When Messenger entered Mercury's orbit, it came within 120 miles of the planet's surface. But since then, its path has shifted so that with each eight-hour orbit cycle, it gets closer. Earlier this month, it came within 100 kilometers — about 62 miles — for the first time. And it will gradually get closer, as engineers slowly use up fuel for brief propulsions that delay the inevitable and prolong observations.
"We'll try to get it all down right before game over," said Andrew Calloway, the mission operation manager at the Hopkins lab in Laurel.
While the mission's extension has been a surprise, engineers nonetheless husbanded Messenger's tank of hydrazine fuel. While the spacecraft orbits in what the engineers call its cruise phase, the pressure of solar radiation can shift its course slightly. During its first year in orbit, this required half a dozen corrective maneuvers.
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On occasion, instead of using correcting bursts from the engines, engineers learned to tilt the spacecraft in different ways or adjust its solar panels to "nudge" it back on track, Calloway said.
"If we hadn't done that, we'd probably be out of fuel already," he said.
Fuel reserves were down to just under 10 kilograms last month, less than 2 percent of what Messenger carried into space in 2004. Plans call for it to be used up in three spurts between now and March, each temporarily rocketing against the pull of the planet's gravity.
But the close-up view will last only as long as the spacecraft can withstand increasing heat as it nears the surface.
The guts of the one-ton spacecraft — instruments including cameras, spectrometers and a magnetometer — are interwoven in a mosaic of electronics that could fit inside a sport utility vehicle. An 8-foot-by-6-foot sun shade protects the instruments from the sun, just 36 million miles away, about a third of the distance from the Earth to the sun. Two solar panel arrays flank the spacecraft's body, soaking up the rays.
Solder on one of Messenger's antennae, used to transmit data back to Earth, is expected to melt at about 320 degrees, while solder used in parts of the solar panel assemblies melts at about 200 degrees — a level the spacecraft could reach next month.
Tests have shown that even after the solder melts and then cools again, it continues to work. But the effects of repeated heating and cooling at those levels are unknown, said Sean Solomon, Messenger's principal investigator and director of Columbia University's Lamont-Doherty Earth Observatory.
There is a chance pieces of the spacecraft could fail in the heat, well before the crash landing, he said. But Solomon is optimistic.
"We've had a lot of bonuses on this mission," Solomon said. "We're in new territory for spacecraft."