The date was 24 April, 1990; “Liftoff of the Space Shuttle Discovery, with the Hubble Space Telescope, our window on the universe”.
Over the next ten days I’ll be reviewing these twenty years, starting with the first two today; I hope you will enjoy the show.
Of course, the Hubble’s history goes back many years before 1990; astrophysicist Lyman Spitzer is credited with the first paper proposing a space-based optical observatory, in 1946! He spent a good half century working on the idea (Trivia fact: Spitzer really knew his plasma physics; among other things he founded the Princeton Plasma Physics Laboratory, in 1951; the PPPL is home to some exciting magnetic reconnection experiments). Not so well-known, in the US at least, is that European involvement in the Hubble – via the European Space Agency (ESA) – dates from 1975, 15 years before its launch (Trivia fact: ESA’s Space Telescope European Coordinating Facility (ST-ECF) issued its first newsletter in March 1985). HST WF/PC first light image (Credit: NASA/ESA/STScI)
For all the brilliant engineering, the best money could buy, the Hubble’s primary mirror was ground to exquisite precision and accuracy … but precisely and accurately wrong; the “presence of significant spherical aberration” was announced by NASA at the end of June, 1990. (Trivia fact: the cause of the mis-grinding was a field lens in the reflective null corrector used to test the figure of the primary mirror; it was “mis-located by about 1.3mm” Did heads roll as a result?)
However, because the primary mirror was ground so precisely and accurately, if wrongly, images sent back from the Hubble could be processed to largely remove the unintended blur, and so after a half year or so of rather intense work, the scientific show did go on. Supernova 1987A (Credit: NASA, ESA, STScI)
And what a show it was! Saturn's North Polar Hood (Credit: NASA, ESA, STScI)
Take a trip down memory lane, check out Hubble’s image of Saturn’s North Polar Hood; it’s zoomable!
But a faulty mirror and image processing are not quite the real thing; sometimes there are image processing artifacts, as this 1991 image of a nearby supernova-to-be shows: Eta Carinae (Credit: NASA, ESA)
Of course it wasn’t only pretty pictures that the Hubble returned to Earth; a great many papers based on the astronomical data from the Hubble were published in its first two years of operation, covering a wide range of topics (perhaps I’ll base a future Universe Puzzle on this, maybe ‘what was the first such paper?’). And it wasn’t only images; the Hubble carried an instrument called the Faint Object Spectrograph, which worked in a part of the electromagnetic spectrum accessible only from space, the far ultraviolet (click on this link to read about limits on He I emission, the He I Gunn-Peterson effect, and Ly-alpha absorption spectrum “at z roughly 0.5”).
What’s your favorite from the first two years? 3C 273, 2003 HST image (Credit: NASA/J.Bahcall(IAS))
Mine’s The Ultraviolet Absorption Spectrum of 3C 273; not only is about the iconic quasar 3C 273, not only is it a classic John Bahcall paper (he writes so well!), not only does it illustrate well the scientific power of spectroscopy, but shines a light on composition of the intracluster medium.
Tomorrow: 1992 and 1993, including COSTAR and the first servicing mission.
Spirit rover, as seen by HiRISE on Feb. 15, 2010. Crop and colorization by Stuart Atkinson, image credit: NASA/JPL, U of AZ
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The HiRISE team released some new images on Wednesday — one of which was another incredible avalanche image. But then there was another shot of Gusev Crater, the home of the Spirit rover. It was a wide shot of the entire region (you can see it below), and visible are the Columbia Hills, and if you look real close you can see the “Home Plate” region where Spirit sits. Our friend Stu Atkinson took a real close look and found Spirit sitting all alone –but very visible in this wonderfully amazing zoom-in closeup! (Click the image for access to a larger version). Stu also colorized it to show almost intricate detail of Spirit’s solar panels. The image was taken on Feb. 15, 2010, and she looks great! She’s in her current stationary position, and even though this image is from before she went into hibernation, it’s great to know she’s still sitting there, waiting for warmer days. “Hang in there rover, hang in there…” Stu said on Twitter, which echoes all our sentiments. Awww, Spirit….
Thanks to Stu and HiRISE for keeping our hopes alive!
