Keeping MEX warm

Today's post was contributed by Luke Lucas, a Mars Express spacecraft operations engineer at ESA's ESOC mission control centre. Read to the bottom for more info and registration for the upcoming Open Data Day, 28 October 2016, at ESOC! Link to live webcast also at foot of this post.

The vacuum of space is a challenging thermal environment. The illuminated side of an object may reach more than 250°C while the non-illuminated side may be less than -150°C.

Without careful consideration such temperature differences could cause parts of the spacecraft to break, twist or fail to function.

For example: the temperature difference between the front and back of the arrays may be hundreds of degrees and when the arrays go from the shadow of an eclipse, sudden and dramatic changes in temperature occur, which can lead to expansion, contraction, torsion and twisting. Selection of correct materials and good structural design are essential.

Mars Express eclipse Credit: ESA

Mars Express eclipse Credit: ESA

Maintaining an optimal thermal environment is the task of the craft's thermal subsystem. The thermal subsystem includes electrical heaters where needed to keep the spacecraft warm (e.g. to prevent fuel lines from freezing) and passive radiators to keep other units cool. In this way Mars Express can perform scientific observations.

Cooling of some units to prevent overheating can be achieved by the clever siting of radiators. By placing radiators where they will always be facing deep space, passive cooling occurs.

The use of multi-layer insulating blankets around the spacecraft promotes a stable thermal environment and minimises the loss of heat to space.

Electrical heaters are used to heat Mars Express and maintain units, structures and instruments within their safe, warm operating range, with more than 200 thermistors continually measuring the temperature at various points around the spacecraft. By monitoring the measured temperatures, the heaters are turned on or off as needed.

As well as keeping MEX warm, it is important to know how much power is needed to achieve this. Power is supplied by either the solar arrays (when exposed to sunlight) by the batteries during an eclipse.

Power produced by either the solar arrays or batteries is used for platform operations, thermal operations and whatever remains is available for science. The energy provided by the batteries during an eclipse is finite, so knowing how much power is needed for the thermal subsystems means mission controllers can know how much power is available for science. Knowing accurately how much power is needed by the thermal subsystem means we can really maximise science observations.

So how much power does the thermal subsystem need to keep MEX warm?

Well it depends how cold the spacecraft is. And that, in turn, depends on many factors. The largest factors include:

  • Mars orbit Credit: ESA

    Mars orbit Credit: ESA

    How far away is MEX from the Sun? The Sun emits solar flux, measured in W/m2, (power per square meter received) and the further away from the Sun, the less flux received – and so the cooler is Mars Express

  • What is the solar aspect angle? This is the angle of the spacecraft with respect to the Sun? Where is the Sun is shining on MEX? On the top (+Z face), the bottom (-Z face) or the +X face, and at what angle to that face? This will affect the craft's temperature.
  • How far is MEX from the surface of Mars? The albedo is the amount of energy reflected off the planet. 'Mars shine' refers to the amount of energy being reflected by the planet and can affect MEX’s thermal condition.
  • Mars Express orbit Credit: ESA

    Mars Express orbit Credit: ESA

    How far away is Mars from the Sun? This affects the Mars albedo, which in turn affects MEX.

  • Is there an eclipse happening? As seen above, during eclipse, MEX is in Mars’ shadow, receiving no illumination from the Sun, and this can cause a dramatic cool down.
  • What operations are on going? Certain operations, such as using the transmitter to communicate with Earth, warm up certain sections of the spacecraft.

Here are two thermally representative images of the -Y face, one seen during a communication pass and one when no communications were happening.

In a communication pass with temperatures up to 16°C

Mars Express -Y face, during a communication pass with a ground station Credit: ESA

Mars Express -Y face, during a communication pass with a ground station Credit: ESA

When not communicating with Earth and temperatures as low as -12°C

Mars Express -Y face, not during a communication pass with a ground station Credit: ESA

Mars Express -Y face, not during a communication pass with a ground station Credit: ESA

That is to say, that the thermal power required changes continuously as the craft orbits Mars and as Mars orbits the Sun; no two days are the same. Predicting the thermal power required is a puzzle of many parts. But it is a very important matter, because we want to perform as much science as possible.

We have a model we use to predict the power required, but wondered if anyone could derive something better.

Our engineering approach is to look at the factors involved and create an equation. But it is always good to look at any puzzle from more than one view. So we asked you, the public to look at this, as the Mars Express Power Challenge.

And – Wow! – we were thrilled by the responses we received, the predictive models that were built and the amount of information shared among this wonderful community of data scientists, researchers, and space fans!

