2012 Earth encounter

2012 TC₄ was discovered on 4 October 2012 at apparent i-band magnitude 20.1 while the asteroid was 0.03 AU (4,500,000 km; 2,800,000 mi) from Earth.^[3] It came within 0.000634 AU (0.247 LD, 94,800 km, 58,900 mi) from Earth on 12 October 2012.^[2]

During the 2012 close approach, the asteroid only had an observation arc of 7 days, between 4 and 11 October 2012, so the exact distance of the 2017 closest approach was poorly constrained. With the 7 day observation arc, the asteroid had a 3-sigma chance of passing between 0.00008818 and 0.002896 AU (0.034 to 1.127 LD, 13,200–433,200 km, 8,200–269,200 mi) from Earth on 12 October 2017.^[15] Astronomers were certain that it would not pass closer than 6,800 km from the surface of Earth, ruling out any possibility that it could hit the Earth in 2017.^[16]

2017 Earth encounter

Orbit in yellow among inner solar system planets before 2017 flyby

Orbit of 2012 TC₄ deflected by Earth's gravity. Earth is blue. For reference, a geosynchronous satellite and the Moon are included in this animation. The green path of 2012 TC₄ turns darker as it dips below the ecliptic plane (click for animation).

Orbits from pre-2012 to post-2017 encounters. The orbit of 2012 TC₄ has changed twice since 2012.

On 12 October 2017 at 5:42 UT, the asteroid passed 0.00033524 AU (50,151 km; 31,163 mi) from Earth.^[2] Observations between July and October reduced the uncertainty region from several hundred thousand kilometers^[15] to about ±140 meters.^[17][2] The asteroid was removed from the Sentry Risk Table on 16 October 2017 using JPL solution #56.^[11] Prior to the encounter, it was rated −4.11 on the Palermo scale, with a 1 in 1,000 chance of impact over the next hundred years.^[12]

Paul Chodas of NASA's Center for near-Earth Object Studies, and Vishnu Reddy of the University of Arizona's Lunar and Planetary Laboratory, viewed the 2017 flyby (inside of the orbit of the Moon) as a way to test and refine the global asteroid detection and tracking network designed to give warning of objects heading toward Earth.^[16] Reddy coordinated the effort, involving over a dozen institutions worldwide.^[16][18][19] In addition to the observation campaign, NASA used this exercise to test communications between the many observers and also to test internal U.S. government messaging and communications up through the executive branch and across government agencies, as it would during an actual predicted impact emergency. Results of the campaign were published on 3 November 2017.^[5]

The asteroid remained too faint to be recovered with automated astronomical surveys until early September,^[20] but a more targeted observation with the Very Large Telescope recovered it on 27 July 2017 at apparent magnitude 26.8, while the asteroid was 0.4 AU (60,000,000 km; 37,000,000 mi) from Earth, making it one of the dimmest asteroid recoveries ever. As such, 2012 TC₄ has become the first known asteroid ever to be observed passing less than 1 Lunar distance from Earth twice in a row.^[21] At the time of recovery the asteroid was about 100 million times fainter than what can be seen with the naked eye^[22] and 500 times fainter than when it was discovered in 2012. As a result of the 2017 recovery observations, it was known that on 12 October 2017 at 5:42 UT, the asteroid would pass 0.0003352 AU (50,150 km; 31,160 mi) from Earth.^[2] Then at 19:19 UT, the asteroid would pass 0.001856 AU (277,700 km; 172,500 mi) from the Moon.^[2] 2012 TC₄ peaked at about apparent magnitude 12.9,^[23] and was too faint to be seen without a telescope. The Earth approach of 2017 increased the asteroid's orbital period from 1.67 years to 2.06 years.^[24]

2012 TC₄ reached a maximum apparent magnitude of 12.9 just prior to its closest approach, soon after which it came too close to the Sun to be seen with telescopes.^[23] It was last observed on 14 December 2017 at an apparent magnitude of 25,^[4]

Fast rotator and tumbler

Studies of the asteroid's light curve in October 2012, found it to have a rotation period of 0.2038 hours (or 12 minutes and 14 seconds) with a brightness variation of 0.93 magnitude (U=3-), which is indicative for a non-spherical shape.^{[9][lower-alpha 1]} 2012 TC₄ is a fast rotator, which is rather typical for its small size. The fastest rotator currently known is 2014 RC, a similarly sized NEO, with a period of only 16 seconds. Lightcurves obtained during the 2017 encounter confirmed that 2012 TC₄ is in a non-principal axis rotation, commonly known as tumbling.^[7][5] The spin axis varies on timescales of minutes, with a second period of 0.142 hours (or 8.5 minutes).^[8] The lightcurve amplitude suggests a ratio of largest to smallest axis of at least 2.3.^[7]

Radar observations

Radar images were taken from Goldstone Observatory and Green Bank Telescope on 12 October 2017. The delay-doppler images had a range resolution of 1.9 meters/pixel, the highest resolution ever obtained using Goldstone transmissions.^[6][1] The images showed that 2012 TC₄ was a very elongated object about 50 feet (15 meters) long and roughly 25 feet (8 meters) wide.^[5] The high circular polarization ratio found for 2012 TC₄ is consistent with results seen from E- and V-type NEAs previously.^[6] Observations from Arecibo Observatory were planned, but had to be cancelled due to damage to the observatory as a result of Hurricane Maria.^[25]

Composition

The spectrum of 2012 TC₄ is that of an E- or Xe-type asteroid.^[7][6] E-type asteroids tend to have a high albedo (>0.30). This agrees with the albedo of 0.35 found for 2012 TC₄.^[1] This type of asteroids is commonly found in the inner Main Belt.^[26]

2012 TC₄ is composed of igneous material.^[5] The short rotation period of 2012 TC₄ implies that it is not a rubble pile but rather a monolithic object of non-negligible strength, which is typical for very small asteroids.^[7]