Skimming Phobos

Inputs from today's blog post were provided by Thomas Duxbury, an interdisciplinary scientist on MEX for the Mars moons and Mars geodesy/cartography (and also a co-investigator on the HRSC scene team), Dmitri Titov, ESA's Mars Express project scientist, and Simon Wood, from the MEX mission operations team at ESOC, ESA's European Space Operations Centre, Darmstadt, Germany.

On Thursday, 14 January, ESA’s Mars Express spacecraft will make an unusually close flyby of the largest Martian moon, Phobos, passing the surface at just 53 km at closet approach at 16:00:21 UTC (17:00 CET) on orbit 15260.

The event will mark the spacecraft’s closest flyby of the moon in 2016, and, as a point of comparison, most of the other almost-60 Phobos flybys this year will occur between several hundred up to almost 2000 km. So it’s a real skimmer!

Phobos flyby 14012016

Predicted view from MEX for the 14 Jan 2016 Phobos flyby. The centre image is the predicted perspective view of Phobos at closest approach. This shows the view along Phobos’ shorter axes and it appears smaller than the other two images, which show the view along Phobos’ longest axis. Credit: T. Duxbury

The flyby will enable Mars Express instruments, especially the HRSC – the High Resolution Stereo Camera – to see points of the moon’s surface that have not previously been observed from such a close range.

“This flyby will provide very good viewing, within 1,000 km, of an area previously not seen well,” Dmitri Titov, ESA's Mars Express project scientist. “HRSC will be taking images; the MARSIS radar and the ASPERA-3 particle instrument will operate as well to sound the subsurface and plasma environment of the moon.”

+ marks the spot

The “+” in the predicted images (see above) indicates a possible landing site for the future Russian Phobos Grunt sample return mission.

“This flyby is important as it will allow us to finally view this area on Phobos that has yet to be seen at high resolution and excellent lighting,” says Thomas Duxbury, professor in planetary science at George Mason University, USA.

In the past, Mars Express has made closer flybys, but not by much. On 29 December 2013, Mars Express flew by at just 45 km, close enough that the moon’s gravity pulled the spacecraft slightly off its course, enabling new estimates of the Phobos mass and density.

Phobos 2010

Phobos as seen by the HRSC nadir channel during Mars Express Orbit 7926 in 2010. Credit: ESA/DLR/FU Berlin (G. Neukum)

The flyby is an operational challenge as well as a scientific opportunity, as the positions of the moon and Mars Express must be known very, very precisely in order to safely make the ‘skim-by’.

Commands on board

Commands to trigger the instruments’ observations were uploaded  Thursday, 7 January, following last-minute optimisation of the expected position of Phobos relative to the spacecraft provided by the flight dynamics team at ESOC , Darmstadt.

“This is needed due to the high level of precision required to target Phobos with the instruments at such a close distance,” says Mars Express Spacecraft Operations Engineer Simon Wood.

“The activity will then take place fully automated and without intervention by the operations team at ESOC, who will be closely monitoring the flyby.”

Deciphering Phobos

Flybys such as this help generate evidence to understand how the moon was formed.

The mass of Phobos is estimated as 1.0603 x 10^16 kg (uncertainty less than 0.5 %) and the density is 1862 kg/m3 (uncertainty less than 2%). For comparison, the density of Mars is about 3930 kg/m3, and Earth has a density of around 5520 kg/m3.

The low density of Phobos is consistent with the moon having a high porosity with an uneven mass distribution; in other words, it is essentially a rubble pile with large empty spaces between the rocky blocks that make up the moon’s interior.

This favours the formation scenario in which Phobos was born in orbit around Mars from a disc of debris and is not a captured asteroid – one of the other leading theories for how Phobos and its sibling Deimos came into existence.

Mars Express in orbit around Mars. Credit: ESA/AOES Medialab

Mars Express in orbit around Mars. Credit: ESA/AOES Medialab

The debris disc could have resulted from a large impact on the surface of Mars, or perhaps Phobos (and maybe Deimos) formed from left-over debris from the formation of Mars itself.

Data from such flybys will also prove valuable in planning future robotic or even human missions to land on the moon, and ideal location from which to observe Mars.

It is expected that the initial results from this flyby will be available in the coming weeks.

