[meteorite-list] Mars Express Heading for Closest Flyby of Phobos

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 2 Mar 2010 14:31:49 -0800 (PST)
Message-ID: <201003022231.o22MVnsD009201_at_zagami.jpl.nasa.gov>

http://www.esa.int/esaCP/SEMFKA6K56G_index_0.html
 
Mars Express heading for closest flyby of Phobos
European Space Agency
1 March 2010

ESA's Mars Express will skim the surface of Mars' largest moon Phobos on
Wednesday evening. Passing by at an altitude of 67 km, precise radio
tracking will allow researchers to peer inside the mysterious moon.
 
Mars Express is currently engaged in a series of 12 flybys of Phobos. At
each close pass, different instruments are trained towards the mysterious
space rock, gaining new information. The closest flyby will take place on 3
March at 21:55 CET (20:55 GMT).

>From close range, Mars Express will be pulled 'off-course' by the
gravitational field of Phobos. This will amount to no more than a few
millimetres every second and will not affect the mission in any way.
However, to the tracking teams on Earth, it will allow a unique look inside
the moon to see how its mass is distributed throughout.

How will the ground teams make these tremendously sensitive measurements?
Ironically, they will turn off all data signals from the spacecraft. The
only thing that the ground stations will listen out for is the 'carrier
signal' -- the pure radio signal that is normally modulated to carry data.

With no data on the carrier signal, the only thing that can modulate the
signal is any change in its frequency caused by Phobos tugging the
spacecraft. The changes will amount to variations of just one part in a
trillion, and are a manifestation of the Doppler effect -- the same effect
that causes an ambulance siren to change pitch as it zooms past.

Two dress rehearsals for this exacting operation have already taken place,
allowing ground station personnel and spacecraft controllers to practise
their various roles. Now it is time to do it for real. Originally planned
for 50 km altitude, Mars Express will now pass Phobos at 67 km.

A slight 'over performance' during a manoeuvre last week had put the
spacecraft on a trajectory that included an occultation by Phobos. This
meant that Mars Express would pass behind Phobos as seen from Earth. As this
would jeopardise the tracking measurements, it was decided to perform
another manoeuvre to position the flyby at a slightly higher altitude than
originally planned.

After the closest flyby, the work is not over. Mars Express will sweep past
Phobos a further seven times before the campaign is complete. In addition to
the tracking experiment, known as MaRS for Mars Radio Science, the MARSIS
radar has already been probing the subsurface of Phobos with radar beams.
"We have performed a preliminary processing of the data and the Phobos
signature is evident in almost all the data set," says Andrea Cicchetti,
Italian Institute of Physics of Interplanetary Space, Rome, and one of the
MARSIS team.

The camera, HRSC, will be used on the 7 March flyby, when Mars Express
passes over the daylight side of Phobos at an altitude of 107 km, and will
continue to be used during all the subsequent flybys, obtaining
high-resolution images of the moon's surface. The other instruments will
also get their chance to work.

ASPERA is already studying the way charged particles from the Sun interact
with the surface of Phobos. SPICAM, PFS, OMEGA are characterising the
surface of the moon, with PFS also aiming to measure the temperature of
Phobos' day and night sides. HRSC will pay particular attention to the
proposed landing site for the Russian Phobos-Grunt mission, which is
expected to launch in 2011/12.

"All the experiments on Mars Express have something to say about Phobos,"
says Olivier Witasse, Mars Express Project Scientist, ESA. This is a bonus
for science, considering that none of them were originally designed to study
Phobos the moon, only Mars the planet. The science results from these flybys
are expected in subsequent weeks or months, when the various teams have had
time to analyse the data.

More information:
 
Updates as the flybys take place will be posted on the Mars Express blog,
     http://webservices.esa.int/blog/blog/7
Received on Tue 02 Mar 2010 05:31:49 PM PST