Without doubt, Phobos is the grooviest moon of the Solar System. By that I mean, that it is covered with a multitude of parallel grooves.

Initially, it was thought that these markings radiated away from the largest crater on Phobos. Called Stickney, the crater has a diameter of 9 km and is the most obvious feature of the moon’s pockmarked surface. Some thought that the grooves were debris ejected across Phobos during Stickney’s creation. In other words, they were similar to the bright rays of material seen emanating from some craters on the Moon. Most thought that they were fractures in the moon, opened up by the mighty impact. But these hypotheses were based upon an incomplete picture of Phobos – literally and metaphorically.

This close-up of the grooves of Phobos was taken on 3 August 2008 at a distance from the moon’s centre of 656 km. Details as small as 6 metres across are shown in this image. Credit: ESA/ DLR/ FU Berlin (G. Neukum). 

Once the High Resolution Stereo Camera on Mars Express had mapped the majority of the moon, a different point of view emerged. The images reinforced an often-overlooked fact that the grooves fall into 12 families, each of a different age and orientation. They also showed that the grooves do not radiate from Stickney at all but from the leading apex of Phobos, the part of the moon that always faces forward. This suggests a much more exotic origin. Instead of being ejecta from Stickney, they could be ejecta from Mars. 

The current thinking is that if a large enough asteroid impacts the surface of Mars, it will throw debris into space. If the ejecta cloud crosses Phobos’s orbit it could strike the moon, like bugs on a car’s windscreen, creating the grooves. If this is the case, it leads to an intriguing possibility.

Collections of martian rocks could be lying on the surface of Phobos, perhaps even dating from all points in the Red Planet’s history. When the Russian mission Phobos-Grunt returns samples from the surface of Phobos in 2012, researchers may be in for a treat. In amongst the Phobos rocks, they may just find some from Mars too. -- Stuart

The top image shows a sketch map of the grooves of Phobos revealed by HRSC, MOC & Viking images. The bottom image shows a model of the grooves expected from Phobos ploughing through debris fields around Mars. Taken from: New evidence on the origin of Phobos’ parallel grooves from HRSC Mars Express by John B. Murray et al.

Of course you don’t, but this latest post will give you a great ‘behind the scenes’ read about the Mars Express mission. In particular, it highlights the role played by the Science Ground Segment at The European Space Astronomy Centre (ESAC) during the Phobos flyby season. It is their job to make sure that the observations requested by the scientists are possible and then to ensure that they are performed. My thanks go to Nicolas Altobelli and Alejandro Cardesin for taking the time to put this together. -- Stuart

Generic role of Science Ground Segment and the Science Operations Centre:

The Science Operations Centre (SOC) is part of the Science Ground Segment (SGS) of the Mars Express mission and is located near Madrid, in Villanueva de la Canada. It provides scientific and technical co-ordination between the instrument teams and the Mission Operation Centre (MOC) located at the European Space Operations Centre (ESOC) in Germany. The SOC is composed of a core team of four scientists and engineers working with the support of a broad group of experts on software, technical and various scientific topics, all of them within the Solar System Science Operations Division.

The main objective of the whole SGS is to co-ordinate the scientific observation requests from the Mars Express instrument teams and to build the final observation plan that fits in terms of available spacecraft resources (mainly battery power, data rate and downlink capacity). Among other tasks, SGS is responsible for defining the final pointing of the spacecraft and planning the operation of the payloads. These planning activities are performed three months in advance to allow the resolution of scientific and technical conflicts. Should two instruments wish to observe in two different directions, obviously they cannot be operated at the same time, and one has to defer to the other.

The SGS also helps teams convert their high-level scientific requirements into geometrical constraints and compute the required spacecraft attitude and associated timings. This latter activity is called Opportunity Analysis and answers the question: When is What feasible and How?

Diagrams like this show the Mars Express team exactly what the spacecraft is doing. 

In a sense, if the MOC can be considered as the skipper of a boat, who pays attention to the 'steering' (spacecraft trajectory control) and the general boat maintenance (fuel available, general health of components...), the SGS can be seen as the navigator and sailor, indicating the directions to follow (spacecraft attitude), looking at all the weather and environmental conditions (planet and spacecraft ephemeris, solar flux, martian season, etc) and carefully taking care of the needs of the passengers (scientific teams) and relying on the rest of the crew (technical experts). As it is not always possible to satisfy all the passengers at the same time, the SGS will receive instructions from the Project Scientist to prioritise the requests. 