The challenge is now over and the winners will present their solution on 28 October 2016, at ESA's ESOC mission control centre, Darmstadt, Germany. On this day, the Centre will host an 'open data day' for the candidates – and anyone interested is welcome to attend!

This will be an exciting, inspiring day, full of great ideas and exchanges!

A few tickets for the open data day are still available here via EventBrite.

Watch live 28 October, 10:00-12:00 and 14:00-16:30 CEST


Update on Phobos flyby science results

Update from ESA's Mars Express project scientist Dmitri Titov on the recent Phobos flyby results.

Unfortunately, HRSC imaging didn't work due to a transient issue with the onboard memory, which meant that no data were saved. This happens from time to time on our 12-year-old spacecraft and unfortunately this time it occurred during a flyby.

The good (excellent!) news is that other instruments did acquire data, particularly ASPERA, the Analyzer of Space Plasma and Energetic Atoms, which studied interactions between the solar wind and Phobos. It will take the instrument team some time to analyse and process their results, but the initial report is that all went very well.

The MARSIS radar (the Subsurface Sounding Radar/Altimeter) also operated during flyby. Although data are still being processed, it was possible to ascertain that Phobos was detected both in subsurface sounding mode and through ionosphere sounding.

Two more close encounters with Phobos will occur in 2016. On 4 July, Mars Express will approach Phobos at ~350 km, and on 16 November the spacecraft will flyby as close as 127 km. Both flybys will be used to continue the programme of moon investigations.

Mars Express continues exploring the Red Planet - soon in the company of ExoMars 2016 Trace Gas Orbiter!

We'll update you here in the blog when we have news.

Controlling Mars Express – How cool is that?

This afternoon, ESA astronaut Samantha Cristoforetti, conducting her #futura42 science mission on board the International Space Station, took a moment to recognise the dozens of students in 25 groups from 12 countries who are taking part in the VMC Schools Campaign. Thank you, astro Sam! Your interest in #STEM and in exploring space is inspiring to everyone – especially young, future planetary scientists/astronauts!

A bit later, @esaoperations replied:

Why conjunction frees up VMC time

You may be wondering why the VMC camera will be free for public imaging requests on 25-28 May – and hence why we can run the VMC Imaging Campaign. Mars Express Spacecraft Operations Engineer Andy Johnstone provided this reply.

The present conjunction period, when the Sun will block the direct line of sight between Mars and Earth, starts on Friday, 28 May, and lasts about five weeks until 1 July; the conjunction point happens on 14 June. As we've described before here in the blog, routine science payload observations are carefully planned well in advance.In this case, there is a boundary on the planning period (which are normally 28 days) that ends four days before 28 May, and it was decided by the operations and science planners not to conduct science during only four days.

ExoMars Trace Gas Orbiter at Mars. TGO will be launched in 2016 with Schiaparelli, the entry, descent and landing demonstrator module. It will search for evidence of methane and other atmospheric gases that could be signatures of active biological or geological processes on Mars. TGO will also serve as a communications relay for the rover and surface science platform that will be launched in 2018. Credit: ESA–D. Ducros

ExoMars Trace Gas Orbiter at Mars. TGO will be launched in 2016 with Schiaparelli, the entry, descent and landing demonstrator module. It will search for evidence of methane and other atmospheric gases that could be signatures of active biological or geological processes on Mars. TGO will also serve as a communications relay for the rover and surface science platform that will be launched in 2018. Credit: ESA–D. Ducros

So, while this frees up a rare time slot when no science will take place, and while VMC therefore may be used for this valuable public and educational outreach activity, this isn't the only activity happening during the four days. We are spacecraft engineers, after all, and our goal is always to test, optimise and maximise the performance of our spacecraft.

We are performing other activities in this 4 day period:

  • Monday-Wednesday:  Tests with MELACOM (our UHF radio used to communicate with landers on the surface) as part of preparations for Mars Express to support ExoMars Entry Descent Module (EDM) landing next year.
  • Tuesday: A test for the Trace Gas Orbiter (TGO) mission, a partnership between ESA and Russia's Federal Space Agency, Roscosmos. They want to prove that we can perform ground station swaps without bringing down the carrier. We are going to let them use MEX as a test vessel.
  • Wednesday: Performance tests on our solar arrays and batteries (last done just after the passage of comet Siding Spring in October 2014) and testing of redundant heater lines. We may also perform a special pointing with all instruments OFF in order to get a more accurate model of how much heating comes from the Sun, and how much is from internal components.
  • Wednesday-Thursday: Loading of the most vital commands to configure the craft for the Solar conjunction period. These relate to our attitude and orbit control system (AOCS). For this conjunction, we will be leaving our X- and S-band transmitters ON throughout as we do not have any power limitations this year.
  • Thursday:  A full performance check of the Transponders (radio transmitter/receivers) in both X- and S-band; this takes advantage of us having some long passes with no science data to be dumped. We can see how they both behave at the same distance on the same station with the same weather.

