2010 GZ60 was originally estimated by JPL to be a near-Earth asteroid approximately 2 kilometers (1.2 miles) in diameter.[4] But is now known to be an asteroid from the inner region of the asteroid belt that does not get closer than 1.5 AU (220 million km) to Earth.[2]
Description
2010 GZ60 was observed for a period of 1.2 days by the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer (WISE) space telescope during 5–6 April 2010. The asteroid has not been observed since.[1] By mid November 2010 the uncertainty in the asteroid's position had grown to ±1 billion km. Based on the exceptionally poor orbit determination, the object was theoretically an Earth impactor because many impact solutions were compatible with the data.[4]2010 GZ60 missed several calculated impact dates.[4]
In February 2018, the NEOWISE team reanalyzed their data and found an additional detection from 5 April 2010 that extended the observation arc by about three hours.[5] As a result of this additional observation, the new orbit is closer to that of a main belt orbit and 2010 GZ60 was removed from the Sentry Risk Table on 23 February 2018.[6]
Size
Not only was the orbit of 2010 GZ60 poorly known, but also its size was largely uncertain. There was no ground-based photometry. Based on the WISE flux, it obtained a size estimate of 2 km, which should be seen as more of an upper limit. The asteroid is now known to be closer to 1 km in diameter.
Pre-2018 orbit solutions
Until February 2018, 2010 GZ60 was identified in only 14 images, and the observations spanned a very short observation arc of 11⁄4 days during 5–6 April 2010.[1][3] On 5 April 2010 the asteroid was estimated to have been 1.8 AU (270,000,000 km; 170,000,000 mi) from Earth with an uncertainty in the asteroid's distance of ±500 million km.[7] With perihelion (closest approach to the Sun) estimated at 1.16 ± 3.5 AU,[8] it was possible this asteroid barely comes inside the orbit of Jupiter which is located 5 AU from the Sun. However, due to the shortness of observations, the object's orbit was only known with the highest possible uncertainty parameter of 9[8] and an orbital note of E[1][a] thus the calculated orbital elements had a large margin of error.[8]
Using the same 14 observations, the 2017 orbit calculations showed 481 potential close approaches to Earth between 2017 and 2116, with a cumulative rating of −0.76 on the Palermo Technical Impact Hazard Scale, which was the highest for any object on NASA's Sentry Risk Table during 2017 and early 2018.[4] This high rating was a result of the object's estimated size and hundreds of potential close approaches to Earth, rather than any confidence in the prediction of any single virtual impactor.[4] As of February 2018[update], there are 144 Near-Earth objects known to have a diameter of at least 2 km.[9]2010 GZ60 was the largest object listed on the Sentry Risk Table.
2010 GZ60 missed 4 virtual impactor dates considered in the 2017 orbit calculations: 22 May 2017, 1 December 2017, 20 December 2017, and 8 January 2018.[4] The next potential close approach by 2010 GZ60 was to be on 17 December 2018 with the odds of an Earth impact being 1 in 91 million.[3]JPL Horizons nominal solution estimated that on 17 December 2018 the asteroid would be 1.9 AU (280,000,000 km; 180,000,000 mi) from Earth with a 3-sigma uncertainty of ±10 billion km.[7][b]
Among the potential close approaches, the one on 10 February 2027 had the highest impact risk with a Palermo Scale rating of −1.98, which was the third-highest for any object on the Sentry Risk Table in early 2018.[3] But the uncertainty in the geocentric distance on 10 February 2027 was ±17 billion km.[7] The odds of this asteroid impacting Earth on 10 February 2027 were 1 in 4.8 million.[3]
In 2017 the Minor Planet Center (MPC) used 11 of the 14 observations and listed 2010 GZ60 as a Mars-crosser with perihelion (closest approach to the Sun) at 1.37 AU.[1]
2018 orbit solution
In February 2018, the NEOWISE team reanalyzed their data and found an additional detection from 5 April 2010 that extended the observation arc by about three hours.[5] This new observation resulted in the JPL Small-Body Database lifting the nominal perihelion (closest approach to the Sun) from 1.2 AU[8] to 2.2 AU.[2] Using the still poorly constrained 21 February 2018 orbit solution,[2]JPL Horizons showed that on the discovery date of 5 April 2010 the asteroid is estimated to have been 2.2 AU (330,000,000 km; 200,000,000 mi) from Earth with an uncertainty in the asteroid's distance of ±350 million km and moving away from Earth at 13±41 km/s.[7] Perihelion (closest approach to the Sun) was estimated to have occurred around September 2010.[2] Aphelion (furthest distance from the Sun) was estimated to have occurred around February 2016. As of March 2018, the asteroid was estimated to be 7.5 AU (1.1 billionkm) from Earth.[c] Perihelion (closest approach to the Sun) may not occur again until August 2021.
Orbits computed with only a handful of observations can be unreliable. Short observation arcs can result in computer generated orbits rejecting some data unnecessarily. The 2018 JPL orbit determination shows a semi-major axis of 4.9 AU (albeit with a large 20 AU uncertainty) which would be near Jupiter's orbit.[2] Such an orbit would be unstable unless it was a Jupiter trojan. Jupiter trojans have (4.6 AU < semi-major axis < 5.5 AU; eccentricity < 0.3).[10] The 2018 nominal semi-major axis of 2010 GZ60 was just outside of the asteroid belt. As more observations came in, it was expected the orbit determination would be that of a main belt asteroid as they are by far the most numerous discovered objects in the solar system and account for 94% of all known Small Solar System bodies.[d] Outer main belt asteroids have a semi-major axis less than 4.6 AU.[11] The orbital inclination is the easiest part of an orbit to determine and both JPL and the MPC list the inclination around 16.4 degrees.[2][1]
The Minor Planet Center's 2018 orbit solution used 12 of the 15 observations and also listed the asteroid as a main belt asteroid.[1] The new observation found in 2018 resulted in the MPC lifting perihelion (closest approach to the Sun) from 1.37 AU[1] to 1.98 AU.[1] The MPC solution estimated the asteroid came to perihelion 1.98 AU from the Sun around February 2009.[12] On the discovery date of 5 April 2010, the asteroid was estimated to have been 2.3 AU from Earth and 2.6 AU from the Sun. As of March 2018[update], the asteroid was estimated to be 1.8 AU (270 million km) from Earth with opposition occurring around 19 March 2018 with a solar elongation of 167 degrees in the constellation of Virgo.[12]
^ abcdJPL Horizons: 2010 GZ60 (Soln.date: 2017-Apr-08) Under "Table Settings" select "39. Range & range-rate". Uncertainty in distance (km) is RNG_3sigma
^ abcdJPL Small-Body Database Browser: (2010 GZ60) solution date: 2017-Apr-08 "2010 GZ60". Archived from the original on 30 January 2018. Retrieved 1 February 2018.{{cite web}}: CS1 maint: bot: original URL status unknown (link)