system

Mariner IV alongside Earth's Moon.]] Hundreds of thousands of asteroids have been discovered within the solar system, and the present rate of discovery is about 5000 per month. As of November 16, 2005, from a total of 305,224 minor planets with calculated orbits, 120,437 asteroids had been calculated well enough to be given official numbers and 12,712 of these had been officially given trivial names to go along with the numbers (at least 610 of which have names requiring diacritics). The lowest-numbered but unnamed minor planet is (3360) 1981 VA; the highest-numbered named minor planet is 99942 Apophis [http://cfa-www.harvard.edu/iau/lists/NumberedMPs095001.html]. The Minor Planet Circular (MPC) of October 19, 2005 was a historical one, as it saw the highest numbered asteroid jump from 99947 to 118161, causing a small "Y2k" like crisis for various automated data services —up until then, only five digits were allowed in most data formats for the asteroid number. This has been addressed in some data fields by having the leftmost digit, the ten-thousands place, use the alphabet as a digit extension. A=10, B=11,…, Z=35, a=36,…, z=61. The highest number 120437 thus is cross-referenced as C0437 on some lists. Also, the fictional asteroid of The Little Prince, B612, now could be connected with the real (110612) 2001 TA142 which is listed as (B0612) 2001 TA142 in the compacted lists —although it is already present as 46610 Bésixdouze (B612 in hexadecimal translates to 46610 in decimal notation). Current estimates put the total number of asteroids in the solar system at several million. The largest asteroid in the inner solar system is 1 Ceres, with a diameter of 900-1000 km. Two other large inner solar system belt asteroids are 2 Pallas and 4 Vesta; both have diameters of ~500 km. Vesta is the only main belt asteroid that is sometimes visible to the naked eye (in some very rare occasions, a near-Earth asteroid may be visible without technical aid; see 99942 Apophis). The mass of all the asteroids of the Main Belt is estimated to be about 2.3x1021 kg, or about 3% of the mass of our moon. Of this, 1 Ceres comprises 940 to 950x1018 kg, some 40% of the total. Adding in the next three most massive asteroids, 4 Vesta (12%), 2 Pallas (9%), and 10 Hygiea (4%), bring this figure up 66%; while the three after that, 511 Davida (1.6%), 704 Interamnia (1.4%), and 3 Juno (1.2%), only add another 4% to the total mass. The number of asteroids then increases exponentially as their individual masses decrease. See also a List of noteworthy asteroids in our Solar System, or a sequentially-ordered List of asteroids.

Asteroid classification

Asteroids are commonly classified into groups based on the characteristics of their orbits and on the details of the spectrum of sunlight they reflect.

Orbit groups and families

Many asteroids have been placed in groups and families based on their orbital characteristics. It is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are much "tighter" and result from the catastrophic break-up of a large parent asteroid sometime in the past. For a full listing of known asteroid groups and families, see minor planet.

Spectral classification

minor planet.]] In 1975, an asteroid taxonomic system based on colour, albedo, and spectral shape was developed by Clark R. Chapman, David Morrison, and Ben Zellner. These properties are thought to correspond to the composition of the asteroid's surface material. Originally, they classified only three types of asteroids:
- C-type asteroids - carbonaceous, 75% of known asteroids
- S-type asteroids - silicaceous, 17% of known asteroids
- M-type asteroids - metallic, most of the remaining asteroids This list has since been expanded to include a number of other asteroid types. The number of types continues to grow as more asteroids are studied. See Asteroid spectral types for more detail or :Category:Asteroid spectral classes for a list. Note that the proportion of known asteroids falling into the various spectral types does not necessarily reflect the proportion of all asteroids that are of that type; some types are easier to detect than others, biasing the totals.

Problems with spectral classification

Originally, spectral designations were based on inferences of an asteroid's composition:
- C - Carbonaceous
- S - Silicaceous
- M - Metallic However, the correspondence between spectral class and composition is not always very good, and there are a variety of classifications in use. This has led to significant confusion. While asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of similar materials. At present, scientists have been unable to agree on a better taxonomic system for asteroids and as a result, the spectral classification has stuck.

Asteroid discovery

Historical discovery methods

Asteroid discovery methods have drastically improved over the past two centuries. In the last years of the 18th century, Baron Franz Xaver von Zach organized a group of 24 astronomers to search the sky for the "missing planet" predicted at about 2.8 AU from the Sun by the Titius-Bode law, partly as a consequence of the discovery, by Sir William Herschel in 1781, of the planet Uranus at the distance "predicted" by the law. This task required that hand-drawn sky charts be prepared for all stars in the zodiacal band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would, hopefully, be spotted. The expected motion of the missing planet was about 30 seconds of arc per hour, readily discernable by observers. Ironically, the first asteroid, 1 Ceres, was not discovered by a member of the group, but rather by accident in 1801 by Giuseppe Piazzi director, at the time, of the observatory of Palermo, in Sicily. He discovered a new star-like object in Taurus and followed the displacement of this object during several nights. His colleague, Carl Friedrich Gauss, used these observations to determine the exact distance from this unknown object to the Earth. Gauss' calculations placed the object between the planets Mars and Jupiter. Piazzi named it after Ceres, the Greek goddess of agriculture. Three other asteroids (2 Pallas, 3 Juno, 4 Vesta) were discovered over the next few years, with Vesta found in 1807. After eight more years of fruitless searches, most astronomers assumed that there were no more and abandoned any further searches. However, Karl Ludwig Hencke persisted, and began searching for more asteroids in 1830. Fifteen years later, he found 5 Astraea, the first new asteroid in 38 years. He also found 6 Hebe less than two years later. After this, other astronomers joined in the search and at least one new asteroid was discovered every year after that (except the wartime year 1945). Notable asteroid hunters of this early era were J. R. Hind, Annibale de Gasparis, Robert Luther, H. M. S. Goldschmidt, Jean Chacornac, James Ferguson, Norman Robert Pogson, E. W. Tempel, J. C. Watson, C. H. F. Peters, A. Borrelly, J. Palisa, Paul Henry and Prosper Henry and Auguste Charlois. In 1891, however, Max Wolf pioneered the use of astrophotography to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This drastically increased the rate of detection compared with previous visual methods: Wolf alone discovered 248 asteroids, beginning with 323 Brucia, whereas only slightly more than 300 had been discovered up to that point. Still, a century later, only a few thousand asteroids were identified, numbered and named. It was known that there were many more, but most astronomers did not bother with them, calling them "vermin of the skies".

Modern discovery methods

Until 1998, asteroids were discovered by a four-step process. First, a region of the sky was photographed by a wide-field telescope. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films of the same region were viewed under a stereoscope. Any body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would appear to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations [http://astrogeology.usgs.gov/About/People/CarolynShoemaker/]. These first three steps do not constitute asteroid discovery: the observer has only found an apparition, which gets a provisional designation, made up of the year of discovery, a code of two letters representing the week of discovery, and of a number so more than the one discovered one took place in this week (example: 1998 FJ74). The final step of discovery is to send the locations and time of observations to Brian Marsden of the Minor Planet Center. Dr. Marsden has computer programs that compute whether an apparition ties together previous apparitions into a single orbit. If so, the object gets a number. The observer of the first apparition with a calculated orbit is declared the discoverer, and he gets the honour of naming the asteroid (subject to the approval of the International Astronomical Union) once it is numbered.

