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2062 Aten
2062 Aten is an asteroid that was discovered at the Palomar Mountain Observatory by Eleanor F. Helin, who is now the principal scientist for the NEAT (Near-Earth Asteroid Tracking) project. Aten was the first asteroid found to have a semi-major orbital axis of less than one astronomical unit. A new category of asteroids was thus created, the Atens, of which 16 are known and numbered, and some 212 awaiting numbering as of July 2004, ranging from (99907) 1989 VA to 2004 MD6.
Aten
Aten
ja:アテン (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 | ...
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ko:소행성
ms:Asteroid
ja:小惑星
simple:Asteroid
th:ดาวเคราะห์น้อย
zh-min-nan:Sió-he̍k-chheⁿ
Eleanor F. HelinEleanor Francis Helin is an American astronomer, who retired in 2002. She was principal investigator of the Near Earth Asteroid Tracking (NEAT) program of NASA's Jet Propulsion Laboratory. Some sources give her name as Eleanor Kay Helin.
She has discovered or co-discovered 863 asteroids, including notably the first two Aten asteroids 2062 Aten and 2100 Ra-Shalom; the Apollo asteroids 4660 Nereus, 4769 Castalia and others; various Amor asteroids; three Trojan asteroids including 3240 Laocoon; and also 9969 Braille.
She also discovered or co-discovered some comets, including periodic comets 111P/Helin-Roman-Crockett, 117P/Helin-Roman-Alu and 132P/Helin-Roman-Alu.
She is also credited as the discoverer of the object now known as both asteroid 4015 Wilson-Harrington and comet 107P/Wilson-Harrington. Although Wilson and Harrington preceded her by some decades, their observations did not establish an orbit for the object, while her rediscovery did.
Asteroid 3267 Glo is named for her ("Glo" is her nickname).
She has been active in planetary science and astronomy at the California Institute of Technology and the Jet Propulsion Laboratory for over three decades. In the early 1970s, she initiated the Palomar Planet-Crossing Asteroid Survey (PCAS) from Palomar Observatory. This program is responsible for the discovery of thousands of asteroids of all types including more than 200 in high inclination orbits, other rare and unique orbital types of asteroids, 20 comets, and approximately 30 percent of the near-earth asteroids discovered worldwide.
She organized and coordinated the International Near-Earth Asteroid Survey (INAS) during the 1980s, encouraging and stimulating worldwide interest in asteroids. In recognition of Ms. Helin's accomplishments, she has received NASA's Exceptional Service Medal. The 1997 JPL Award for Excellence was presented to Ms. Helin in recognition of her leadership of the Near-Earth Asteroid (NEAT) program. She has also received NASA's Group Achievement Award for the NEAT Team.
After conducting the PCAS photographic search program from Palomar for nearly 25 years, Ms. Helin concentrated on a new, upgraded search program using electronic sensors on a large aperture telescope: NEAT. She is the principal investigator for this program operating from JPL.
In operation since December 1995, NEAT is the first autonomous observing program; no JPL personnel are on-site, only the JPL Sunspark computer which runs the observing system through the night and transmits the data back to JPL each morning for team member review and confirmation. NEAT has detected over 26,000 objects, including 31 near-earth asteroids, two long period comets and the unique object, 1996 PW, the most eccentric asteroid known (e = 0.99012940), which moves in a long-period (4110.50 a), comet-like orbit (semi-major axis 256.601 AU).
External Links
[http://www.witi.com/center/witimuseum/womeninsciencet/1998/061298.shtml WITI Museum Women in Science & Technology Month 1998 June 12]
Eleanor Francis Helin]
Helin, Eleanor F.
Helin, Eleanor F.
Astronomical unitThe astronomical unit (AU or au or a.u. or sometimes ua) is a unit of distance, approximately equal to the mean distance between Earth and Sun. The currently accepted value of the AU is 149 597 870 691 ± 30 metres (about 150 million kilometres or 93 million miles).
The symbol "ua" is recommended by the Bureau International des Poids et Mesures [http://www.bipm.org/en/si/si_brochure/chapter4/table7.html], but in the United States and other anglophone countries the reverse usage is more common. The International Astronomical Union recommends "au" [http://www.iau.org/IAU/Activities/nomenclature/units.html] and international standard ISO 31-1 uses "AU".
