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


Spacecraft in good shape

Update from James Godfrey in the Mars Express control room at ESOC about 1 hour ago.

The signal from the spacecraft is now being modulated with telemetry – meaning that housekeeping information on the status and health of Mars Express is being downloaded.

During the flyby this morning, this modulation was turned off so as to concentrate all of the energy transmitted by the spacecraft into the main carrier radio signal and maximise the possibility of detecting the (very small!) changes in frequency as the spacecraft was accelerated by Phobos' gravity.

Now that the most critical radio science measurements have been completed, we have been able to turn the modulation back on so that we can receive the telemetry data.

As expected: everything looks OK!

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.


Update at 06:00CET

This sent in about an hour ago by Andy Johnstone at ESOC.

Update at 05:00GMT

We are now transmitting from NASA's 70m deep-space station in Madrid, DSS-63, and have released ESA's 35m station at New Norcia.

The hand-over between the two networks was very smooth with both DSN [NASA] and ESTRACK [ESA] performing their duties exactly on time despite an unusually tight schedule and a hand-over method that is not routinely used.

The telemetry modulation was switched off again at 03:46GMT (04:46CET) and will remain off until shortly after we begin tracking with NASA's 70m station at Goldstone, California, DSS-14, at 11:23GMT (12:32CET). Until then we will be relying on a system that the NASA tracking stationcolleagues have very helpfully provided for us, in which we can monitor directly their spectrum analysers and check on the quality of the signal.

Everything is currently running as planned and the next event will be the fly-by itself!

Saturday evening update from the MEX team

This sent in from Andy Johnstone at ESOC, ESA's Space Operations Centre in Darmstadt, just a few minutes ago.

Everything is running smoothly at the moment! James Godfrey and I have been at ESOC since about 17:00GMT (18:00 local); I will stay on through the night and he will come back in with Erhard Rabenau at around 06:00GMT (07:00CET) on Sunday morning.

Acquisition of signal by ESA's New Norcia ground tracking station was on time today at 18:19GMT (19:19 CET) via both X- and S-Band frequencies.

This animation shows the flyby on 29 Dec 2013 (speeded up) as if you were standing on Phobos, with Mars in the background and Mars Express sweeping out an arc above. Toward the end of the sequence, Mars Express disappears behind Mars and out of sight of Phobos. Thanks to the MEX team and Simon Wood at ESOC for generating this!

We have had our first period where we switch off the telemetry modulation [that is, downloading information from MEX] and only receive a carrier signal [just a raw radio signal] from the spacecraft.

Setting Mars Express to transmit carrier only increases the strength and quality of the signal, which therefore gives us better radio science, But this is always a worrying period for the Flight Control Team as we cannot tell what is happening on board the spacecraft. All we have to go on during these periods is a spectrum analyser and a display from the ground station saying that a signal is still being detected.

The next scheduled event will be loss of signal at 00:25GMT (01:25CET) when Mars Express passes behind Mars; it will reappear at the end of this occultation at 00:53GMT (01:53CET). This in itself is a good opportunity for radio science as our scientists can obtain information on Mars' atmosphere by the way the radio signal is affected as it briefly passes through en route to Earth.

We are looking forward to a quiet night with everything running as planned!

Best regards

-- Andy Johnstone
Spacecraft Operations Engineer, TT&C, Mars Express



Timeline: Mars Express makes closest-ever flyby of Phobos

Find below an outline timeline for the flyby of Phobos around 29 December 2013.

Initial tracking will be provided by DSA 1, ESA's 35m deep-space ground station at New Norcia, Australia. Then NASA will provide support with two of their 70m stations, DSS-63 near Madrid, Spain, and then DSS-14 in Goldstone, California. Finally, Mars Express will be tracked again by New Norcia.

