(725095) 2008 TN208
was originally estimated by JPL to be a near-Earth asteroid approximately in diameter. But is now known to be an asteroid from the inner region of the asteroid belt that does not get closer than to Earth.
Description
was observed for a period of 1.2 days by the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer space telescope during 5–6 April 2010. The asteroid has not been observed since. 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. missed several calculated impact dates.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. As a result of this additional observation, the new orbit is closer to that of a main belt orbit and was removed from the Sentry Risk Table on 23 February 2018.
Size
Not only was the orbit of 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, was identified in only 14 images, and the observations spanned a very short observation arc of 1 days during 5–6 April 2010. On 5 April 2010 the asteroid was estimated to have been from Earth with an uncertainty in the asteroid's distance of ±500 million km. With perihelion estimated at 1.16 ± 3.5 AU, 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 and an orbital note of E thus the calculated orbital elements had a large margin of error.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. 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., there are 144 Near-Earth objects known to have a diameter of at least 2 km. was the largest object listed on the Sentry Risk Table.
missed 4 virtual impactor dates considered in the 2017 orbit calculations: 22 May 2017, 1 December 2017, 20 December 2017, and 8 January 2018. The next potential close approach by was to be on 17 December 2018 with the odds of an Earth impact being 1 in 91 million. JPL Horizons nominal solution estimated that on 17 December 2018 the asteroid would be from Earth with a 3-sigma uncertainty of ±10 billion km.
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. But the uncertainty in the geocentric distance on 10 February 2027 was ±17 billion km. The odds of this asteroid impacting Earth on 10 February 2027 were 1 in 4.8 million.
In 2017 the Minor Planet Center used 11 of the 14 observations and listed as a Mars-crosser with perihelion at 1.37 AU.
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. This new observation resulted in the JPL Small-Body Database lifting the nominal perihelion from 1.2 AU to 2.2 AU. Using the still poorly constrained 21 February 2018 orbit solution, JPL Horizons showed that on the discovery date of 5 April 2010 the asteroid is estimated to have been from Earth with an uncertainty in the asteroid's distance of ±350 million km and moving away from Earth at. Perihelion was estimated to have occurred around September 2010. Aphelion was estimated to have occurred around February 2016. As of March 2018, the asteroid was estimated to be from Earth. Perihelion 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 which would be near Jupiter's orbit. Such an orbit would be unstable unless it was a Jupiter trojan. Jupiter trojans have. The 2018 nominal semi-major axis of 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. Outer main belt asteroids have a semi-major axis less than 4.6 AU. 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.
The Minor Planet Center's 2018 orbit solution used 12 of the 15 observations and also listed the asteroid as a main belt asteroid. The new observation found in 2018 resulted in the MPC lifting perihelion from 1.37 AU to 1.98 AU. The MPC solution estimated the asteroid came to perihelion 1.98 AU from the Sun around February 2009. 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., the asteroid was estimated to be from Earth with opposition occurring around 19 March 2018 with a solar elongation of 167 degrees in the constellation of Virgo.