The distance
Earth's orbit is not a circle but an ellipse; originally, the AU was defined as the length of the semimajor axis of said orbit. For greater precision, the International Astronomical Union in 1976 defined the AU as the distance from the Sun at which a particle of negligible mass, in an unperturbed circular orbit, would have an orbital period of 365.256 898 3 days (a Gaussian year). More accurately, it is the distance such that the heliocentric gravitational constant (the product GM☉) is equal to (0.017 202 098 95)² AU³/d².
At the time the AU was introduced, its actual value was very poorly known, but planetary distances in terms of AU could be determined from heliocentric geometry and Kepler's laws of planetary motion. The value of the AU was first estimated by Jean Richer and Giovanni Domenico Cassini in 1672. By measuring the parallax of Mars from two locations on the Earth, they arrived at a figure of about 140 million kilometers.
The first good measurement on the distance between Earth and the Sun was made by Eratosthenes in around 200 BC. By studying lunar eclipses, his result was 804 000 000 stadia. If we use the common Attic stadion this translates to roughly 150 million km.
A somewhat more accurate estimate can be obtained by observing the transit of Venus.
This method was devised by Edmond Halley, and applied to the transits of Venus observed in 1761 and 1769, and then again in 1874 and 1882.
Another method involved determining the constant of aberration, and Simon Newcomb gave great weight to this method when deriving his widely accepted value of 8.80" for the solar parallax (close to the modern value of 8.794 148").
The discovery of the near-Earth asteroid 433 Eros and its passage near the Earth in 1900–1901 allowed a considerable improvement in parallax measurement. More recently very precise measurements have been carried out by radar and by telemetry from space probes.
While the value of the astronomical unit is now known to great precision, the value of the mass of the Sun is not, because of uncertainty in the value of the gravitational constant. Because the gravitational constant is known to only five or six significant digits while the positions of the planets are known to 11 or 12 digits, calculations in celestial mechanics are typically performed in solar masses and astronomical units rather than in kilograms and kilometres. This approach makes all results dependent on the gravitational constant. A conversion to SI units would separate the results from the gravitational constant, at the cost of introducing additional uncertainty by assigning a specific value to that unknown constant.
It is known that the mass of the Sun is very slowly decreasing, and therefore the orbital period of a body at a given distance is increasing. This implies that the AU is getting smaller (by about one centimetre per year) over time.
Examples
The distances are approximate mean distances. It has to be taken into consideration that the distances between celestial bodies change in time due to their orbits and other factors.
- The Earth is 1.00 ± 0.02 AU from the Sun.
- The Moon is 0.0026 ± 0.0001 AU from the Earth.
- Mars is 1.52 ± 0.14 AU from the Sun.
- Jupiter is 5.20 ± 0.05 AU from the Sun.
- Pluto is 39.5 ± 9.8 AU from the Sun.
- 90377 Sedna's orbit ranges between 76 and 942 AU from the Sun; Sedna is currently (2005) about 90 AU from the Sun.
- As of November 2005, Voyager 1 (the farthest human-made object) is 97 AU from the Sun.
- The mean diameter of the Solar system, including the Oort cloud, is approximately 105 AU.
- Proxima Centauri (the nearest star) is ~268 000 AU away from the Sun.
- The mean diameter of Betelgeuse is 2.57 AU.
- The distance from the Sun to the centre of the Milky Way is approximately 1.7×109 AU.
Some conversion factors:
- 1 AU = 149 597 870.691 ± 0.030 km ≈ 92 955 807 miles ≈ 8.317 light minutes ≈ 499 light-seconds
- 1 light-second ≈ 0.002 AU
- 1 light-minute ≈ 0.120 AU
- 1 light-hour ≈ 7.214 AU
- 1 light-day ≈ 173 AU
- 1 light-year ≈ 63 241 AU
- 1 pc ≈ 206 265 AU
See also
- Conversion of units
- Light year
- Orders of magnitude
- Parsec
References
- E. Myles Standish. "Report of the IAU WGAS Sub-group on Numerical Standards". In Highlights of Astronomy, I. Appenzeller, ed. Dordrecht: Kluwer Academic Publishers, 1995. (Complete report available online: [http://ssd.jpl.nasa.gov/iau-comm4/iausgnsrpt.ps PostScript]. Tables from the report also available: [http://ssd.jpl.nasa.gov/astro_constants.html Astrodynamic Constants and Parameters])
- D. D. McCarthy ed., IERS Conventions (1996), IERS Technical Note 21, Observatoire de Paris, July 1996
External links
- [http://physics.nist.gov/cuu/Units/outside.html Units outside the SI] (at the NIST web site)
- [http://www.iau.org/IAU/Activities/nomenclature/units.html Recommendations concerning Units] (at the IAU web site)
- [http://home.comcast.net/~pdnoerd/SMassLoss.html Solar Mass Loss, the Astronomical Unit, and the Scale of the Solar System] (a discussion of the relation between the AU and other quantities)
- [http://www.ex.ac.uk/trol/scol/ccleng.htm Conversion Calculator for Units of LENGTH]
Category:Celestial mechanics
Category:Astronomical units of length
ko:천문 단위
ja:天文単位
th:หน่วยดาราศาสตร์
zh-min-nan:Thian-bûn tan-ūi
Aten asteroidThe Aten asteroids are a group of near-Earth asteroids, named after the first of the group to be discovered (2062 Aten, discovered January 7 1976 by Eleanor F. Helin). They have semi-major axes of less than one astronomical unit, placing them inside the orbit of Earth.
