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.

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.

Mars Express team readies for Siding Spring

One of the most interesting events in planetary exploration in 2014 is potentially also one of the most threatening for spacecraft orbiting Mars. This post was contributed by the MEX operations team here at ESOC and marks the start of our coverage of their efforts to safeguard the mission during the close flyby of Comet Siding Spring in October – while doing some unique science.

On Sunday, 19 October 2014, at around 18:30 UTC (20:30 CET), comet C/2013 A1 – known widely as ‘Siding Spring’ after the Australian observatory where it was discovered in January 2013 – will make a close fly-by of Mars.

This graphic depicts the orbit of comet C/2013 A1 Siding Spring as it swings around the sun in 2014. Credit: NASA

This graphic depicts the orbit of comet C/2013 A1 Siding Spring as it swings around the sun in 2014. Credit: NASA

It will be the second comet to visit the Red Planet in 12 months, following Comet ISON in October 2013. However, where ISON passed some 10 000 000 km from the planet, current estimates put Siding Spring’s miss distance at just 136 000* km from the surface.

To give some perspective, Siding Spring will approach Mars by about 1/3 the average distance from Earth to the Moon (about 385,000 km).

At the scale of our Solar System, this is a very, very close shave…

Siding Spring seen from ESA's Optical Ground Station, Tenerife, Spain, 31 January 2014, 19:50 UTC Credit: ESA

Siding Spring seen from ESA’s Optical Ground Station, Tenerife, Spain, 31 January 2014, 19:50 UTC Credit: ESA

While we know the comet will not hit Mars, nor our spacecraft, Mars Express, initial observation data lead us to expect that the coma (the cloud of dust particles surrounding the comet’s nucleus) will be big enough to envelop Mars and therefore the spacecraft orbiting it.

Three orbiters are currently active at Mars: NASA’s Mars Reconnaissance Orbiter (MRO) and Mars Odyssey, and our Mars Express. Two more that departed Earth in late 2013 are due to enter orbit around Mars about three weeks before the comet Siding Spring flyby: NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) and India’s Mars Orbiter Mission.

Further observation of the comet will allow better predictions of the actual size of the coma, and the resulting level of risk to Mars-orbiting spacecraft, but this may not come for several months. Nonetheless, the mission operations team at ESOC have already begun considering ways to best protect Mars Express from the cloud of cometary dust.

The particles in the coma – ranging from 1 to 1/10,000th of a cm in diameter – are not expected to be large. However, they will be travelling toward Mars Express at a staggering 56 km/second (200 000 kph!).

At these speeds, even dust can be dangerous.

Consider that man-made space debris in orbit around Earth, where the relative velocities are ‘merely’ 7 km/second, can seriously harm satellites. The relative velocity for the Siding Spring dust particles will be about eight times faster – but the energy of an impact goes up with the square of the speed, meaning that the energy levels are 64 times higher!

Plus, it is not only the risk of physical damage from an impact the must be considered. Hyper-velocity impacts such as these can generate plasma clouds and electromagnetic pulses, which can cause disruptions with the many electronic systems onboard Mars Express.

The team have been doing a great deal of brainstorming to ‘work the issue’, and one of the obvious solutions lies in how we could adjust our orbit to shield the spacecraft behind the bulk of Mars, for at least part of the encounter if not all.

We also need to determine how can we best orient the spacecraft to reduce the exposure of instruments and critical systems to the coma and comet debris.

The team are also looking at how the many subsystems on board the spacecraft can be configured to ensure the highest possible resilience to the potential risks. Finally, given this opportunity to observe a comet as it passes so close to a rocky planet, we must co-ordinate spacecraft operations with the ESA science teams to accommodate as much science observations of this unique event as possible, consistent with safety.

Our close encounter with Siding Spring is still over nine months away, but the Mars Express team have already begun preparing for it, consulting with experts, industry and scientists and researching a complex set of details, possibilities and what-ifs.

It’s a major challenge, and even if we design and implement the best possible way to deal with the close approach, there’s no guarantee that Mars Express remains unaffected.

We’ll keep you updated here in the blog (and in the main ESA website) to share how we’re tackling these issues, ensuring that both the team and the spacecraft are ready for this incredibly challenging, once-in-a-lifetime encounter.

Siding Spring is currently around 670 million km from Mars, a distance it will cover in just nine months.

The countdown has begun!

Ed: Thanks to Andy, Kees, Simon, Luke and Michel for this great report.

Siding Spring by the numbers

Source: ESA and NASA

  • Date of comet closest approach (CA): 19 October 2014
  • Time of CA: ~18:28 UT
  • *Estimated distance of comet from Mars at CA: 136 000 km from centre || 132 000 km from surface
  • Comet nucleus diameter: Unknown
  • Coma radius: Likely to engulf Mars
  • Time for Mars to pass through coma: Several hours (MEX now orbits Mars every 7 hours)
  • Velocity of cometary dust particles: 56 km/second
  • Dust particles produced by comet (as of 28/1/14):
    100 kg/second

 

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.

 

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

Hey! Phobos is APOD for today!

ESA’s great Phobos animation – showing a 360-degree view of Phobos – is today’s APOD! What a great way to celebrate the holiday (and MEX’s 10th anniversary of arrival at Mars)! Better yet: Mars Express is lined up to make the closest-ever flyby of Phobos on 29 December, just 45 km from the surface. (There won’t be any images, but we’ll keep you updated here in the blog.)

What does the Martian moon Phobos look like? To better visualize this unusual object, images from ESA’s Mars Express orbiter have been combined into a virtual rotation movie. The rotation is actually a digital illusion – tidally-locked Phobos always keeps the same face toward its home planet, as does Earth’s moon. The above video highlights Phobos’ chunky shape and an unusually dark surface covered with craters and grooves. What lies beneath the surface is a topic of research since the moon is not dense enough to be filled with solid rock. Phobos is losing about of centimeter of altitude a year and is expected to break up and crash onto Mars within the next 50 million years. To better understand this unusual world, Mars Express is on course to make the closest flyby ever on Sunday.

Mars Express heading towards daring flyby of Phobos

Late this month, ESA’s Mars Express will make the closest flyby yet of the Red Planet’s largest moon Phobos, skimming past at only 45 km above its surface.

The flyby on 29 December will be so close and fast that Mars Express will not be able to take any images, but instead it will yield the most accurate details yet of the moon’s gravitational field and, in turn, provide new details of its internal structure.

“At just 45 km from the surface, our spacecraft is passing almost within touching distance of Phobos,” says Michel Denis, Mars Express Operations Manager.

Full report via ESA web