Discovered on New Year's Eve by a telescope in Arizona, a small asteroid struck Earth somewhere over the Atlantic Ocean — apparently unnoticed — about 21 hours later.It was New Year's Eve, but that didn't stop observer Richard Kowalski from scanning the sky for near-Earth objects (NEOs).
He hadn't been using the 60-inch telescope on Arizona's Mount Lemmon for long when he noticed a 19th-magnitude blip skimming through northern Orion in a seven-image series begun at 11:18 p.m. MST (6:18 Universal Time on January 1st). After confirming that it was a new find, Kowalski dutifully submitted positions and times to the IAU's Minor Planet Center. Then he went back to the night's observing run.
Thus did the Mount Lemmon reflector, part of the Catalina Sky Survey, discover 2014 AA, the first asteroid found this year. But at the time neither Kowalski nor anyone else realized that the little intruder was only 300,000 miles (500,000 km) from Earth and closing fast.As announced by the MPC earlier today, it's "virtually certain" that 2014 AA hit Earth. According to calculations by dynamicist Stephen Chesley (Jet Propulsion Laboratory), the impact occurred somewhere between Central America and East Africa. Chesley's "best-fit" collision is over the Atlantic Ocean just off the coast of West Africa at roughly 2:30 Universal Time this morning.
More precision has come from an analysis of infrasound data by Peter Brown (University of Western Ontario). Infrasound is extremely low-frequency acoustic energy (20 hertz or less) created, for example, during energetic explosions. A global network of detectors, maintained by the Comprehensive Nuclear-Test-Ban Treaty Organization, can pinpoint the location and energy of any powerful detonation — including airbursts from meteoric blasts.According to Brown, 2014 AA triggered very weak detections at three infrasound stations. His triangulation from those records, shown in the graphic at right, indicates that the space rock slammed into the atmosphere near 40° west, 12° north. That location, about 1,900 miles (3,000 km) east of Caracas, Venezuela, is far from any landmass.
"The energy is very hard to estimate with much accuracy — the signals are all weak and buried in noise," Brown explains. And yet, he adds, we're lucky that the event happened just after local midnight, when winds are calmest. "Had this occurred in
the middle of the day I doubt we would see any signals at all," he says.
Brown's rough guess is that the impact energy was equivalent to the explosive power of 500 to 1,000 tons of TNT — which, though powerful in human terms, implies the object was no bigger than a small car. "It was no Chelyabinsk," he says.
So 2014 AA was too small to reach the ground intact. But it must have created one heck of a fireball! The skies over the Atlantic were relatively clear last night. Alas, a search of ship- and plane-tracking websites turned up no vessels in that area — it seems that no one was positioned to witness 2014 AA's demise.
"I'm not aware of any visual sightings," says William Cooke of NASA's Meteoroid Environment Office in Huntsville, Alabama. "Looks like it was too far away from human eyes."
The impact occurred a little after 3h UT, Brown says. That's only about 19 hours after Kowalski's initial report to the MPC, and it's giving me déjà vu all over again. It's been just five years since another small asteroid called 2008 TC3 struck Earth over Sudan just 19 hours after its discovery by the same telescope.
The difference between these events is that astronomers had nearly a day of advance warning regarding the 2008 impact. Telescopes worldwide amassed hundreds of observations before the object slammed into the atmosphere, and eventually many fragments were recovered.There was no heads-up alert this time. "I'm kicking myself for not having spotted this," admits amateur NEO sleuth Bill Gray (Project Pluto). Most mornings Gray downloads "and yes, for me, it was holiday-related."
Most mornings, he downloads the circumstances for recent discoveries and computes "what ifs" for potential impactors and near-misses. "However, on New Year's Day, I'd made arrangements to go with my family to visit my sister, go for a walk, stop off for a doughnut, shovel snow, etc., etc." He didn't realize an impact was imminent until last night — only a couple of hours before the impact.
Let's cut Gray some slack and instead give him, Chesley, and Gareth Williams at the MPC a collective pat on the back. All three were able to conclude — based on just seven images taken within 69 minutes — not only that 2014 AA was going to strike Earth, but also roughly where and when. Mad props for that impressive number-crunching!