[meteorite-list] Small Asteroid 2014 AA Hits Earth

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 2 Jan 2014 20:34:49 -0800 (PST)
Message-ID: <201401030434.s034Yn0G026936_at_zagami.jpl.nasa.gov>

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!
Received on Thu 02 Jan 2014 11:34:49 PM PST