[meteorite-list] Mars Express to Make Closest Ever Approach to Phobos

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 26 Feb 2010 10:18:54 -0800 (PST)
Message-ID: <201002261818.o1QIIspO025091_at_zagami.jpl.nasa.gov>

http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=46571

Mars Express to make closest ever approach to Phobos
European Space Agency
24 Feb 2010

On 3 March 2010 Mars Express will make its closest ever approach to
Phobos, the larger of the two Martian moons. During a series of flybys,
spanning six weeks, all seven instruments onboard Mars Express will be
utilised to study Phobos. The close approach provides a first
opportunity to perform a unique gravity experiment that may reveal the
distribution of mass within this intriguing moon.

ESA's Mars Express spacecraft orbits the Red Planet in a highly
elliptical, polar orbit that brings it close to Phobos every five
months. It is the only spacecraft, currently in orbit around Mars, whose
orbit reaches far enough away from the planet to provide a close-up view
of Phobos. Over the course of twelve flybys, taking place between 16
February and 26 March 2010, Mars Express will pass within 1400 km of the
surface of Phobos. The Mars orbiter will make its closest ever approach
to Phobos- just 50 km above the surface - on 3 March 2010.

The suite of seven experiments onboard Mars Express are primarily used
to study the atmosphere, surface and subsurface of the Red Planet. These
science instruments can also be used to investigate Phobos. During this
current series of flybys all Mars Express instruments will be used to
study Phobos, taking advantage of not only the close approach to the
moon but also, for the gravity experiment during the closest flyby, the
proximity of Mars to the Earth.

Phobos flyby campaign 2010

Date Altitude (km)* Instruments used during this flyby
16 February 991 PFS, SPICAM, ASPERA
22 February 574 PFS, SPICAM, ASPERA
25 February 398 PFS, MARSIS
28 February 226 PFS, MARSIS
03 March 50 MaRS, ASPERA
07 March 107 HRSC, OMEGA, MARSIS, SPICAM, ASPERA
10 March 286 HRSC, OMEGA, MARSIS, ASPERA
13 March 476 HRSC, SPICAM, PFS, ASPERA
16 March 662 HRSC, SPICAM, PFS, ASPERA
19 March 848 HRSC, SPICAM, PFS, ASPERA
23 March 1341 Not used
26 March 1304 HRSC, SPICAM, PFS, ASPERA
* Distance from surface of Phobos

Phobos - a moon of unknown origin

Phobos, the larger of the two Martian moons, remains one of the few
objects in the Solar System whose location cannot be easily explained.
By studying Phobos with the Mars Express instruments scientists are
hoping to contribute to the understanding of the moon's nature and
origin. Phobos (and Deimos) could be captured asteroids - early
measurements of the composition of both moons were compatible with this
idea - or they could have formed from material that was ejected
following a large collision with Mars. Additional theories are that the
moons could be survival planetisimals, or formed from the break-up of a
moon that was created early in the formation of the Solar System.
Knowing how the mass is distributed within Phobos is an important step
in understanding the interior of the moon and this in turn will provide
crucial insight into the moon's origin.

Studying Phobos close-up - unique science

At a distance of just 50 km above the surface of Phobos Mars Express
will make the most precise measurements to date of the moon's gravity
field using the X-band (8.4 GigaHertz) channel of the Mars Radio Science
(MaRS) instrument. This instrument relies on the observation of the
phase, amplitude, polarisation and propagation times of radio signals
transmitted from the spacecraft and received at ground station antennas
on Earth. The radio signals are affected by the medium through which the
signals propagate, by the gravitational influence of Mars on the
spacecraft and finally by the performance of the various systems
involved both on the spacecraft and on ground. In addition, during this
series of close flybys, the gravitational attraction of Phobos will
slightly disturb the trajectory of the spacecraft. The difference
between predicted trajectory (without Phobos) and the actually observed
trajectory will lead to the determination of the forces acting on the
spacecraft and from them the gravity field of the moon. To make these
measurements, the spacecraft operates in two-way link mode with an
X-band uplink and downlink.

This current series of flybys happen to occur when the orbits of Earth
and Mars bring them close together which means that Mars Express will be
ideally positioned to maximize the signal-to-noise ratio of the two-way
X-band radio-link. Nasa???s Deep Space Network (DSN) 70 metre radio
station at Robledo, Spain, will track the radio signal from Mars Express
and will pick up the subtle changes in the signal due to the doppler
effect as the gravity of Phobos affects the spacecraft's velocity. In
addition, the ESA Cebreros station will also be listening to the signal.

Mapping the mass distribution of Phobos

Analysis of Mars Express data will provide key coefficients of the
gravity field. The most important coefficient, the mass of Phobos, has
been determined from previous flybys at higher altitudes, but it does
not provide any information about how the mass is distributed.
Calculation of the density of Phobos, using the mass and volume, gives a
value too low to be consistent with a solid, non-porous body, which has
led to speculation about the composition of the moon and about how its
mass is distributed.

