MESSENGER finds a 716-kilometer scar on Mercury's southern hemisphere
MESSENGER was more than four years into a three-pass approach to Mercury, trading speed for a safe orbit three years later. On October 6, 2008, during its second flyby, the probe's cameras caught a basin 716 kilometers across in the planet's southern hemisphere, the second-largest impact structure on Mercury after Caloris. Scientists named it Rembrandt. Inside its rim are volcanic plains, a long scarp, and craters cut nearly four billion years ago.
A NASA probe, launched on an American Delta II rocket, found a feature named after a Dutch painter rather than an American figure, a reminder that space science runs on international naming conventions even when the hardware and funding are domestic. The discovery also marked MESSENGER as only the second American mission ever to study Mercury up close, after Mariner 10 in 1975.
QHow did engineers manage to skip some of MESSENGER's planned course-correction burns?
Several thruster firings scheduled for the cruise to Mercury turned out to be unnecessary because sunlight did the work instead. Flight controllers used solar radiation pressure acting on MESSENGER's solar panels to make fine trajectory adjustments, effectively steering the spacecraft with photons rather than burning propellant. The technique mattered because MESSENGER carried only 607.8 kilograms of propellant and helium pressurizer for its entire seven-year journey, and every gram saved extended what the mission could do once it reached Mercury orbit. It is a small, almost accidental efficiency that let the probe arrive with more fuel margin than the original flight plan assumed, buying room for the reboosts needed later to survive its degrading orbit.
QWhat strange science opportunity did MESSENGER's first Venus flyby accidentally rule out?
During the October 24, 2006 Venus flyby, MESSENGER passed behind the planet and entered superior conjunction, meaning the Sun sat directly between Earth and the spacecraft, blocking radio contact entirely. With no way to downlink data, the team conducted zero scientific observations during that particular encounter, a rare case of a flyby with no data reaching Earth at all. Contact was not reestablished until late November, more than a month later. The second Venus flyby in June 2007 made up for it, delivering the mission's greatest velocity reduction and full instrument coverage, including simultaneous measurements with ESA's Venus Express orbiter already stationed there.
QWhy did MESSENGER's own instruments nearly become unreliable before the mission even properly began?
After launch in August 2004, the team used the August 2005 Earth flyby as a deliberate shakedown, pointing MDIS at the Earth and Moon and running checks across the other instruments measuring atmosphere, surface composition, and magnetic fields. The goal was to catch calibration problems years before they could corrupt science data at Mercury. The flyby also doubled as a test for the so-called flyby anomaly, a puzzling trajectory deviation seen in some past spacecraft encounters with Earth, but MESSENGER showed no such anomaly. Catching instrument quirks this early meant minor errors could be corrected over the following five years rather than discovered too late, after orbit insertion in 2011.
