Mission controllers at NASA's Ames Research Center, Moffett Field, CA, commanded the Lunar Prospector spacecraft into a 30-kilometer (19-mile) mapping orbit over the poles of the moon at 11 p.m. PST on Thursday.

The new lower orbit, down from the current 40-kilometer (25-mile) transition orbit that began on December 19, 1998, will provide an unprecedented close-up "view" for the spacecraft's science instruments over the next six months. The orbit adjustment maneuver officially completes the very successful primary mission of Lunar Prospector that began with its launch on Jan. 6, 1998.

"Lunar Prospector's five instruments gathered such superior data in the one-year primary mission at the 100-kilometer (63-mile) orbit that we are very excited to get an even closer look," said Dr. Alan Binder, the mission's principal investigator and director of the Lunar Research Institute, Gilroy, CA.

"In the six weeks at the lower transitional 40-kilometer orbit, the data return is already looking tremendous. This raises our expectations about getting an even closer look at the lunar surface, collecting data at higher resolutions, and gaining further insights about the Moon," added Dr Binder.

The extended mission is expected to continue through July 1999. The enhanced data it will gather should enable scientists to refine their estimates concerning the concentration and form of the hydrogen detected at the north and south lunar poles, which mission scientists interpret as deposits of water ice. Mapping of the Moon's magnetic and gravity fields will also benefit greatly from the lower orbit. Additionally, initial global maps of the Moon's elements will be confirmed with the close-up data.

According to project officials, the lower orbit of the extended mission does have an element of risk. The 30-kilometer average altitude orbit is designed to clear all known terrain by at least nine kilometers, even as the orbit becomes less circular because of asymmetries in the lunar gravity field. Attitude control maneuvers are scheduled every 28 days to maintain this orbit. If one of these maneuvers cannot be conducted as scheduled because of problems in the ground system or on the spacecraft, Lunar Prospector would impact only two days later. In the 100-km orbit of the primary mission, the spacecraft would continue to orbit for many months before impacting.

"We are accepting a higher level of risk now that we have met the nominal mission objectives because of the high science payback," said Marcie Smith, Lunar Prospector operations manager. "We have no reason to think we will have problems, especially based on the excellent performance during the nominal mission."

"This one year anniversary of the Lunar Prospector mission represents an opportunity to reflect on the wonderful science accomplishments of the mission, the spacecraft and the dedicated individuals that made it all possible," said Sylvia Cox, NASA's mission manager for Lunar Prospector. "Lunar Prospector, across the board, has been a complete success."

NASA's Ames Research Center, Moffett Field, CA, manages the Lunar Prospector mission for NASA. The spacecraft was built by Lockheed Martin Missiles and Space, Sunnyvale, CA, and successfully launched on an Athena 2 spacecraft built by Lockheed Martin Corporation, Denver, CO.

Further information on the success of this maneuver and additional information about Lunar Prospector, its science data return, and other charts and graphics can be found on the following Web sites: