Computer animation depicts the NEAR spacecraft resting on the surface of Eros. Photo: JHU TV/Spaceflight Now

NASA granted an additional four-day extension for the Near Earth Asteroid Rendezvous (NEAR) Shoemaker mission on Friday so that the spacecraft can return additional data from the surface of the asteroid Eros through the end of the month.

The extension will allow the spacecraft's gamma-ray spectrometer to collect more data on the composition of the asteroid's surface and subsurface regions as well as provide valuable time on NASA's Deep Space Network (DSN) to transmit the data back to Earth.

NEAR was approaching the end of a ten-day extension granted on February 14, the first anniversary of NEAR's arrival at Eros and the original termination date for the mission. The extension was granted after the spacecraft made a surprisingly successful landing on the surface of Eros February 12.

Scientists planned to use the extension to measure the composition of the asteroid's surface to an accuracy up to ten times greater than possible from orbit. First, however, engineers had to calibrate the instrument to work on the surface and compensate for issues like an increased temperature of the instrument since it can no longer radiate heat into space from its position on the surface.

"We optimized the instrument for collecting science in its new environment," said John Goldsten, lead engineer of the spectrometer. The instrument is now collecting "prime" science data and transmitting it to Earth. "We've received the first of three prime data sets and the data is outstanding. This tells us the new commands are working perfectly," Goldsten added.

The new four-day extension will allow the other two data sets to be returned during DSN passes scheduled for February 26 and 28. Scientists hope that the data sets should allow scientists to accurately measure the composition of the surface down to a depth of about 10 centimeters (4 inches).

The mission extension "allows us to build a much better sample," said Jacob Trombka, team leader for NEAR Shoemaker's X-ray/Gamma-ray Spectrometer. "The longer you accumulate data the more you can reduce the uncertainty of your results. When you look at a little bit of data you see clues, but when you get more data down you can be a bit more definitive."

The spectrometer is the only one of the spacecraft's main instruments still returning data. Officials decided to turn off the spacecraft's magnetometer after it failed to detect any evidence of a magnetic field from orbit, during its descent to the surface, and on the surface itself. "The landing site shows no evidence for an intrinsic magnetic field," said Brian Anderson, magnetometer instrument scientist. "Since the sensor is only two meters above the surface this is a pretty definitive measurement."

The spacecraft's camera was also turned off since it was not designed to take images while on the surface, and the radio science and laser rangefinder instruments were also designed to be used only from orbit. A sixth instrument, an infrared spectrometer, failed last June, four months after NEAR Shoemaker entered orbit around Eros.

News of the latest mission extension leaked out via the Internet Thursday afternoon when a release announcing the extension was prematurely posted on the project's web site. Helen Worth, a project spokesperson at the Applied Physics Laboratory of The Johns Hopkins University, explained that final approval of the four-day extension was not granted by NASA until Friday morning.