So, VMC is not the only valuable activities taking place in the run up to the conjunction period. We are using this opportunity to carry out lots of tests and it is likely that we may end up with more. But, with VMC, we are using some spare time in between operational activities to give something back to the public – especially students and teachers – who are some of our strongest supporters!

 

VMC Imaging Campaign

Welcome to the VMC Imaging Campaign!

Information for schools, astronomy clubs, science centres and any other eligible group wishing to take part. Official hashtag: #vmcschools

NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft successfully entered Mars’ orbit at 04:24 CEST on 22 September 2014. This image was acquired by the low-resolution VMC camera on board Mars Express at 14:50 CEST on 20 September, when MAVEN was an estimated 312,000 km from Mars. Credit: ESA/MEX/VMC

NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft successfully entered Mars’ orbit at 04:24 CEST on 22 September 2014. This image was acquired by the low-resolution VMC camera on board Mars Express at 14:50 CEST on 20 September, when MAVEN was an estimated 312,000 km from Mars. Credit: ESA/MEX/VMC

Mars is approaching solar conjunction where it will be on the opposite side of the Sun from Earth; this will affect communication with the spacecraft for a period of about five weeks and so science observations have to be stopped.

For this particular Solar conjunction, running for about five weeks between 28 May and 1 July, the Mars Express team will be stopping science four days earlier than usual for operational reasons. Part of this time is to be used to run tests on spacecraft subsystems, but we have an exciting plan with what to do with the remainder! (See details via Why conjunction frees up VMC time?)


How would you like to be a scientist on a Mars mission?


 

We would like to offer the opportunity for about eight (final number depends on the proposed targets) schools or other youth clubs/organisations to propose observations to be performed with the VMC camera (in principle, almost any large feature on the martian surface can be imaged) and then complete and submit a project report with their results; we'll publish them here in the MEX blog. (See official announcement  plus link to terms, conditions and the registration form here.)

The closing date for proposals is 12:00 CET on 27 March 2015 – which is not far off, so you’ll need to work quickly if you wish to be involved.

So what do you need to know?

First of all, you need to understand a bit about the VMC camera. It is our most basic instrument, being basically a low-resolution webcam that was originally only to be used to record the release of the Beagle 2 lander. Since then, we have used it to take some very impressive images of Mars, its moons and atmosphere as well as other planets. Although lacking the extreme resolution of the professional HRSC camera on board Mars Express, it does allow the entire martian disk to be observed in a single image. Go through our Flickr library to get a good idea of what we can do with it.

The VMC webcam provides images of Mars having about the same quality as those provided by the ESA/NASA HUbble telescope. Image credit: ESA/Mars Express/VMC/ Humboldt Gymnasium, Vaterstetten

The VMC webcam provides images of Mars having about the same quality as those provided by the ESA/NASA HUbble telescope. Image credit: ESA/Mars Express/VMC/ Humboldt Gymnasium, Vaterstetten

In fact, VMC provides images of Mars having about the same resolution and quality as those obtained by the 'professional' ESA/NASA Hubble orbiting observatory!

MEX Orbit

Next, you need to know a bit about the orbit of Mars Express. We don’t expect you to attempt any of the incredible mathematics that our Flight Dynamics team here at ESOC perform on a routine basis, do but you need to understand that Mars Express has a highly elliptical orbit, which – combined with the rotation of Mars – means that not all of the planet's surface will be visible to the camera during the available observing slots during 25-27 May.

We’ll make this easier for you by supplying orbit files to use with the fantastic (and free) Celestia software. These will help you in working out which observations are possible (Editor's note: download a RAR compressed archive here - open with any common compression tool like WINZIP – detailed installation instructions here).

Take a look at the VMC full-orbit animation, derived from Celestia, which is a great way to visualise what VMC can see during 25-27 May.

MEX operations

You will also need to know a bit (but not too much!) about Mars Express. Keep in mind that although we are inviting you to point Mars Express at a target of your choice (the VMC camera is fixed in position, so to point it, we slew the entire craft), we have many rules and restrictions for ensuring the safety of the spacecraft that cannot be violated.