The Spirit rover landing region. Credit: NASA/JPL/University of Arizona
Lunar Landscape. Credit: LROC; Goddard Space Flight Center and U of AZ, 3-D modeling and rendering by Bernhard Braun.
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You may recall Bernhard Braun as the wizard from UnmannedSpaceflight.com who created the amazing 3-D images of the Mars avalanche. Now he’s created incredible planetary landscapes for a different world: the Moon. “Actually, this has been my very first attempt with lunar imagery after my previous work has almost been exclusively devoted to Mars,” Braun said. The special software he developed can create three dimensional images from one 2-dimensional picture, but he says the real stars are the spacecraft that gather the data, the Lunar Reconnaissance Orbiter and the Mars Reconnaissance Orbiter. “It is the unprecedented quality together with the unprecedented availability of the raw data that opens the door for everyone to explore new ideas and processing techniques,” Braun said.
See below for more stunning from-the-surface 3-D looks at the Moon – no special 3-D glasses needed!
Lunar Landscape. Credit: LROC; Goddard Spaceflight Center, U of AZ, 3-d rendering and modeling by Bernhard Braun.
I asked Braun if working with images from the Moon was different than working with Mars images. “Creating the single-image shading-derived DEMs from the Moon imagery is both easier and more difficult at the same time when compared to the same process applied to Mars images,” he said. “It’s easier because the lunar surface does not vary as much in its intrinsic albedo, i.e. the visible brightness variations are almost exclusively caused by variations in surface topography, especially at low illumination angles, which can be exploited by the reconstruction algorithm to derive high precision 3D geometry.”
But the work is more difficult because of the totally black shadows on the Moon due to lack of any atmosphere. “So on the Moon any shadows are virtually featureless areas where the 3-D reconstruction algorithm cannot infer anything about the structure of the invisible shadowed surface,” Braun said. “This is different on Mars, where the shadowed areas are usually lit indirectly by considerable amounts of ambient light scattered by dust particles suspended in the atmosphere. So the 3-D models of the Mars surface can be more complete, showing surface details even in shadowed areas.”
Lunar Landscape. Credit: LROC; Goddard Spaceflight Center, U of AZ, 3-d rendering and modeling by Bernhard Braun.
“All in all it’s a lot of fun to play around with both camera and sun positions until an interesting landscape shot is found,” Braun said. “I would like to add that much of the credit must really go to those true wizards at NASA/JPL for not only making and bringing to orbit these almost unearthly powerful cameras like LROC and HiRISE … but also for sharing the whole image catalog via the internet with everyone in the world!”
Braun said he hopes to tackle 3-D views of the Apollo landing sites — which we cannot wait to see!
This new image from Planck spans about 50° of the sky. Credits: ESA/HFI Consortium/IRAS
Dust has never looked so beautiful! This new image from the Planck spacecraft shows giant filaments of cold dust stretching through our galaxy. The image spans about 50 degrees of the sky, showing our local neighborhood within approximately 500 light-years of the Sun. “What makes these structures have these particular shapes is not well understood,” says Jan Tauber, ESA Project Scientist for Planck. Analyzing these structures could help to determine the forces that shape our galaxy and trigger star formation. Continue reading “Planck Reveals Giant Dust Structures in our Local Neighborhood”
This image, taken by Lunar Reconnaissance Orbiter's high-resolution camera, shows the final resting place of Lunokod 2, as well as the crater that caused its death. Credit: NASA / GSFC / Sergei Gerasimenko / Sasha Basilevsky, via the Planetary Society Blog
On Monday, NASA released the complete set of science data from the Lunar Reconnaissance Orbiter Camera’s first six months of observations, consisting of more than 100,000 lunar images. Straight away, Phil Stooke from the University of Western Ontario began scanning the images to help find a “missing” Russian rover on the lunar surface, the Lunokhod 2. It didn’t take him long to discover the tracks left by the lunar sampler 37 years ago after it made a 35-kilometer trek. “The tracks were visible at once,” said Stooke.