Phobos specials

The Mars Express spacecraft is on a highly inclined and eccentric orbit around

Mars and periodically crosses Phobos’ equatorial and nearly circular orbit. With a slight offset of the spacecraft position on its orbit, performed by the Flight Dynamics Team at ESOC, the spacecraft can make multiple close Phobos encounters. During this period of time, called the 'Phobos Flyby Season', the routine co-ordination task of the SGS becomes more hectic.

Co-ordination meetings and teleconferences are set up between the instrument teams and the SGS to collect special requirements that have to be applied during the flyby measurements. The SGS provides a detailed geometrical description of the events, relevant for any type of experiment performed. These include the minimum approach distance, the solar phase angle value (which determines the illumination conditions), the spacecraft velocity profile, whether Phobos will be in Mars' shadow, whether an occultation of radio links is expected. In addition to the geometrical studies, the SGS performs an estimate of the spacecraft resources that will be needed by the experiments, and judges the overall feasibility of the observation plan before sending it to MOC for implementation.

Because the precise position of the spacecraft on its orbit cannot be known to any high precision (within 1 second) until a few days before the Phobos flybys, the SGS co-ordinates specific observation requests from the High Resolution Stereo Camera (HRSC) directly with Flight Dynamics. For example, when the spacecraft gets very close to Phobos, (say 100 km), the high relative velocity would smear the images and so this must be compensated by spacecraft motion (slews). A complicated slew pattern is computed by the HRSC team and communicated to Flight Dynamics, who will make certain that it is executed in a timely manner by the spacecraft, using the latest orbital determination result. The SGS supervises this information exchange and ensures that the instrument commanding will stay in line with the final spacecraft orientation strategy. 

Nicolas Altobelli, Alejandro Cardesin

We’re all thrilled to see the Phobos image from the 10 March Mars Express flyby featured on the Astronomy Picture of the Day website today.  Even if you saw the image yesterday, do go and have a look, the APOD team have provided an excellent set of links in the caption for you to explore more about the mysterious origin of Phobos. Check it out here. -- Stuart

Just wanted to keep you all informed. The camera team are busy processing the flyby images. We'll be posting them as soon as we have them. Stay tuned! -- Stuart

A short video clip recorded 9 March 2010, 19:50 CET, showing ESA's giant 35m deep space antenna at Cebreros station (Spain) - part of the Agency's ESTRACK network - swinging into position to start a ground station pass. The weather tonight in central Spain is crystal clear and cold (below freezing) - perfect!

CEB was used last week to track Mars Express during Phobos closest approach... -- Daniel

Just had the following message through from Mars Express Project Scientist Olivier Witasse. -- Stuart

“The Mars Express Radio-Science team, led by Martin Pätzold (Cologne University), has performed a preliminary analysis of the radiometric data recorded during the evening of closest approach, 3 March 2010. 

The NASA ground-station DSS-63 near Madrid recorded the frequency of the transmitted signal, at about 8.4 GHz and 2.3 GHz, which contains the signature of Phobos’ gravity field. To be able to decipher this weak signature, the team has subtracted all known variations, which would have been measured even in the absence of Phobos. What remains is produced by the gravity of Phobos pulling Mars Express.

Credit: ESA/ Department of Planetary Research at the University of Cologne (M. Pätzold). 

The grey line in the image shows the frequency change due to Phobos during a 20-minute window, centred on the closest approach. Before closest approach, the effect of Phobos on the spacecraft is negligible. Then there is a clear jump in frequency at closest approach. This is Phobos slightly changing the orbit of Mars Express.

The blue line is the expected frequency change assuming the mass of Phobos, as measured during a previous flyby, is evenly distributed throughout the moon’s interior. There are clearly small differences between the blue and grey lines. The challenge now for the Radio-Science team is to dig into these small differences to prise out information on the mass distribution. “The real work starts right here,” says Pätzold.

“It may take a few weeks for the extraction of precise information on the interior of Phobos,” says Tom Andert, from Munich University.

The Mars Express close encounter with Phobos was also observed by three European VLBI network stations: the 20 m Wettzell radio telescope (Bundesamt für Kartographie und Geodäsie, Forschungseinrichtung Satellitengeodäsie, Germany), the 14 m Metsähovi (Aalto University - School of Science and Technology, Finland), and 40 m Yebes (Observatorio Astronómico Nacional, Instituto Geográfico Nacional, Spain). Data processing was performed at the Metsähovi Radio Observatory, and analysis at the Joint Institute for VLBI in Europe (The Netherlands).

