[meteorite-list] Surprising Recent Discoveries of Three Large Near-Earth Objects

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 5 Nov 2013 18:16:46 -0800 (PST)
Message-ID: <201311060216.rA62Gl4R027967_at_zagami.jpl.nasa.gov>

http://neo.jpl.nasa.gov/news/news181.html

Surprising Recent Discoveries of Three Large Near-Earth Objects
Don Yeomans and Paul Chodas
NASA/JPL Near-Earth Object Program Office
November 5, 2013

Two surprisingly large Near-Earth Asteroids have been discovered in just
the last week or so, as well as a third moderately large asteroid which
surprisingly has also gone undetected until now, even though it can pass
close enough to the Earth to be classified as "potentially hazardous".
Not since 1983 has any near-Earth asteroid been found as large as the
approximately 20-kilometer (12-mile) size of the two new large ones. In
fact, there are only three other known near-Earth asteroids that are of
comparable size or larger than the two new large ones.

It is important to note that none of these three new large near-Earth
asteroids can come close enough to the Earth to represent a near-term
threat to our planet.

The first of the new large near-Earth asteroid discoveries is named 2013
UQ4, and it is perhaps the most unusual. This approximately 19-kilometer
(12-mile) wide object was spotted by the Catalina Sky Survey on Oct. 23
when the asteroid was 435 million kilometers (270 million miles) away
from Earth. Not only is this object unusually large, it follows a very
unusual highly inclined, retrograde orbit about the Sun, which means it
travels around the Sun in the opposite direction of all the planets and
the vast majority of asteroids.

The only objects usually found in retrograde orbits are comets, which
suggests that 2013 UQ4 may be the remains of an old comet that no longer
possesses the near-surface ices required for it to become active while
near the Sun. Comets that have exhausted most, or all, of their volatile
ices do not spew dust during sweeps through the inner-solar system like
their less-seasoned, more hyperactive space kin. Without the telltale
comet tails or atmospheres, dead comets look like, and in fact for all
practical purposes are, asteroids.

As reported on Circular No. 9262 of the International Astronomical
Union, MIT's Richard Binzel, David Polishook and Rachel Bowens-Rubin
observed this object on October 31 with NASA's 3-meter Infrared
Telescope Facility in Hawaii and determined this object belongs to the
so-called X-type spectral class and exhibits no obvious comet-like
activity. This implies about a 4 percent reflectivity, from which they
estimate a diameter of approximately 19 kilometers (12 miles).

The second very large near-Earth object, named 2013 US10, was discovered
on October 31 by the Catalina Sky Survey. While the reflectivity of this
object has not yet been determined, and hence its diameter is still
uncertain, it is also likely to be about 20 kilometers (12 miles) in
size. Only three near-Earth asteroids (1036 Ganymed, 433 Eros and 3552
Don Quixote) are of comparable size or larger.

Why has it taken so long to discover these large near-Earth asteroids?
The delay in discovering 2013 UQ4 is more easily understood because it
has a very long orbital period that has kept it out of the Earth's
neighborhood for centuries. But the delayed discovery of 2013 US10 is a
bit harder to explain, since current population models suggest that
almost all near-Earth asteroids of this size and orbit should have
already been found. A contributing factor may be that this object's
orbit does not allow it to get closer than 80 million kilometers (50
million miles) of the Earth's orbit, so the asteroid seldom gets close
enough to the Earth to become easily detectable. However, NASA-supported
telescopic surveys are now covering more sky and looking "deeper" than
they ever have before, and in fact, 2013 US10 was first detected where
it spends much of its time, well beyond the orbit of Jupiter.

The third of the recent discoveries is the approximately two-kilometer
near-Earth asteroid 2013 UP8, found on October 25 by the Pan-STARRS
group in Hawaii. This asteroid can approach quite close to the Earth's
orbit, within 5.5 million kilometers (3.4 million miles), which makes it
a "potentially hazardous asteroid" (PHA). 2013 UP8 is in the top 5th
percentile of the largest PHAs, most of which were found much earlier
during NASA's asteroid survey program. Like the other new discoveries,
this asteroid has gone undetected for a long time because it has not
approached the Earth closely for decades. But the increasingly capable
NASA-supported asteroid surveys finally found this object while it was
still at a large distance from the Earth, well beyond the orbit of Mars.

[Figure 1]
Figure 1: The orbits of near-Earth asteroids 2013 UQ4, 2013 US10 and
2013 UP8 are shown in a view looking down on the plane of the solar
system. While 2013 US10 and 2013 UP8 orbit the sun in a counter
clockwise direction (so called "direct" orbits like all the planets and
most asteroids), 2013 UQ4 orbits in a clockwise (retrograde) direction.
The positions of the asteroids and planets are shown for Nov. 5, 2013.
The positions of other large asteroids in the belt between the orbits of
Mars and Jupiter and the so-called Trojan asteroids that lead and follow
Jupiter in its path about the sun are also shown for that date, but
their sizes are not shown to scale so the density of these objects in
these areas is greatly exaggerated. The orbital periods for 2013 US10
and 2013 UP8 are 6.2 and 4.1 years respectively. The orbital period for
2013 UQ4 is currently very uncertain, but is likely to be a several
hundred years.
Image credit: NASA/JPL-Caltech

[Figure 2]
Figure 2: The orbits of 2013 UQ4, 2013 US10 and 2013 UP8 are shown as
viewed from within the plane of the solar system (ecliptic plane), which
makes clear their highly inclined orbits relative to Earth's orbit.
Image credit: NASA/JPL-Caltech
Received on Tue 05 Nov 2013 09:16:46 PM PST