The Search for Asteroid Moons –First Discovered in 1994

Vesta NASA's Galileo spacecraft took this image of asteroid Ida and its moon Dactyl in 1994. The image was the first conclusive evidence that natural satellites of asteroids exist. We still don't know whether Vesta has a moon. The answer may be coming soon: NASA's Dawn spacecraft is closing in on Vesta, and from now until the ion-powered spacecraft goes into orbit in mid-July, the imaging of the giant asteroid will be non-stop.

 "For starters," says Dawn chief engineer Marc Rayman, "we're going to look for an asteroid moon."

It's entirely possible: the 19-mile-wide Ida, 90-mile-wide Pulcova, 103-mile-wide Kalliope, and 135-mile-wide Eugenia each have a moon. And 175-mile-wide Sylvia has two moons. Measuring 330 miles across,

"When another large body collides with an asteroid, the resulting debris is sprayed into orbit around the asteroid and can gradually collapse to form a moon."

Another theory is "gravitational pinball," when a moon formed elsewhere in the asteroid belt might, interact with various bodies, end up captured by the gravity of one of them.

Hubble and ground based telescopes have looked for Vesta moons before, and seen nothing. Dawn is about to be in position for a closer look. This Saturday, July 9th, just one week before Dawn goes into orbit around Vesta, the moon hunt will commence.2 The cameras will begin taking images of the space surrounding the asteroid, looking for culprits.

"If a moon is there, it will appear as a dot that moves around Vesta in successive images as opposed to remaining fixed, like background stars," says Dawn Co-investigator Mark Sykes, who is also director of the Planetary Science Institute. "We'll be able to use short exposures to detect moons as small as 27 meters in diameter. If our longer exposures aren't washed out by the glare of nearby Vesta, we'll be able to detect moons only a few meters in diameter."

"We'll use the spacecraft's radio signal to measure its motion around Vesta," added Sykes."This will give us a lot of detailed information about the asteroid's gravitational field. We'll learn about Vesta's mass and interior structure, including its core and potential mascons (lumpy concentrations of mass)."
"The pictures are beginning to reveal the surface of this battered, alien world," says Rayman. "They're more than enough to tantalize us. We've been in flight for four years, we've been planning the mission for a decade, and people have been looking at Vesta in the night sky for two centuries. Now, finally, we're coming close up to it, and we'll be getting an intimate view of this place.

In past explorations, flyby missions occurred first, providing a good estimate of the target's gravity along with information on other aspects of its physical environment, including whether any moons are present. This time we're much less certain what we'll find."

The map of Vesta below created using the Hubble Space Telescope was released showing a rugged surface highlighted by a single crater spanning nearly the entire length of the asteroid. The large crater dominates the lower part of the false-color conglomerate image: blue indicates low terrain, while red indicates raised terrain. Evidence indicates that Vesta underwent a tremendous splintering collision about a billion years ago. In October 1960, a small chunk of this rock believed to have originated on Vesta fell to Earth and was recovered in Australia.


The Daily Galaxy via Science@NASA

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Image Credit: B. Zellner (GSU), P. Thomas (Cornell), et al., WFPC2, HST, NASA

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