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Because Mercury is so close to the Sun (about 58 million kilometers), the Sun’s gravity pokes at MESSENGER, changing its orbit. It uses hydrazine propellant to alter its orbit, but it's out of fuel. The last orbital maneuver on April 6 used up the remaining hydrazine in its tanks. All that's left is helium (used to keep the tanks pressurized); that can be used as a propellant, but it's not terribly efficient. A few more "burns" are planned to raise its minimum distance above the surface, but they will only add a bit of time to MESSENGER's life. It's not clear when exactly, but sometime in late April or early May the probe is expected to crash into the planet’s surface.