Asteroid 2015 TB145
© ShutterstockAsteroid 2015 TB145 (not pictured) will pass our planet on October 31, 2015 at approximately 1.3 times the distance from the Earth to the Moon.
An asteroid designated 2015 TB145 will pass by the Earth at around 1.3 lunar distances (approximately 310,000 miles or about 499,000 km) on October 31 this year. Estimated to be anywhere between 280 to 620 m (918 to 2,034 ft) in diameter and traveling in excess of 126,000 km/h (78,293 mph), the asteroid was discovered less than two weeks ago using the Pan-STARRS array in Hawaii and is the largest object to so closely approach our planet in recent times.

Before you start panicking, NASA says that the object is expected to safely pass by the Earth and is following an eccentric and high-inclination orbit, which may help explain why it was not discovered until October 10 of this year.

Much closer than a "near miss" of 3.1 lunar distances by another recent asteroid, 2004 BL86, 2015 TB145 is of a similar size or greater than that object, but without an accompanying moon. Nevertheless, this now qualifies as the latest known close encounter, after 2004 XP14 in July 2006 at 1.1 lunar distances, and until the asteroid 1999 AN10 swings by Earth at less than one lunar distance sometime in August 2027.

Though possessing an absolute maximum magnitude of around 19.9, with which astronomers were originally able to determine its size to within a factor of 2, the relative magnitude when observed from the Earth at its closest approach is expected to be less than 10. As such, the asteroid won't be visible with the naked eye, but should be observable using an amateur telescope of reasonable size and magnification.
Asteroid 2015 TB145_1
© GizmagIllustration showing the approximate path of asteroid 2015 TB145 as viewed from the central United States on the evening of 30 - 31 October 2015.
Expected to travel through the well-known constellation of Orion across the evening of October 30 and the early hours of October 31, California time, residents of the United States should be able to observe the transit of this object in the night sky as it appears to move slowly through the star field.

As unexpected as this encounter is, however, it also provides a perfect opportunity for many observatories to try out their imaging and radar-tracking capabilities before the next large rock encounter with 1999 AN10 in 2027. The radio astronomy installations at both DSN Goldstone, California and the Arecibo array in Puerto Rico intend to track and record the path of 2015 TB145 via radar, which should provide invaluable data for future near-Earth asteroid encounters (of which there are quite a few known according to NASA's Near-Earth Object Program).

This data may also help improve the earlier detection of close-encounter objects and enhance warning times, as well as contribute to the knowledge development of the proposed NASA Asteroid Redirect Mission that is designed to divert any such celestial object whose trajectory may put it on a collision course with our planet.

Source: NASA JPL