We will take care of this within the MEX flight control team here at ESOC for you, but there are a few obvious things that you need not request, such as pointing toward the Sun or asking for two targets in quick succession (we avoid turning the spacecraft too quickly). Also, as Mars (and hence Mars Express) is almost at its furthest distance from Earth, the amount of data we can return is very limited (which is why the professional instrument payload is being switched off in the first place), and so we will not accept any long observation proposals (this also enables a larger number of short observation slots, giving as many schools or clubs as possible an opportunity to carry out observations).

The Red Planet

Some knowledge of Mars is also important – as we assume that is at what you will be pointing VMC. In principle, you could request to point VMC away from Mars, but, as it is a low-resolution device, we don't think you'll see that much (we did get a misty shot of Earth one time!). We will leave this for you to research on your own. There are many sources of information on the Main ESA website, the Internet and in your library that you will want to look in to in order to come up with a good proposal.

You can also find many examples of past school and astronomy club projects completed using VMC images, many of which are excellent! (Here's one from the Humboldt Gymnasium, Vatterstetten, Germany) The difference with this campaign is that you can request pointings at specific features from much lower altitudes than has ever been done before, so yours might be even better! 🙂

Tutorials

Emily Lakdawalla, from the Planetary Society, has posted online a series of excellent tutorials on working with space images, including the VMC. And you can find all archived VMC images for practice via the Mars Webcam blog and Flickr.

... and the fine print

Mars Express is an operational mission, and considerations of spacecraft safety and the primary professional science mission always come first. We may have to amend, change, or cancel the VMC Imaging Campaign at any time, or there may be some other reason why we can't carry out your requested observation(s). But the slots on 25-27 May are looking good and we will do our best!

REGISTER ONLINE BY 12:00 CET, 27 MARCH 2015

Questions?
Tweet with the hashtag #vmcschools or post a query in the blog

So, what can you propose?

What do you think you can do? Would you like to get a close-up image of a certain feature (Olympus Mons?), or observe the whole of Mars? Are you going to work with raw VMC data or use the processed images? Can you identify certain features or landforms and explain what is going on? What caused them? We aren't necessarily looking for the cleverest or most innovative observation proposals, but we will select eight (or so) good ones that we can fit together in to our observation window and that provide the best scientific, artistic or educational merit.

So, if you would like to take part in this extremely rare opportunity to briefly 'take charge' of a spacecraft around another world, make a plan and submit your proposals. Time is short and we know that there are many enthusiastic people – teachers, students, artists, young amateur astronomers and many more – out there with great ideas. Best of luck and we look forward to hearing from you!

Editor's note: Thanks to Andy Johnstone & Michel Denis for this post


Timeline/details

  • 6 March – Call for proposals open; all interested groups must register their interest
  • 19 March – #ESAHangout via Google+ – Mars Express mission team will provide a tutorial on the VMC and how its images are planned & acquired
  • 27 March – Deadline for registered groups to submit final proposal (12:00 CET)
  • 8 May – #ESAHangout via Google+ –  the Mars Express mission team will announce accepted observation targets
  • 25-27 May – VMC imaging!
  • 28 May (+/-) – VMC images downloaded and delivered to participant groups
  • End of current academic year or 31 July, which ever comes first – All participant groups must submit project report

We asked Michael Khan, working at ESA's Mission Analysis Office at ESOC, what he would select as targets for VMC. His comments and some very useful charts are below – Ed.

Potential observation targets

Here are a series of charts that indicate when/where MEX will be in relation to a selection of nine surface features (click for full size). These indicate the ground track, time, the range and the elevation for Mars Express (and hence the VMC) with respect to nine select features.

The ground track of the Mars Express spacecraft from 25 through 27 May. Where the red line is vertical, the spacecraft is passing its closest point to Mars, at around 250 km over the surface. Conversely, where the line is canted, the spacecraft is near the farthest point out on its elliptical orbit. This diagram shows the entire ground track - however, some of the ground track also passes over the Martian night, when the regions directly below are dark. Credit: ESA/M. Khan

The ground track of the Mars Express spacecraft from 25 through 27 May. Where the red line is vertical, the spacecraft is passing its closest point to Mars, at around 250 km over the surface. Conversely, where the line is canted, the spacecraft is near the farthest point out on its elliptical orbit. This diagram shows the entire ground track - however, some of the ground track also passes over the Martian night, when the regions directly below are dark. Credit: ESA/M. Khan

The local solar time is the current actual time at a given Mars location. In late May, it just so happens that the orbit is oriented such that most passes occur in the local morning hours, with very few passes (those that occur when the spacecraft is closest to Mars) in the late afternoon. Credit: ESA/M. Khan

