Have satellites visited Mercury

Mercury gets the first earthly satellite: Messenger

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After more than six and a half years of flight time, the American space probe Messenger reaches an orbit around the planet Mercury, the closest to the sun. What started as planning in 1995 has now been successfully put into orbit around Mercury.

The Messenger space probe started on August 3, 2004 on a Delta II 7925H for its mission into the interior of the solar system. The aim is to place the probe in a highly elliptical orbit around the planet Mercury, the closest to the sun.

The project to bring a space probe to Mercury was already on the so-called Discovery List in 1997, but initially failed to gain acceptance. The Messenger project was not financially approved until July 1999.

For technical reasons, the start window had to pass in March 2004 (19 days) and mid-May 2004 (12 days). The third, 15-day, starting window 2004, which opened on July 30th, could be approached with more confidence. If bad weather at Cape Canaveral AFS Launch Complex 17 on August 2, 2004 forced a launch delay, the probe finally lifted off on a Delta II 7925H launch vehicle on August 3, 2004 at 02:15:56 local time (EDT).

The postponements had their price. If the arrival of the probe at Mercury was originally planned for 2009, a new trajectory lasting two years longer had to be expected due to the takeoff delay and the available fuel.

A long way without a gas station

Getting to Mercury from Earth is a big change in speed. Carrying out this only by means of the probe's drive simply blows up the amount of fuel, which now makes up almost 60% of the total weight. So other solutions had to be found to save weight and fuel.

The celestial bodies Venus and Mercury, but also the earth, were used to slow down the probe by means of so-called flyby or swingby maneuvers. One year after the launch, the earth was used to be diverted into the interior of the solar system. Two flights past Venus (October 24, 2006 and June 5, 2007) caused the Messenger to fly past Mercury for the first time on January 14, 2008. But at this point the probe was still too fast.

Only two further fly-bys on Mercury (October 6th, 2008 and September 29th, 2009) slowed the probe down so far and brought it so close to Mercury that it only required a controlled 15-minute braking maneuver (change in speed 862.4 m / s), to put them in a 12 hour orbit with a minimum approximation of 200 km over 60 degrees north of Mercury. The fuel economy went so far that anomalies in flight were compensated for by changing the orientation of the solar cell panels.

Today at 1:45 a.m. CET the time had come - the almost 8 billion kilometers and 2,417 days long journey had reached another critical mission point. After the tanks and systems were heated successfully, the probe's engines fired to slow it down so that it "fell" into orbit.

After swiveling in and the successful transmission with the high gain antenna, the star trackers (star alignment) were put into operation to ensure the further alignment of the space probe.

The first visitor only shot still pictures

Mariner 10, the last of NASA's Mariner series, was Mercury's first visitor. After taking off on November 3, 1973, it passed Mercury for the first time on March 29, 1974 at a distance of around 705 km and entered a 176-day orbit around the sun. Mercury, which itself orbits the Sun in 88 days with an equatorial diameter of around 4,880 km, and Mariner 10 met again after around 176 days, at the same point in Mercury's orbit. For this reason, Mercury, which rotates around its own axis in 59 days, turned Mariner 10 to the already known side. Thus, Mariner 10 could only cover around 45% of Mercury's surface.

The successful launch of the Messenger space probe into orbit around Mercury today will thus help to gain further views of the sun's direct companion.