Four years after the launch of the two Phobos spacecraft, the U.S. resumed its exploration of Mars with the Mars Observer spacecraft. This was the first spacecraft the U.S. had launched toward Mars in seventeen years (and sixteen days). Mars Observer was designed to study the geoscience and climate of Mars. However, three days prior to orbital insertion, Mission Control lost contact permanently with the spacecraft for unknown reasons. Whether Mars Observer went into orbit around Mars or flew by the planet, going into heliocentric orbit, is unknown. One possible reason for the loss of contact may have been an explosion in a propellant line during pressurization procedures just before the orbital insertion engine burn. Hypergolic fuel may have leaked past valves in the system during the cruise to Mars, allowing the fuel and oxidizer to combine prematurely before reaching the combustion chamber. The engine was not designed to lie dormant for months before being fired. Despite the loss of the spacecraft, several science instruments originally developed for Mars Observer are (or were) being used by three other orbiters, the Mars Global Surveyor, Mars Odyssey and the Mars Reconnaissance Orbiter.
Two launch windows later, on November 7, 1996, the U.S. followed up by launching Mars Global Surveyor (MGS). The primary mission of MGS was to map the surface of Mars; however, it also carried a laser altimeter (Mars Orbiter Laser Altimeter or MOLA) that allowed the spacecraft to determine the altitudes of all points on the Martian surface. The data from MOLA has resulted in a very colorful and popular map of Mars (see the image above). MGS was also used to track the Martian weather and act as a communications relay for the Mars Exploration Rovers, Spirit and Opportunity.
After a series of aerobraking maneuvers that lowered the apoapsis (highest point of the orbit) from 54,026 km to 450 km, MGS began its primary mission of mapping the surface of Mars. The spacecraft circled Mars once every 117.65 minutes at an average altitude of 378 kilometers (235 miles). It was in a near-polar orbit (inclination = 93°) which was almost perfectly circular, moving from the south pole to the north pole in just under an hour. The altitude was chosen to make the orbit sun-synchronous, so that all images taken by the spacecraft of the same surface features on different dates would be under identical lighting conditions. After each orbit, the spacecraft viewed the planet 28.62° to the west because Mars had rotated underneath it. In effect, it was always 14:00 for MGS as it moved from one time zone to the next. After seven sols and 88 orbits, the spacecraft would approximately retrace its previous path, with an offset of 59 km to the east. This ensured an eventual full coverage of the entire surface.
MGS Mission Control lost contact with the spacecraft on November 2, 2006 after being ordered to perform a routine adjustment of its solar panels. The spacecraft reported a series of alarms but, in a final transmission, indicated it had stabilized. According to NASA, the MGS loss was likely due to faulty computer code uploaded five months before that ultimately caused one of the spacecraft's battery to overheat. The spacecraft had reoriented to an angle that exposed one of its two batteries to direct sunlight. The battery overheated and, over the course of 11 hours, depleted the other battery as well. An incorrectly oriented antenna prevented the spacecraft from communicating its status to controllers. The board investigating the loss of MGS concluded that the MGS team followed existing protocols correctly, but that the procedures were insufficient to spot the errors that occurred.
The contribution MGS made to our understanding of Mars is enormous. Using the Mars Orbiter Camera, MGS took over 212,000 images. I hope to highlight a number of the accomplishments MGS made in future posts. Of all the spacecraft sent to Mars, MGS was in operation the longest, having survived five days short of ten years since its launch.