[meteorite-list] Small Asteroid 2014 AA Hits Earth

[Ron Baalke]

http://www.skyandtelescope.com/news/home/Small-Asteroid-2014-AA-Hits-Earth-238481431.html

Small Asteroid 2014 AA Hits Earth
Kelly Beatty
Sky & Telescope
January 2, 2014

Discovered on New Year's Eve by a telescope in Arizona, a small asteroid 
struck Earth somewhere over the Atlantic Ocean - apparently unnoticed 
- about 25 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 5:16 p.m. (1:16 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.

[Graphic]
Impact possibilities for 2014 AA	
This plot shows the range of possible locations where the small asteroid 
2014 AA struck Earth's atmosphere early on January 2, 2014.
Bill Gray / Project Pluto

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 over the Atlantic Ocean 
somewhere between Central America to East Africa. Chesley's "best-fit" 
collision is 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.

[Graphic]
Pinpointing 2014 AA's impact	
The overlap of the white curves, from three marginal infrasound detections, 
shows where the small asteroid 2014 AA likely hit. However, this preliminary 
plot does not take winds into account, which might shift the true impact 
point somewhat further east.
Peter Brown

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 
22 hours after Kowalski's initial report to the MPC, and it's giving me 
deja 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.

[Graphic]
Orbit of asteroid 2014 AA	
Based on images taken in the hours before its impact, asteroid 2014 AA 
averaged 110 million miles (175 million km) from the Sun in a low-inclination 
orbit that crossed paths with Mars and Earth. It was only a matter of 
time before it encountered our planet. Click on the image for an interactive 
version.
JPL Horizons

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 3 minutes - not only that 2014 
AA was going to strike Earth, but also roughly where and when. Mad props 
for that impressive number-crunching!