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  ESA's Mars Express closest flyby to Phobos

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Author Topic:   ESA's Mars Express closest flyby to Phobos
Robert Pearlman

Posts: 27328
From: Houston, TX
Registered: Nov 1999

posted 03-04-2010 09:13 AM     Click Here to See the Profile for Robert Pearlman   Click Here to Email Robert Pearlman     Edit/Delete Message   Reply w/Quote
According to the European Space Agency's (ESA) blog, Mars Express' closest-ever flyby of Mars' moon Phobobs went "smoothly":
Spacecraft in excellent health

According to the bits of telemetry that just came through we have a healthy spacecraft all-around! Despite the heavy use of our batteries with long transmitter times during a season of long eclipses, the spacecraft is in good shape, showing that the flight control team's careful planning of the past weeks paid off.

With a new bag full of high-fidelity radio science data, researchers can now feed their number crunchers for weeks to come. Stay tuned, however, as we have more exciting flybys coming up. The next one as early as Sunday!

ESA's fly-by preview press release (March 3, 2010):
Mars Express heading for closest flyby of Phobos

ESA's Mars Express will skim the surface of Mars' largest moon Phobos this evening. Passing by at an altitude of 67 km, precise radio tracking will allow researchers to peer inside the mysterious moon. Closest approach is at 21:55 CET. Live updates on the Mars Express blog.

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 tonight 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.

Updates as the flybys take place will be posted on the Mars Express blog.

Robert Pearlman

Posts: 27328
From: Houston, TX
Registered: Nov 1999

posted 03-04-2010 10:19 AM     Click Here to See the Profile for Robert Pearlman   Click Here to Email Robert Pearlman     Edit/Delete Message   Reply w/Quote
ESA release
Phobos Flyby Success

Mars Express encountered Phobos last night, smoothly skimming past at just 67 km, the closest any manmade object has ever approached Mars' enigmatic moon. The data collected could help unlock the origin of not just Phobos but other 'second generation' moons.

Something is not right about Phobos. It looks like a solid object but previous flybys have shown that it is not dense enough to be solid all the way through. Instead, it must be 25-35% porous. This has led planetary scientists to believe that it is little more than a 'rubble pile' circling Mars. Such a rubble pile would be composed of blocks both large and small resting together, with possibly large spaces between them where they do not fit easily together.

Last night's flyby was close enough to give scientists their most exquisite data yet about the gravitational field of Phobos. Mars Express locked onto the radio signal from Earth at around 21:20 CET (20:20 UT). The radio frequency oscillators on the ground are 100 000 times more stable than those on the spacecraft, so for this experiment, which required the best precision possible, the signal was sent up to Mars Express and then returned by the spacecraft to the ground.

The radio waves travel at the speed of light and took 6 minutes 34 seconds to travel from Earth to the spacecraft last night. So the round trip time was 13 minutes 8 seconds. Once the signal was received back at Earth, it was clearly strong and good. So strong that radio amateurs were also able to lock onto the signal, although their equipment would not be able to detect the subtle variations induced by the gravity of Phobos.

Now that the data are all collected, the analysis can begin.First will be an estimate of the density variation across the moon. This will tell scientists just how much of Phobos' interior is likely to be composed of voids.

"Phobos is probably a second-generation Solar System object," says Martin Paetzold, Universitat Koln, Cologne, Germany, and Principal Investigator of the Mars Radio Science (MaRS) experiment. Second generation means that it coalesced in orbit after Mars formed, rather than forming concurrently out of the same birth cloud as the Red Planet. There are other moons around other planets where this is thought to have been the case too, such as Amalthea around Jupiter.

Whatever the precise origin, Phobos will eventually crumble back into this disrupted state. It is gradually spiralling towards Mars and will eventually be pulled apart. "It came from debris, it will return to debris," says Paetzold. In the meantime, it is there to be studied and explored.

Last night's flyby was just one of a campaign of 12 Mars Express flybys taking place in February and March 2010. For the previous two, the radar was working, attempting to probe beneath the surface of the moon, looking for reflections from structures inside. In the coming flybys, the Mars Express camera will take over, providing high resolution pictures of the moon's surface.


Posts: 972
From: Lincolnshire IL USA
Registered: Aug 2000

posted 11-25-2010 05:25 PM     Click Here to See the Profile for DChudwin   Click Here to Email DChudwin     Edit/Delete Message   Reply w/Quote
The Mars Express satellite orbiting Mars will soon have an opportunity for another set of close encounters with Phobos during December and January. On January 9 the ESA satellite will come within 96 km of Phobos.

ESA has released new recent photos of Phobos with the surface of Mars as the background. Here are two images 1 | 2

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