Latest technology: detecting hazardous asteroids

There is increasing interest in identifying asteroids whose orbits cross Earth's orbit, and that could, given enough time, collide with Earth (see Earth-crosser asteroids). The three most important groups of near-Earth asteroids are the Apollos, Amors, and the Atens. Various asteroid deflection strategies have been proposed. The near-Earth asteroid 433 Eros had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects. In order of discovery, these were: 1221 Amor, 1862 Apollo, 2101 Adonis, and finally 69230 Hermes, which approached within 0.005 AU of the Earth in 1937. Astronomers began to realize the possibilities of Earth impact. Two events in later decades increased the level of alarm: the increasing acceptance of Walter Alvarez' theory of dinosaur extinction being due to an impact event, and the 1994 observation of Comet Shoemaker-Levy 9 crashing into Jupiter. The U.S. military also declassified the information that its military satellites, built to detect nuclear explosions, had detected hundreds of upper-atmosphere impacts by objects ranging from one to 10 metres across. All of these considerations helped spur the launch of highly efficient automated systems that consist of Charge-Coupled Device (CCD) cameras and computers directly connected to telescopes. Since 1998, a large majority of the asteroids have been discovered by such automated systems. A list of teams using such automated systems includes [http://neo.jpl.nasa.gov/programs]:
- The Lincoln Near-Earth Asteroid Research (LINEAR) team
- The Near-Earth Asteroid Tracking (NEAT) team
- Spacewatch
- The Lowell Observatory Near-Earth-Object Search (LONEOS) team
- The Catalina Sky Survey (CSS)
- The Campo Imperatore Near-Earth Objects Survey (CINEOS) team
- The Japanese Spaceguard Association
- The Asiago-DLR Asteroid Survey (ADAS) The LINEAR system alone has discovered 50,484 asteroids as of May 24, 2005 [http://cfa-www.harvard.edu/iau/lists/MPDiscSites.html]. Between all of the automated systems, 3353 near-Earth asteroids have been discovered [http://cfa-www.harvard.edu/iau/lists/Unusual.html] including over 600 more than 1 km in diameter.

Naming asteroids

The naming format

Newly discovered asteroids are given a provisional designation consisting of the year of discovery and an alphanumeric code, such as 2001 FH. When its orbit is confirmed, it is given a number, and later may also be given a name (e.g. 1 Ceres). The formal naming convention uses parentheses around the number (e.g. (433) Eros), however, dropping the parentheses is quite common. Informally, especially when a name is repeated in running text, it is common to drop the number altogether, or to drop it after the first mention.

Unnamed asteroids

Unnamed asteroids that have been given a number keep their provisional designation, e.g. (29075) 1950 DA. As modern discovery techniques have discovered vast numbers of new asteroids, they are increasingly being left unnamed. The first asteroid to be left unnamed was (3360) 1981 VA. On rare occasions, an asteroid's provisional designation may become used as a name in itself: the still unnamed (15760) 1992 QB₁ gave its name to a group of asteroids which became known as cubewanos.

Sources for names

The first few asteroids were named after figures from Graeco-Roman mythology, but as such names started to run out, others were used —famous people, literary characters, the names of the discoverer's wives, children, and even television characters. The first asteroid to be given a non-mythological name was 20 Massalia, named after the city of Marseilles. For some time only female (or feminized) names were used; Alexander von Humboldt was the first man to have an asteroid named after him, but his name was feminized to 54 Alexandra. This unspoken tradition lasted until 334 Chicago was named; even then, oddly feminised names show up in the list for years afterward. As the number of asteroids began to run into the hundreds, and eventually the thousands, discoverers began to give them increasingly frivolous names. The first hints of this were 482 Petrina and 483 Seppina, named after the discoverer's pet dogs. However, there was little controversy about this until 1971, upon the naming of 2309 Mr. Spock (which was not even named after the Star Trek character, but after the discoverer's cat who supposedly bore a resemblance to him). Although the IAU subsequently banned pet names as sources, eccentric asteroid names are still being proposed and accepted, such as 6042 Cheshirecat, 9007 James Bond, or 26858 Misterrogers. For a full list, see meanings of asteroid names.

Special naming rules

Asteroid naming is not always a free-for-all: there are some types of asteroid for which rules have developed about the sources of names. For instance Centaurs (asteroids orbiting between Saturn and Neptune) are all named after mythological centaurs, Trojans after heroes from the Trojan War, and trans-Neptunian objects after underworld spirits.

Asteroid symbols

The first few asteroids discovered were assigned symbols like the ones traditionally used to designate Earth, the Moon, the Sun and planets. The symbols quickly became ungainly, hard to draw and recognise. By the end of 1851 there were 15 known asteroids, each (except one) with its own symbol. The first four's main variants are shown here: :1 Ceres 1851 1851 1851 1851 :2 Pallas 1851 1851 :3 Juno 1851 1851 :4 Vesta 1851 1851 1851 Johann Franz Encke made a major change in the Berliner Astronomisches Jahrbuch (BAJ, "Berlin Astronomical Yearbook") for 1854. He introduced encircled numbers instead of symbols, although his numbering began with Astraea, the first four asteroids continuing to be denoted by their traditional symbols. This symbolic innovation was adopted very quickly by the astronomical community. The following year (1855), Astraea's number was bumped up to 5, but Ceres through Vesta would be listed by their numbers only in the 1867 edition. A few more asteroids (28 Bellona, 35 Leukothea, and 37 Fides) would be given symbols as well as using the numbering scheme. The circle would become a pair of parentheses, and the parentheses sometimes omitted altogether over the next few decades. For details, see James L. Hilton, 2001, [http://aa.usno.navy.mil/hilton/AsteroidHistory/minorplanets.html When Did the Asteroids Become Minor Planets?].

Asteroid exploration

Until the age of space travel, asteroids were merely pinpricks of light in even the largest telescopes and their shapes and terrain remained a mystery. The first close-up photographs of asteroid-like objects were taken in 1971 when the Mariner 9 probe imaged Phobos and Deimos, the two small moons of Mars, which are probably captured asteroids. These images revealed the irregular, potato-like shapes of most asteroids, as did subsequent images from the Voyager probes of the small moons of the gas giants. gas giant The first true asteroid to be photographed in close-up was 951 Gaspra in 1991, followed in 1993 by 243 Ida and its moon Dactyl, all of which were imaged by the Galileo probe en route to Jupiter. The first dedicated asteroid probe was NEAR Shoemaker, which photographed 253 Mathilde in 1997, before entering into orbit around 433 Eros, finally landing on its surface in 2001. Other asteroids briefly visited by spacecraft en route to other destinations include 9969 Braille (by Deep Space 1 in 1999), and 5535 Annefrank (by Stardust in 2002). In September 2005, the Japanese Hayabusa probe started studying 25143 Itokawa in detail and will return samples of its surface to earth. Following that, the next asteroid encounters will involve the European Rosetta probe (launched in 2004), which will study 2867 Šteins and 21 Lutetia in 2008 and 2010. NASA is planning to launch the Dawn Mission in 2006, which will orbit both 1 Ceres and 4 Vesta in 2010-2014.

