NASA’s Messenger spacecraft, in orbit around Mercury the past four years, will come to an abrupt end on Thursday.
Messenger’s path will intersect with the surface of the planet. The impact of the 1,100-pound spacecraft at 8,750 miles per hour is expected to gouge a crater some 50 feet wide.
That will bring to a close a mission that has painted an unexpected portrait of Mercury, once thought a boring round rock not much different from Earth’s moon. Mercury, the smallest planet in our solar system, is only slightly larger than the moon, although it has more drastic temperature swings — 800 degrees Fahrenheit during the day, minus 300 degrees at night.
“It’s really been exciting to see a planet unfold, a planet that is one is our neighbors,” said Sean C. Solomon, the principal investigator for the mission. “Almost every aspect of Mercury has had its share of surprises.”
Messenger — a shortening of Mercury Surface, Space Environment, Geo-chemistry and Ranging — discovered that Mercury shrank slightly as it cooled over billions of years; mapped ancient lava flows; found enigmatic shallow depressions that are some of the youngest and brightest features on the surface; and confirmed the presence of ice in perpetually dark craters near the poles.
The water ice, perhaps a billion tons, was not a complete surprise. Radio telescope observations from Earth had hinted at something reflective, and the shadowed craters are extremely cold. Beyond confirming the suspected, Messenger also made a new discovery: The ice was covered by an unexplained dark layer.
“We have a reasoned hypothesis,” Dr. Solomon said. The material could be carbon-rich compounds, similar to substances found in certain meteorites and in comets. “It’s tarlike in its consistency,” he said. “It’s as dark as tar.”
The biggest scientific finding of the mission, Dr. Solomon said, was that Mercury was rich in “volatiles,” elements like chlorine, sulfur, potassium and sodium that easily evaporate at moderate temperatures. Mercury is small and dense, full of iron, and the thinking about how it formed was that it must have been heated to temperatures that should have boiled off the volatiles. Yet the volatiles are still there.
Scientists now have to come up with new ideas to explain how Mercury formed.
When NASA launched Messenger in 2004, knowledge of Mercury was sparse. The only close-ups had come in three flybys by NASA’s Mariner 10 in the 1970s.
For more than six years, Messenger took a circuitous route through the inner solar system, making flybys of Earth, Venus and Mercury, to slow itself down enough to enter orbit around Mercury. On March 18, 2011, it finally arrived. The orbital portion of the mission was originally planned for one year, then was extended twice.
But now, the spacecraft has run out of fuel for the thrusters. Engineers came up with a clever trick to use pressurized helium, which pushes fuel to the thrusters, to provide three additional boosts that kept it in orbit a while longer.
A push on Friday lifted the minimum altitude to 11.3 miles from 5.1 miles, a bit less than engineers had hoped. One more maneuver is planned for Tuesday. But the helium is almost exhausted, and the low point of the orbit will decrease again. Messenger, still gathering data, is expected to hit at 3:30 p.m. Eastern time on Thursday.
Dr. Solomon will be at the missions operations center, at the Johns Hopkins Applied Physics Laboratory in Laurel, Md. “Unfortunately, it’s going to be a little anticlimactic,” he said.
When it crashes, Messenger will be behind Mercury, out of view from Earth. Dr. Solomon and the rest of the team will not be certain of Messenger’s demise until hours later, when the craft does not emerge from behind Mercury.
“It will be a somber moment,” he said.
Even its destruction could provide insights by digging up what lies beneath the surface — but it will be some time before anyone will see what that is.
NASA does not have any follow-up Mercury missions set, but an ambitious European and Japanese collaboration called BepiColombo is scheduled to launch in 2017. It is to arrive at Mercury in 2024.
That mission consists of two orbiters, one focusing on the planet, the other on the surrounding space environment, in greater and finer detail than Messenger.
Johannes Benkhoff, the project scientist for BepiColombo at the European Space Agency, described Messenger as a “really a fantastic mission.”
“It’s perfect that we have BepiColombo to follow on” Messenger’s findings, Dr. Benkhoff said. “They have provided a lot of new results, unexpected results.”
BepiColombo is named after Giuseppe Colombo, an Italian scientist nicknamed Bepi who came up with the orbital trajectories that enabled NASA’s Mariner 10 flybys.
Dr. Solomon is also looking forward to the next trip to Mercury. “There are many things about Mercury I still want to learn more about,” he said.
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