UPDATE: It turns out the original image that showed what Dr. Stooke thought was the Lunokhod 2 rover’s location was not quite correct. Emily Lakdawalla posted a story about it on The Planetary Society Blog, and so I checked with Stooke. He replied: “After I posted my “discovery” Sasha Basilevsky, a veteran Russian planetary scientist, sent me and Emily an image – the one she put on her blog – which shows the true situation. My dark spot is a dark marking the rover made as it turned in place before heading out on one last short drive. That took it out beyond the edge of my image. That new image shows the rover as a bright spot. Yes, I concur with their interpretation. My spot was made by the rover but it’s not actually the rover itself.”
So, I have updated the image above to show the actual final resting spot. The black arrow shows the spot that Stooke originally thought was the rover, where the white arrow shows the real rover. The smaller white arrows point out the rover’s tracks. (end of update)
Phobos, as seen by Mars Express on March 7, 2010. Credits: ESA/DLR/FU Berlin (G. Neukum)
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ESA released new images of Mars’ moon Phobos, taken during the Mars Express March 7, 2010 flyby, showing the rocky moon in exquisite detail and also in 3-D. Mars Express orbits the Red Planet in a highly elliptical, polar orbit that brings it close to Phobos every five months, and it is the only spacecraft currently in orbit around Mars whose orbit reaches far enough from the planet to provide a close-up view of Phobos. Like our Moon, Phobos always shows the same side to the planet, so only by flying outside the orbit is it possible to observe the moon’s far side. Mars Express did such flybys on March 7, 10 and 13. Get out your 3-D glasses for a great look at Phobos, below.
Phobos in 3-D. taken by the High Resolution Stereo Camera (HRSC) onboard the ESA spacecraft Mars Express. Credits: ESA/DLR/FU Berlin (G. Neukum)
Phobos is an irregular body measuring some 27 × 22 × 19 km. Its origin is debated. It appears to share many surface characteristics with the class of ‘carbonaceous C-type’ asteroids, which suggests it might have been captured by Mars. However, it is difficult to explain either the capture mechanism or the subsequent evolution of the orbit into the equatorial plane of Mars. An alternative hypothesis is that it formed around Mars, and is therefore a remnant from the planetary formation period.
The Phobos/Grunt landing site, as seen by Mars Express on March 7, 2010. Credits: ESA/DLR/FU Berlin (G. Neukum)
In 2011 Russia will send a mission called Phobos–Grunt (meaning Phobos Soil) to land on Phobos, and an experiment will collect a soil sample and return it to Earth for analysis.
Phobos-Grunt will also carry with it The Planetary Society’s LIFE experiment which will test the survivability of microorganisms in the conditions of deep space. The experiment is a study of the panspermia hypothesis, which posits that microorganisms have traveled between planets sheltered deep inside space rocks.
For operational and landing safety reasons, the proposed landing sites were selected on the far side of Phobos within the area 5°S-5°N, 230-235°E. But new HRSC images showing the vicinity of the landing with better illumination from the Sun that previous images, which will provide valuable views and information for mission planners.
Mars Express will continue to encounter Phobos until the end of March, when the moon will pass out of range. During the remaining flybys, the high-resolution camera and other instruments will continue to collect data.
Representing what may be the first long term lunar environmental impact study, recent laser ranging data from the Apache Point Observatory in New Mexico suggests the Lunar Ranging Retro Reflectors (LRRRs) left on the Moon by Apollo missions 11, 14 and 15 are beginning to shows signs of age.
Apache Point Observatory’s Lunar Laser-ranging Operation (the acronym says it all) has been collecting ranging data from the LRRRs since 2006, using a 3.5 metre telescope and a 532 nm laser.
A typical APOLLO observing session involves shooting the laser at the largest of the LRRRs (Apollo 15’s) over a ‘run’ of four to eight minutes. Each shot sends about 1017 photons to the Moon, from which only one returned photon per shot may be detected. This is why the laser is shot thousands of times at a 20 Hz repetition rate during each run.
If the return signal from the Apollo 15 LRRR is good, the laser is then directed to fire at the Apollo 11 and 14 reflectors. The laser can even be directed to the Russian Lunokhod 2 reflector, landed on the Moon in 1973, although this reflector does not return a reliable signal if it is in sunlight, probably because heating affects the reflectors’ refractive index and distorts the return signal.
