As this particular Phobos flyby season comes to an end, I caught up with Michel Denis, spacecraft operations manager; Nicolas Altobelli, scientist with the science ground segment; and Olivier Witasse, project scientist, to ask how it all went and what we can look forward to in the future. -- Stuart
This has been the most ambitious Phobos flyby campaign yet attempted, how did it go?
Michel Denis: “After almost 8000 orbits which have all been a bit different from each other, and after about 10 Phobos campaigns, we had mixed feelings between the old-sailor's confidence "it will be all-right, we have mastered much more complex activities already" and the juvenile excitement of the beginners "it's so close, and we must turn the spacecraft swiftly, and what happens if...?"
With today's flyby at 12:17 UTC (13:17 CET), the current Phobos flyby campaign will be complete.
Given the present polar and elliptical orbit of Mars Express (meaning that it orbits over the Red Planet's North and South poles and varies in altitude from fairly close to quite far away), and the near-circular and equatorial orbit of Phobos, encounters between the bodies can be expected every five months. However, the number of flybys, the geometry and the closest distances are always different.
The current campaign was unique: It stands out with a total of 12 fly-bys (more than any campaign before) and a minimum approach distance of 77 km from the centre of the moon, which corresponds to approx. 67 km from the surface (the closest flyby of any spacecraft to date).
So, what about the next campaigns? Let's look ahead to the next two flyby 'seasons.'
The next one will come in August 2010, with nine flybys within 1200 km from Phobos. The closest approach will be about 403 km over Phobos' night side, on 24 August 2010. Then, between December 2010 and January 2011, 10 flybys are planned, with the closest one coming within 96 km of the moon's dayside, on 9 January 2011. (This is, by the way, roughly four weeks before Mars passes behind the Sun (as seen from Earth), and only a few days before Mars Express science operations will stop for approximately 6 weeks due to this so-called 'Solar conjunction'.)
Quite interesting observations can be made from such distances indeed! The powerful cameras, spectrometers and other instruments on Mars Express can glean amazing details; repeated flybys will augment coverage of the surface of Phobos, help confirm or improve previous findings, complement existing data sets, and possibly even make new discoveries.
So we hope you'll stay with us as we set out to explore this mysterious, small world even further. This month's closest-ever flyby lies behind us, but the excitement is far from over. On the contrary, it has only just begun!
I’ve just heard that the technical paper discussing the mass and density of Phobos, as determined during the 2008 flyby, has been accepted by Geophysical Research Letters. The abstract is:
We report independent results from two subgroups of the Mars Express Radio Science (MaRS) team who independently analyzed Mars Express (MEX) radio tracking data for the purpose of determining consistently the gravitational attraction of the moon Phobos on the MEX spacecraft, and hence the mass of Phobos. New values for the gravitational parameter (GM=0.7127 ± 0.0021 x 10-³ km³/s²) and density of Phobos (1876 ± 20 kg/m³) provide meaningful new constraints on the corresponding range of the body's porosity (30% ± 5%), provide a basis for improved interpretation of the internal structure. We conclude that the interior of Phobos likely contains large voids. When applied to various hypotheses bearing on the origin of Phobos, these results are inconsistent with the proposition that Phobos is a captured asteroid.
If you are a subscriber to the journal, you can access the full paper here.
For a less technical treatment, see the piece I wrote for New Scientist, where some of the authors talk about this work in the section under the 'Space oddity' heading.
(Click on 'Full story' for full GRL journal reference).
I have just received the following information from Andrea Cicchetti of the MARSIS radar team. Congratulations to all of those involved; I look forward to the full analysis. -- Stuart
MARSIS, the radar aboard the Mars Express orbiter, has successfully observed Phobos again. MARSIS is a multi-frequency, synthetic aperture, orbital sounding radar that operates in the range 1.3-5.5 MHz with a 1 MHz bandwidth.
The closest flyby in which MARSIS operated during this fruitful Phobos season took place on March 7th 2010, during the 7915th orbit of Mars Express, when the orbiter reached a minimum distance from the surface of Phobos of about 112 km. For instrument safety reasons, the radar software blocks operations when the target is closer than 240 km. Therefore the team had to devise a new set-up of the main navigation parameters, allowing them to reduce the minimum operational distance down to 175 km, while maintaining a high level of safety for the instrument’s hardware.
During this flyby, MARSIS successfully collected 6478 echoes from Phobos in just 72 seconds. The carrier frequency was centred at 4.0 MHz with a 1 MHz bandwidth. Taking advantage of the instrument’s internal mass-memory facility, it was possible to store and then downlink the raw unprocessed echoes.
Credit: ESA and MARSIS team.
After the ground-processing of science data, it was found that the radar worked successfully during the flyby. The figure above shows echoes reflected by Phobos as the highest peak in the signal, clearly above the noise level. Scientific analysis of the results is still ongoing. The main quest is the determination of the origin of detected echoes: are they reflections from various surface features of Phobos, or have they been produced by the internal structure of the moon?
The MARSIS radar was originally designed solely for the observation of Mars. However, thanks to the Italian operations team, working in collaboration with the international instrument science team, it was possible to re-configure it to allow the observation of Mars’ moon Phobos, a unique target, thereby expanding the scientific capabilities of the mission.
In the first phase of the data analysis, the main goal was to validate a new operative configuration of the onboard software and hardware. The scientific analysis of existing and future data will provide us with new and unique insights on the nature of Phobos’ interior.
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
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.
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.
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
The Mars Express HRSC team sent in a pair of new images earlier today based on imagery acquired on 10 March 2010. (Click on images to access full-size versions.) -- Daniel
Phobos as seen by the HRSC nadir channel during Mars Express Orbit 7926. This image was
enhanced photometrically to better bring out features in the less-illuminated part.
Resolution: about 9 metres/pixel. Credits: ESA/DLR/FU Berlin (G. Neukum)
The High Resolution Stereo Camera (HRSC) on board the ESA spacecraft Mars Express
took the images for this 3D stereo composite of the surface of the moon Phobos on
10 March 2010. The image data were taken from a distance of 278 km with a spatial
resolution of about 9 m/pixel in orbit 7926. All images are contrast-enhanced and
geometrically adjusted for flight movements. The anaglyph has also been slightly
geometrically adjusted at the left rim of the Phobos image to correct for viewing conditions.
A pair of excellent animations sent in by the HRSC team showing the Phobos flyby on 3 March 2010. The first shows Mars Express on its closest-approach orbit, with first Mars than Phobos passing below. The second is an overview showing the relative orbit orientations of Mars, Phobos and Mars Express on 3 March (first animation is below; click on 'Full story' for second...). -- Daniel
Copyright: ESA/DLR/FU Berlin (G. Neukum) - Credit: S. Walter/Celestia/NAIF/SPICE
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