The residual Doppler frequency pattern, as detected by the Wettzell radio telescope is shown on the plot. The sharp swing of the carrier line frequency at 21:03 UT corresponds to the closest proximity of MEx spacecraft to Phobos. The people who contributed to this project are Guifre Molera Calves and Jan Wagner (MRO, Finland), Gerhard Kronschnabl (BKG, Germany), Pablo de Vicente (OAN-IGN, Spain), and Sergei Pogrebenko (JIVE, The Netherlands).

Just spoken to Prof. Thomas Duxbury, of the Physics and Astronomy Department, George Mason University, Fairfax, USA, about the current Phobos flybys, when the camera will be used. Tom is an interdisciplinary scientist on Mars Express, and a specialist on Phobos. Note that Tom refers to Phobos-Grunt as 'PhSRM', for 'Phobos Sample Return Mission'. -- Stuart

Q: How do images of Phobos help 'map out' a possible landing place for the Russian Phobos-Grunt mission? 

A: One HRSC 9-channel (stereo-plus-colour-plus-SRC) image sequence within 100 km of the Russian PhSRM proposed landing site would be an excellent start. Then, additional images at higher resolution, different viewing and lighting would be desired - but not required. The images provide information on geology, composition and topography that are needed to select a safe landing site of high scientific value.

Q:  How will the Mars Express studies, of the mass and composition of Phobos, help Phobos-Grunt?

A:  The PhSRM spacecraft will be in the near vicinity of Phobos for many months before landing. The Phobos mass and gravity field will move the spacecraft orbit around, causing the project [team] to perform orbit correction manoeuvres to keep the spacecraft in the desired orbit. The more accurately the mass and gravity field are known now, the more accurately the PhSRM mission can be designed. Fewer orbit corrections would make the flight operations prior to landing significantly simpler. The Phobos-Grunt Spacecraft

Q: What other information from Mars Express will be useful to Phobos-Grunt?

A: Typically, before landing on a planetary body, one has a series of flyby missions and then orbiting missions. Mars Express is providing the equivalent of these to PhSRM with the multiple close flybys - previously, now and in the future. Mars Express can provide information to PhSRM that ranges from crucial to very helpful: such as improved knowledge of the Phobos orbit; the Phobos rotation; the Phobos mass and gravitational field; the global size, shape and topography; and, the dust and plasma environment around Phobos and near its orbit. It may also provide knowledge of the regolith thickness, and whether Phobos is formed from accreted chunks or is solid.

I have just heard some more news about Sunday’s Phobos flyby from Mars Express Project Scientist Olivier Witasse. He says, “We are now entering a new phase for the Phobos flybys. The dayside encounter phase means that remote sensing can proceed at full speed!”

The MARSIS, SPICAM, OMEGA, ASPERA experiments will all be working, as will, of course, the camera (HRSC). The Sunday flyby will take place at an altitude of 107 km, and provide the opportunity for high-resolution imaging. It is a delicate operation.

The camera is fixed in position on the spacecraft and cannot move independently. So, to keep tracking Phobos the whole spacecraft will have to turn. Because of the large MARSIS antenna, which measures 40 metres end-to-end, the spacecraft is usually only turned at a rate of once every 40 minutes. On Sunday, the team will exceed this a little to keep Phobos centred in the camera. But the tracking does means that we will all have to be patient before seeing the images.

The spacecraft cannot point in two directions at once. It cannot track Phobos and keep pointing its high gain antenna to Earth. So the images will be stored onboard and then downlinked at the next available ground station pass. “The images will arrive on the ground on Monday,” says Witasse. The data will then pass straight to the camera team, who will begin the processing.

Witasse suggests that images may be available by Wednesday, once the processing is complete. Watch the blog for updates to this schedule. -- Stuart

Mars Express has just locked onto the radio signal from the ground station that it will use to trace the gravity field of Mars.  The radio frequency oscillators on the ground are about 100 000 times more stable than those on the spacecraft, so a signal is sent up to the Mars Express and this is returned by the spacecraft to the ground.  Variations in the frequency of this signal will then be used to calculate the gravity field of Mars.  The more stable the original frequency the better the data.

The light travel time is 6 mins 34 seconds one way (radio waves also travel at the speed of light).  So round trip time is 13 mins 8 seconds. -- Stuart

There is one instrument that can be left on during the close flyby of Phobos. ASPERA (Analyzer of Space Plasma and Energetic Atoms) was created to study the interaction between the constant stream of charged particles from the Sun and the Martian atmosphere. 

