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.'
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