The local solar time is the current actual time at a given Mars location. In late May, it just so happens that the orbit is oriented such that most passes occur in the local morning hours, with very few passes (those that occur when the spacecraft is closest to Mars) in the late afternoon. Credit: ESA/M. Khan

For the nine sample locations, the range (distance from the location to Mars Express) is shown for 25-27 May. Ideally, to obtain bright, high-resolution images, the elevation (see http://bit.ly/1MbUteQ) should be high and the range should be low, though this combination may be difficult to obtain. Credit: ESA/M. Khan

For the nine sample locations, the range (distance from the location to Mars Express) is shown for 25-27 May. Ideally, to obtain bright, high-resolution images, the elevation (see http://bit.ly/1MbUteQ) should be high and the range should be low, though this combination may be difficult to obtain. Credit: ESA/M. Khan

For nine sample locations on Mars, the elevation at which the spacecraft passes overhead, 25-27 May, is shown. Only those overflights where the Sun is up at each of the respective locations are taken into account. The higher the elevation, the better the observation conditions. For 90-deg elevation, Mars Express would be directly overhead. Credit: ESA/M. Khan

For nine sample locations on Mars, the elevation at which the spacecraft passes overhead, 25-27 May, is shown. Only those overflights where the Sun is up at each of the respective locations are taken into account. The higher the elevation, the better the observation conditions. For 90-deg elevation, Mars Express would be directly overhead. Credit: ESA/M. Khan

Two proposals from my side, based on my results:

  1. Eos Chasma on 2015/5/28, around 07:00 UTC at <2000 km range and up to 65 deg elevation. Arguably, pretty!
  2. Elysium Planitia and Elysium Mons on 2015/5/26 around 16:00 at <1200 km range and up to 45 deg elevation, and again on 2015/5/27 at 18:00 UTC at <2000 km range and up to 55 deg elevation. This area is the one where Mars Express saw the 'frozen sea' 10 years ago. It is also the landing region of NASA's Insight Spacecraft in September 2016.

There are also numerous opportunities to observe Meridiani Planum, the target location for ESA's 2016 Mars lander, the EDM Schiaparelli. VMC imaging opportunities occur on 2015/5/26, at 15:00 UTC, and 2015/5/28, at 18:00 UTC. This also applies to Oxia Planum, currently designated as reference landing site for ESA's 2018 Mars rover, ExoMars.

 

Comet flyby timeline

The timeline of major events related to Mars Express during the passage of Siding Spring on 19 Oct.Mars Express timeline around Siding Spring

Mars Express timeline around Siding Spring

  • ERT: Earth receive time
  • LOS: Loss of signal
  • AOS: Acquisition of signal
  • Beacon: Continuous 'beacon' signal transmitted via MEX low gain antenna to confirm spacecraft functioning as expected

Ya gotta have a little ‘tude

This week's report on how the Mars Express flight control team is planning to deal with Comet Siding Spring is all about attitude – Ed.

We have now finalised our choice for spacecraft attitude through the comet encounter. As we’re sure many of you have also worked out, our chosen attitude is with the High Gain Antenna (HGA) facing the comet.

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

Mars Express - spacecraft directions

This was identified early on as a likely attitude as there are no internal components mounted directly on the front wall, plus the HGA should act as an improvised Whipple shield.

 

 

I'm in control, my worries are few
'Cause I've got love like I never knew
Ooo, ooo, ooo, ooo, ooo
I got a new attitude

– Patti Labelle, 'New Attitude'

It is not perfect, however, as there are still several components in the 'firing line' of cometary dust particles. All antennas will be facing the incoming dust particles, but one or two holes in the parabolic reflector dish of the HGA shouldn’t prevent it from functioning. The ASPERA instrument is also exposed, as is the forward Sun Acquisition Sensor (SAS) and two of the thruster pairs.

This diagram shows the major components in the spacecraft body. Credit: ESA

This diagram shows the major components in the spacecraft body. Credit: ESA

As the angle between the comet and the Sun will be around 89°, we also had to decide which of the faces (i.e sides of the spacecraft) should point towards the Sun.

As the solar panels are mounted on the left and right sides, if they were pointed at the Sun only the array on the side facing the Sun would be illuminated – and only on its end, so Mars Express would not be able to rely on solar power. The batteries are not able to support this configuration sufficiently long (up to 10 hours).

Pointing the top surface – where the instruments are located  – toward the Sun is generally not a good idea, but pointing the base – where the thrusters are – toward the Sun does not cause any problem (see our diagramme of MEX sides here  – Ed.).

Actually, this would provide some extra heat to the spacecraft fuel tanks and lines so we can save some power by not needing to use the on-board heaters as often. This angle also works out well for our solar arrays. They can still be facing the Sun (for full power) and yet lie edge-on to the expected particle 'flux' (stream of incoming particles), therby presenting the smallest target.

