An Astronomical First
In March of 1994 scientists at the Jet Propulsion Laboratory announced the discovery of a moon
of asteroid Ida. This marks the first discovery of an asteroid moon. The moon, since named Dactyl,
is only 1.5 kilometers (1 mile) across and appeared at a distance of about 100 kilometers (60 miles)
from Ida's center. This discovery brings many ideas about asteroids into question. The fact that one
of the very few asteroids to have been examined up-close has a moon, led one scientist to remark
that asteroid moons are "probably more common than previously thought." If they are not common,
finding one so early would be a remarkable coincidence. This discovery was made by the Galileo
spacecraft, which took pictures of Ida and its moon in August 1993 as it flew to within 1,500 miles
of Ida. In 1991 Galileo took pictures of asteroid Gaspra, but it found nothing as remarkable as
Galileo scientists believe the moon may have been created at the same time as Ida -- when an older,
larger asteroid was shattered in a collision with yet another asteroid, fragmenting into dozens of
smaller asteroids. So Ida and Dactyl could have formed together from one larger asteroid. It is also
possible that Ida was hit by a smaller object recently, leaving a crater on Ida and throwing off
material into space that became Dactyl, the small moon. Like most asteroids, Ida is generally potato-shaped. It measures about 56 by 24 by 21 kilometers (35 by 15 by 13 miles) in size. Essentially all
asteroids are irregular in shape. This is due to in part to their small size and from the long history
of collisions that asteroids and all planetary bodies have suffered. One thing that we do know is
that Ida and Dactyl look to be made up of the same kind of stuff.
Other Asteroid Oddities?
In 1989 astronomers using the radio telescope at the Arecibo Observatory in Puerto Rico bounced
radar beams off of asteroid Castalia as it flew past Earth. What they got was a detailed three
dimensional picture of the asteroid. They also found that Castalia seems to be not one but two
asteroids! Less that two kilometers across (about one mile), Castalia represents the first contact
double asteroid found. One factor that made these images possible was its close pass to Earth, only
5.7 million kilometers (3.5 million miles) away. In 1992 astronomers used this same technique to
discover that asteroid Toutatis is also a double asteroid. Still others are possibly dumb-bell shaped.
Very few asteroids have been seen in such detail which raises the point that possibly a high
percentage of asteroids are actually one or more bodies traveling together.
Trying to understand how asteroids got to be this way is an important challenge for astronomers. It is possible that the double-lobed shape of Castalia is the result of a gentle collision between two separate asteroids some time in the past. To truly understand this and other asteroids, further studies, which should include spacecraft flybys, radar imaging, and ground based telescope observations, need to be made. While is it unlikely that Ida, Dactyl, or Castalia will be seen up-close again, there are other space missions planned for asteroids. NASA hopes to launch the Near Earth Asteroid Rendezvous (NEAR) mission in February 1996. This would give us a close look at two more asteroids, Iliya and Eros. NEAR is expected to make a flyby of asteroid Iliya and then move on to make a year-long study of Eros - giving us our closest, most detailed look at an asteroid yet.
Astronomy Magazine April 1994, January 1995.
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