Nearly all known Aten asteroids have their aphelion greater than one AU. Those that have their aphelion entirely within the Earth's orbit are known as Apohele asteroids. As of May 2004 there are only two known Apoheles: and .
The smallest semi-major axis is that of , at 0.642 AU (its eccentricity of 0.688 takes it from a perihelion of 0.200 AU —well within Mercury's orbit!— to an aphelion of 1.084 AU), although seems to have an even smaller one (0.635 AU; eccentricity 0.532 ranging from 0.297 to 0.973 AU —enough to cross Venus' orbit but not Mercury's).
For a brief time near the end of 2004, the asteroid 99942 Apophis (then known only by its provisional designation ) appeared to pose a threat of causing an Earth impact event in 2029, but earlier observations were found that eliminated that possibility, although a very small possibility still exists for the years 2035 and 2036.
Related topics
- Aten asteroids (category)
- List of numbered Aten asteroids
- Aten asteroid records
External links
- [http://cfa-www.harvard.edu/iau/lists/Atens.html List of Aten Minor Planets]
-
(99907) 1989 VA(99907) 1989 VA is an Aten_asteroid located in Venus' zone of influence that has frequent close encounters with the Earth. It was discovered on November 2, 1989 by C. S. and E. M. Shoemaker at Mount Palomar and was the eighth Aten asteroid discovered.
1989 VA was the first asteroid discovered with such a small semi-major axis (0.729 au, about the same as Venus), breaking 2100 Ra-Shalom's distance record (0.832 au), which had held for over a decade. It remained the asteroid with the smallest known semi-major axis for five years until the discovery of 1994 GL (0.684 au), which was the first asteroid discovered closer to the Sun than Venus.
Being so close to Venus, it is also the first asteroid discovered within Venus' zone of influence. This means that it is close enough to Venus for the planet to capture 1989 VA into a co-orbital relationship. Though it is not a Venus co-orbital at the moment, it may become one in the future and may have been one in the past. Currently, the only known Venus co-orbital is 2002 VE68. Of the six known objects in Venus' zone of influence, 1989 VA is the largest at about 800 metres. All of these objects, like 1989 VA, have eccentric orbits that cross Mercury's and Earth's orbits.
1989 VA briefly held the title to Aten asteroid with the highest eccentricity (0.595), which was higher than the record the Earth co-orbital 3753 Cruithne (0.516) had set a few months earlier. 1991 VE (0.701) claimed that record two years later.
The combination of a small semi-major axis and high eccentricity made 1989 VA the first Aten asteroid discovered to get closer to the Sun (0.295 au) than Mercury ever does. 2340 Hathor (the second Aten discovered, in 1976) had the smallest perihelion (0.464 au) earlier, which was about the same distance as Mercury's aphelion (0.467 au). It was not until (66063) 1998 RO1 (0.277 au) was discovered that an Aten asteroid with a lower perihelion was found.
1989 VA's eccentric orbit takes it out past the Earth, where it has encounters of about 0.15 to 0.20 astronomical units about every 3 to 5 years around October-November. It was discovered during its 1989 encounter and was about 0.17 au away at the time. Its last observation was made during its October 2002 encounter. Its next close encounter will be in November of 2007.
1989 VA is an 800 meter diametre type Sq asteroid with a rotation period of 2.5 hours.
1989 VA
Category:Earth-crosser asteroids
Category:Near-Earth asteroids Category:SquaresoftSquare Co., Ltd. was a Japanese video game developer, most famous for its computer role-playing games, especially the Final Fantasy series.
On April 1, 2003, Square merged with its long-time competitor, Enix, and was absorbed into a new company, Square Enix. For all articles relating to Square Enix, please see :Category:Square Enix.
Category:Defunct computer and video game companies
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