NASA’s MESSENGER space probe has just had its third and closest flyby of Mercury, and has sent back pictures and data that will help to fill in our understanding of the solar system’s innermost planet. In this encounter, the probe imaged five percent of Mercury’s surface that had never been seen before, performed various observations and successfully accomplished a course-changing maneuver with a gravity assist from Mercury.
A sudden loss of signal unexpectedly took place just before the closest approach, but this now appears to have been a safety measure taken by the onboard fault-management system, and did not result in a loss of important data, as was originally feared. It happened during an eighteen minute period when the probe was being eclipsed by the shadow of Mercury. With solar power temporarily cut off, the probe was supposed to operate on its own internal batteries during this time, but ten minutes after entering eclipse and four minutes before the closest approach point, the signal from the probe stopped.
According to Eric Finnegan of the Johns Hopkins Applied Physics Laboratory, the spacecraft had autonomously switched itself to a safe operating mode due to an unexpected configuration of the power system during the eclipse. While in this safe mode, the probe preserved all data on its solid-state recorder. When the probe came out of eclipse, it returned to operational mode and began transmitting the stored data to Earth. All the data have now been received, and are being analyzed to confirm the full sequence of events.
It now appears that the main purpose of the flyby, the gravity assist to alter the probe’s orbit, was completely successful. In addition, all of the observational data from the approach have been retrieved, including pictures of previously unseen terrain.
This was MESSENGER’s final flyby of Mercury. For the next 18 months, the probe and the planet will move in their respective orbits, and when they approach each other again, MESSENGER will not flyby, but instead will go into orbit around the planet. From that vantage point, it will be able to observe the planet in unprecedented detail.
It will be the culmination of a mission that has been a model of success from the start. MESSENGER was launched on August 3, 2004, the first probe to be sent to Mercury since the Mariner 10 flyby 33 years earlier. The planned course would be an ambitious voyage through the inner solar system, with course modifications provided by one flyby of Earth, two flybys of Venus and three flybys of Mercury.
The first of the Mercury encounters happened on January 14, 2008. In it, MESSENGER passed 200 kilometers from the planet, imaging portions of the surface that had never been seen before and collecting a wide array of scientific observations, including magnetometer readings and spectrographic data. The second flyby took place on October 6, 2008. Again the probe passed about 200 kilometers from the surface, and took advantage of the opportunity to make further observations.
The third flyby was the closest, bringing the probe within about 140 miles of Mercury’s surface. On this one, Mercury still had a few surprises. One of these was a double-ringed impact basin 180 miles across. The shape of the feature is remarkably well preserved, indicating that it is probably relatively young, perhaps only a billion years old. That’s unusual, since most impact craters date to the early days of the solar system, some four billion years ago. The inner floor of this basin is of a different color from its surroundings, and appears to be even younger than the rest of the feature. This may be the youngest volcanic material on Mercury.
Spectrographic observations performed on the third flyby also revealed that the surface material on Mercury is rusty. It contains a high percentage of iron oxide, which provides the answer to an earlier puzzle. Previous observations had indicated that there was a low amount of iron on the surface, which was a surprise for a small, rocky planet. Now we know that the iron is there after all; it’s just locked up in those oxides.
This flyby also provided the chance for further observations of Mercury’s magnetic field and its exosphere (extremely thin atmosphere) and the first detailed scans of the north and south poles. These polar regions will be a great source of interest when MESSENGER settles into its final orbit around the planet, since there is at least a slight possibility that they could contain water. This amazing possibility is due to a paradox: since Mercury keeps the same face toward the sun at all times, its dark side- and particularly the polar regions- may have some of the coldest real estate in the solar system. As incredible as it seems, Mercury may have ice that has been frozen since the planet’s birth.
That’s just one of the many questions that will be addressed when MESSENGER and Mercury come back together and the probe goes into its permanent orbit. When that happens, we will start to really get acquainted with the sun’s closest planet, until recently the least explored body in the solar system.