Over a decade ago, NASA shot Messenger into space, and in 2011, it became the first probe to orbit around Mercury, sending back our first closeups of the planet. But all good things must come to an end, and Messenger is running out of fuel. In one last tour, Messenger will fly lower than ever over Mercury—so close that the solder on its instruments could melt.
Because Mercury has no atmosphere to trap heat, temperature on the planet varies wildly. At night and inside craters in permanent shadow, it's more than cold enough for frozen water. But under the sun, it can get as hot as 426 C.
Messenger does have a sunshade that protects its instruments from the sun's brutal rays. The problem, then, lies in heat radiating up from Mercury and heating up the unprotected underbelly of the probe. Messenger engineer Dan O'Shaughnessy tells New Scientist that the solder will melt at around 185 C, when the probe gets to 16 miles above the surface of the planet. It's risky to fly this low, but Messenger could send back its best images of Mercury yet.
After its low-flying trip, Messenger will get one last boost into orbit on January 21st, when it will continue gathering data with the instruments that survived. It's expected to run out of fuel and crash land on Mercury some time March, finally touching the planet it had been circling for so long. [New Scientist]