Mars Express with the solar arrays edge on - as they are only 20mm thick they had to be drawn larger to even be visible in this picture

Mars Express with the solar arrays edge on - as they are only 20mm thick they had to be drawn larger to even be visible in this picture

Mars Express with the solar arrays face on - the change in area is dramatic"

Mars Express with the solar arrays face on - the change in area is dramatic

 

So now that we have chosen our attitude, we now have to ensure that we stay so oriented!

Our current modelling shows that it is unlikely that an impact from the types of particles we expect could disturb the spacecraft's attitude. Even if it did, the on-board systems should be able to compensate. What we are more concerned about is if an impact were to cause a component to fail or behave strangely. This could then cause the on-board systems to think that the spacecraft is at risk and trigger a 'safe mode'.

Safe mode can be considered a spacecraft’s survival instinct; it's a mode that MEX enters automatically if it detects a condition or event that indicates loss of control or damage to the spacecraft. Usually the trigger is a system failure or detection of operating conditions considered dangerously out of the normal ranges. All non-essential systems are shut down and those that are vital will switch to their backup way of functioning; this is to try and isolate any suspected problem and prevent it from causing damage.

When a safe mode is triggered, the spacecraft automatically uses its SAS to point the front of the spacecraft and the solar arrays towards the Sun (ensuring that MEX has power). Next, the active Star Tracker (STR) makes a scan to determine in which attitude the spacecraft has ended up. With this knowledge the spacecraft consults an internally stored table containing the position in the sky of the Earth at that moment to determine in which direction the HGA needs to be pointed to re-establish communications. The spacecraft body is then rotated to point the HGA in that direction while simultaneously keeping the arrays facing the Sun.

The craft then starts sending a signal to Earth and waiting for a reply.

There are two transmitter types on MEX: X-Band and S-Band (we’ll explore why in a later post), but in safe mode, the spacecraft uses the lower bandwidth (and less complex) S-Band system at its lowest transmission rate, which results in a painfully slow communication rate of 9 bits per second (in comparison: in X-Band the maximum rate is 228 thousand bits per second!)

Furthermore, in entering safe mode, a small amount of fuel is consumed and the communications are a bit annoying (until we can restore the faster X-Band) but safe mode is by definition 'safe.'

So, two questions (you may have to go back a few posts for clues):

  • What do you think the problem would be if this were to happen on 19 October?
  • What are the weak points on the front of Mars Express?

(Click below on 'Page 2' for answers – Ed.)

We're also working on another plan to avoid nasty bits of comet, but we’ll save that for next time...

Andy, Michel, Kees, Simon, James and Luke

Hypervelocity cratering and riding out the risk

You know any blog post that includes the term 'hypervelocity cratering' has got to relate to some pretty serious stuff! Today's update from the Mars Express team contains the realisation that, for some of the risks associated with October's Siding Spring flyby, there may not be much the team can do. This is as close to real-life spacecraft operations you can get without actually sitting on console at ESOC – Ed.

Hubble solar array impact crater. Credit: ESA/NASA

Hubble solar array impact crater. Credit: ESA/NASA

Last week, we considered the spacecraft structure and how it might be affected by any impact of particles – even tiny ones – from the comet's coma.

Whilst it is clear that a particle striking the spacecraft has the ability to cause physical damage to either the structure or components, what is not necessarily obvious is the potential for it to cause disruption to the spacecraft’s many and delicate electrical units. Why is this?

As the velocity (and therefore kinetic energy) of these particles is (very, very) high, there can be electromagnetic effects resulting from these impacts too.

When the particle strikes the body of the spacecraft, not only is the particle itself vaporised, but also some of the material from the part of the spacecraft that has been struck,  an effect called 'hypervelocity cratering' (this has been well investigated during space debris studies in low-Earth orbit – Ed.)

This plume of vaporised material is so hot that it forms a plasma (an ionised gas) and it is this charged plasma that has the ability to cause issues for the spacecraft’s electrical systems.

Here are three examples of the type of effects we've been considering.

  1. The conductive plasma can act a like a wire and cause short circuits by electrically connecting two different components/units together that are normally electrically isolated from one another.
  2. The outer surface of the spacecraft become electrically charged due to light from the Sun knocking electrons off the surface (the photoelectric effect) and by being hit by charged particles from the Solar Wind. If an impact were to puncture into the spacecraft, the plasma produced could provide an electrical connection from the outer body to a unit/component inside, allowing the electrical charge to flow from the spacecraft surface into the unit in question.
  3. The plasma has a magnetic and electrical fields associated with it (due to the difference in velocities of its component ions and electrons) moving at a similar speed to the original impact velocity; these moving fields potentially have the ability to induce large currents in cables or components).

