|Discovery site||Haleakala Obs.|
|Discovery date||24 January 2011|
|(471240) 2011 BT15|
|NEO · PHA · Apollo|
|Epoch 4 September 2017 (JD 2458000.5)|
|Uncertainty parameter 0|
|Observation arc||9.63 yr (3,519 days)|
|1.47 yr (537 days)|
|0° 40m 13.08s / day|
|Earth MOID||0.0008 AU · 0.3 LD|
|0.136 km (calculated)|
|Mass||4.9×109 kg (assumed)|
(471240) 2011 BT15, provisional designation 2011 BT15, is a stony, sub-kilometer sized asteroid and fast rotator, classified as a near-Earth object and potentially hazardous asteroid of the Apollo group. It had been one of the objects with the highest impact threat on the Palermo Technical Impact Hazard Scale.
It was discovered on 24 January 2011, by a team of astronomers at Pan-STARRS, the Panoramic Survey Telescope and Rapid Response System at Haleakala Observatory on Hawaii, United States. The discovery was made using a 1.8-meter Ritchey–Chrétien telescope. At the time of discovery, the object had an apparent magnitude of 22.
Before the 2013 recovery it had an observation arc of 41 days with an uncertainty parameter of 7. Due to precovery images from 2007 it now has an observation arc of more than 5 years. It makes close approaches to Earth and Mars.
While listed on the Sentry Risk Table, virtual clones of the asteroid that fit the uncertainty region in the known trajectory showed a 1 in 71,000 chance that the asteroid could impact Earth on 5 January 2080.
With a 2080 Palermo Technical Scale of −3.58, the odds of impact by 2011 BT15 in 2080 were about 3800 times less than the background hazard level of Earth impacts which is defined as the average risk posed by objects of the same size or larger over the years until the date of the potential impact. JPL Horizons shows that the nominal pass will be on 17 January 2080 at a distance of 0.125 AU (18,700,000 km; 11,600,000 mi) from Earth.
In January 2014, a rotational lightcurve of 2011 BT15 was obtained from photometric observations by American astronomer Brian Warner at the CS3-Palmer Divide Station (U82) in California. Lightcurve analysis gave a well-defined rotation period of 0.109138 hours (393 seconds) with a brightness amplitude of 0.61 magnitude (U=3).
According to the CALL and JPL's impact-risk table, this near-Earth object measures 136 and 150 meters, respectively. For its size estimate, CALL uses a standard for stony asteroids of 0.20 with an absolute magnitude of 21.7.