Full article via ESA.

• The final pre-flyby orbit determination was concluded earlier this week; this precise calculation of the Mars Express orbit was used to generate the commands for the flyby
• These commands include all necessary pointing and instrument instructions so that the flyby and science observations can be conducted automatically
• They were uploaded to the spacecraft this morning during a ground station pass at 07:00 UT (08:00 CET)
• Mars Express is now in an orbit with respect to Mars ranging from approximately 400 km to 10 000 km; it will pass by the centre of Phobos on Sunday at just 111 km
• On Sunday, 9 January, Mars Express will automatically slew to point instruments toward the expected direction of Phobos starting at 13:55 UT (14:55 CET) - it will stay in this pointing mode for 5 minutes and 46 seconds
• Closest approach occurs at 14:06:30.016 UT (15:06:30.016 CET)
• A few minutes later, after closest approach, it will slew back to Earth pointing and the next ground station pass, via ESA's 35m DSA-2 deep space station at Cebreros, Spain, will begin at 14:57 UT (15:57 CET). Data will be downloaded progressively over the following days(see "When can we expect first images" below)

"The simple fact that this flyby is being done automatically and with little human activity on the days around and during closest approach hides the large amount of work done by the operations team, the flight dynamics team, the mission scientists and the principle investigators to get ready. We are looking forward to a smooth flyby and some very interesting results."

Overall, it's a quiet but nonetheless exciting phase of the mission that promises to provide some excellent and valuable Phobos images and data from close up! We'll update you on Monday as soon as we hear any word from the team at ESOC.  -- Daniel

Our long-time colleague Emily Lakdawalla over at the Planetary Society blog sent in a Twitter query last night: "Would love to see a blog entry explaining more details of how the MEX orbit shifts with time."

Her question, while general, is directly related to this week's Phobos fly-by activity as precise knowledge of the spacecraft's orbit is vital for planning observations (Mars Express will zip by tiny Phobos is just a few seconds - see my earlier post below).

I passed her query to the Mars Express operations team here at ESOC, and received the following reply from Spacecraft Operations Engineer Hannes Griebel. His colleague Thomas Ormston, who also works on the Mars Webcam Blog, additionally took some time to generate a fantastic animation showing how the Mars Express orbit changes over time. First, from Hannes:

Mars Express is on a highly eccentric, polar orbit around Mars. This means that our spacecraft flies over the poles during each orbit, as can be seen in this full-orbit movie captured in May 2010.

However, due to perturbation forces, resulting chiefly from Mars' inhomogeneous gravity field as well as gravity from other celestial bodies and also from light and solar wind pressure, the orbit rotates slightly so that periapsis (point of closest approach to the planet) drifts around Mars.

This is actually a big plus, because over the course of several months, the spacecraft gets a close-up view of all regions - from the snow-covered poles over the windswept plains of the north to the heavily cratered highlands of the south. In other words, as Mars rotates beneath our 7-hour orbit, we get full close-up coverage of virtually the entire surface over time.

And, once in a while, our orbit intersects that of Phobos. The Flight Control and Flight Dynamics teams always take great care to ensure that whenever this happens, Mars Express and Phobos are never at that point of intersection at the same time (this would be sub-optimal -- Ed. :-)).

But, when the geometry is favourable and the two are going to pass close by, a carefully timed manoeuvre, provided by firing the thrusters, can bring our satellite close enough to Phobos to take pictures and other measurements of this extremely interesting moon.

More info available here: http://webservices.esa.int/blog/post/7/1084

Hi Daniel, all,

Please find attached an animation of the Mars Express orbit progression over time. The video runs from the middle of our first year at Mars, 2004, all the way through to 2011... sped up 10,000,000 times! You can see the red path of the Mars Express orbit as it swings around Mars throughout the Martian year (Earth dates are in the top right corner). Every few months this intersects with the orbit of Phobos (the inner green ring, the outer one is Deimos), giving us an opportunity for a Phobos flyby.

The lines don't always display in the correct order on this video, so sometimes it looks like we also cross the orbit of Deimos; this is just an optical illusion on the video (in reality, it orbits too far out from Mars for a close encounter with Mars Express).

Thomas, Hannes: Thanks for the excellent update!

ACCESS VIDEO

Watch the animation in your browser here, or

Download original file (AVI) here, or

Watch it in the 'Friends of ESA' page in Facebook here, or 

See it in the Planetary Society YouTube channel: 

Click for larger view

For the teams working on the Mars Express mission, one of the most important activities during the current fly-by phase is generating and refining estimates of the spacecraft's orbit, which must be as accurate as possible.