What protection do the spacecraft’s electrical systems have?

Interior view of Mars Express, seen during construction. Credit: ESA/Astrium

Interior view of Mars Express, seen during construction. Credit: ESA/Astrium

As you can see from the image (above) taken during the construction of MEX, the individual electrical units are contained in their own protective housing and the cables are all wrapped in an electrically conductive screen. This provides protection against electromagnetic (EM) effects both from other units inside the spacecraft and from external sources.

Additionally, the electrical interfaces of each unit are provided with protection against excess electrical currents. The power connections are fitted with current limiters that will cut the power to the unit if the current flow exceeds a given value. The data connections are also provided with protection in the form of opto-isolators and electrical filters.

Will this protection be enough?

This appears to be a difficult question to answer...!

As noted, the spacecraft’s electrical systems have safety measures built in, but if an induced current were large enough, or the short circuit happened in the wrong place, it is theoretically possible that these safeguards could be defeated.

There is a complex interconnectivity to the electrical systems on MEX, which means that induced currents have many possible paths to take. The effects are also highly dependent on the properties of the particle impacting the spacecraft, where on the spacecraft the hit occurs, the properties of the produced plasma, which components the plasma interacts with, what state the components in question are in, &etc.

As you can see, there are so many variables governing what might happen that trying to anticipate specific problems can become almost meaningless, as adjusting any of these variables even slightly can vastly effect the eventual outcome.

The question, then, is: What can we do?

A obvious possibility is to switch units off. This won’t always protect against induced currents, but it can reduce the risk/effects of short circuits.

So if we assume that is the way to go, the next question is what realistically can we switch off?

As has been discussed in the earlier blog posts, we will be required to maintain a specific pointing during the encounter to best protect the spacecraft. And as we cannot spin Mars Express, this means the Attitude & Orbit Control System (AOCS) must be used to keep the spacecraft correctly oriented. Therefore the AOCS (and all its component units) must also be left on.

For the AOCS to function, this then requires that the main computer is also on – which means the power control units must also be on.

We cannot disconnect the solar arrays, so they will be electrically active throughout. As the Reaction Control System (RCS) thrusters may be called upon, then the thermal control systems also need to be on, so as to regulate tank and fuel-line temperatures. So as you can see, there are not a lot of units left to consider. Almost everything has to stay on!

This whole issue is one we’re actively considering right now, so we have yet to come to any formal conclusions as yet.

This is a good example of a situation in which we will likely have to make an assessment of what to do even though we don’t have a lot of data on which to base a decision and for which it would appear, at the moment at least, that our options are limited.

It is conceivable that we may decide that there is little we can do to significantly reduce the risk of EM effects and this may be something we simply have to live with.

Andy, Michel, Kees, Simon, James and Luke

Space is really, really big – except sometimes it isn’t

Editor's note: Here's the next installment in the continuing story of how the Mars Express team is preparing for Comet Siding Spring flyby, 19 October 2014. This week: introducing the spacecraft's subsystems and structure – and wondering how we can absorb impacts.

Now that we have looked at some of the external factors affecting Mars Express, let’s take a look inside and see how the spacecraft was built and what it's made from.

This diagram shows the major components in the spacecraft body. Credit: ESA

This diagram shows the major components in the spacecraft body. Credit: ESA

This diagram shows the major components in the spacecraft body. There are a lot of acronyms, which we will explain in more detail in future postings. For now, briefly:

  • AOCS (blue): Attitude and Orbit Control System – this controls where Mars Express is pointing (the attitude) and can change the speed of the spacecraft to modify its orbit.
  • DMS (pink): Data Management System (sometimes also called OBDH – On-Board Data Handling) – The computers and storage that interpret commands from Earth, collect data from sensors and transmit telemetry back to Earth.
  • Instruments (purple): The payload. The sole purpose of Mars Express is to carry provide support to these by pointing them at their targets, collecting their data, keeping them at the correct temperature and feeding them with power.
  • Power/Thermal (green): Generating, storing and distributing electricity throughout the spacecraft and maintaining the temperature within acceptable limits.
  • TT&C (yellow): Tracking, Telemetry and Control - the radio communications system of Mars Express.

There is one other subsystem that we will look at in a little more depth today – Structure.

This 'system' is the only subsystem that we cannot change in flight – but with the upcoming comet encounter, and the possibility of any sort of comet dust impact, we have been looking at the structural design in much detail!

