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
I’ve just come back from the Mars Express DCR (Dedicated Control Room) where everything is green across the board. In other words, the spacecraft is operating within normal conditions and is in great shape for tonight’s flyby. This ‘pass’ as the engineers call it was the last one before they tell the spacecraft to shut up and keep quiet in preparation for its close pass. During the flyby itself, no data of any kind will be placed on the spacecraft’s radio signals as this would interfere with the accuracy of the sensitive radio tracking. -- Stuart
40m de diámetro del Observatorio Astronómico Nacional en Yebes
I spoke this morning with Mars Express Project Scientist Olivier Witasse, who explained details on an unofficial - but very cool! - radio science tracking activity being conducted by the JIVE (Joint Institute for Very Long Baseline Interferometry in Europe) Institute.
This project is an excellent addition to the 'official' radio science tracking campaign organised by ESA using a number of ESA and NASA ground stations and centred on use of the 70m Deep Space Network station at Robledo, Spain, later this evening during closest approach.
Olivier says that scientists and engineers from JIVE learned about the Phobos flyby campaign and promptly organised a radio tracking campaign of their own, using three stations: Metsahovi (14m, Helsinki, Finland), Wettzell (20m, Munich, Germany) and Yebes (40m, Madrid, Spain).
"JIVE tracking will run for 2 hours around 21:00 UT tonight," says Olivier. "The goal is to record Mars Express signal Doppler with these three stations as an enhancement to ESA's own tracking."
NASA's big dish at Robeledo is the biggest possible 'ear' that we can use to listen to the Mars Express radio signal. But by correlating the data captured by JIVE's smaller stations, it may be possible to extract very precise Doppler data - which is the goal of the Phobos radio science activity.
"A baseline of ~2500 km between Robledo and Cebreros and Helsinki could provide another vantage point on the Phobos-induced Doppler variation; although Metsahovi is just a 14-metre dish, they expect to obtain good Doppler accuracy," JIVE institute Sergei Pogrebenko says.
Afterwards, the JIVE team will share their results with the Mars Express project team and the MaRS radio science team.
Olivier adds: "This is a very nice contribution, very much appreciated! We know JIVE very well, as they also tracked the descent of ESA's Huygens on to Titan, back in January 2005."
Here's an mp3 audio of our discussion earlier today:
Daniel and I have just been speaking to Thomas Ormston, Spacecraft Operations Engineer, and contributor to this blog. We wanted the definitive answer on why the Mars Express camera (HRSC) would not be taking images at closest approach. It turns out that there are three reasons – each of them alone would be show stoppers for the camera.
Phobos on 30 August 2007. Credits: ESA/ DLR/ FU Berlin (G. Neukum)
First is that Mars Express will fly between Phobos and Mars, placing it on the nightside of Phobos. In fact, for a few seconds at closest approach Phobos will actually totally eclipse the Sun. So the hemisphere of Phobos that HRSC would see will be in darkness, apart from the tiny amount of light reflected from the surface of Mars back onto the moon.
Second, at 67 km, the surface of Phobos will be moving through the camera’s field of view too quickly. To track the surface, Mars Express would have to turn or slew as the engineers like to call it. That is not a problem usually as Mars Express is designed to do this. But at the speed at which Mars Express will pass Phobos, the spacecraft will have to slew faster than safety allows. One factor determining the maximum speed is the fragile MARSIS experiment. This is a 40-metre-wide antenna that could break if the spacecraft turns too quickly. And MARSIS is still needed to probe beneath the surface of Mars and Phobos.
Thirdly, even if it were daylight and possible to turn the spacecraft fast enough to take the necessary images, this close flyby is specifically designed to probe the gravity field of Mars. This is a unique experiment and requires the spacecraft to be entirely passive, so that the only deviations to its motion are produced by the gravitational field of Phobos (which for Mars Express is just one billionth the strength of Earth’s gravity at the surface of our planet). Mars Express will therefore point its high gain antenna fixedly at the Earth for the duration of the flyby.
HRSC will swing into action at the next flyby on 7 March, when Mars Express will pass Phobos at around 107 km. The picture above was taken at a previous Phobos flyby, 30 August 2008, at a distance from the moon's centre of 2366 km. -- Stuart
Stuart and I just spoke with Mars Express Spacecraft Operations and Mission Planning Engineer Hannes Griebel (who also contributes to the blog - Thanks!) who is on console in the Mars Express Dedicated Control Room (DCR) here at ESOC. The first track of recorded signals for radio science using NASA's DSS-15 finished this morning just after 08:00 UT (09:00 CET), and the next recording, via DSS-43, started around 09:20 UT and will run for almost two hours. For radio science, the more tracking data, the better. -- Daniel
Martin Pätzold, MaRS Radio Science Principle Investigator, was here at ESA/ESOC, Darmstadt, a few days ago to prepare for this week's Phobos flyby activities.
I had the chance to ask him how the current Mars Express flyby studies could potentially help Russia's planned Phobos-Grunt ('Phobos soil') sample-return mission, expected in the 2012 timeframe.
His answer was interesting, and the activity will also directly benefit ESA's own Rosetta mission (full info after the jump). -- Daniel
Engineering drawing of Russia's Phobos-Grunt spacecraft
In an email earlier today, ESA's Flight Dynamics team here at ESOC provided their latest Mars Express orbit prediction for tomorrow's Phobos flyby.
It is based on ground station tracking data received up until yesterday, and includes data from the NASA Deep Space Network (DSN) station at Goldstone, California, and from ESA's own Deep Space Antenna (DSA) station at New Norcia. The predicted closest approach time to Phobos remains unchanged at 20:55:40 UTC (21:55:40 CET) on 3 March 2010, with a miss distance of 77 km from the centre of Phobos. This implies that Mars Express will pass 67 km above the surface of Phobos (which has an average radius of 11.1 km).
The prediction also confirms that there will be no occultation by Phobos on 3 March, meaning that Mars Express will not pass 'behind' Phobos as seen from the Earth. There will be a very brief eclipse caused by Phobos passing in front of the Sun (as seen from Mars Express) - in other words, Mars Express will enter the shadow cast by Phobos. Thanks to Frank Budnik for the details! -- Daniel
ESA's Mars Express will skim the surface of Mars' largest moon Phobos on Wednesday evening. Passing by at an altitude of 67 km, precise radio tracking will allow researchers to peer inside the mysterious moon. Latest update is now available in the ESA website. -- Daniel
An excellent overview of orbits in space and featuring "Mars Express and the mystery of Phobos" has just been published in the ESA Kids pages. The folks over at the Kids site do an excellent job of making space science accessible to all ages, and in particular those in the elementary school age bracket. The pages are available in English, German, Spanish, French, Italian and Dutch.
If you know of any up-and-coming space fans, be sure to tell them about Mars Express and Phobos and point them toward the ESA Kids site! -- Daniel
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