Asteroids in fiction and film

Understandably, most fictional depictions of asteroids focus on their potential risk of striking Earth. Representations of the asteroid belt in film tend to make it unrealistically cluttered with dangerous rocks; in reality asteroids, even in the main belt, are spaced extremely far apart.
- Professor Moriarty, Sherlock Holmes' arch-enemy, "is the celebrated author of "The Dynamics of an Asteroid", a book which ascends to such rarefied heights of pure mathematics that it is said that there was no man in the scientific press capable of criticizing it" (The Valley of Fear, 1914, set in 1888).
- In The Little Prince, a 1943 novel by Antoine de Saint-Exupéry, the title character lives on an asteroid named "B-6-12". The asteroid moon Petit-Prince was named after the character, and 46610 Bésixdouze after his asteroid.
- 'Catch that Rabbit', one of the short stories in Isaac Asimov's collection I, Robot (1950), takes place on an asteroid.
- The Japanese science fiction film The Mysterians aka Chikyu Boeigun (1957) reveals the solar system's asteroid belt as the remnants of the Mysterian's home planet, Mysteroid, after a nuclear war broke out.
- In Green Slime (1968), a masterpiece of B-movies, a rogue asteroid hurtles toward Earth. The astronauts leave Space Station Gamma 3 and place bombs on the asteroid, finding it inhabited by strange blobs of glowing slime that are drawn to the equipment. Unfortunately for everyone some of the slime was carried back on a space suit and soon evolves into tentacled creatures! See the review: [http://www.badmovies.org/movies/greenslime/]. The movie inspired the classic board game Awful Green Things from Outer Space.
- In the classic science-fiction movie 2001: A Space Odyssey (1968), the Discovery has a scientifically accurate "close approach" by a binary asteroid whilst en route to Jupiter. The scene simply cuts briefly to two lone rocks passing by the ship, with tens of thousands of kilometres to spare.
- The disaster movie Meteor (1979) depicts an asteroid named Orpheus hurtling toward Earth after its orbit is deflected by a comet.
- Atari released the arcade game Asteroids in 1979.
- In The Empire Strikes Back (1980), Han Solo escapes Imperial spacecraft by hiding the Millennium Falcon on an asteroid; The ship is then attacked by a vast monster that lives (inexplicably) within the asteroid in the vacuum of space.
- Arthur C. Clarke's novel 2061: Odyssey Three (1986) depicts a journey through the asteroid belt and its ominous parallels with the journey of the RMS Titanic.
- L. Neil Smith's novel Pallas (Tor Books, 1993) depicts a modernized hunting based life on the terraformed asteroid Pallas and introduces Emerson Ngu. The book was partly insired by the 1987 article "The Worst Mistake in the History of the Human Race" written by Jared Diamond. The book also includes a brief description of a way to encapsulate the entire surface of a small body such as an asteroid to enable creating an Earthlike environment.
- Arthur C. Clarke's novel The Hammer of God (1993) depicts mankind's efforts to stop an asteroid named Kali from hitting the Earth. The film Deep Impact (1998) was based on Clarke's novel, although in the movie, the asteroid becomes a comet.
- In the LucasArts game The Dig (originally released in 1995) and its novelization, the impact-threatening asteroid Attila turns out to be an alien probe.
- In the 1998 movie Starship Troopers, aliens launch an asteroid at Earth, completely wiping out Buenos Aires. This is the opening move in the war.
- The film Armageddon (1998) is also about efforts to stop an asteroid hitting Earth. Its representation of an asteroid (and of space travel in general) is deeply unrealistic.
- Ben Bova's novel series The Asteroid Wars (2001-2004) focuses on a war over the mining of the asteroid belt.
- An episode of the political television drama, The West Wing entitled "Impact Winter" included a subplot in which the White House staff prepared for a possible asteroid strike on the Earth. (First broadcast on December 15, 2004).

See also


- List of noteworthy asteroids
- List of asteroids
- List of asteroids named after important people
- List of asteroids named after places
- Meanings of asteroid names
- Near-Earth object
- Pronunciation of asteroid names
- Minor Planet Center
- Asteroid groups and families
- Asteroids

References


- McSween and McSween,

External links


- [http://www.armageddononline.org/asteroid.php Known Asteroid Impacts & Their Effects]
- [http://cfa-www.harvard.edu/iau/lists/MPNames.html Alphabetical list of minor planet names (ASCII)] (Minor Planet Center)
- [http://www.ipa.nw.ru/PAGE/DEPFUND/LSBSS/englenam.htm Alphabetical and numerical lists of minor planet names (Unicode)] (Institute of Applied Astronomy) (Warning: some designation here might be incorrect)
- [http://newton.dm.unipi.it/cgi-bin/neodys/neoibo Near Earth Objects Dynamic Site]
- [http://hamilton.dm.unipi.it/cgi-bin/astdys/astibo Asteroids Dynamic Site ]
- [http://quasar.ipa.nw.ru/PAGE/DEPFUND/LSBSS/statmpn.htm Asteroid naming statistics]
- [http://neat.jpl.nasa.gov/ Near Earth Asteroid Tracking (NEAT)]
- [http://www.spaceguarduk.com/ Spaceguard UK]
- [http://aa.usno.navy.mil/hilton/AsteroidHistory/minorplanets.html When Did the Asteroids Become Minor Planets?]
(asteroid navigator) | First asteroid | ...

- ko:소행성 ms:Asteroid ja:小惑星 simple:Asteroid th:ดาวเคราะห์น้อย zh-min-nan:Sió-he̍k-chheⁿ



Delta-v

In general physics, delta-v is simply the change in velocity. Depending on the situation delta-v can be referred to as a spatial vector (\Delta \mathbf\,) or scalar (\Delta\,). In both cases it is equal to the acceleration (vector or scalar) integrated over time: :\Delta \mathbf = \mathbf_1 - \mathbf_0 = \int^_ \mathbf \, dt (vector version) :\Delta = _1 - _0 = \int^_ \, dt (scalar version) where:
- \mathbf\, or \, is initial velocity vector or scalar at time t_0\,,
- \mathbf\, or \, is target velocity vector or scalar at time t_1\,.

Astrodynamics

In astrodynamics delta-v is a scalar measure for the amount of "effort" needed to carry out an orbital maneuver, i.e., to change from one orbit to another. A delta-v is typically provided by the thrust of a rocket engine. The time-rate of delta-v is the magnitude of the acceleration, i.e., the thrust per kilogram total current mass, produced by the engines. The actual acceleration vector is found by adding the gravity vector to the vector representing the thrust per kilogram. Without gravity, delta-v is, in the case of thrust in the direction of the velocity, simply the change in speed. However, in a gravitational field, orbits which are not circular involve changes in speed without requiring any delta-v, while gravity drag can cause the change of speed to be less than delta-v. When applying delta-v in the direction of the velocity and against gravity the specific orbital energy gained per unit delta-v is equal to the instantaneous speed. For a burst of thrust during which both the acceleration produced by the thrust, and the gravity, are constant, the specific orbital energy gained per unit delta-v is the mean value of the speed before and the speed after the burst. The rocket equation shows that the required amount of propellant can dramatically increase, and that the possible payload can dramatically decrease, with increasing delta-v. Therefore in modern spacecraft propulsion systems considerable study is put into reducing the total delta-v needed for a given spaceflight, as well as designing spacecraft that are capable of producing a large delta-v. For examples of the first, see Hohmann transfer orbit, gravitational slingshot; also, a large thrust reduces gravity drag. For the second some possiblities are:
- staging
- large specific impulse
- since a large thrust can not be combined with a very large specific impulse, applying different kinds of engine in different parts of the spaceflight (the ones with large thrust for the launch from Earth).
- reducing the "dry mass" (mass without propellant) while keeping the capability of carrying much propellant, by using light, yet strong, materials; when other factors are the same, it is an advantage if the propellant has a high density, because the same mass requires smaller tanks. Delta-v is also required to keep satellites in orbit and is expended in orbital stationkeeping maneuvers.