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Editor's comment: It is interesting to note that, because the polar orbit of Mars Express intersects the equatorial orbit of Phobos, at some point in the future – long after Mars Express has depleted its fuel and has been shut down – the spacecraft is likely to impact the moon.

Listening to Syzygy from a garden in France

This morning – Bertrand Pinel, a keen amateur radio-astronomer who's shared his work with us before – was awake to listen to Phobos making is Syzygy occultation of Mars Express.

Bertrand Pinel's 3.5m backyard antenna. Image credit: B. Pinel

Bertrand Pinel's 3.5m backyard antenna. Image credit: B. Pinel

At 01:11 UT (03:11 CEST), Mars Express was occulted by tiny Phobos, a roughly potato-shaped body measuring 27 km across its longest axis and 18 km across its shortest.

Bertrand (the same gentleman who gave us the Phobos sound recording) captured the drop in signal (as Phobos blocked the direct line of sight between MEX and Earth) using his own radio dish antenna in his garden in France. Some screen shots are included below from his signal processing equipment – you can see the gap and the times of occultation.

MEX-Phobos occultation Credit: Bertrand Pinel

MEX-Phobos occultation Credit: Bertrand Pinel

Thanks, Bertrand, for sharing!

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Mars Express, Phobos and the occult … ation

Today's post contributed by Michael Khan, a mission analyst at ESOC and an avid amateur astronomer – Ed.

Imagine you're playing Scrabble. The tiles on your rack don't look as if there is much you could do with them. No vowels. Three 'Y's, a 'Z', and a 'G'. Oh no! You might as well pass and exchange all tiles for new ones, Right?

You think you're good at online Scrabble? Well, I just put "syzygy" on the board - see if you can beat that! Credit: E. Yourdon CC BY-NC-SA

You think you're good at online Scrabble? Well, I just put "syzygy" on the board - see if you can beat that! Credit: E. Yourdon CC BY-NC-SA

Wrong! (Don't quit yet!)

Just look for a letter 'S' somewhere on the game board and you can append five of your tiles to spell the word SYZYGY (preferably such that it covers a triple word score). Thus you get rid of the 'Z' and the 'Y's and you earn heaps of points. There will be complaints around the table, but you can relax and let them complain: You really cleaned their clocks with this one. You win!

In astronomy, a syzygy defines a situation where at least three bodies are aligned. Eclipses, such as the 15 April lunar eclipse, are a perfect example of a syzygy in action.

Via Flickr E. Yourdon

In spacecraft operations, syzygies abound, and our Mars Express is no exception. Often, they are regarded as an unavoidable nuisance. When the spacecraft passes through the shadow of Mars and has to run on batteries, that is a syzygy of Mars Express, Mars and the Sun. When MEX is occulted by Mars and communications are interrupted, that is a syzygy of MEX, Mars and the Earth.

Other syzygies, however, offer real science opportunities. Stellar occultations by bodies in the Solar System have led to very important scientific discoveries.

To name but a few:

When will we see the results?

ESA's Olivier Witasse provided this update on Phobos flyby results.

The flyby data analysis should give 2 information:

  1. The mass of Phobos and
  2. A gravity coefficient called "J2" (sometimes called C20), that will tell us something about the interior structure of Phobos

The mass of Phobos is already known with a great accuracy. From this flyby, we might expect a better accuracy on the mass determination, but that will not change the big picture.

Same for the density: The density is calculated from the mass and the volume, therefore the new density estimated from the flyby won't be that different compared to the known Phobos density. We can expect a number before the end of the month for sure.

In contrast, the most interesting and new result would be about the J2 gravity field coefficient. The data analysis to get the value will be very complex, and the first result may take a few months.

And the crater is…

On Christmas Eve we posted a new image of a small portion of Phobos taken by Mars Express only a few days before, on 22 December, and gave you the challenge of working out where on Phobos you thought it was located.

As correctly commented by JSch, the crater in the bottom left corner is indeed Wendell crater!

Here’s part of the image again, rotated and with coordinates overlaid:

Wendell crater on Phobos

Wendell crater on Phobos. Credits: ESA/DLR/FU Berlin (G. Neukum)

Thanks to our DLR colleagues Marita Waehlisch and Juergen Oberst for confirming the location!

