Near-Earth Asteroids

The majority of asteroids within our solar system reside in the Asteroid Belt, orbiting the sun between Mars and Jupiter. A large number of asteroid groupings, however, have their orbits elsewhere. This includes the Trojan asteroids, which orbit the sun in the Lagrangian points of Jupiter’s orbit; the Apollo asteroids, which cross Earth’s orbit; the Amor asteroids, which cross Mar’s orbit, and the Aten asteroids, which cross Venus’s orbit and approach from within Earth’s orbit (JPL). Vulcanoids have aphelia completely within the orbits of Mars or Venus, but as of this date no confirmed Vulcanoid asteroids have been found (Wikipedia.org). Alindas are in a 1:3 orbital resonance with Jupiter, resulting in a 1:4 orbital resonance with Earth. Most Alindas, due to their highly eccentric orbits and orbital resonance, pass near the inner planets (especially Earth) every 4 years (Wikipedia.org). Many but not all Alindas are also considered Atens.

Most known Aten asteroids have aphelion greater than 1AU and semi-major axes less than 1AU (within Earth’s orbit), however, some have their entire orbits (perihelion and aphelion) within the orbit of Earth. These subclasses of Aten asteroids are known as Apohele Asteroids (Wikipedia.com). The Apohele¹ – which is Hawaiian for “orbit” - class of asteroids are considered to be potentially dangerous to Earth as they cross within Earth’s orbit and are very difficult to find since they tend to be in the daylight sky most of the time (Lowell.edu. Wikipedia.org). While it is estimated that there may be as many as 50 or more Apohele asteroids, currently there are only two confirmed (Wikipedia.com, Lowell.edu).

The first Apohele, 1998DK36, never had its existence confirmed and is now considered lost. The two known, confirmed Apohele asteroids were both found in 2003 and 2004 (Wikipedia.com). Known as 2003CP20 and 2004JG6, both have their entire orbits within the orbit of Earth.

2003CP20, discovered in 2003 by the Lincoln Near-Earth Asteroid Research Project in Socorro, New Mexico, is about two kilometers in diameter (Wikipedia.org). While its orbit is within the orbit of Earth, it also crosses the orbit of Venus (but not “as close to the sun as Mercury”) (Wikipedia.org). It is one of the larger Aten-class asteroids, the largest of the Apohele-class, and orbits the sun in about 233 days (Wikipedia.org). 2003CP20’s orbit is very eccentric, with a perihelion of 0.5023 and aphelion of 0.9798 (NeoDys).

2004JG6, the only other known, confirmed Apohele-class asteroid, was discovered in May, 2004 by Brian A. Skiff of the Lowell Observatory (Lowell.edu). 2004JG6 has the shortest orbital period of any known object to date, rotating the Sun in 184 days, and other than Mercury, has the closest orbit to the Sun (Lowell.edu. NeoDys). 2004JG6 crosses the orbits of both Venus and Mercury, coming within two million miles of Mercury and 3.5 million miles of Earth (Lowell.edu). It is estimated to be between 500 meters and one kilometer in size (Lowell.edu). Like 2003CP20, its orbit is very eccentric with a perihelion of 0.297AU and aphelion of 0.973 (NeoDys).

While 2004JG6 and 2003CP20 are the only currently known Apohele-class asteroids, neither asteroid is considered a potential risk to Earth. There are, however, several Aten asteroids which are considered “Potentially Hazardous Objects.” In 2001, 1998WT24 came within 1,194,285 miles of Earth. Several times during the next ten years, 1998WT24 will approach Earth closer than the Moon (Wikipedia, NeoDys). In September 2004, 4179 Toutatis (an Aten-Alinda class) came within 955,428 miles of Earth. While other asteroids have come closer to Earth than these, they were of much smaller size and would not cause the potential world-wide damage that an asteroid the size of 1998WT24 could cause. (They would likely burn up harmlessly in the atmosphere). For example, in March 2004 a meteoroid known as 2004FU162 passed within 7,000 miles of Earth. Tracking 2004FU162 was lost, and it is not known if it burned up in the atmosphere or “glanced off” and continued past Earth (Wikipedia.org). Also in March 2004, meteoroid 2004FH passed within 26,875 miles of Earth and is expected to return (although not as closely) in 2044 (Wikipedia.org). Finally, 69230 Hermes, a binary asteroid “rediscovered” in 2003, is a recurring visitor to Earth (Wikipedia.org). Hermes has been estimated to come within 360,000 miles of Earth in 1937, 1942, 1954, 1974, 1986, and 2003 (NeoDys). 69230 Hermes is next expected to come within .028AU of Earth in 2040 (NeoDys).

As observation and tracking technology improves, there is no doubt that many more asteroids will be discovered at a record-setting pace. New classes of asteroids will likely be created, and improved methods of tracking the potentially hazardous asteroids will be found. Many organizations, observatories, governments and companies are currently involved in discovering and tracking asteroids not just for the purposes of pure knowledge, but also to hopefully find and prevent potentially Earth-impacting asteroids in time.

¹The word Apohele also has roots in Greek: “Apo” for outside and “heli” for sun (Lowell.edu).

Works Cited

JPL: Jet Propulsion Laboratory. “Near-Earth Asteroid Tracking.” Jet Propulsion Laboratory. 6 Dec. 2004. .

Wikipedia.org. “Apohole Asteroids.” Wikipedia.org. June 2004. 6 Dec. 2004. .

Also many sublinks of this page, including:

http://en.wikipedia.org/wiki/Category:Aten_asteroids

http://en.wikipedia.org/wiki/Aten_asteroid

http://en.wikipedia.org/wiki/69230_Hermes

http://en.wikipedia.org/wiki/List_of_noteworthy_asteroids#Record-setting_close_approaches_by_asteroids_to_Earth

http://en.wikipedia.org/wiki/List_of_noteworthy_asteroids

http://en.wikipedia.org/wiki/4179_Toutatis

http://en.wikipedia.org/wiki/%2833342%29_1998_WT24#Record_close_encounter

http://en.wikipedia.org/wiki/%2833342%29_1998_WT24

NeoDys: Near Earth Objects Dynamic Site. “NeoDys Object List.” Space Mechanics Group, Department of Mathematics, University of Pisa, Italy. 8 Dec. 2004. < http://newton.dm.unipi.it/cgi-bin/neodys/neoibo?objects:2003CP20;main>.

Lowell.edu. “LONEOS Discovers Asteroid With The Smallest Orbit.” Lowell Observatory. 20 May 2004. 7 Dec. 2004. < http://www.lowell.edu/press_room/releases/recent_releases/2004JG6_rls.html>.

Written for Professor Murch's Astronomy I class at Pikes Peak Community College, 9th December 2004.