Nearly a decade after the spacecraft launched and four years after it settled into orbit around the inner-most planet of the solar system, NASA’s Mercury Surface, Space Environment, Geochemisty and Ranging probe, Messenger for short, is expected to and crash-land on Mercury. Since running out of fuel the spacecraft has slowly sunk lower in orbit. NASA predicts the crash will happen around 3:30 pm EDT today, April 30.
It is only slowly losing altitude but will hit at 8,750mph (14,000km/h).
That means the 513kg craft, which is only 3m across, will blast a 16m crater into an area near the planet's north pole, according to scientists' calculations.
All of Messenger's fuel, half its weight at launch, is completely spent; its last four maneuvers, extending the flight as far as possible, have been accomplished by venting the helium gas normally used to pressurize actual rocket fuel into the thrusters.
The collision marks the end of the spacecraft’s history-making lifetime of scientific observation. Messenger successfully gathered enough images and data to make a full map of Mercury’s pock-marked surface. Researchers learned that the closest planet to the Sun shrank by almost seven miles in diameter a few billion years ago and gained an understanding of the tiny planet’s structure, history and tectonic activity. Messenger also spotted ice lurking in the shadows of polar craters. JoAnna Wendel writes for EOS:
This finding forces scientists to rethink how the solar system evolved to bring volatiles to the inner planets. MESSENGER also discovered coatings of organic-rich material on some of the ice within these craters, a finding that excited astrobiologists everywhere.
Don't get too excited just yet though."I don’t think anybody could count Mercury as habitable," says Messenger mission head Sean Solomon, in an interview with Alexandra Witze for Nature News. "But it is a witness to the delivery of the ingredients for habitability, from the outer Solar System to the inner Solar System."
Solomon elaborates on the the most surprising Messenger findings:
The big surprise was the high abundances of volatile elements. All of the ideas for how Mercury got put together predicted that it would be depleted in volatiles, much like the Moon. But instead, we see sulphur [at] ten times the average for Earth. We see sodium and potassium. We see chlorine, one of the most volatile elements that we have the ability to measure.
That means we really didn't understand the particular way that Mercury became an iron-rich planet. It wasn't a process with sustained high temperatures that drove off the volatile elements. I don't think the final chapter has been written on what the most likely explanation is for the formation of Mercury.
He also explains what will happen when the spacecraft finally meets its end:
The last couple of hours will probably be pretty quiet. There will be a final orbit when the spacecraft passes behind the planet and we won't hear from it again. We think that we know where the impact will be, near the crater Shakespeare. [When it hits], it will become one of the youngest, albeit one of the tiniest, impact features on Mercury. And that will be the end of MESSENGER.
I have worked on the mission for 19 years. It's like losing a member of the family. Even pre-knowledge doesn't prepare you completely for the loss.
Messenger won't be able to send final images because it will be on the back side of Mercury for the crash. However, keep an eye on the spacecraft’s Twitter persona for farewell messages.