Games

Delta-V is a trenchrunner game published by Bethesda Softworks in the 1980's

See also


- Delta-v budget
- Gravity drag
- Orbital maneuver
- Orbital stationkeeping
- Spacecraft propulsion
- Specific impulse
- Tsiolkovsky rocket equation Category:Astrodynamics Category:Celestial mechanics Category:Spacecraft propulsion

External link


- http://www.pma.caltech.edu/~chirata/deltav.html

Moon

:For other moons in the solar system see natural satellite. For the astrological meaning of the Moon, see Solar system in astrology. For other uses see Moon (disambiguation). The Moon is the planet Earth's only natural satellite. It has no formal name other than "The Moon", although it is occasionally called Luna (Latin for moon), or Selene, to distinguish it from the generic "moon" (natural satellites of other planets are also called moons). Its symbol is a crescent (Unicode: ☾). The terms lunar, selene/seleno-, and cynthion (from the Lunar deities Selene and Cynthia) refer to the Moon (aposelene, selenocentric, pericynthion, etc.). The average distance from the Moon to the Earth is 384,403 kilometers (238,857 miles). The Moon's diameter is 3,476 kilometers (2,160 miles). The first manmade object to land on the Moon was Luna 2 in 1959, the first photographs of the otherwise occluded far side of the Moon were made by Luna 3 that same year, and the first people to land on the Moon came aboard Apollo 11 in 1969.

The two sides

The far side is sometimes called the "dark side". In this case "dark" means "unknown and hidden" and not "lacking light" as percieved by the name; in fact the far side receives (on average) as much sunlight as the near side, but at opposite times. Spacecraft are cut off from direct radio communication with the Earth when on the far side of the Moon. One distinguishing feature of the far side is its almost complete lack of maria (singular: mare), which are the dark albedo features.

Orbit

The Moon makes a complete orbit about once every 28 days. Each hour the Moon moves relative to the stars by an amount roughly equal to its angular diameter, or by about 0.5°. The Moon differs from most satellites of other planets in that its orbit is close to the plane of the ecliptic and not in the Earth's equatorial plane. Several ways to consider a complete orbit are detailed in the table below, but the two most familiar are: the sidereal month being the time it takes to make a complete orbit with respect to the stars, about 27.3 days; and the synodic month being the time it takes to reach the same phase, about 29.5 days. These differ because in the meantime the Earth and Moon have both orbited some distance around the Sun. The gravitational attraction that the Moon exerts on Earth is the cause of tides in the sea. The tidal flow period, but not the phase, is synchronized to the Moon's orbit around Earth. The tidal bulges on Earth, caused by the Moon's gravity, are carried ahead of the apparent position of the Moon by the Earth's rotation, in part because of the friction of the water as it slides over the ocean bottom and into or out of bays and estuaries. As a result, some of the Earth's rotational momentum is gradually being transferred to the Moon's orbital momentum, resulting in the Moon slowly receding from Earth at the rate of approximately 38 mm per year. At the same time the Earth's rotation is gradually slowing, the Earth's day thus lengthens by about 15 µs every year. A more detailed discussion follows in the section titled Earth & Moon. The Moon is in synchronous rotation, meaning that it keeps the same face turned to the Earth at all times. This synchronous rotation is only true on average because the Moon's orbit has definite eccentricity. When the Moon is at its perigee, its rotation is slower than its orbital motion, and this allows us to see up to an extra eight degrees of longitude of its East (right) side. Conversely, when the Moon reaches its apogee, its rotation is faster than its orbital motion and reveals another eight degrees of longitude of its West (left) side. This is called longitudinal libration. Because the lunar orbit is also inclined to the Earth's equator, the Moon seems to oscillate up and down (as a person's head does when nodding) as it moves in celestial latitude (declination). This is called latitudinal libration and reveals the Moon's polar zones over about seven degrees of latitude. Finally, because the Moon is only at about 60 Earth radii distance, an observer at the equator who observes the Moon throughout the night moves by an Earth diameter sideways. This is diurnal libration and reveals about one degree's worth of lunar longitude. Earth and Moon orbit about their barycenter, or common center of mass, which lies about 4700 km from Earth's center (about 3/4 of the way to the surface). Since the barycenter is located below the Earth's surface, Earth's motion is more commonly described as a "wobble". When viewed from Earth's North pole, Earth and Moon rotate counter-clockwise about their axes; the Moon orbits Earth counter-clockwise and Earth orbits the Sun counter-clockwise. It may seem curious that the inclination of the lunar orbit and the tilt of the Moon's axis of rotation are listed as varying considerably. One must be reminded here that the orbital inclination is measured with respect to the primary's equatorial plane (in this case the Earth's), and that the axis of rotation's tilt is measured with respect to the normal to the satellite's orbital plane (the Moon's). For most planetary satellites, but not for the Moon, these conventions model physical reality and the values are therefore stable. The plane of the lunar orbit maintains an inclination of 5.145 396° with respect to the ecliptic (the orbital plane of the Earth), and the lunar axis of rotation maintains an inclination of 1.5424° with respect to the normal to that same plane. The lunar orbital plane precesses quickly (i.e. its intersection with the ecliptic rotates clockwise), in 6793.5 days (18.5996 years), mostly because of the gravitational perturbation induced by the Sun. During that period, the lunar orbital plane thus sees its inclination with respect to the Earth's equator (itself inclined 23.45° to the ecliptic) vary between 23.45° + 5.15° = 28.60° and 23.45° - 5.15° = 18.30°. Simultaneously, the axis of lunar rotation sees its tilt with respect to the Moon's orbital plane vary between 5.15° + 1.54° = 6.69° and 5.15° - 1.54° = 3.60°. Note that the Earth's tilt reacts to this process and itself varies by 0.002 56° on either side of its mean value; this is called nutation. The points where the Moon's orbit crosses the ecliptic are called the "lunar nodes": the North (or ascending) node is where the Moon crosses to the North of the ecliptic; the South (or descending) node where it crosses to the South. Solar eclipses occur when a node coincides with the new Moon; lunar eclipses when a node coincides with the full Moon.

