[meteorite-list] Mars Express Heading Towards Daring Flyby of Phobos

[Ron Baalke]

http://www.esa.int/Our_Activities/Space_Science/Mars_Express/Mars_Express_heading_towards_daring_flyby_of_Phobos

Mars Express Heading Towards Daring Flyby of Phobos
European Space Agency
December 23, 2013

Late this month, ESA's Mars Express will make the closest flyby yet of 
the Red Planet's largest moon Phobos, skimming past at only 45 km above 
its surface.

The flyby on 29 December will be so close and fast that Mars Express will 
not be able to take any images, but instead it will yield the most accurate 
details yet of the moon's gravitational field and, in turn, provide new 
details of its internal structure.

As the spacecraft passes close to Phobos, it will be pulled slightly off 
course by the moon's gravity, changing the spacecraft's velocity by no 
more than a few centimetres per second. These small deviations will be 
reflected in the spacecraft's radio signals as they are beamed back to 
Earth, and scientists can then translate them into measurements of the 
mass and density structure inside the moon.
Mars Express HRSC image of Phobos, taken on 7 March 2010

Earlier flybys, including the previous closest approach of 67 km in March 
2010, have already suggested that the moon could be between a quarter 
and a third empty space - essentially a rubble pile with large spaces 
between the rocky blocks that make up the moon's interior.

Knowing the structure of the roughly 27 x 22 x 18 km Phobos will help 
to solve a big mystery concerning its origin and that of its more distant 
sibling, Deimos, which orbits Mars at approximately three times greater 
distance.

The leading theories propose that the duo are either asteroids captured 
by Mars, or that they were born from debris thrown up from giant impacts 
on Mars.

"By making close flybys of Phobos with Mars Express in this way, we can 
help to put constraints on the origin of these mysterious moons," says 
Olivier Witasse, ESA's Mars Express project scientist.

In addition to probing the gravitational field of Phobos during its close 
approach, Mars Express will be making measurements of how the solar wind 
influences the moon's surface.

"At just 45 km from the surface, our spacecraft is passing almost within 
touching distance of Phobos," says Michel Denis, Mars Express Operations 
Manager.

"We've been carrying out manoeuvres every few months to put the spacecraft 
on track and, together with the ground stations that will be monitoring 
it on its close approach, we are ready to make some extremely accurate 
measurements at Phobos."

Both the position of the spacecraft and the moon must be known to high 
precision in order to make the most accurate calculations of the moon's 
internal characteristics. To improve the positional data, the spacecraft's 
high-resolution stereo camera has been capturing images of Phobos set 
against the background star field in the weeks leading up to closest approach 
and will continue to do so afterwards.

Furthermore, ground stations around the world will track the spacecraft 
for a total of 35 hours in the lead up to, during, and after the flyby 
to ensure that the position of Mars Express is precisely known.

"Mars Express entered orbit around the Red Planet exactly ten years ago 
this week - this close flyby of Phobos is certainly an exciting way to 
celebrate!" adds Olivier.