Mars Express Archives

March 5, 2010December 24, 2015 by Nancy Atkinson

Could Phobos Be Hollow?

A mosaic image of Phobos composed by 53 pictures. Credits: ESA/ DLR (S. Semm, M. Wählisch, K.Willner)/ FU Berlin (G. Neukum)

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Back in the 1950s and 1960s, there was some speculation that Mars’ moon Phobos could possibly be hollow due to the its unusual orbital characteristics. While scientists now agree that the moon is very likely not hollow, vast caverns may exist within the moon, and it might be a porous body instead of solid. The Mars Express spacecraft made a close flyby of Phobos on Wednesday to help provide more data on the interior of Phobos, and all indications are the event was a big success. The spacecraft skimmed smoothly over the odd-shaped moon at just 67 km, the closest any manmade object has ever been. No images were taken from this flyby. Instead all the instruments were turned off so that ground stations could listen for a pure radio signal of how Phobos “tugged” on the spacecraft. Scientists say the data collected could help unlock the origin of Phobos and other ‘second generation’ moons.

“Phobos is probably a second-generation Solar System object,” said Martin Pätzold, 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 at the same time 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.

Previous flybys of Phobos 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.

The March 3rd flyby was close enough to give scientists the best data yet about the gravitational field of Phobos.

The radio waves travel at the speed of light and took 6 minutes 34 seconds to travel from Earth to the spacecraft on Wednesday night, and by analyzing the data on Phobos’ gravity field, scientists should be able to estimate of the density variation across the moon and detect just how much of Phobos’ interior is likely to be composed of voids.

This 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.

Source: ESA

March 2, 2010April 26, 2016 by Nancy Atkinson

Follow Closest Flyby of Phobos in Real Time

Phobos from Mars Express. Credit: ESA/DLR/FU Berlin (G. Neukum).

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Mars Express will skim over the surface of Mars’ largest moon on Wednesday, making the closest flyby of Phobos by any spacecraft. Passing at just 67 km above the surface, precise radio tracking will allow researchers to virtually peer inside the mysterious moon. You can follow the flyby in “real time,” — allowing for the current 6 minute and 30 second light time delay from Mars (13 minutes round trip) – on the Mars Express blog. The flyby will take place on March 3, at 20:55 GMT.

The straight-line distance between Mars Express and Earth is now about 116 million km.

Flying by at such close range, Mars Express will be pulled ‘off-course’ by the gravitational field of Phobos. This will amount to no more than a few millimeters 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. Phobos’ shape is 27 km × 22 km × 19 km, and has a mass of 1.072 x 1016 kg, or about one-billionth the mass of Earth.

To make the very sensitive measurements of Phobos’ interior, all the data signals from the spacecraft will be turned off. 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 practice.

Originally, the closest flyby was going to only 50 km above the surface, but a slight ‘over performance’ during a maneuver 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 jeopardize the tracking measurements, it was decided to perform another maneuver to position the flyby at a slightly higher altitude than originally planned.

An illustration showing the ESA's Mars Express mission. Credit: ESA/Medialab)

Mars Express will zoom past Phobos seven more times after Wednesday’s closest approach. The first planned High Resolution Stereo Camera (HRSC) observations will be on March 7, when the spacecraft will be at 107 km altitude above Phobos.

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.

Source: ESA

December 11, 2009December 24, 2015 by Nancy Atkinson

Phobos and Deimos Together At Last!

ESA’s Mars Express orbiter took images last month of Mars two moons, Phobos and Deimos. This is the first time the moons have been imaged together in high resolution, but as Emily Lakdawalla points out on Planetary Blog, not the first time the two have been imaged together: the Spirit rover did it back in 2005! But these new image definitely provide a ‘wow’ factor, as well as helping to validate and refine existing orbit models of the two moons.

Phobos and Deimos together for the first time in high resolution. Credits: ESA/DLR/FU Berlin (G. Neukum)

“It doesn’t happen very often that both Martian moons are right in front of the camera, directly one behind the other,” said Harald Hoffmann from the German Aerospace Center (DLR).

“During the now more than six-year long project, we have had several opportunities to photograph the two moons together,” said Klaus-Dieter Matz, who worked with Hoffmann to plan the acquisition of these images. “The geometry of the constellation during Orbit 7492 on 5 November 2009 was especially favorable, so this time we wanted to try taking a sequence of photographs – and this first attempt has delivered the expected result!”

The geometrical relationships between Mars, its moons and the Mars Express probe at the time of the sequence. Credit: DLR

Phobos, the larger of the two moons, orbits closer to the Red Planet, circling it every 7 hours and 39 minutes. It travels faster relative to Mars than the Moon relative to Earth. It was 11,800 km from Mars Express when the images were taken. Deimos was 26,200 km away.

The images were acquired with the Super Resolution Channel (SRC) of the High Resolution Stereo Camera (HRSC). The camera took 130 images of the moons on 5 November at 9:14 CET over period of 1.5 minutes at intervals of 1s, speeding up to 0.5-s intervals toward the end. The image resolution is 110 m/pixel for Phobos and 240 m/pixel for Deimos — since Deimos was more than twice as far from the camera.

Sources: DLR, ESA