Messenger Readies For its Last Weeks at Mercury

Low Flyovers Will Give the Best Close-up Views

Sometime in the next few months, the long-running MESSENGER mission at Mercury is going to take its last swoop over the planet, and then angle into Mercury’s surface. The long-planned Hover Campaign, uses orbit-correction maneuvers to delay the spacecraft’s final plunge for a few weeks so that they can send it on some low-pass runs and get some last good birds-eye views of the cratered terrain. They’ll also be using the spacecraft’s magnetometer (which senses the magnetic fields and magnetic anomalies in the surface), and the neutron spectrometer, which will let them get more data on the crustal composition.

In particular, the mission scientists want to zero in on those mysterious shadowed craters at Mercury’s poles, to study the water ice that exists within their chilly dark areas. If it does, then they’ll have some proof that water can exist in this region, and try to figure out just how the water got there. One idea is that comet bombardments could have deposited ice in these regions. Since the walls of the craters do not get sunlight, the ice could have been safely locked away there for a while.

How long? It’s a good question. The images and data returned about Mercury ice so far indicate that it is relatively young, meaning it was delivered (or uncovered) in recent geological time.  Mercury ice is a mystery, and although it sounds crazy that ice could exist so close to the Sun, the MESSENGER images and data have shown that it’s there and sitting there quite happily NOT getting melted by sunlight. It’s yet another dazzling result delivered from a very successful mission.

Why the rush to get MESSENGER in low swooping and controlled orbits? Well, the spacecraft is running out of propellant, and its subject to the gravitational pull of the Sun, both of which will combine to send it to its final resting place on Mercury. Mission controllers are taking advantage now of the spacecraft’s continuing good health to plan the final views and studies, and giving it the best altitude to do so. Once the propellant is gone, that’s the end of the road for a mission that has lasted since its 2004 launch, and has been orbiting Mercury since March 18, 2011. I remember the evening it went into orbit; we went down to the University of Colorado’s Laboratory for Atmospheric and Space Physics (my old employer) to sit with friends who are on the spacecraft team and watch as the mission slipped into orbit around the planet after a nearly five-year trip to get there.

The MESSENGER spacecraft is the first ever to orbit the planet Mercury, and the spacecraft’s seven scientific instruments and radio science investigation have done a spectacular job of studying this world, and helping us to understand its history and evolution. MESSENGER has acquired over 250,000 images and extensive other data sets. I suspect that there will be many incredible papers written about this treasure trove of information about the solar system’s smallest rocky planet. Stay tuned as MESSENGER spends her final weeks cranking out some spectacular science!