On 29 September 2015 the near-Earth object (NEO) 2015 SZ2 passed within roughly 500,00 km or 305,000 miles from Earth (that's only 1.3 times farther away than the Moon). At LCOGT, we monitored the NEO as it flew by. The movie shows images taken from our 1-m telescope in Chile during its closest approach, spanning about 45 minutes. Because of its very close proximity to the Earth, 2015 SZ2 traveled a distance across the sky of roughly 3.5 times the diameter of the Moon in that time!
2015 SZ2 is about as big across as the length of a blue whale (roughly 28 m or 90 ft wide) and is about 1.4 times larger than the meteor that hit Chelyabinsk, Russia on 15 February 2013. It is not uncommon for asteroids of this size to come close to the Earth, and quick detection and follow-up of such objects is important to determine the probability that they could hit us. 2015 SZ2 was brought to our attention by a new experimental system at NASA's Jet Propulsion Laboratory (JPL) that determines the impact probability for newly discovered objects in need of confirmation as NEOs. When the early orbit determination measurements showed that 2015 SZ2 was likely to come very close to Earth, our LCOGT team was able to preform follow-up observations within 2 hours of the initial alert. These observations helped confirm that 2015 SZ2 is in fact an NEO and were used to compute its closest approach date and distance.
With the images taken on 29 September 2015, the LCOGT team hopes to produce a lightcurve showing 2015 SZ2's brightness variation as it rotates as well as compute its rotation period. The small size of 2015 SZ2 along with its rotation period could tell us something about its material strength. Objects that spin very quickly (with a less than 2 hour rotation period) are likely to be made of solid rock and not a pile of pebbles that stick together by gravity since such a loosely bound object would likely break apart if spinning too quickly. Fast-spinning NEOs tend to be small in size (less than about 200 m or 650 ft in diameter), so 2015 SZ2 is a good candidate for such a determination. Early measurements point to a roughly 2 minute rotation period, which is not uncommon for NEOs as small as 2015 SZ2.
At LCOGT, we continue to monitor the NEO population through follow-up observations of objects discovered by asteroid surveys such as the Catalina Sky survey, the PanSTARRS(1) survey, and NEOWISE, particularly those objects that come close to Earth and those that need confirmation as NEOs. In the past 12 months, our follow-up observations have resulted in 11,900+ measurements on 2,300+ objects and confirmed 250+ new NEOs, 8 comets, and 690+ other objects from these and other surveys. Future plans include employing a way for citizen scientists to become involved in the work being done at LCOGT for follow-up and even discovery of new NEOs.