Lunokhod 2 (moon walker in Russian), an 840 kg rover that landed on the Moon on January 15, 1973 and undertook scientific investigations on the lunar surface until May 1973.
The Apollo LRRRs were designed to remain isothermal, even in direct sunlight, to avoid the problem apparently suffered by Lunokhod 2. But a review of current and historical data has revealed a noticeable decline in their performance at each Full Moon. Since the reflectors are directed straight at Earth, they experience the most direct sunlight at a Full Moon.
Recent Apache Point Observatory data has been compared to historical data collected by earlier observatories involved in lunar laser ranging. For the period 1973 to 1976, no Full Moon deficit was apparent in data records, but it began to emerge clearly in a 1979 to 1984 data set. The research team estimate that return signal efficiency at Full Moon has degraded by a factor of 15 over the approximately forty years since the Apollo reflectors were placed on the Moon.
While heating effects may play a part in the performance degradation of the LRRRs, lunar dust is suggested to be the more likely candidate, as this would be consistent with the very gradual performance degradation – and where the most substantial performance loss occurs right on Full Moon. These findings may require careful consideration when designing future optical devices that are intended to remain on the lunar surface for long periods.
On the bright side – all the reflectors, including Lunokhod 2’s, are still functioning on some level. Hopefully, decades before their slow and steady decline progresses to complete failure, even more efficient replacement devices will be landed on the lunar surface – perhaps carefully positioned by a gloved hand or otherwise by robotic means.
Mosaic of microscopic images of Spirit underbelly on Sol 1925 (June 2009) showing the predicament of being stuck at Troy with wheels buried in the sulfate-rich martian soil. The sulfate deposits formed by aqueous (water-related) processes when this area dubbed “Home Plate’ was volcanically active. This false color mosaic has been enhanced and stretched to bring out additional details about the surrounding terrain and embedded wheels and distinctly show a pointy rock perhaps in contact with the underbelly. Spirit fortuitously discovered extensive new evidence for an environment of flowing liquid water at this location on Mars adjacent to ‘Home Plate’, an eroded over volcanic feature. Credit: Marco Di Lorenzo, Ken Kremer - NASA/JPL/Cornell
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The plucky Mars rover ‘Spirit’ may yet rove again !
She’ll just have to outwit and outlast the unavoidably harsh cold and desperately low power levels of the looming winter in Mars southern hemisphere. Rather long odds to be sure – but she’s done it before. Remember – at this moment on Sol 2192, Spirit is 75 months into her 3 month mission ! That’s 25 times beyond her “warrenty” as Rover Principal Investigator Prof. Steve Squyres of Cornell University is extremely fond of saying.
Spirit has been stuck in a rut since becoming mired in a sand trap of soft soil in April 2009. While driving on the western edge of ‘Home Plate’, she unknowingly broke through a hard surface crust (perhaps 1 cm thick) and sank into hidden soft sand beneath. Her wheels churned, sinking deeply – and became embedded in the sand trap at the place called ‘Troy’.
See the underbelly mosaic above, created by Marco Di Lorenzo and Ken Kremer, to get a glimpse of the predicament Spirit lurched into with her sunken wheels. Note: we have significantly enhanced and stretched the mosaic to extract out hard to see details.
As luck would have it, “Troy is a great place to be stuck. It’s like getting stuck in front of Disneyland”, says John Callas the Mars Rover Project manager at the Jet Propulsion Laboratory.
Spirits wheels are buried in sulfate-rich Martian soil. “The sulfate deposits formed by aqueous [water-related] processes when volcanism was active at this area dubbed Home Plate”, says Roy Arvidson, the deputy Rover Principal Investigator. “Spirit has uncovered evidence for two timescales of water related processes”.
“The Troy area is one of the most scientifically interesting ones that we’ve found the whole mission, and we rarely have enough time to study something this thoroughly. So we’re putting that to good use”, Squyres told me.
