UPDATE (Jan. 15): According to a Twitter update on Thursday, the Mars Express orbiter successfully completed its Phobos flyby. "Today's #Phobos flyby complete. Science data will come down from #MarsExpress in the next few days," -- @ESAOperations.
On Thursday (Jan. 14), the European Space Agency's Mars Express orbiter is set to pay Phobos a visit in what will be the mission's closest flyby of the Martian moon this year.
Flyby will occur at 16:00:21 UTC (10 a.m. EST) when the mission will come within 53 kilometers (33 miles) of Mars' largest moon. According to a Mars Express blog update, in the nearly-60 planned Phobos flybys this year, this event is a "real skimmer" (the other flybys will come no closer than a few hundred miles), allowing mission scientists to carry out some invaluable science.
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Principally, key scientific instruments, including the High Resolution Stereo Camera (HRSC), will be able to view a region of the moon that has been poorly observed in the past.
"This flyby will provide very good viewing, within 1,000 km (620 miles), of an area previously not seen well," said ESA's Mars Express project scientist Dmitri Titov. "HRSC will be taking images; the MARSIS radar and the ASPERA-3 particle instrument will operate as well to sound the subsurface and plasma environment of the moon."
Within the flyby region is the proposed landing site for the future Russian lander and sample-return mission, Phobos Grunt. The mission was first launched in 2011, but failed when the spacecraft became stranded in low-Earth orbit and ultimately reentered the atmosphere. It is hoped that a repeat attempt may happen by 2026.
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"This flyby is important as it will allow us to finally view this area on Phobos that has yet to be seen at high resolution and excellent lighting," said Thomas Duxbury, professor in planetary science at George Mason University, Va.
In addition, the incredibly close encounter will cause Phobos' weak gravitational field to slightly alter Mars Express' orbital trajectory. In 2013, another close flyby (passing within 45 km or 28 miles) allowed scientists to detect the gravitational deflection so a precise measurement of Phobos' mass and average density could be arrived at. Thursday's flyby will be used to confirm and tightly refine these measurements.
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These observations will add to the growing wealth of information about the irregularly-shaped moon that only measures 22 miles across at its widest section. Recently, new theories as to the origin of Phobos' trademark lines and likely doomed future have seen the headlines. Through tidal sheer with Mars, the moon is expected to be pulled apart in less than 40 million years, likely creating a ring system that could persist in Mars orbit for a few million years more.
For more about this fascinating flyby, read Wednesday's mission blog update.