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IDA has a moon. New images of the pocked, potato-shaped asteroid, which
were captured by the spacecraft Galileo last August, reveal that a much
smaller rock is floating nearby. The photograph, released last week by
NASA’s Jet Propulsion Laboratory in Pasadena, shows ‘1993 (243) 1’, the
first asteroid moon ever detected.

The image was taken as Galileo passed through the asteroid belt on its
way to Jupiter. But troubles with Galileo’s main antenna mean that the spacecraft
transmits data very slowly, and images of Ida’s moon have only just arrived
at Earth. From this angle, the moon appears to be about 1.5 kilometres across.
Ida is about 56 kilometres long. The moon is orbiting roughly 100 kilometres
from the asteroid’s surface. Images still to be transmitted will fine-tune
these estimates and tell scientists more about the chemical composition
of Ida and its satellite.

Ida and its moon were probably formed when two larger asteroids collided,
breaking into pieces, say scientists working on the Galileo mission. ‘It
was previously thought that natural satellites of asteroids could form,
but they probably weren’t common,’ says Torrence Johnson, a scientist on
the Galileo project. ‘Having found one fairly quickly, we can say that they’re
probably more common that we previously thought.’