Galileo Transmits First-Ever Close-Up Photo of Asteroid

Scientists at the Jet Propulsion Laboratory on Thursday released history’s first close-up photo of an asteroid, a tattered, tumbling refugee from the early solar system that has been bombarded and blasted throughout its 4.5-billion-year history.

The image was snapped as the Galileo spacecraft zipped within 1,000 miles of the asteroid Gaspra last month, but scientists had not expected to see it until late next year because Galileo’s main antenna has refused to open. But it turned out that the photographic mission was such a technological triumph that engineers were able to sort through the data in Galileo’s tape recorder, extract one image and send it back ahead of time using a smaller antenna.

The photo shows what many would consider an unremarkable piece of rock, but to scientists working on the Galileo project at JPL in Pasadena, it is a thing of beauty.

“We just went crazy,” Michael Belton, chief of Galileo’s imaging team, said of the moment when scientists realized they had an image.

Until Galileo’s encounter with the 12-mile-long metallic rock, Gaspra had been seen only as a “point of light” in images from ground-based telescopes, so this picture has been awaited eagerly by scientists. The photo revealed that astronomers who have studied Gaspra from the ground have been remarkably accurate in determining what the asteroid is like, even down to its counterclockwise rotation once every seven hours.

But the photo also reveals details about the asteroid’s past.

“It has suffered a lot during its history,” Joseph Veverka of the imaging team said during a press conference at JPL. He speculated that Gaspra is all that is left of a much larger body that has been smashed repeatedly by other objects, including other asteroids.

“Large chunks of Gaspra have disappeared” because they were blasted free during violent collisions. Gaspra is so small that its gravity is 2,000 times weaker than Earth’s, and thus nearly any material blasted free by an impact can escape the asteroid’s gravity and drift off into space.

Veverka demonstrated by throwing a small ball across the auditorium at JPL. Had he been standing on Gaspra, he said, the ball would have had enough velocity to continue on into space. Thus Gaspra has been nicked and chipped away, and today it may be 10 times smaller than it was when it formed during the infancy of the solar system.

“Gaspra is the survivor of a series of catastrophic collisions,” Veverka said.

The asteroid bears the scars of those collisions in the form of impact craters, and some of its surface seems to have been blasted off, leaving it looking a little like a badly mangled potato.

Belton described it as looking like “the head of a salamander crawling out of space, or a great interplanetary ghoul, or maybe a shark’s head.”

It appears light gray in the black-and-white photo, and that is not expected to change much when scientists at JPL put together a color image in a couple of weeks.

The photo surprised scientists in that it suggests that much of Gaspra is covered by a layer of “soil” made up of tiny rocks and grains of sand. Scientists have known for years that larger bodies, such as the moon, have a soil-like surface, but objects as small as Gaspra were thought to have rock-hard surfaces.

“We see a layer of rubble, but we don’t know how deep it is,” Veverka said.

Galileo project scientist Torrence Johnson said the clue that tipped scientists off to the soil-like layer is the “sandblasted, rounded appearance” of some of the areas of Gaspra, especially some of its impact craters. A soft, sandly surface tends to cover ridges, for example, making them appear rounded.

Perhaps the biggest surprise, however, was that scientists were able to see the photo at all this soon. Using the smaller “engineering data” antenna aboard Galileo, engineers were able to transmit the image back to Earth in a little less than 20 hours, according to project manager William J. O’Neil. If the main antenna had opened properly, the same operation would have taken about one minute, he said.

Despite the fact that it would tie up the National Aeronautics and Space Administration’s deep space communications antennas for a considerable period, engineers decided it would be possible to retrieve the one image when they learned that they had been right on target during the Gaspra flyby.

Gaspra was 231 million miles away from Earth during the flyby, and no one could be certain precisely where the asteroid was in relation to the spacecraft. That left engineers with the problem of making sure that their camera was pointed toward the asteroid at least part of the time.

They did that by scanning the sky as the camera snapped 150 overlapping images, ensuring that Gaspra would be in at least some of the photos. All of those images were to have been transmitted back via the spacecraft’s 16-foot-wide antenna. But that plan went out the window when the antenna jammed. It was not feasible to transmit them all back with the smaller antenna.

But engineering data from the spacecraft indicated that ground controllers had been extremely precise in predicting the relative position of Gaspra, leading some to wonder if Gaspra had been right in the middle of the montage of 150 images.

Engineers ordered Galileo to transmit a few lines of data--somewhat like lines of type--running through the middle image. And there they found Gaspra, “smack in the middle,” Belton said.

“The key word that will come out of all this is precision,” he said. The camera was pointed so precisely that it was comparable to “pointing at a house in San Francisco” while standing in Pasadena, he added.

Since they knew which of the 150 images included the asteroid, engineers could transmit just one picture and be sure they had the asteroid. It worked so well they eventually transmitted three, but only one has been released. Later, when Galileo passes closer to the Earth, they plan to transmit all the images, including some that should be even sharper than the one released Thursday.