Spirit Traverse close-up map shows route rover has traveled from Sol 200 to today. Spirit is located at black dot, adjacent to western edge of ‘Home Plate’ volcanic feature. Husband Hill is upper left of center. Click to enlarge. Credit: NASAAt a press briefing on 26 January 2010, NASA declared Spirit would henceforth be a “stationary lander” after exhaustive extrication efforts failed to ‘Free Spirit’. So the small remaining team of rover handlers are now busily getting Spirit ready for sleeping – and surviving – through plunging winter temperatures at her current parked position. Spirit may enter a ‘hibernation’ mode for weeks or even months at a time as she struggles to survive through the freezing winter cold of her upcoming 4th Martian winter.
Remarkably, just as Spirit was concluding her escape efforts and being put into her “stationary” winter parked position to achieve a more optimal tilt for power generation, she finally managed to move about 34 centimeters (13 inches) toward the south southeast in a series of drives beginning on Sol 2145 (Jan. 15, 2010). Her final movement was on Sol 2169 (Feb. 8, 2010). The left-rear wheel even moved out of a rut that it had dug into back in April 2009.
Spirit Traverse close-up map shows entire route rover has traveled until today. Spirit is located at black dot, adjacent to western edge of ‘Home Plate’ volcanic feature. Husband Hill is upper left of center. Click to enlarge. Credit: NASABut with the onset of winter and dropping power levels, time had just run out for further escape attempts. The team was forced to halt extrication efforts and focus instead on maximizing chances for mere survival. The goal was to adjust the tilt of the rover more towards the sun in order to increase the energy output generated from the wing like solar arrays. This strategy had been successfully implemented and saved Spirit during the last two Martian winters as she eked out just enough power to endure – and make new breakthrough (pun intended) science discoveries !
Given that Spirit did move 13 inches in the final drive attempts, I enquired whether NASA is reevaluating to try more driving IF she survives winter ?
Yes.
“The rover team does plan to try driving Spirit out of the immediate sand trap if the rover survives the winter. That was always a possibility”, Guy Webster informed me. He is the Public Affairs Officer at the Jet Propulsion Laboratory which manages the Mars rover project for NASA. Webster cautioned that, “With only four working wheels, the expectation is that even if Spirit gets out of Troy, the rover will not be able to rove significant distance, but might reposition itself to reach different targets in the immediate vicinity”.
Due to the extremely low power levels, the team is implementing plans to minimize energy usage by shutting down almost all functions except keeping a master clock running and checking its power status periodically until it has enough power to reawaken. Even communications will only be on an sporadic basis.
As of Feb. 23, the power output was down to 163 watt-hours. That compares to roughly 900 watt hours at landing. Earlier in the mission, “the line of death” was considered to be in the 200-250 watt hour range. Now, out of necessity, the team has developed ways to operate the rover on somewhat less power.
To get an idea of just how deeply Spirit was embedded and better evaluate her chances to escape, the team seized on an idea that was completely different. For the very first time, they commanded Spirit to carefully maneuver the robotic arm to peek underneath the rover and image her underbelly using the microscopic imager (MI) mounted at the end of the arm’s ‘hand’.
“We used the MI in a new way, because we have no bellycam”, explained Callas.
This action was outside the design envelope of the arm and not previously contemplated partly because the MI is a short focuser, built to focus on objects only 6 centimeter (2.4 inches) away – as well as concerns for damaging the rover and arm assembly. Nonetheless it was hoped that the expected fuzzy pictures would clarify the situation somewhat and perhaps elucidate clues about the wheels and terrain.
Indeed, the underbelly images from Sol 1925 in June 2009 revealed significant new details on how deeply the wheels had sunk and also discerned an upward pointing rock, possibly in contact with the rovers belly. See our mosaic (above) assembled from the MI images. If the rover was caught on the rock, the wheels could be spinning aimlessly if not in firm contact with the soil and thereby obstruct potential escape movements.
At the time of her embedding, Spirit was making great progress towards her next science targets, ‘Von Braun’ mound and the ‘Goddard’ depression in hopes of further elucidating the historical record of flowing liquid water in the Columbia Hills region where Spirit safely landed on Jan 3, 2004.
‘Von Braun’ is the intriguing feature some 100 meters distant at the top left of the mosaic (below) created by Ken Kremer and Marco Di Lorenzo for Spaceflight Now. ‘Troy’ is located approximately in the left of center foreground of the mosaic which was assembled from images taken by the Pancam imager just a few sols – martian days – before she became stuck. Pancam is bolted to the top of Spirits head like mast.