Earth & Moon

The tides on Earth are generated by the Moon's gravitation (see tide and tidal force for a more detailed discussion). There are two tidal bulges, one in the direction of the Moon, and one in the opposite direction (figure 1). The buildup of these bulges and their movement around the earth causes an energy loss due to friction. The energy loss decreases the rotational energy of the Earth. Since the Earth spins faster than the Moon moves around it, the tidal bulges are dragged along with the Earth's surface faster than the Moon moves, and move "in front of the Moon" (figure 2). Because of this, the Earth's gravitational pull on the Moon has a component in the Moon's "forward" direction with respect to its orbit. This component of the gravitational forces between the two bodies acts like a torque on the Earth's rotation, and transfers angular momentum and rotational energy from the Earth's spin to the Moon's orbital movement. angular momentum Because the Moon is accelerated in forward direction, it moves to a higher orbit. As a result, the distance between the Earth and Moon increases, and the Earth's spin slows down (figure 3). Measurements reveal that the Moon's distance to the Earth increases by 38 mm per year (lunar laser ranging experiments with laser reflectors are used to determine this). Atomic clocks also show that the Earth's day lengthens by about 15 µs every year. However, the formation of tidal bulges on Earth is irregular and not directly related to the frictional energy loss which accompanies the tides. For example, continents on Earth may cause an increase in frictional energy losses and hamper the buildup of tidal bulges (figure 4). The energy loss of the Earth's spin (loss of rotational energy of the Earth) is related to both the energy transfer to the Moon, which depends on the geometry of the mass distributions on Earth (causing a gravity component which pulls the Moon forward), and also to frictional losses, which depends on the properties of the material moving around within tides. The transfer of angular momentum to the Moon's orbit, in contrast, depends only on the geometry of the mass distribution. In general, the angular momentum transferred to the Moon will not correspond to an equivalent energy transfer. There will be a surplus or a deficit in the transfer of angular momentum to the Moon, compared to the energy transfer (figure 5). Since both angular momentum and energy are conserved, there must be a mechanism on earth to store a surplus or a deficit of angular momentum. Candidates for this mechanism are the Earth's magnetic field and internal material currents of the Earth (figure 6). The lunar surface is also subjected to tides from earth, and rises and falls by around 10 cm over 27 days. The lunar tides comprise a mobile component, due to the Sun, and a selenographically fixed one, due to Earth (the Moon keeps the same face turned to the Earth, but not to the Sun). The vertical motion of the Earth-induced component comes entirely from the Moon's orbital eccentricity; if the Moon's orbit were perfectly circular, there would be solar tides only. The magnitude of the Moon's tides corresponds to a Love number of 0.0266, and supports the idea of a partially melted zone around its core. Moonquake waves lose energy below 1000 km depth, and this may also show that the deep material is at least partially melted. The Earth’s Love number is 0.3, corresponding to a movement of 0.5 metres per day; for Venus the Love number is also 0.3. (Source: Patrick Moore, The Data Book of Astronomy - June 2003 Updates)

Origin and history

magnetic field The inclination of the Moon's orbit makes it implausible that the Moon formed along with the Earth or was captured later; its origin is the subject of some scientific debate. Early speculation proposed that the Moon broke off from the Earth's crust due to centrifugal force, leaving an ocean basin (presumed to be the Pacific) behind as a scar. This concept requires too great an initial spin of the Earth. Others speculated the Moon formed elsewhere and was captured into its orbit. Two of the other theories include the coformation or condensation theory and the impact theory, which speculates that the Moon formed from the debris that resulted from a collision between the early Earth and a planetesimal. The Coformation or Condensation hypothesis posits that the Earth and the Moon formed together at about the same time from the primordial accretion disk, the Moon forming from material surrounding the coalescing proto-Earth, similar to the way the planets formed around the Sun. Some suggest that this hypothesis fails to adequately explain the depletion of iron in the Moon. Recently, the Giant Impact theory has been considered a more viable scientific theory for the moon's origin than the coformation or condensation theory. The Giant Impact theory holds that the Moon formed from the ejecta resulting from a collision between a semi-molten Earth and a planet-like object the size of Mars, which has been referred to as Theia. The geological epochs of the Moon are defined based on the dating of various significant impact events in the Moon's history. Analysis of craters and Moon rocks show that there was a late heavy bombardment by asteroids around the period 4000 to 3800 million years ago. Tidal forces deformed the once molten Moon into an ellipsoid, with the major axis pointed towards Earth.

Physical characteristics

Composition

More than 4.5 billion years ago, the surface of the Moon was a liquid magma ocean. Scientists think that one component of lunar rocks, KREEP (K-potassium, Rare Earth Elements, and P-phosphorus), represents the last chemical remnant of that magma ocean. KREEP is actually a composite of what scientists term "incompatible elements": those which cannot fit into a crystal structure and thus were left behind, floating to the surface of the magma. For researchers, KREEP is a convenient tracer, useful for reporting the story of the volcanic history of the lunar crust and chronicling the frequency of impacts by comets and other celestial bodies. The lunar crust is composed of a variety of primary elements, including uranium, thorium, potassium, oxygen, silicon, magnesium, iron, titanium, calcium, aluminium and hydrogen. When bombarded by cosmic rays, each element bounces back into space its own radiation, in the form of gamma rays. Some elements, such as uranium, thorium and potassium, are radioactive and emit gamma rays on their own. However, regardless of what causes them, gamma rays for each element are all different from one another — each produces a unique spectral "signature", detectable by a spectrometer. A complete global mapping of the Moon for the abundance of these elements has never been performed. However, some spacecraft have done so for portions of the Moon; Galileo did so when it flew by the Moon in 1992. [http://photojournal.jpl.nasa.gov/catalog/PIA00131] The overall composition of the Moon is believed to be similar to that of the Earth other than a depletion of volatile elements and of iron.

Selenography

1992 photo.]] When observed with earth based telescopes, the moon can be seen to have some 30,000 craters having a diameter of at least 1 kilometers, but close up observation from lunar orbit reveals a multitude of ever smaller craters. Most are hundreds of millions or billions of years old; the lack of atmosphere or weather or recent geological processes ensures that most of them remain permanently preserved. In the lunar terrae, it is indeed impossible to add a crater of any size without obliterating another; this is termed saturation. The largest crater on the Moon, and indeed the largest known crater within the solar system, forms the South Pole-Aitken basin. This crater is located on the far side, near the south pole, and is some 2,240 km in diameter, and 13 km in depth. The dark and relatively featureless lunar plains are called maria, Latin for seas, since they were believed by ancient astronomers to be water-filled seas. They are actually vast ancient basaltic lava flows that filled the basins of large impact craters. The lighter-colored highlands are called terrae. Maria are found almost exclusively on the Lunar nearside, with the Lunar farside having only a few scattered patches. Scientists think that this asymmetry of lunar features was caused by the synchronization between the Moon's rotation and orbit about the Earth. This synchronization exposes the far side of the Moon to more asteroid and meteor impacts than the near, thereby allowing the maria on the near side to remain relatively undisturbed for many hundreds of millennia. Blanketed atop the Moon's crust is a dusty outer rock layer called regolith. Both the crust and regolith are unevenly distributed over the entire Moon. The crust ranges from 60 km (38 mi) on the near side to 100 km (63 mi) on the far side. The regolith varies from 3 to 5 m (10 to 16 ft) in the maria to 10 to 20 m (33 to 66 ft) in the highlands. In 2004, a team led by Dr. Ben Bussey of Johns Hopkins University using images taken by the Clementine mission determined that four mountainous regions on the rim of the 73 km wide Peary crater at the Moon's north pole appeared to remain illuminated for the entire Lunar day. These unnamed "mountains of eternal light" are possible due to the Moon's extremely small axial tilt, which also gives rise to permanent shadow at the bottoms of many polar craters. No similar regions of eternal light exist at the less-mountainous south pole, although the rim of Shackleton crater is illuminated for 80% of the lunar day. Clementine's images were taken during the northern Lunar hemisphere's summer season, and it remains unknown whether these four mountains are shaded at any point during their local winter season.