Each wall of the square, box-like Mars Express is made from aluminium sandwich panel. This comprises two sheets of thin aluminium separated by a honeycomb of aluminium.

These panels are very popular in many aerospace and motorsport applications as they have fantastic strength-to-weight ratios and are incredibly stiff, which is extremely important when factors like the alignment of instruments is concerned. The trade-off in this case is that we are using thin materials with thicknesses similar to that of a carbonated drink can, which – while very strong – does not provide much protection from hypervelocity impact penetration.

Inside the right wall of Mars Express, looking in from the front of the spacecraft. Credit: ESA

Inside the right wall of Mars Express, looking in from the front of the spacecraft. Credit: ESA

This picture is of the right wall, looking in from the front. The aluminium sandwich panel is visible on the left of the photo and is 20mm thick.

The three black boxes are the CDMU2 (bottom), RTU (top) and the RFDU (right). A reaction wheel is also visible, at bottom right.

The other thing you probably noticed is the harness – the huge mass of cables that connect the different parts of the subsystems together.

The solar arrays are of the same construction and the high-gain antenna is based on an aluminium core but is has an additional skin on either side with six layers of carbon-fibre composite.

Now, we're sure that some of you are thinking that this is mad – how could we possibly send such a valuable spacecraft out with so little protection? Well, the first answer comes from the Hitchhiker’s Guide to the Galaxy:

"Space is big. Really big. You just won't believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space..."

In normal circumstances, the chances of our spacecraft being hit by anything significant is quite small. For a spacecraft, the worst place to be (with the exception of a comet coma) is in low-Earth orbit, and even in this relatively cluttered environment, only a few spacecraft have ever suffered enough damage due to impacts to have their missions affected.

Unfortunately, while the chances of an impact are normally very, very low, should an impact happen, it can be quite devastating. Why? Here's the other thing to remember: in space, collisions tend to be fast – very fast – and the energy of a collision increases with the square of the speed.

At such energies, impacting particles/objects and any part of a satellite they hit behave more like liquids than solids, and break up violently. Spacecraft that have been designed to operate in environments where they need to be protected from impacts use a system called a Whipple shield for protection – serving basically as armour plating (see "Hypervelocity impacts and protecting spacecraft" for much more detail – Ed.).

In Whipple shielding, a thin plate is mounted some distance offset from one or more additional shield plates. The first one will cause any impacting object to break up into fragments, and then the multiple layers behind this absorb the remaining energy of the fragments.

One of the best examples of this was ESA’s Giotto probe that flew just 596km from Halley’s comet in 1986.

Mars Express was not built with a Whipple shield and as it was not expected to face such a fierce environment as Giotto, but we're sure you can work out from the image at the top of this post (and from last week’s post on pointing restrictions) which side is the least vulnerable (we think it's the front of MEX – with the big radio antenna acting as a Whipple shield! Ed.).

Of course every decision we make is a trade-off, and we will see why in later weeks.

Andy, Michel, Kees, Simon and Luke

Waking up with a little help from our friends – Part 2

Not only is NASA helping Rosetta exit hibernation: ESA's very own Mars Express has been standing in for Rosetta in a series of crucial tests to ensure the NASA ground stations are ready to track the comet chaser. Andy Johnstone, from the Mars Express team here at ESOC, sent in this report.

Although all the attention for Rosetta wake up is mainly on the spacecraft itself, the other half of the equation is the ground stations that will be used to listen for the signal, NASA's DSS-14 in Canberra and DSS-63 in Goldstone.

If, by chance, no signal were to be detected on 20 January, this could mean that either (a) Rosetta has a problem, or that (b) possibly there is something wrong at the ground station.

Mars Express Credit: ESA/Alex Lutkus

Mars Express Credit: ESA/Alex Lutkus

Therefore, to reduce the possibility that there are any problems on ground, and since the radio systems on our two spacecraft are very similar, a test campaign was carried out using Mars Express; MEX 'pretended' to be Rosetta transmitting to the ground stations to ensure they are in perfect working condition.

The testing involved us, the MEX team, setting Mars Express to use its S-band transponders (which are normally only used for radio science or for emergency communications) to transmit at a very low bit rate, as Rosetta will on Monday.

This involved a lot of behind-the-scenes work from both ESA's Mars Express team and our colleagues at NASA DSN (including having them come in to work on weekends and on US Thanksgiving). But it paid off: a series of five test passes demonstrated to us that the 70m antennas and the teams manning them do a great job and are ready for Rosetta's wake up.

Best of luck to the Rosetta team and we're looking forward to the event on Monday!