[meteorite-list] Tiny Icy Objects Revealed In Outer Solar System

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Aug 10 12:08:15 2006
Message-ID: <200608101535.IAA15523_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn9708-tiny-icy-objects-revealed-in-outer-solar-system.html

Tiny icy objects revealed in outer solar system
David Shiga
New Scientist
09 August 2006

Dozens of small icy objects have been detected in the far reaches of the
solar system. Based on this sample, the objects appear far more abundant
than astronomers had thought.

The objects, between 20 and 100 metres in size, are the smallest known
members of a class called trans-Neptunian objects (TNOs), which reside
beyond the orbit of Neptune. TNOs are thought to be similar in
composition to comets, which are made mostly of ice, dust, and rock.

Most of the 1000 or so TNOs known are hundreds of kilometres across. The
smallest previously seen was a 25-kilometre wide object spotted by the
Hubble Space Telescope in 2003.

Now, astronomers have discovered 58 miniscule TNOs. They were discovered
by a team led by Hsiang-Kuang Chang of National Tsing Hua University in
Hsinchu, Taiwan.

The objects are much too small to show up in images, even with the most
powerful telescopes. Instead, the researchers detected them when they
passed in front of a distant X-ray source called Scorpius X-1, causing
sudden dips in its brightness.

Quadrillion objects

Chang's team found the objects by combing through 89 hours of archived
Scorpius X-1 observations from NASA's Rossi X-ray Timing Explorer (RXTE).

To calculate the sizes of the objects, the researchers had to assume
they are all 43 Astronomical Units from the Sun (1 AU the distance from
the Sun to Earth). Most of the X-ray dips lasted 2 or 3 milliseconds,
which they calculate would be caused by objects 50 metres across. Some
of the dips appear to be due to objects as small as 20 metres.

Based on the numbers they saw, they estimate that there are a
quadrillion (one million billion) TNOs of 100 metre size in the frigid
realm beyond Neptune.

There is some uncertainty in the numbers as it is possible that a
smaller, closer object could mimic the effect of a TNO. The most likely
candidate would be an object in the asteroid belt but the researchers
calculate that such passes in front of Scorpius X-1 would be very rare.

More resilient

However, it is harder to rule out larger, more distant objects causing
the dips.

"They make a convincing case that they're not closer than 40 AU, but
they could be at 100, 1000 or 10000 AU," says Scott Kenyon, of the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts,
US.

Chang admits that some of the objects could indeed be farther away: "But
our estimate of a quadrillion small bodies of 100 metre size will
probably not change by orders of magnitude."

If he is right, then it challenges the conventional thinking about TNOs.
The X-ray results suggest there are about 1000 times more 10 to 100
metre TNOs than the models predict.

The number of TNOs of different sizes astronomers expect to be out there
depends on how easily they break apart when they smash into each other.
The discrepancy could be resolved by assuming these smaller objects are
more resilient. "The objects that are tens of metres across would have
to be a lot stronger than we think they are," Kenyon says.

Journal reference: Nature (vol 442, p 660)
Received on Thu 10 Aug 2006 11:35:06 AM PDT