MESSENGER gains critical gravity assist for Mercury orbital observations

With more than 90 percent of the planet's surface already imaged, MESSENGER's science team had drafted an ambitious observation campaign designed to tease out additional details from features uncovered during the first two flybys. But an unexpected signal loss prior to closest approach hampered those plans.

At approximately 21:55 UTC, the spacecraft passed by Mercury at an altitude of 142 miles and at a relative velocity of more than 12,000 miles per hour according to Doppler residual measurements logged just prior to the closet approach point. As the spacecraft approached the planet, MESSENGER's Wide Angle Camera captured this striking view, which shows portions of Mercury's surface that had remained unseen by spacecraft even after the three flybys by Mariner 10 in 1974 and 1975 and MESSENGER's two earlier flybys in 2008.

"This third and final flyby was MESSENGER's last opportunity to use the gravity of Mercury to meet the demands of the cruise trajectory without using the probe's limited supply of on-board propellant," says MESSENGER Mission Systems Engineer Eric Finnegan of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md.

A portion of the complicated encounter was executed in eclipse, when the spacecraft is in Mercury's shadow and the spacecraft -- absent solar power -- was to operate on its internal batteries for 18 minutes. Ten minutes after entering eclipse and four minutes prior to the closet approach point, the carrier signal from the spacecraft was lost, earlier than expected.

High resolution image

Yesterday, as the spacecraft approached Mercury for the mission's third flyby of the Solar System's innermost planet, MESSENGER captured this striking view. This WAC image shows portions of Mercury's surface that had remained unseen by spacecraft even after the three flybys by Mariner 10 in 1974-75 and MESSENGER's two earlier flybys in 2008. In this image, just returned to Earth early this morning, the newly imaged terrain is located in a wide vertical strip near the limb of the planet (on the left side of Mercury's partially sunlit disk). Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington




High resolution image

In this image, Mercury's northern horizon cuts a crisp line against the blackness of space. The surface in the lower right corner of the image is near Mercury's terminator, the line between the light dayside and dark night side of the planet. Looking toward the horizon, smooth plains extend for large distances. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington




High resolution image

This unnamed impact basin was seen for the first time yesterday during MESSENGER's third flyby of Mercury. The outer diameter of the basin is approximately 260 kilometers (160 miles). This basin has a double-ring structure common to basins with diameters larger than 200 kilometers (about 125 miles). The floor of the basin consists of smooth plains material. Concentric troughs, formed by surface extension, are visible on the basin floor, similar to those seen in Raditladi basin. Such troughs are rare on Mercury, and the discovery of such features in this newly imaged basin is of great interest to members of the MESSENGER Science Team. Crater chains produced during ejecta emplacement also can be seen emanating from the basin. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington




The unnamed crater in the center of the image, viewed at close range for the first time yesterday during MESSENGER's third flyby of Mercury, displays an arc-shaped depression known as a pit crater on its floor. Impact craters on Mercury that host pit craters in their interiors have been named pit-floor craters. Unlike impact craters, pit craters are rimless, often irregularly shaped, and steep-sided, and they display no associated ejecta or lava flows but are typically distinctive in color. Thought to be evidence of shallow magmatic activity, pit craters may have formed when subsurface magma drained elsewhere and left a roof area unsupported, leading to collapse and the formation of the pit. In this example, the southern area of the pit appears to have two or more floor levels. The discovery of multiple pit-floor craters augments a growing body of evidence that volcanic activity was widespread in the geologic evolution of Mercury's crust. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

According to Finnegan, the spacecraft autonomously transitioned to a safe operating mode, which pauses the execution of the command load and "safes the instruments," while maintaining knowledge of its operational state and preserving all data on the solid-state recorder.

"We believe this mode transition was initiated by the on-board fault management system due to an unexpected configuration of the power system during eclipse," Finnegan says. MESSENGER was returned to operational mode at 04:30 UTC with all systems reporting nominal operations. All on-board stored data were returned to the ground by early morning and are being analyzed to confirm the full sequence of events.

"Although the events did not transpire as planned, the primary purpose of the flyby, the gravity assist, appears to be completely successful," Finnegan adds. "Furthermore, all approach observing sequences have been captured, filling in additional area of previously unexplored terrain and further exploring the exosphere of Mercury."

"MESSENGER's mission operations and engineering teams deserve high commendation for their professional and efficient approach to last night's spacecraft safe-mode transition," says MESSENGER Principal Investigator Sean Solomon, of the Carnegie Institution of Washington. "They quickly diagnosed the initial problem, restored the spacecraft to its normal operating mode, and developed plans to recover as much of our post-encounter science observations as possible. Most importantly, we are on course to Mercury orbit insertion less than 18 months from now, so we know that we will be returning to Mercury and will be able to observe the innermost planet in exquisite detail."

Source: Johns Hopkins University Applied Physics Laboratory
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