The satellite's oblong orbit around the solar system's innermost planet brings it gradually closer and closer as it looks into Mercury's mysterious volcanoes, craters and magnetic field. With dwindling fuel to counteract the dense planet's pull, the scientists managing the mission at Johns Hopkins Applied Physics Laboratory in Laurel can only delay its fall for so long.
It's now orbiting as close as several dozen miles above the planet's gray, dimpled crust — and soon, closer. Data collected in the final months of the decadelong mission to explore the inner solar system could help prove the presence of ice in polar craters and provide more detailed accounts of what volatile elements are contained in lava flows or the mysterious depressions on the planet's surface.
Concern about instruments
Those “bonus” observations depend on Messenger's instruments holding up to the 800-degree heat radiating from Mercury's surface during the slow-rotating planet's prolonged days. The planet, named for the Roman god of messages, makes one revolution around the sun every 88 Earth days, and yet it rotates so slowly that it takes 176 Earth days for one solar day to pass on Mercury.
As long as the instruments hold up, they could add detail and nuance to data that is expected be used by scientists for decades to come, providing insights into how planetary systems form around stars across the universe.
“Every time we've gone somewhere in the solar system and looked with higher resolution, we've made new discoveries,” said Larry Nittler, a scientist at the Carnegie Institution for Science who is deputy principal investigator of the $500 million project's science team.
Since its launch in 2004, Messenger has made plenty of discoveries.
Much of what was known about the planet closest to the sun came from NASA's Mariner 10 mission, which flew past Mercury and Venus in 1974 and 1975. Mariner found early signs of Mercury's iron core, a small magnetic field and minimal atmosphere.
Messenger began sending photographs and other observations of the planet in January 2008, eventually cataloging the entire surface for the first time. The closer it got to the planet, the more theories it confirmed and disproved.
When Messenger began an elongated, elliptical orbit around Mercury in 2011, the data were even more dramatic.
Scientists predicted they would find few “volatile” elements — abundant here on Earth but with boiling points too low to be expected to withstand Mercury's heat — in the planet's crust. But they found astonishingly high levels of sodium and potassium, as well as surprisingly low levels of iron. They also found signs of significant volcanic activity, perhaps not as distant in the past as once expected.
They mapped its magnetic field and found it to be asymmetrical, unlike Earth's, with its magnetic equator located about 20 percent of the way toward the pole.
And they believe they have confirmed that some craters at the planet's poles contain ice — the holes are deep enough to remain in permanent shadow.
“In many cases, a lot of our original ideas about Mercury were just plain wrong,” Nittler told the Baltimore Sun in June 2011, after just three months of data collection in orbit.
Now scientists hope to prove more theories wrong, or right, as the case may be.
A closer view could prove the presence of ice in the polar craters and provide greater detail of what elements are in different geographic features. Images from the first year of orbit showed, at best, 10 meters per pixel, but are now approaching 2 meters per pixel. Observations of magnetic fields and elemental composition are similarly getting increasingly detailed.
New details emerge
One feature scientists are particularly interested in getting a closer look at is what are known as hollows, irregularly shaped depressions with bright, flat floors and halo-like markings around them. They believe a combination of the planet's heat, its constant bombardment by tiny meteors, and the powerful effects of charged solar particles could be causing materials in surface rocks, likely sulfur or potassium, to sublimate — transforming from solid to gas without first becoming liquid — leaving the curious craters behind.
“We've always had to squint at the images to see the details,” said Nancy Chabot, instrument scientist for Messenger's cameras, the Mercury Dual Imaging System. “We're getting new insights into the depths of these features and what the edges look like in more detail.”
Any more data the mission is able to collect before Messenger crashes or is fried by the heat will be the last gathered on Mercury for years. Last year, NASA signed on to the European Space Agency's BepiColombo mission, which plans to send two spacecraft to Mercury in 2022 to gather more data on the planet's composition, core and magnetic field.
The past two years of Messenger's observations weren't assured — the orbit portion of the mission was intended to last one year, but was extended once for an additional year and then again, through Messenger's crash landing, and then a year beyond for assessing the data.
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 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.
Nudging the spacecraft
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 1-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.”
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