And congratulations to JSch for the quick and correct call!

Alien audio: Listen to Mars Express skim by Phobos

An excellent result from the dedicated efforts of amateur* radio astronomer Bertrand Pinel, who lives near near Castelnaudary, France.

This sound file is a transposed version of the Doppler signal transmitted at radio frequencies by Mars Express on 29 December as it sped past Phobos. Its orbit was altered a tiny amount due to Phobos' (tiny) gravity, and this was reflected in the Doppler shift, which you can hear.

Bertrand says: "For the 29 December Phobos flyby, the favourable view over the horizon from my [home] station towards Mars was ideal to attempt recording the MEX X-Band radio signals."

Bertrand Pinel's 3.5m backyard antenna. Image credit: B. Pinel

Bertrand Pinel's 3.5m backyard antenna. Image credit: B. Pinel

The 'station' he refers to is his home-built 'satellite tracking station'.

It comprises an old HP network analyser and spectrum analyser, two Rubidium atomic clocks, a Wandel and Goltermann Selective Level Meter and a 3.5m dish antenna salvaged from the Aussagel tracking station (Issus-Aussagel, near Toulouse) when CNES decommissioned it.

He sent in the audio recording, acquired around 07:20 GMT (08:20 CET) on 29 December, together with a number of screen shots from his spectrum analyser, showing his (excellent) results tracking the Mars Express Phobos flyby (click on link below for images in the full post).

He writes:

Le fichier Audio est l 'authentique signal radio du satellite Mars Express au cours de la manoeuvre. On notera que le récepteur doit être recalé 2 ou 3 fois , causé par un effet Doppler important  (et mesurable).

The audio file is the authentic Mars Express satellite radio signal during the manoeuver. Note that the receiver had to be readjusted two or three times, due to the large Doppler effect (which was measurable).

Thank you, Bertrand for sharing your results (which have already been sent to the Mars Express science team). These are a wonderful New Year's present! Congratulations for an excellent effort!

Happy New Year to everyone from all of us at the MEX Blog!

Continue reading

Phobos: Like a Phoenix from the ashes

While we wait for the scientific results of Sunday’s close flyby to be analysed, we can take a look at the results from the previous closest flyby, in 2010, some results from which are currently in press in the journal Icarus ("Phobos Mass Determination from the very Close Flyby of Mars Express in 2010" M. Pätzold et al.).

Phobos as seen by the HRSC nadir channel during Mars Express Orbit 7926. Credit: ESA/DLR/FU Berlin (G. Neukum)

Phobos as seen by the HRSC nadir channel during Mars Express Orbit 7926. Credit: ESA/DLR/FU Berlin (G. Neukum)

In short, the paper summarises the geophysical parameters of Phobos and discusses their implications. The mass of Phobos is estimated as 1.0603 x 10^16 kg (uncertainty less than 0.5 %) and the density is 1862 kg/m3 (uncertainty less than 2%). For comparison, the density of Mars is about 3930 kg/m3, and Earth has a density of around 5520 kg/m3.

The low density of Phobos is consistent with the moon having a high porosity with an uneven mass distribution; in other words, it is essentially a rubble pile with large empty spaces between the rocky blocks that make up the moon’s interior.

This favours the formation scenario in which Phobos was born in orbit around Mars from a disc of debris and is not a captured asteroid – one of the other leading theories for how Phobos and its sibling Deimos came into existence.

The debris disc could have resulted from a large impact on the surface of Mars, or perhaps Phobos (and maybe Deimos) formed from left-over debris from the formation of Mars itself.

Whatever the source of the material, the paper concludes: "Phobos must be seen as a second-generation Solar System object born like a Phoenix from the ashes."

Sunday’s flyby brought Mars Express to just 45 km the moon, enabling even more accurate measurements to be made – some of which could not be derived even during 2010’s close pass – so there is still more incredible scientific knowledge to look forward to!

Last update from Mars Express team

Update on Phobos flyby from Mars Express engineer James Godfrey at the end of Sunday evening.

Erhard Rabenau and I have come tonight in for the last leg of the flyby operations.