ESA's 35m New Norcia station in action

Phobos flyby timeline

13-362T 17:40:00Z New Norcia (NNO) station starts tracking Mars Express
13-363T 01:30:00Z DSS-63 acquires the signal from MEX
13-363T 03:21:52Z DSS-63 takes over as primary station
13-363T 03:44:05Z NNO stops tracking MEX
13-363T 07:09:00Z Mars Express – closest approach to Phobos (45km from surface)
13-363T 07:20:35Z Signals from closest approach reach Earth (due to the one-way light time)
13-363T 08:55:00Z DSS-14 begins tracking MEX
13-363T 11:02:50Z DSS-14 takes over as primary station
13-363T 11:25:00Z DSS-63 stops tracking MEX
13-363T 17:06:02Z NNO begins tracking MEX
13-363T 18:37:53Z NNO takes over as primary station
13-363T 19:00:00Z DSS-14 stops tracking MEX
13-364T 01:12:30Z The transmitters on MEX start switching off; flyby 'mission' ends

This timeline was taken directly from the operations plan used by the MEX flight team at ESOC; the '13-262' and '13-263' annotation simply refers to the year and day of the year. So, 362 is 28 December, 363 is 29 December and so on. 'Z' time (Zulu-time) refers to GMT time; CET time is +1 hour. All times subject to change.

Tracking stations

The station coverage must be uninterrupted (except during occultation by Mars) for the entire 35-hour period of the flyby mission. Therefore, an overlap in coverage between stations has been intentionally planned so as to be able to detect and eliminate any inaccuracies due to the stations themselves.

"This overlap imposes a very specific handover procedure between the ESA Estrack and NASA DSN networks; this handover has been rehearsed numerous times with the actual ground stations," says Michel Denis, the MEX Operations Manager at ESOC.

The sequence of station hand-over – from ESA to NASA to ESA – makes this a truly international, intercontinental, inter-agency endeavour!

Michel also points out that the use of NASA's 70m stations is critical to the accuracy of the measurements. "We'd like to thank our US colleagues for the provision of some very specific station passes; this is due to the very cooperative efforts of the DSN station scheduling team."

NASA 70m tracking station. Credit: NASA

NASA 70m tracking station. Credit: NASA

Should any of the station listed above become unavailable at the last minute, a backup has been booked in advance using ESA's 35m Estrack stations, which could take over to 'save' the Phobos flyby; this would incur a penalty in reduced measurement accuracy.

Phobos flyby

Early in the morning (GMT time) on Sunday, 29 December, ESA's Mars Express will make the closest-ever flyby of Phobos, one of Mars' two moons.

The breathtakingly close passage will see Mars Express skim past the moon just 45 km from its surface and promises to provide valuable scientific insight into the unresolved origins of the two Martian moons, Phobos and Deimos (see Mars Express heading toward daring flyby of Phobos).

As the spacecraft passes close to Phobos, it will be pulled slightly off course by the moon’s gravity, changing the spacecraft’s velocity by no more than a few centimetres per second. These small deviations will be reflected in the spacecraft’s radio signals as they are beamed back to Earth, and scientists can then translate them into measurements of the mass and density structure inside the moon.

Earlier flybys, including the previous closest approach of 67 km in March 2010, have already suggested that the moon could be between a quarter and a third empty space – essentially a rubble pile with large spaces between the rocky blocks that make up the moon’s interior.

Knowing the structure of the roughly 27 x 22 x 18 km Phobos will help to solve a big mystery concerning its origin and that of its more distant sibling, Deimos, which orbits Mars at approximately three times greater distance.

Artist’s impression of Mars Express set against a 35 km-wide crater in the Vastitas Borealis region of Mars at approximately 70.5°N / 103°E.

Artist’s impression of Mars Express set against a 35 km-wide crater in the Vastitas Borealis region of Mars at approximately 70.5°N / 103°E. Credit: ESA/DLR/FU-Berlin-G.Neukum

The flyby is not only a scientific challenge, but also an operations one as well, which led our director general earlier this year to mention, 'I hope that my colleagues at ESOC will prove that they are the best pilots'.

In fact, Mars Express Spacecraft Operations Manager Michel Denis and the extended 'team of teams' responsible for MEX flight operations at ESOC – including the flight operations team, the flight dynamics experts and the ground tracking station specialists – are treating the flyby as a mini 'mission within a mission'.

"For 35 hours around the time of closest approach [08:09 CET 29-12-13], MEX will conduct a science mission completely different from its routine and highly automated operations for observing Mars," says Michel.

"Months of preparation will come to fruition, but the scientific prize will be worth the work."

Here's the skinny on how the flyby will work.

Continue reading