A NASA probe hit an interplanetary bullseye during its close flyby of Mercury on Tuesday, but the spacecraft unexpectedly went into safe mode during the encounter and did not achieve all of its scientific goals.

MESSENGER took this picture of Mercury with its wide-angle camera on Tuesday at a range of about 16,700 miles. The image shows part of Mercury never seen before. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The MESSENGER spacecraft flew 142 miles above Mercury's tortured surface at 2155 GMT (5:55 p.m. EDT) Tuesday, hitting its precise aimpoint at the appointed time.

The primary objective of the flyby was to use Mercury's gravity to alter the craft's trajectory, setting the probe on a course to intercept the innermost planet again in March 2011.

"Although the events did not transpire as planned, the primary purpose of the flyby, the gravity assist, appears to be completely successful," said Eric Finnegan, mission systems engineer at the Johns Hopkins Applied Physics Laboratory in Laurel, Md.

Scientists carefully crafted a series of science observations using MESSENGER's eight instruments to take advantage of the encounter.

But four minutes prior to closest approach, ground stations lost tracking signals from the spacecraft earlier than expected. The loss of communications occurred as MESSENGER was flying in the shadow of Mercury without solar power.

After passing behind Mercury, MESSENGER's signals were reacquired and engineers returned the probe to normal operations overnight.

MESSENGER automatically transitioned to a safe mode, halting its programmed command sequence and putting its instruments in a safe state, according to Finnegan.

"We believe this mode transition was initiated by the on-board fault management system due to an unexpected configuration of the power system during eclipse," Finnegan said.

About 150 megabytes of stored science and engineering data were beamed back to Earth early Wednesday morning. Officials will know more about the cause of the glitch after analyzing the data.

Before Tuesday's flyby, engineers uplinked about 7,000 commands for MESSENGER to execute during the encounter.

Scientists identified nine features, mostly impact craters, for MESSENGER's camera and surface composition spectrometer to closely study during the flyby sequence.

Those observations were not completed because they were scheduled to occur after MESSENGER went into safe mode, according to Finnegan.

Plans called for the spacecraft to take 1,559 pictures during the core imaging sequence. All of MESSENGER's instruments were expected to gather 50 megabytes of data during the most intensive 37 hours of the maneuver.

Finnegan said all of the science activities during the probe's approach to the planet were completed, including imaging of 5 percent of Mercury's surface never seen before.

MESSENGER also collected important data on Mercury's tenuous atmosphere and an enigmatic comet-like tail of particles streaming away from the planet.

This picture from Tuesday shows Mercury's northern hemisphere through MESSENGER's narrow-angle camera. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

"MESSENGER's mission operations and engineering teams deserve high commendation for their professional and efficient approach to last night's spacecraft safe-mode transition," said Sean Solomon, the mission's principal investigator from the Carnegie Institution of Washington.

"They quickly diagnosed the initial problem, restored the spacecraft to its normal operating mode, and developed plans to recover as much of our post-encounter science observations as possible," Solomon said.

Finnegan said science observations could begin again early Saturday morning.

Tuesday's flyby was MESSENGER's third and final temporary visit to Mercury since early 2008. Along with earlier flybys of Earth and Venus, the encounters changed the craft's orbit in gravity assist maneuvers that would otherwise have to be conducted using MESSENGER's limited supply of chemical propellant.

The flybys set up MESSENGER for insertion in orbit around Mercury the next time it returns to the planet in 2011.

"Most importantly, we are on course to Mercury orbit insertion less than 18 months from now, so we know that we will be returning to Mercury and will be able to observe the innermost planet in exquisite detail," Solomon said.