Mars Express orbiting the Red Planet - artist's impression Credit: ESA/Alex Lutkus

Mars Express orbiting the Red Planet - artist's impression Credit: ESA/Alex Lutkus

NASA's Goldstone 70m antenna has just finished tracking Mars Express, which
concludes the NASA deep-space network support to the Mars Express Phobos flyby. Many thanks to NASA for their support! The remaining 6 hours of radio science operations – that is, tracking Mars Express as its orbit is perturbed by Phobos – will be performed using ESA's 35m antenna at New Norcia in Australia (which is routinely used for Mars Express tracking).

At 17:06 GMT (18:06 CET), we started receiving telemetry via New Norcia in
parallel with Goldstone, which still had a command connection to the spacecraft. At 18:37 GMT (19:37 CET), we brought down the uplink from Goldstone and one minute later started to uplink from New Norcia.

The rest of tonight's operations will be run by the on-shift Spacecraft Controllers, Achim Zschaege and later Darrel Barrowman.

In order to perform the Phobos flyby radio science measurements, the spacecraft needed to have its high gain antenna dish pointed at Earth for the entire duration of the flyby operations. This meant that we were not able to conduct observations with any of the other instruments (which would need to be pointed at Mars). However, we have been able to take advantage of the flyby time to transmit to Earth an additional several hundred megabits of science data that had been recorded on board the spacecraft during the past few days. This included some more pictures of Phobos that were taken when the spacecraft passed 500km from the moon one week ago.

Ed's note: Figure for amount of data downloaded corrected 31.12

 

Update at 10:32 CET

James Godfrey is working this morning in the Mars Express dedicated control room at ESOC and he's sent in this update.

[10:32 CET] NASA DSN have reported that they collected good data at their Madrid 70m station during the flyby and saw "a slight effect in the Doppler residuals, as expected." This slight effect is caused by the gravitational field of Phobos accelerating the Mars Express spacecraft as it flies past the moon and is the signal that we are trying to measure! NASA DSN are now receiving signals from MEX via their 70m dish at Goldstone.

Now it is up to the radio scientists to examine the measurements made on the radio signals and to determine the effect of the uneven (non-spherical) mass distribution of this oddly shaped moon, Phobos, on Mars Express.

This is a screen shot from a spectrum-analyser display at NASA's 70m DSN station in Madrid (as received at ESOC). The left-hand panel shows the S-band channel and the right-hand shows the X-band channel.

This is a screen shot from a spectrum-analyser display at NASA's 70m DSN station in Madrid (as received at ESOC). The left-hand panel shows the S-band channel and the right-hand shows the X-band channel.

 View of the signal strength coming from MEX as received via
NASA's 70m antenna at Madrid, just after closest approach

Phobos flyby now

As you read this, at 08:09 CET, Mars Express will be making its closest-ever flyby of Phobos!

Earth-Mars relative positions 28 Dec 2013 Credit: NASA/MGCMG/H. Houben

Earth-Mars relative positions 28 Dec 2013 Credit: NASA/MGCMG/H. Houben

MEX is transmitting a continuous radio signal across 208 million km of space, which is being tracked and received by NASA's 70m station at Madrid. The recording will enable scientists to precisely reconstruct the spacecraft's trajectory and, hence, know the gravitational influence of Phobos.

The one-way signal time is 11 minutes, 35.4 seconds right now.

If you were standing on the (lumpy) surface of Phobos and looked up, the animation below shows more or less what you would see: ESA's spacecraft as a pinpoint of light slowly but steadily advancing across the sky.

This animation shows the view in real time from the surface of Phobos.

A few minutes ago, I got an update from the Mars Express Dedicated Control Room at ESOC: everything is going as planned!

The tracking today will continue through until this evening; NASA Goldstone takes over tracking at 11:02 GMT (12:02 CET) until 19:00 GMT (20:00 CET). ESA's New Norcia station will also shadow track starting at 17:06 GMT (18:06 CET). The MEX transmitter will remain on until tomorrow at 01:12 GMT (02:12 CET).

Months of preparation have culminated in a successful and – As hoped for! – relatively quiet tracking and flyby operation. Best wishes and well done to the ESA and NASA pros who made this work.

This animation shows how the flyby would appear from a vantage point near Mars Express, with Phobos passing by beneath. The relative movement between the spacecraft and moon has been speeded up 10x faster than actual.