Presence of water

Over time, comets and meteorites continuously bombard the Moon. Many of these objects are water-rich. Energy from sunlight splits much of this water into its constituent elements hydrogen and oxygen, both of which usually fly off into space immediately. However, it has been hypothesized that significant traces of water remain on the Moon, either on the surface, or embedded within the crust. The results of the Clementine mission suggested that small, frozen pockets of water ice (remnants of water-rich comet impacts) may be embedded unmelted in the permanently shadowed regions of the lunar crust. Although the pockets are thought to be small, the overall amount of water was suggested to be quite significant — 1 km³. Some water molecules, however, may have literally hopped along the surface and gotten trapped inside craters at the lunar poles. Due to the very slight "tilt" of the Moon's axis, only 1.5°, some of these deep craters never receive any light from the Sun — they are permanently shadowed. Clementine has mapped ([http://www.lpi.usra.edu/research/clemen/clemen.html]) craters at the lunar south pole ([http://www.lpi.usra.edu/research/clemen/2polar.gif]) which are shadowed in this way. It is in such craters that scientists expect to find frozen water if it is there at all. If found, water ice could be mined and then split into hydrogen and oxygen by solar panel-equipped electric power stations or a nuclear generator. The presence of usable quantities of water on the Moon would be an important factor in rendering lunar habitation cost-effective, since transporting water (or hydrogen and oxygen) from Earth would be prohibitively expensive. Clementine twisting the shadow due to the fact that cosmic rays are charged particles.]] The equatorial Moon rock collected by Apollo astronauts contained no traces of water. Neither the Lunar Prospector nor more recent surveys, such as those of the Smithsonian Institution, have found direct evidence of lunar water, ice, or water vapor. Lunar Prospector results, however, indicate the presence of hydrogen in the permanently shadowed regions, which could be in the form of water ice.

Magnetic field

Compared to that of Earth, the Moon has a very weak magnetic field. While some of the Moon's magnetism is thought to be intrinsic (such as a strip of the lunar crust called the Rima Sirsalis), collision with other celestial bodies might have imparted some of the Moon's magnetic properties. Indeed, a long-standing question in planetary science is whether an airless solar system body, such as the Moon, can obtain magnetism from impact processes such as comets and asteroids. Magnetic measurements can also supply information about the size and electrical conductivity of the lunar core — evidence that will help scientists better understand the Moon's origins. For instance, if the core contains more magnetic elements (such as iron) than Earth, then the impact theory loses some credibility (although there are alternate explanations for why the lunar core might contain less iron).

Atmosphere

The Moon has a relatively insignificant and tenuous atmosphere. One source of this atmosphere is outgassing — the release of gases, for instance radon, which originate deep within the Moon's interior. Another important source of gases is the solar wind, which is briefly captured by the Moon's gravity.

Eclipses

The angular diameters of the Moon and the Sun as seen from Earth overlap in their variation, so that both total and annular solar eclipses are possible. In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye. Since the distance between the Moon and the Earth is very slightly increasing over time, the angular diameter of the Moon is decreasing. This means that several million years ago the Moon always completely covered the Sun on solar eclipses so that no annular eclipses occurred. Likewise, in several million years the Moon will no longer cover the Sun completely and no total eclipses will occur. Eclipses happen only if Sun, Earth and Moon are lined up. Solar eclipses can only occur at new moon; lunar eclipses can only occur at full moon. See also Solar eclipse and Lunar Eclipse.

Observation of the Moon

Lunar Eclipse During the brightest full moons, the Moon can have an apparent magnitude of about −12.6. For comparison, the Sun has an apparent magnitude of −26.8. The Moon appears larger when close to the horizon. This is a purely psychological effect (see Moon illusion). The angular diameter of the Moon from Earth is about one half of one degree. Various lighter and darker colored areas (primarily maria) create the patterns seen by different cultures as the Man in the Moon, the rabbit and the buffalo, amongst others. Craters and mountain chains are also prominent lunar features. From any location on Earth, the highest altitude of the Moon on a day varies between the same limits as the Sun, and depends on season and lunar phase. For example, in winter the Moon is highest in the sky when it is full, and the full moon is highest in winter. The orientation of the Moon's crescent side also depends on the latitude of the observing site. Close to the equator an observer can see a boat Moon. [http://curious.astro.cornell.edu/question.php?number=393] Like the Sun, the Moon can also give rise to an optical effect known as a halo. For more information on how the Moon appears in Earth's sky, see Lunar phase.

Exploration of the Moon

Lunar phase prepares to descend towards the surface of the Moon. NASA photo.]] NASA standing next to boulder at Taurus-Littrow during third EVA (extravehicular activity). NASA photo.]] The first leap in Lunar observation was caused by the invention of the telescope. Especially Galileo Galilei made good use of this new instrument and observed mountains and craters on the Moon's surface. The Cold War-inspired space race between the Soviet Union and the United States of America led to an acceleration. What was the next big step is politically laden. In the US (and the West in general) the landing of the first humans on the moon in 1969 is seen as a culmination, indeed of the space race in general. But from a scientific point of view the first photographs of the until then unseen far side of the moon in 1959 constituted the second leap in Lunar observation. 1959 and Luna missions]] The first man-made object to reach the Moon was the unmanned Soviet probe Luna 2, which made a hard landing on September 14, 1959, at 21:02:24 Z. The far side of the Moon was first photographed on October 7, 1959 by the Soviet probe Luna 3. Luna 9 was the first probe to soft land on the Moon and transmit pictures from the Lunar surface on February 3, 1966. It was proven that a lunar lander would not sink into a thick layer of dust, as had been feared. The first artificial satellite of the Moon was the Soviet probe Luna 10 (launched March 31, 1966). The first robot lunar rover to land on the Moon was the Soviet vessel Lunokhod 1 on November 17 1970 as part of the Lunokhod program. On December 24, 1968 the crew of Apollo 8, Frank Borman, James Lovell, and William Anders became the first human beings to see the far side of the Moon with their own eyes (as opposed to seeing it on a photograph). Humans first landed on the Moon on July 20, 1969. The first man to walk on the lunar surface was Neil Armstrong, commander of the American mission Apollo 11. The last man to stand on the Moon was Eugene Cernan, who as part of the mission Apollo 17 walked on the Moon in December 1972. See also: A full list of lunar astronauts. Moon samples have been brought back to Earth by three Luna missions (nrs. 16, 20, and 24) and the Apollo missions 11 through 17 (minus Apollo 13, which almost ended in a disaster). On January 14 2004, US President George W. Bush called for a plan to return manned missions to the Moon by 2020. NASA's [http://www.nasa.gov/missions/solarsystem/cev.html plan] to accomplish that goal was announced on March 19 2005, and was promptly dubbed Apollo 2.0 by critics. The European Space Agency has plans to launch probes to explore the Moon in the near future, too. European spacecraft Smart 1 was launched September 27, 2003 and entered lunar orbit on November 15 2004. It will survey the lunar environment and create an X-ray map of the Moon. [http://news.bbc.co.uk/2/hi/science/nature/2818551.stm] [http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=36091] The People's Republic of China has expressed ambitious plans for exploring the Moon and is investigating the prospect of lunar mining, specifically looking for the isotope Helium-3 for use as an energy source on Earth [http://space.com/missionlaunches/china_moon_030304.html]. Japan has two planned lunar missions, LUNAR-A and Selene; even a manned lunar base is planned by the Japanese Space Agency (JAXA). India will also try an unmanned orbiting satellite, called Chandrayan. From the mid-1960's to the mid-1970's there were 65 moon landings (with 10 in 1971 alone), but after Luna 24 in 1976 it suddenly stopped. The Soviet Union started focusing on Venus and space stations and the US on Mars and beyond. In 1990 Japan visited the moon with the Hiten spacecraft, becoming the third country to orbit the moon. The spacecraft released the Hagormo probe into lunar orbit, but the transmitter failed rendering the mission scientifically useless.

