After 4,104 orbits of Mercury and billions of miles of space travel, NASA's Messenger orbiter will end its mission with a quiet bang on Thursday. Messenger will crash into the planet it has been orbiting for four years.
NASA says the orbiter will begin the process of lithobraking at 3:26 p.m. ET — meaning that Messenger will essentially scrape to a stop after hitting the planet's surface traveling at thousands of miles an hour. The Oxford English Dictionary reminds us that litho is the combining form for the Greek word for "stone."
Messenger was launched from Earth in 2004. It took years just to get to Mercury and years more to reach orbit around Mercury in 2011. Now, after studying Mercury's craters, it will make a new one — NASA says its impact crater should be about 52 feet wide.
"Orbiting Mercury is really tough, because it's so close to the sun," NPR's Geoff Brumfiel reports. "The sun's gravity is actually pulling the Messenger spacecraft into the planet. Messenger has used its engines to push itself away — but now those engines are out of fuel, so it's going to plow into Mercury at over 8,000 miles per hour."
The orbiter has added a wealth of research to the data gleaned by Earth's first probe to visit Mercury — the Mariner 10, which launched in 1973 and performed a flyby two years later.
Geoff says, "I think we can call this mission a big success — it spent four years gathering really good data about the planet. It gave us our first solid maps of Mercury, and it provided evidence for water on its surface, which is really surprising, given how hot Mercury is."
NASA says the orbiter has sent back more than 255,000 images to Earth, and that it created more than 10 terabytes of data that have been publicly released. One of those images included Mercury's neighbors.
Messenger's name is shorthand for its official title: the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft. It has its own Twitter feed, which will likely go dark sometime Thursday.
NASA Mission Systems Engineer Dan O'Shaughnessy tells Air and Space that researchers have been squeezing the last bits of usefulness out of the Messenger mission — particularly as the craft has gotten closer to Mercury's surface in recent weeks.
The terminal impact will be hidden from Earth's view. With no other craft nearby to witness its final seconds, NASA officials say they'll know Messenger is no more when they can no longer track the orbiter.
"We'll just be looking for a signal, and the absence of signal will sort of confirm our demise," O'Shaughnessy tells Air and Space.
As Scott reported for the Two-Way this month, Messenger will join the elite "Touchdown Club" of man-made objects that ended their missions on other planets. Member station KQED has a roundup that includes several objects on the moon as well as spacecraft on Mars, Venus and Jupiter, along with the moon Titan, an asteroid and a comet.