Mosaic of the area adjacent to Home Plate where Spirit remains stuck was made especially for Spaceflight Now, and is used by permission. It shows smooth area, foreground, that concealed slippery water related sulfate material where rover became stuck. Credit: Kenneth Kremer, Marco DiLorenzo, NASA/JPL/Cornell/Spaceflight Now
Teams had spent many months developing and testing numerous drive strategies to escape by using nearly identical replicas of the rovers at a test bed at the Jet Propulsion Lab. They solicited ideas worldwide from outside experts. No clear answers emerged, according to Project manager John Callas.
At last Spirit was commanded to move her 5 still functioning wheels. The 6th wheel had broken long ago after climbing down from Husband Hill (see our mosaic below). After a somewhat hopeful start, actual progress could only be measured in millimeters of movement. And her wheels began to sink deeper. Then another wheel broke, leaving only 4. Thus an already desperate situation became much worse with diminished wheel capacity. At last she moved that last foot, slightly improving her chances. And that’s the State of Spirit today.
Self portrait of Spirit atop Martian mountain top on Sol 618, September 2005. Spirit climbed for more than 1 year to reach the summit of Husband Hill where she made crucial discoveries related to water flowing on Mars. This mosaic was assembled by a group of mars enthusiasts at Unmannedspaceflight.com and originally published on the cover of Aviation Week & Space Technology magazine on 14 November 2005. Credit: Marco Di Lorenzo, Doug Ellison, Bernhard Braun, and Kenneth Kremer – NASA / JPL/Cornell/Aviation Week & Space Technology
The solar powered Spirit is now tilted unfavorably, about 9 degrees to the south. The team stove mightily to obtain the desired northward tilt to achieve a better attitude for generating energy from the sun in the northern Martian sky.
Only time will tell the outcome. Let’s pray for clear skies for Spirit.
Squyres has often been quoted to say, “Never bet against the rovers. Those who have bet against the rovers have been repeatedly proven wrong !”
Spirit recorded this fisheye view with its rear hazard-avoidance camera after completing a drive during the 2,169th Martian day, or sol, of Spirit's mission on Mars (Feb. 8, 2010). The drive left Spirit in the position where the rover will stay parked during the upcoming Mars southern-hemisphere winter. The top of the image shows the underside of Spirit's solar array. Credit: NASA/JPL-Caltech
A radar on NASA's Mars Reconnaissance Orbiter has detected widespread deposits of glacial ice in the mid-latitudes of Mars.NASA/JPL-Caltech/ASI/University of Rome/Southwest Research Institute
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Sure, it’s big news the Moon has water ice on the north pole, but Mars is loaded! A new extensive radar mapping of the middle-latitude region of northern Mars shows that thick masses of buried ice are quite common beneath the surface. The Mars Reconnaissance Orbiter’s Shallow Radar instrument has detected subsurface ice deposits that extend for hundreds of kilometers in the rugged region called Deuteronilus Mensae, about halfway from the equator to the Martian north pole. “We have mapped the whole area with a high density of coverage,” said Jeffrey Plaut from JPL. “These are not isolated features. In this area, the radar is detecting thick subsurface ice in many locations.”
The Shallow Radar instrument has been charting the locations of these hidden glaciers and ice-filled valleys, finding that the most common locations are around the bases of mesas and scarps, and confined within valleys or craters. After obtaining more than 250 observations of the study area, which is about the size of California, the science team is beginning to understand how these deposits may have been left as remnants when regional ice sheets retreated.
“The hypothesis is the whole area was covered with an ice sheet during a different climate period, and when the climate dried out,” Plaut said, “these deposits remained only where they had been covered by a layer of debris protecting the ice from the atmosphere.”
These buried masses of ice are a significant fraction of the known non-polar ice on Mars. The ice could contain a record of environmental conditions at the time of its deposition and flow, making the ice masses an intriguing possible target for a future mission with digging capability.
MRO will continue mapping the area to provide more insight into the buried ice.
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.
