A Close Encounter with Asteroid Eros

NASA Space Science News
NASA's NEAR Shoemaker spacecraft swooped 3 miles above the surface of 433 Eros on Oct 26th, marking its closest-ever approach to the tumbling space rock.

Early this morning, NASA's NEAR Shoemaker spacecraft, which has been in orbit around asteroid 433 Eros since February, swooped just 3 miles (about 5 km) over the tumbling space rock. The elevation of the flyby was similar to the cruising altitude of a commuter jet on Earth. No space probe has ever been so close to a minor planet.

"Although NEAR was very close to Eros -- the closest we've been before was about 35 km in July -- the spacecraft was never in any danger," says Andrew Cheng, the NEAR Shoemaker project scientist at Johns Hopkins University. "We chose to fly over an area of the southern hemisphere where, if we were off-target, the uneven gravity of the irregular asteroid would actually kick us back into a higher orbit." Compared to a commercial airliner flying hundreds of miles per hour above Earth, NEAR traveled slowly through Eros's weak gravitational field. Its maximum speed was only 14 miles per hour (23 kph).

Above: This image was taken in the early hours of October 26, 2000 as NEAR Shoemaker was skimming over the surface of Eros. Most of the 350-meter wide scene is covered in rocks of all sizes and shapes. The large boulder just below the center of the picture is about 15 meters (50 feet) wide. The smallest visible rocks are about 1.4 meters (5 feet) across.

The Johns Hopkins University Applied Physics Lab, which manages the NEAR mission for NASA, announced on Wednesday that the flyover had gone as lanned and that the spacecraft was heading back to a higher orbit.

While the dangers from skimming so close to Eros were slight, the potential rewards were great.

"One of the mysteries we've encountered on Eros is a curious deficiency of small craters," explained Cheng. "Something seems to be obliterating impact features smaller than a few tens of meters across."

On worlds that are peppered with impact scars (like the Moon or Mercury) there are always many small craters for each large one. That's true on Eros, too, but images of the asteroid collected during the first 8 months of the NEAR mission reveal fewer small craters than researchers expected. On Earth small impact scars wear away because of weather, but there is no weather on airless Eros. Some other process must be at work and scientists would like to know what it is.

"The high-resolution pictures we captured today will show these small scales very clearly," says Cheng. "They may give us some hints about what's going on."

While Eros seems to be running low on diminutive craters, it boasts a surprising surplus of boulders.

"Clark Chapman of the Southwest Research Institute has noticed that the surface of Eros is littered with 10- to 20-meter wide boulders, many more than we would expect [by simply extrapolating the number of large boulders to smaller sizes]," continued Cheng. "This is telling us that there's something funny about Eros's cratering history in the 'recent' geological past.

Above: Another image from NEAR Shoemaker's Oct. 26 low-altitude flyover of Eros. The large boulder near the bottom of the image is about 25 meters (82 feet) across.

"One possibility is that the cratering rate plummeted a billion or so years ago when Eros exited the main asteroid belt between Mars and Jupiter to become a near-Earth asteroid. After that, there may have been too few impacts to pummel these boulders into smaller pieces. These close-ups of Eros may tell us if the overabundance of 10-meter rocks extends to smaller sizes as well -- that would be an important clue."

Four months from now, NEAR Shoemaker will be poised to record an even closer view of Eros. "We're considering landing on the asteroid at the end of NEAR's one-year mission," says Cheng. "The spacecraft would touch down near the south pole of Eros where the rotational surface velocity is low."

Fans of Arthur C. Clark's science fiction novel "Rendezvous with Rama" might recall that explorers in that story landed near the pole of an asteroid-sized cylindrical spaceship, a spinning behemoth about the same size and shape as Eros. They chose to touch down near Rama's spin axis for the same reason that NEAR would settle near Eros's south pole; it's easier to land where the ground is moving slowly.

Mission scientists are still reviewing various end-of-mission scenarios and expect a final decision on whether the spacecraft will land and how by the first week of December.

"NEAR was designed to orbit Eros, not to land on it," says Cheng. "Most of the science instruments won't even work so close to the asteroid's surface. We want to do this as a proof of concept, to show that a spacecraft can land on an asteroid." Future missions to explore and possibly return samples from the minor planets will depend on maneuvers that NEAR might soon try for the first time.

"Things could go wrong," Cheng stressed, like crashing into one of Eros's many boulders. But if NEAR touches down without mishap and can still communicate with Earth, scientists will enjoy a brief close-up of Eros that will make today's flyby seem remote by comparison.