This stream of particles is called the solar wind and the data ASPERA collects addresses important questions in Martian science.

Graphic illustration of Phobos and Mars Express just prior to closest approach this evening. Credit: ESA/ESOC Flight Dynamics. More in ESA Flickr

For example, how strongly is the Martian atmosphere affected by the solar wind?  Can this interaction explain where the Martian water went? In other words, was the water lost to space? Since liquid water is the fundamental requirement for life, a clear understanding of the fate of the Martian water supply is crucial to resolving the mystery of whether life ever existed on Mars.

During the Phobos flyby, ASPERA will detect and characterise the particle population around Phobos.  The instrument is comprised of four sensors that passively collect any particle that happen to hit it.  So as not to jeopardize the tracking signal accuracy, ASPERA’s detections will be recorded on board and beamed back to Earth later.

At Phobos there is no atmosphere to interact with, and so the solar wind will slam directly into the rocks on the surface of the moon.  This happens to any airless body.  Our own Moon is constantly struck by solar wind particles.  Each impact moves the lunar dust by a tiny amount.  Eventually the process will even destroy the footprints left by the Apollo astronauts – but not for millions of years. At Phobos there are no footprints (yet)! -- Stuart

Those of you who have been following the blog will know that on Monday the estimated flyby distance of 50 km was revised to 67 km.  This will not affect the science investigation and has come about because of what spacecraft engineers call an “over performance”.  This means that during the manoeuvre to place Mars Express on its flyby trajectory, the engine expended just a little bit too much thrust.

The over performance was well within the normal limits of uncertainty but meant that the spacecraft would arrive at Phobos 20 seconds late.  In this time, Phobos would have moved on a little and so the closest distance that Mars Express would find itself to Phobos was going to be 60 km.

If you could stand on the nightside of Phobos tonight, you would see Mars Express zoom past in front of Mars. Credits: Alex Lutkins

The flight team at ESOC then began their calculations to understand exactly what this new scenario would mean and they discovered that the new trajectory would cause Mars Express to pass behind Phobos as seen from the Earth.  So Phobos would block the radio signal to Earth just at the moment when the most precise tracking data was being gathered.  That simply could not happen.

So the spacecraft was manoeuvred once more to delay its closest approach by a little more, meaning that it would remain visible from the Earth at all times.  The drawback is that the altitude would rise more, by 7 km.  However, this is not expected to affect the science – which is what this flyby is all about.

“It will mean the Doppler signal is a little smaller than at 50 km but it will not affect the science.  The main point is that we are significantly closer than 100 km,” says Hannes Griebel of the Mars Express flight control team, and one of our blog contributors. -- Stuart

Mars Express is currently working through its series of Phobos flybys, heading for its closest approach on 3 March 2010.  Different instruments are used on different flybys to gain different information about the mysterious moon.

ASPERA is studying the interaction between the sleet of electrically charged particles given out by the Sun, called the solar wind, and the surface of Phobos. HRSC will produce high resolution images of surface, paying particular attention to the Phobos-Grunt landing site.

MaRS will determine the Phobos gravity field allowing the internal distribution of mass to be determined. MARSIS is studying the sub-surface of Phobos, seeking indications of structure and internal composition. SPICAM, PFS, OMEGA are characterising the surface of the moon, with PFS aiming to measure the day and night side temperature. 

Digital terrain model of Phobos derived from HRSC data. Published in M. Wählisch et al., "A new topographic image atlas of Phobos", Earth Planet. Sci. Lett. (2009), doi:10.1016/j.epsl. 2009.11.003 Credits: ESA/ DLR/ FU Berlin (G. Neukum)

The full list of flybys, altitudes and instruments is as follows:

Date Altitude (km) * Instruments used during 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 the surface of Phobos

You can read a detailed rundown of the flyby campaign here. -- Stuart

Phobos observed by the HRSC. Credits: ESA/ DLR/ FU Berlin (G. Neukum) 

New webpages went online today allowing anyone to download data from the digital terrain model of Phobos created by the EuroPlanet project, hosted by the German Aerospace Center.  Images and data from the High Resolution Stereo Camera (HRSC) and the Super Resolution Channel (SRC) aboard Mars Express were used to create the three-dimensional representation of the moon. -- Stuart

More information is available here.