 | |
| Discovery[1] | |
|---|---|
| Discovered by | LINEAR |
| Discovery site | Lincoln Lab ETS |
| Discovery date | 11 February 2003 |
| Designations | |
| (163693) Atira | |
| Pronunciation | /əˈtɪrə/ Pawnee:[ətíɾəʔ] |
Named after | Atíraʼ "my/our mother"[1](Pawnee epithet of the earth goddess)[2] |
| 2003 CP20 | |
| Orbital characteristics[3] | |
| Epoch 23 March 2018 (JD 2458200.5) | |
| Uncertainty parameter 0 | |
| Observation arc | 14.21 yr (5,192 d) |
| Aphelion | 0.9798 AU |
| Perihelion | 0.5024 AU |
| 0.7411 AU | |
| Eccentricity | 0.3221 |
| 233 days | |
| 30.769° | |
| 1° 32m 41.64s / day | |
| Inclination | 25.618° |
| 103.90° | |
| 252.93° | |
| Knownsatellites | 1[4][5][6][7] |
| Earth MOID | 0.2076 AU (80.88 LD) |
| Physical characteristics | |
| 4.8±0.5 km[4][5][6][7] | |
| 3.3984±0.0006 h[7] | |
| 192°[8] | |
| −50°[8] | |
| 0.0231(derived)[7] | |
| S(assumed)[7] | |
| 16.3[3][7][9] | |
163693 Atira (/əˈtɪrə/;provisional designation2003 CP20) is a stonyasteroid, dwelling in the interior of Earth's orbit. It is classified as anear-Earth object. Atira is abinary asteroid, a system of two asteroids orbiting their commonbarycenter. The primary component with a diameter of approximately 4.8 kilometers (3 miles)[7] is orbited by aminor-planet moon that measures about 1 km (0.6 mi).[4] Atira was discovered on 11 February 2003, by astronomers with theLincoln Near-Earth Asteroid Research atLincoln Laboratory's Experimental Test Site nearSocorro,New Mexico, in theUnited States.[1][10] To date, it is the only confirmed binary asteroid that orbits inside Earth's orbit.[11]
It is the namesake and the first numbered body of theAtira asteroids, a new subclass of near-Earth asteroids, which have their orbits entirely within that of Earth and are therefore alternatively called Interior-Earth Objects (IEO).[3][12][13] As of 2019, there are only 36 known members of the Atira group of asteroids.[14] Atiras are similar to the larger group ofAten asteroids, as both are near-Earth objects and both have asemi-major axis smaller than that of Earth (< 1.0 AU). However, and contrary to Aten asteroids, theaphelion for Atiras is always smaller than Earth'sperihelion (< 0.983 AU),[15] which means that they do not approach Earth as close as Atens do in general. Atira has an Earthminimum orbit intersection distance of 0.2059 AU (30,800,000 km) or approximately 80.1lunar distances.[3]
Atira is aS-type asteroid and orbits the Sun at a distance of 0.5–1.0 AU once every 8 months (233 days). Its orbit has aneccentricity of 0.32 and aninclination of 26° with respect to theecliptic.[3] With a perihelion of 0.50 AU the body also classifies as aVenus-crosser – as Venus orbits the Sun at a distance of 0.72–0.73 AU – but does not get as close to the Sun as Mercury (which orbits between 0.31 and 0.47 AU). As noprecoveries were found,Atira'sobservation arc begins with its discovery observation in 2003.[1] It has arotation period of 3.3984 hours with a brightness variation of 0.36magnitude (U=2) and a very lowalbedo of 0.0231.[7]
With a diameter of 4.8 kilometers, Atira is one of the largest Near-Earth objects. Early estimates of its size ranged from 1 to 2 kilometers,[10] but those were based on an assumed higher albedo of 0.20. Its larger size and low albedo were discovered when Atira was imaged by radar in early 2017.[4] These radar images also revealed that Atira is a binary asteroid.
Atira came within 0.207 AU (31,000,000 km) from Earth in January 2017, the closest since its discovery in 2003.[3] This provided an opportunity to study the asteroid byradar. Images taken atArecibo Observatory on 20 January 2017 revealed that Atira is a synchronousbinary asteroid with aminor-planet moon.[4][6] The primary with a diameter of4.8±0.5 km is possibly elongated and very angular in shape. The secondary is tidally locked and has a diameter of1.0±0.3 km. Additional images taken on 23 January 2017 showed that the two components are orbiting each other at a distance of about 6 km with an orbital period of 15.5 hours.[4][5] It is currently the only confirmed binary asteroid that orbits inside Earths orbit.[16]
Because the first known object in a new class of asteroids will become the name of the new class, due consideration was given to the name for (163693). The other classes of near-Earth asteroids,Amors,Apollos, and Atens, are named after a Roman, Greek, and Egyptian god respectively, each of which begins with the letter 'A'. Atira follows this pattern, being named afterAtíraʼ [Pawnee pronunciation:[ətíɾəʔ]], an epithet of the Earth goddess of thePawnee people. Atíraʼ is the wife of the creator god, Tirawa, and goddess of Earth and the evening star.[1][17] The official naming citation was published by theMinor Planet Center on 22 January 2008 (M.P.C. 61768).[18]
