[meteorite-list] More Asteroids Pair Up
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:47:11 2004
More Asteroids Pair Up
Astronomers have found four more binary asteroid systems.
by Vanessa Thomas
November 10, 2001
October was a busy month for binary asteroid observers. Four teams using
various observation methods found that four asteroids, once thought
solitary, all have traveling companions.
The four asteroid pairs reside throughout the solar system - from Earth's
neighborhood to the Kuiper belt.
On October 29, a team led by William Merline of the Southwest Research
Institute announced that asteroid 617 Patroclus is the first known Trojan
binary. Discovered in 1906, Patroclus shares Jupiter's nearly 12-year trip
around the sun, following 57 degrees (or about one-sixth of Jupiter's orbit)
behind the giant planet. When Merline and his team observed Patroclus with
the 8.1-meter Gemini North Telescope on September 22, they found that there
were two near-equal objects in their field of view.
Astronomers, assuming that Patroclus was a single body, had estimated that
the asteroid was 141 kilometers across. "To get the same brightness with two
objects of equal size, they would be about 100 km diameter each," Merline
explains. However, one component is slightly (0.2 magnitude) brighter than
the other, so Merline imagines the partners' sizes differ by about 10 km.
While conducting follow-up observations of objects discovered by the Deep
Ecliptic Survey Team, astronomers aimed the Magellan Project's 6.5-meter
Baade Telescope at Kuiper belt object 2001 QT297. Images taken October 11
and 12 show the object attended by a fainter companion. Additional Magellan
observations last week confirm the two bodies are bound by gravity and
didn't just happen to be near one another in mid-October.
The smaller component is 0.55 magnitude fainter than the primary. Because
2001 QT297 is quite distant (about 44.5 astronomical units) and astronomers
don't know each object's albedo, it's difficult to estimate their sizes.
According to MIT astronomer Jim Elliot, who reported the discovery, the
larger component is likely between 150 and 350 km across. "If they have the
same albedo, the diameter of the larger one would be 1.3 times that of the
smaller one," he writes.
Closer to home, near-Earth asteroid 1998 ST27 also appears to have a smaller
companion. A team led by Lance Benner of the Jet Propulsion Laboratory used
the world's largest single-dish telescope to observe the asteroid in early
October. According to Benner, the team's best preliminary estimate from the
Arecibo radar observations places the primary's diameter between 500 and 600
meters and the secondary's under 100 m.
The fourth binary detected with radar since September last year, 1998 ST27
was no longer observable from Arecibo after October 11. Subsequent optical
images have so far failed to resolve the two components, but Benner's team
is confident of the discovery, having imaged the separated pair over several
days. "The binary nature of 1998 ST27 is unambiguous from the Arecibo images
... because we see the two objects separated by several kilometers and we
see their positions changing with time," Benner explained.
The latest binary announcement also concerned a near-Earth object.
Astronomers at the Ondrejov Observatory and in Colorado observed the
Apollo-type asteroid 2001 SL9 between October 11 and 21. They found that the
minor planet's light curve was composed of two periods: one lasting about
2.4 hours and the other about 16.4 hours.
The astronomers believe that the roughly 2-hour cycle reveals the rotational
period of 2001 SL9 and that a smaller companion orbits the asteroid
approximately every 16 hours. Two sharp dips in the light curve are probably
occultation events, they say. Assuming the components have albedos typical
of near-Earth asteroids, team leader Petr Pravec estimates that the primary
is about 1 km wide while the secondary is likely near 0.3 km across. As of
November 6, efforts to confirm 2001 SL9's binary nature had been thwarted by
clouds, but Pravec was optimistic that later attempts would have better
Received on Thu 15 Nov 2001 12:36:50 PM PST