Human understanding of the Moon

Myth and folk culture

The Moon as muse

The Moon has been the subject of many works of art and literature and the inspiration for countless others.

Astrology

Scientific understanding

A 5,000 year old rock carving at Knowth, Ireland may represent the Moon, which would be the earliest depiction discovered. In many prehistoric and ancient cultures, the Moon was thought to be a deity or other supernatural phenomenon. Among the first in the Western world to offer a scientific explanation for the Moon was the Greek philosopher Anaxagoras, who reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former. His atheistic view of the heavens was one cause for his imprisonment and eventual exile. By the Middle Ages, before the invention of the telescope, more and more people began to recognize the Moon as a sphere, though they believed that it was "perfectly smooth". sphere In 1609, Galileo Galilei drew one of the first telescopic drawings of the Moon in his book Sidereus Nuncius and noted that it was not smooth but had craters. Later in the 17th century, Giovanni Battista Riccioli and Francesco Maria Grimaldi drew a map of the Moon and gave many craters the names they still have today. Francesco Maria Grimaldi. Surprisingly, the Moon is actually brighter than the Sun at gamma ray wavelengths.]] On maps, the dark parts of the Moon's surface were called maria (singular mare) or "seas", and the light parts were called terrae or continents. The possibility that the Moon could contain vegetation and be inhabited by "selenites" was seriously considered by some major astronomers even into the first decades of the 19th century. In 1835, the Great Moon Hoax fooled some people into thinking that there were exotic animals living on the Moon. Almost at the same time however (during 1834–1836), Wilhelm Beer and Johann Heinrich Mädler were publishing their four-volume Mappa Selenographica and the book Der Mond in 1837, which firmly established the conclusion that the Moon has no bodies of water nor any appreciable atmosphere. There remained some controversy over whether features on the Moon could undergo changes. Some observers claimed that some small craters had appeared or disappeared, but in the 20th century it was determined that these claims were illusory, due to observing under different lighting conditions or due to the inadequacy of earlier drawings. It is however known that the phenomenon of outgassing occasionally occurs. During the Nazi era in Germany, the Welteislehre theory, which claimed the Moon was made of solid ice, was promoted by Nazi leaders. The far side of the Moon remained completely unknown until the Luna 3 probe was launched in 1959, and was extensively mapped by the Lunar Orbiter program in the 1960s. From the 1950s through the 1990s, NASA aerodynamicist Dean Chapman and others advanced the "lunar origin" theory of tektites. Chapman used complex orbital computer models and extensive wind tunnel tests to support the theory that the so-called Australasian tektites originated from the Rosse ejecta ray of the large crater Tycho on the Moon's nearside. Until the Rosse ray is sampled, a lunar origin for these tektites cannot be ruled out. In 1997 the asteroid 3753 Cruithne was found to have an unusual Earth-associated orbit, and has been dubbed by some to be a second "moon" of Earth. It is not considered a moon by astronomers, however, and its orbit is not stable in the long term.

Legal status

Though several flags of the United States have been symbolically planted on the moon, the U.S. government makes no claim to any part of the Moon's surface. The U.S. is party to the Outer Space Treaty, which places the Moon under the same jurisdiction as international waters (res communis). This treaty also restricts use of the Moon to peaceful purposes, explicitly banning weapons of mass destruction (including nuclear weapons) and military installations of any kind. A second treaty, the Moon Treaty, was proposed to restrict the exploitation of the Moon's resources by any single nation, but it has not been signed by any of the space-faring nations. Several individuals have made claims to the Moon in whole or in part, though none of these claims are generally considered credible (see Moon for sale).

Satellites


- Clementine mission - Observation and research satellite
- Smart 1 (or SMART-1) - a European Space Agency research satellite

Surface installations

Multiple scientific instruments were installed during the Apollo missions, some of them still function today. Among those were seismic detectors and reflecting mirrors for laser ranging. laser ranging laser ranging

See also


- Apollo moon landing hoax accusations
- Blue moon
- Chang'e (mythology), Chinese moon goddess
- Crescent
- Colonization of the Moon
- Detailed image of an almost full Moon
- Earthshine
- Lunar effect
- Lunar geologic timescale
- Lunar mare
- Lunar meteorite
- Lunar phase
- Moon landing
- Selene, Greek moon goddess
- Transient lunar phenomenon

Lunar location listings


- List of artificial objects on the Moon
- List of craters on the Moon
- List of features on the Moon
- List of maria on the Moon
- List of mountains on the Moon
- List of valleys on the Moon

References


- Ben Bussey and Paul Spudis, The Clementine Atlas of the Moon, Cambridge University Press, 2004, ISBN 0521815282.
- Patrick Moore, On the Moon, Sterling Publishing Co., 2001 edition, ISBN 0304354694.
- Paul D. Spudis, The Once and Future Moon, Smithsonian Institution Press, 1996, ISBN 1-56098-634-4.

External links

Moon phases


- [http://tycho.usno.navy.mil/vphase.html US Naval Observatory: phase of the Moon for any date and time 1800-2199 A.D.]
- [http://www.moonphaseinfo.com/ Current Moon Phase]
- [http://www.bapuli.co.nr/moon.htm Display current moon phase as wallpaper in Windows]

Space missions


- [http://www.lpi.usra.edu/research/lunar_orbiter/ Digital Lunar Orbiter Photographic Atlas of the Moon]
- [http://www.apolloarchive.com/apollo_archive.html The Project Apollo Archive]
- [http://www.cmf.nrl.navy.mil/clementine/clib/ Clementine Lunar Image Browser]

Scientific


- [http://www.solarviews.com/eng/moon.htm The Moon - by Rosanna and Calvin Hamilton]
- [http://seds.lpl.arizona.edu/nineplanets/nineplanets/luna.html The Moon - by Bill Arnett]
- [http://www.inconstantmoon.com Inconstant Moon - by Kevin Clarke]
- [http://www.moonsociety.org The Moon Society (non-profit educational site)]
- [http://cps.earth.northwestern.edu/GHM/ Geologic History of the Moon by Don Wilhelms]
- [http://isthis4real.com/orbit.xml Can you put the moon into orbit? An interactive simulation - (Needs Firefox 1.5)]