This is mandated by the fact that Phobos is really not that big - its mean radius is just 11 km - and the fly-by at closest approach happens rather quickly. It will be over in just a few moments as Mars Express zips by at around 3 km/second.

As a result, the camera and other instruments on board the spacecraft - and indeed the spacecraft itself - must be pointed in precisely the correct direction at precisely the correct time in order to get any useful results at all.

These pointings and timings, in turn, depend on knowing the spacecraft's trajectory very accurately - and estimating this is at least as much an art as it is a science.

The trajectory estimates make use of astrophysical data including the gravity of Mars and Phobos as well as data from the star trackers on board Mars Express (which provide a picture, literally, of the background star field) and radiometric data provided by the ground tracking stations that communicate with the spacecraft, currently ESA's giant 35m station at Cebreros, Spain.

Fixing the orbit is also very much a team effort involving experts from the flight dynamics team here at ESOC, the scientists who operate the instruments (including the HRSC camera and the MARSIS radar), and the flight operations team.

The next set of orbit calculations for closest approach on 9 January will be generated later today, and will be refined later in the week. -- Daniel

A very effective animation showing Phobos flyby on 3 March created by the team at the Boxx blog (audio in French).

This animation shows the Mars Express orbit very well. The animation uses our original, slightly closer approach (50km instead of the actual 67km) because that's what is in the Celestia files that we posted earlier (unfortunately, we haven't yet had time to upload the 'real' files after measuring the effects of the engine burn earlier in the week - we will do so soon!). The orientation of the spacecraft isn't quite correct because throughout the closest approach MEX stayed pointed at Earth so we could receive the radio signal. Nonetheless, the animation is still excellent, and the illustration of how our orbit crosses Phobos is excellent!

Thanks to the Association Planete Mars (French section of the Mars Society) and their newly created "Atelier 3D" (free 3D for Mars exploration) for an excellent job! -- Daniel

Kester Habermann, one of the flight dynamics team members here at ESOC, kindly sent in the data files for anyone who uses the popular (and free) Celestia real-time, 3D visualisation software. With these files, you can play back the entire flyby sequence (details and links after the jump). Thanks, Kester! -- Daniel

Martin Paetzold, Principle Investigator of the Mars Radio Science observation is smiling.  This morning, before the flyby, he admits to having been anxious.  “There was a very small probability of an occultation at closest approach,” he says.  It was a very small possibility but if it happened, Phobos would have blocked the signal with Earth at the critical moment.

There is nothing to be anxious about any more.  Closest approach took place at 20:55 CET.  It took 6 minutes and 34 seconds for signals to cross the volume of space between Phobos and Earth, and be received on Earth.

If the occultation had taken place, it would have created a gap in the data of five to ten minutes whilst the signal link was re-established with the spacecraft. “We would have had to extrapolate between the two data sets and that would not have been good.  Now we have continuous data,” says Paetzold.

The team will receive the data on Friday, and they will begin the full analysis once they receive the precise orbit determination of Mars Express on its way into the flyby.  They hope to assess the data and the preliminary results in about two weeks time.

Let the analysis begin! -- Stuart

Animation showing Mars, Phobos, Diemos and Mars Express in their relative orbits this evening is now live in ESA Flickr (click on 'HD' at bottom right of Flickr playback window to watch in high resolution):

http://www.flickr.com/photos/europeanspaceagency/4403651689/ 

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

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 

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

A few key details gleaned from various sources:

• Planned date/time of closest approach: 3 March 21:55 CET (20:55 GMT)
• Planned altitude above Phobos surface: 67 km
• First planned High Resolution Stereo Camera (HRSC) observations: 7 March, at 107 km altitude
• Phobos shape/size & mass: 27 km × 22 km × 19 km || 1.072 x 10¹⁶ kg, or about one-billionth the mass of Earth
• One-way radio signal travel time between Mars Express and ground stations on Earth: 6 minutes, 30 seconds (6:30)
• Straight-line distance between Mars Express and Earth: 116 919 058 620 m = ~116 million km

Will update this post a little later with more... -- Daniel

Hello once again from the Mars Express flight deck!

OMEGA Visible and Infrared Mineralogical Mapping Spectrometer on board Mars Express - Credits: ESA

The first of the two radio-sounding proficiency tests (see my earlier blog entry on the critical radio science experiments) was completed in the night from 22/23 Feb, and went very well. The second test is scheduled for tonight (24.02), and if no problems are encountered, then Mars Express is all set for probing the moon's gravity field with unprecedented accuracy.

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