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Messenger settles into new orbit to probe Mercury

Paul Sutherland, Feature writer
Apr 24, 2012, 7:00 UTC

Sen—NASA's Messenger spacecraft is set to build on its impressive list of discoveries about Mercury after being successfully put into a new orbit about the innermost planet.

The robotic probe, launched in 2004, has obeyed two special commands that slowed the time it takes to circle the planet by almost a third, preparing it to make more useful scans of its surface.

Mission controllers at the John Hopkins University Applied Physics Laboratory in Laurel, Maryland, ordered the manoevres, which were carried out 133 million kilometres (83 million miles) across space.

The first of the manoevres happened on April 16 when Messenger fired its main thruster, using up the rest of that engine's oxidizer propellant, and cutting the time taken to make one orbit from 11.6 to 9.1 hours.

Then on April 20, the probe's four medium-sized thrusters were fired for four minutes to complete the adjustment, putting it in an eight-hour orbit.

Mission controllers got the good news that all had gone well seven minutes and 23 seconds later after Messenger's signals were picked up by NASA's Deep Space Network tracking station in Canberra, Australia.

Now planetary scientists hope that they can discover more to answer tantalising questions already raised by Messenger about Mercury's make-up, evolution and the nature of its environment today. It is the densest of the rocky terrestrial planets.

Patrick Peplowski, from mission control, said: "The eight-hour orbit gives us more observing time at low altitudes, which will permit measurements of variations in surface composition on shorter spatial scales. Such information will give us new insight into the chemical and geological processes by which Mercury's crust was formed."

Mercury was a mystery until 1974 when NASA's Mariner 10 made three flybys, sending back images of a rugged, heavily cratered landscape that, at first glance, seemed like the long-dead, sterile Moon. But we have subsequently learned that there are clear differences with Mercury apparently a dynamic world showing signs of recent volcanic activity.

It has also returned data about a magnetic field around Mercury, indicating some geological activity, plus a tenuous atmosphere, made up mainly of hydrogen and helium from the solar wind. In 2001 a solar telescope detected that Mercury has a "tail" stretching more than 25,000 miles into space.