Myth and folklore


- [http://www.straightdope.com/classics/a2_337.html Do things get crazy when the moon is full? by Cecil Adams]
- [http://www.infoplease.com/spot/bluemoon1.html Once in a Blue Moon - What is a blue moon? by Ann-Marie Imbornoni]
- [http://www.suite101.com/article.cfm/folklore/10667 The Moon In Folklore - by Virginia Marin]
- [http://www.laputanlogic.com/articles/2004/04/05-0001.html The Rabbit in the Moon - by John Hardy]

Others


- [http://webgis.wr.usgs.gov/the_moon.htm USGS Planetary GIS webserver - the Moon]
- [http://www.perseus.gr/Astro-Lunar-Scenes-Apo-Perigee.htm The Moon at Apogee and Perigee] (striking photographic comparison)
- [http://www.perseus.gr/Astro-Lunar-Scenes-Sounion-01.htm The Full Moon Rising: I] (striking photo - NOT a composite)
- [http://www.perseus.gr/Astro-Lunar-Scenes-Sounion-02.htm The Full Moon Rising: II] (striking photo - NOT a composite)
- [http://www.perseus.gr/Astro-Lunar-Scenes-Sounion-03.htm The Full Moon Rising: III] (striking photo - NOT a composite)
- [http://www.straightdope.com/classics/a2_110.html Why does the Moon appear bigger near the horizon?] (from The Straight Dope)
- [http://www.badastronomy.com Bad Astronomy]: Dr. Philip Plait, an astronomy professor at Sonoma State University, California, runs this site to explain the many cases of incorrect astronomy (and physics) available to the public, including astrology and the Apollo moon landing hoax accusations.
- [http://www.lunarrepublic.com/atlas/index.shtml The Lunar Navigator: Interactive Maps Of The Moon] features free, interactive online access to maps of the Moon's surface
- [http://www.moonpeople.com A comprehensive guide to the Earth's Moon] (Includes a discussion forum)
- [http://www.traipse.com/earth_and_moon/index.html Distance from the Earth to the Moon, illustrated]
- [http://www.ibiblio.org//e-notes/VRML/Globe/Globe.htm 3D VRML Moon globe] zh-min-nan:Go̍eh-niû ko:달 ms:Bulan (satelit) ja:月 simple:Moon th:ดวงจันทร์

433 Eros

The asteroid 433 Eros (eer'-os) was named after the Greek god of love Eros. It is an S-type asteroid approximately 13 × 13 × 33 km in size, the second-largest near-Earth asteroid. It is also a Mars-crosser asteroid. Mars-crosser asteroid Eros was visited by the NEAR Shoemaker probe, which orbited it, taking extensive photographs of its surface, and then, on February 12 2001 at the end of its mission, landed on the asteroid's surface using only its maneuvering jets.

Physical characteristics

Surface gravity depends on the distance from a spot on the surface to the center of a body's mass. The Erotian surface gravity varies a lot, since Eros is not a sphere but an elongated peanut-shaped (or potato- or shoe-shaped) object. The daytime temperature on Eros hovers at about 100 °C and nighttime measurements at −150 °C. Eros's density is 2,400 kg/m3, about the same as the density of Earth's crust. It rotates once every 5.27 hours. NEAR scientists have found that most of the larger rocks strewn across Eros were ejected from a single crater in a meteorite collision perhaps 1 Ga (1 billion years) ago. This impact may also be responsible for the 40 percent of the Erotian surface that is devoid of craters smaller than 0.5 kilometers across. It was originally thought that the debris thrown up by the collision filled in the smaller craters. An analysis of crater densities over the surface indicates that the areas with lower crater density are within 9 kilometers of the impact point. Some of the lower density areas were found on the opposite side of the asteroid but still within 9 kilometers. It is theorized that seismic shockwaves propgated through the asteroid, shaking smaller craters into rubble. Since Eros is irregularly shaped, a 9 kilometer straight line through the asteroid can reach locations that would be further away if travelling across the surface, thus leading to the uneven pattern of crater density on the surface. (Thomas & Robinson, 2005)

Legal controversy

In an experimental legal case, Eros was claimed as property by Gregory W. Nemitz of OrbDev. According to the Homestead principle, Nemitz argued that he had the right to claim ownership of any celestial body that he made use of; he claimed he had designated Eros a spacecraft parking facility and wished to charge NASA a parking and storage fee of 20 cents per year for NEAR Shoemaker. Nemitz's case was dismissed and an appeal denied. [http://www.erosproject.com/appeal/apindex.html]

Aspects

Eros in fiction

Eros is also mentioned in Orson Scott Card's novel Ender's Game. It used to be an outpost for the aliens known as Formics who installed artificial gravity but was taken over by humans and a Command School was built there. This is where Ender was sent after he graduated from Battle School. 433 Eros also plays an important role in the future evolution of life on Earth in Stephen Baxter's novel Evolution. Millions of years after being perturbed into a new orbit, the asteroid collides with Earth, bringing about another mass extinction. The micrometeoroid-ravaged shell of NEAR Shoemaker still stands on the surface of Eros until seconds before the impact.

See also


- List of geological features on 433 Eros

References


- PMID 16034412

External links


- [http://near.jhuapl.edu/ NEAR Shoemaker spacecraft]
- [http://near.jhuapl.edu/iod/archive.html NEAR image of the day archive]
- [http://near.jhuapl.edu/iod/20010731/index.html Movie: NEAR Shoemaker spacecraft landing]
- [http://www.erosproject.com The Eros Project] (OrbDev's attempts at litigation over their property claim) Eros Eros ja:エロス (小惑星)

Near Earth Asteroid Rendezvous

The Near Earth Asteroid Rendezvous - Shoemaker (NEAR Shoemaker), renamed after its launch in honor of planetary scientist Eugene M. Shoemaker, is an unmanned spacecraft designed to study the near-Earth asteroid Eros from close orbit over a period of a year. The primary scientific objectives of NEAR were to return data on the bulk properties, composition, mineralogy, morphology, internal mass distribution and magnetic field of Eros. Secondary objectives include studies of regolith properties, interactions with the solar wind, possible current activity as indicated by dust or gas, and the asteroid spin state. These data will be used to help understand the characteristics of asteroids in general, their relationship to meteorites and comets, and the conditions in the early solar system. To accomplish these goals, the spacecraft was equipped with an X-ray/gamma ray spectrometer, a near infrared imaging spectrograph, a multi-spectral camera fitted with a CCD imaging detector, a laser rangefinder, and a magnetometer. A radio science experiment was also performed using the NEAR tracking system to estimate the gravity field of the asteroid. The total mass of the instruments was 56 kg, and they required 81 W power.

Mission profile


- Launch date/time: 1996-02-17 at 20:43:27 UTC
- On-orbit dry mass: 487 kg
- Nominal power output: 1800 W

Summary

The primary goal of the mission was to study the near Earth asteroid 433 Eros from orbit for approximately one year. Eros is an S-type asteroid approximately 13 × 13 × 33 km in size, the second largest near-Earth asteroid. Initially the orbit was circular with a radius of 200 km. The radius of the orbit was brought down in stages to a 50 × 50 km orbit on 30 April 2000 and decreased to 35 × 35 km on July 14