Sen—It's close to the end for the MESSENGER mission to Mercury. Its orbit often carries it to within just a few kilometers of Mercury's surface. The Sun's gravity constantly tugs at the spacecraft, nudging its periapsis ever lower. By firing its rocket at apoapsis, MESSENGER can raise its periapsis, but each time it does that it uses up irreplaceable fuel. Over the course of its decade-long mission, MESSENGER's engineers have proven creative at fuel conservation, using solar sailing to trim its trajectory.
Their efforts have given MESSENGER a much longer life than expected, but on April 6, the spacecraft burned the last hydrazine fuel that remained in its auxiliary fuel tank. The engineers are not quite out of tricks; they are now using helium brought along for the purpose of pressurizing the fuel tanks as a form of cold-gas propellant. Because of its low mass, helium is not a particularly effective cold-gas propellant, but having used all its fuel MESSENGER is considerably lighter than it was when it launched; of its launch mass of 1100 kilograms, 600 were propellant.
MESSENGER's low-altitude flight is providing unique opportunities for science, including higher-resolution imaging of Mercury than ever before, showing how Mercury's surface has evolved over time. In this recent photo, many generations of impact craters are visible. The largest one, occupying most of the frame, is a "ghost" crater, covered over long ago by volcanic flows. But the structure of the crater underlying the flows became visible as a ghost as Mercury's surface contracted and buckled along zones of weakness around the crater rim. Since then, innumerable other craters have followed, singly or in sprays of secondaries.
MESSENGER took this photo on Jan. 23, 2015. The large ghost crater is about 8 km in diameter. Image credit: NASA / JHUAPL / CIW
The high-resolution observations give MESSENGER an opportunity to examine some of Mercury's most unusual features at higher resolutions than ever before. Mercury's "hollows," discovered in MESSENGER images, are flat-floored depressions surrounded by deposits of bright material, having a Swiss cheese appearance. Scientists' best guess for how they form is that a relatively volatile material—perhaps a sulfur-rich volcanic flow—is sublimating into gas, leaving behind collapsed pits. In this image, taken during the low-altitude campaign, many of the hollows seem to be forming in rims around circular crater floors. But why do only some craters in the image show hollow morphology? It's still a mystery.
MESSENGER took this photo on March 2, 2015. It covers an area 20 km wide within the Zeami impact basin. Image credit: NASA / JHUAPL / CIW
High-resolution photos are not the only reason for MESSENGER's low-altitude flights. MESSENGER has a total of seven science instruments. Almost all of them benefit from the extra-low altitude, but there are three investigations in particular that will make the most of MESSENGER's final days. One is the Gamma-Ray and Neutron Spectrometer, which measures the elemental composition of the surface. The neutron spectrometer is particularly sensitive to the presence or absence of hydrogen, so as MESSENGER's orbit carries it low over Mercury's north pole, the neutron spectrometer is mapping out the location of hydrogen sequestered in water ice in permanently shadowed regions of Mercury's poles.
Another beneficiary of low-altitude flights is the magnetometer. It has been mapping Mercury's global magnetic field, but with the low-altitude flights it has the opportunity to catch more subtle fluctuations in the field that could indicate remanent magnetization in the rocks—recordings of past magnetic fields, locked into rocks, which may or may not match the present field.
Finally, there is an experiment that isn't bookkept as a science instrument, but can peer deeper into Mercury than any other: radio science. As MESSENGER orbits Mercury, small inhomogeneities in Mercury's mass tug the spacecraft in different ways. By precision tracking of the Doppler shift of MESSENGER's radio signal, we can map the distribution of anomalously low or high mass. Whatever produces these mass anomalies is buried within the planet's crust—extra thickness of iron-rich lava flows can locally increase the force of gravity, while shattered and porous impact ejecta can decrease it. The closer MESSENGER flies to Mercury, the smaller the features it can discern in Mercury's gravity field, and the more we can learn about its interior and the sequence of events that built its present surface.
But it's all going to end before the month is over. By April 30, MESSENGER will crash into Mercury. Unfortunately, it will happen on the far side of Mercury from Earth, out of sight. It will pass over Mercury's limb and not come out the other side.
With the recent loss of Venus Express (after a similarly extended mission), that leaves us without an active science mission at either planet interior to Earth.
MESSENGER has spent its last breath of fuel returning a treasure trove of data to Earth, data that we'll be analyzing for decades to come.
MESSENGER has accomplished what it set out to do—perform the first detailed global survey of Mercury's surface, exosphere, and magnetic field.
In two years, ESA and JAXA, working together, will launch the Bepi-Colombo mission toward a Mercury arrival in January 2024, and Mercury science will march on.