Measurement of the gravity field coefficients from a lower altitude, as
will be achieved during this series of flybys, will provide increased
accuracy of the mass and allow the subsequent, smaller coefficients,
such as the J_2 coefficient, to be determined for the first time. To
determine the mass distribution of Phobos these coefficients are
required along with the libration, a measure of how Phobos rotates -
this has already been determined from Mars Express HRSC images.
Knowledge of these various parameters allows the three principle moments
of inertia to be derived - these in turn describe the mass distribution
of Phobos. Models of Phobos' interior are being developed and will be
tested against the findings of the current and future close flybys.

Studying Phobos close-up ??? continuing investigation

In addition to the new science performed during the gravity experiment,
this series of flybys will see Mars Express build on knowledge gained
from previous flybys. HRSC data obtained during previous flybys has led
to the development of a new topographical atlas of Phobos (see M.
W??hlisch et al. (2009) for further details and the Phobos atlas
website). On-going investigations include: improving the accuracy of the
location of Phobos (see J. Oberst et al. (2006), V. Lainey et al.
(2007), K. Willner et al. (2008), P. Rosenblatt et al. (2008)) and
therefore knowledge of its constantly changing orbit as it spirals
slowly towards Mars; measurements of the surface to determine its
composition (see B. Gondet et al. (2008), S. Perrier et al. (2004)),
study of the origin of grooves (see J. Murray et al. (2006)), shape (see
K. Willner et al. (2010)) and sub-surface properties; as well as
studying how the surface interacts with the solar wind.

Scientific objectives of the 2010 Phobos flyby campaign

Instrument Objective
ASPERA Study the interaction between the solar wind and the surface of Phobos
HRSC Produce high resolution images of surface and characterise the Phobos-Grunt landing site
MaRS Determine the Phobos gravity field
MARSIS Study the sub-surface seeking indications of structure and internal composition
SPICAM, PFS, OMEGA Characterise the surface

Mars Express data will provide an important contribution to
understanding the nature and origin of Phobos but this alone will not
provide a definitive answer. Further exploration is required, and in
2011 the Russian Phobos-Grunt (Phobos-Soil) mission is scheduled to
launch to retrieve a sample from Phobos to return for study on Earth.
Images taken by the Mars Express HRSC instrument during this series of
flybys will be used to support the final selection of the Phobos-Grunt
landing site.

Reference publications

M. W??hlisch et al., "/A new topographic image atlas of Phobos/", Earth
Planet. Sci. Lett. (2009), doi:10.1016/j.epsl.2009.11.003

K. Willner et al., "/Phobos control point network, rotation, and
shape"/, Earth Planet. Sci. Lett. (2009), doi:10.1016/j.epsl.2009.07.033

J. Oberst et al., "/Astrometric observations of phobos and deimos with
the SRC on Mars Express/", Astronomy and Astrophysics, Volume 447,
Number 3, pp 1145 - 1151, 2006, doi:10.1051/0004-6361:20053929

V. Lainey et al., "/First numerical ephemerides of the Martian moons/",
Astronomy and Astrophysics, Volume 465, Number 3, pp 1075 ??? 1084, 2007,
doi:10.1051/0004-6361:20065466

K. Willner et al., "/New astrometric observations of Phobos with the SRC
on Mars Express/", Astronomy and Astrophysics, Volume 488, Number 1, pp
361 ??? 364, 2008, doi:10.1051/0004-6361:200809787

P. Rosenblatt et al., "/Accurate Mars Express orbits to improve the
determination of the mass and ephemeris of the Martian moons/",
Planetary and Space Science, Volume 56, Issue 7, pp 1043 ??? 1053, 2008
doi:10.1016/j.pss.2008.02.004

J. Murray et al., "/New Evidence on the Origin of Phobos' Parallel
Grooves from HRSC Mars Express/", 37th Annual Lunar and Planetary
Science Conference, March 13-17, 2006, League City, Texas, abstract no. 2195

B. Gondet et al., "/Phobos Observations by the OMEGA/Mars Express
Hyperspectral Imager"/, 39th Lunar and Planetary Science Conference,
March 10-14, 2008, League City, Texas, abstract no. 1832

S. Perrier et al., "/Spatially Resolved UV albedo spectra of PHOBOS with
SPICAM on Mars Express/", American Astronomical Society, DPS meeting 36,
Session 31.09, Bulletin of the American Astronomical Society, Volume 36,
p 1137, 2004

Contact

Olivier Witasse, ESA Mars Express project scientist
Olivier.Witasseesa.int
Received on Fri 26 Feb 2010 01:18:54 PM PST