One plan becomes two plans

Editor's note: Those who have been following our blog will know that the MEX Flight Control Team at ESOC have been actively preparing for the flyby of comet C/2013 A1/Siding Spring on 19 October. Initial estimates gave the possibility that Mars Express might have to contend with a large particle flux – and that several (2? 3?) very high-speed (~56 km/sec!) particles might bash into the spacecraft. Happily, additional observations by ground and space telescopes (including the ESA/NASA Hubble Space Telescope) have allowed initial estimates to be refined and the risk is now understood to be much lower – and perhaps even as low as zero. In today's blog post, the team explain how this (happy!) real-life, real-time development is affecting their preparations for fly-by.

Comet C/2013 A1 Siding Spring seen on 6 September 2014 from Argentina. Image credit: César Nicolás Fornari https://www.facebook.com/cesar.fornari

Comet C/2013 A1 Siding Spring seen on 6 September 2014 from Argentina. Image credit: César Nicolás Fornari https://www.facebook.com/cesar.fornari

Late last year, estimates given in scientific papers estimated that over the duration of the encounter, the number of large cometary particles per square metre would be around 1. As MEX’s area in the most protected attitude is about 3m2, we could then expect about 3 potentially significant impacts. Not good!

By the middle of this summer, published estimates (based on new images and additional modelling) were indicating a flux of around 10-6 particles per m2, which, for Mars Express, very roughly equates to a 1-in-300,000 chance of being hit. It's starting to look like our comet C/2013 A1/Siding Spring will manifest itself as a more friendly passer-by than initially thought and that it won't be hurling clouds of large particles at unthinkable speeds towards Mars and its man-made satellites.

Closest approach: If Mars were Earth. Credit: NASA/JPL

Closest approach: If Mars were Earth. Credit: NASA/JPL

So why the big change?

Continue reading

Comet Siding Spring flyby cancelled?

In the closing weeks of March, astronomers worldwide were surprised to see a sudden increase in brightness for comet C/2013 A1 (Siding Spring). At the same time, try as they might, it was not possible to detect even the remnant of a nucleus in the rapidly expanding cloud of dust that had replace the previously still very small tail.

Is this a comet breaking apart? Credit: NASA, ESA, H. Weaver (APL/JHU), M. Mutchler and Z. Levay (STScI)

Is this a comet breaking apart? Credit: NASA, ESA, H. Weaver (APL/JHU), M. Mutchler and Z. Levay (STScI)

At the same time, the control team in Baikonistan reported that contact was lost with the Uranus-bound international spacecraft Behemoth, the first truly global attempt to explore our Solar System.

Aptly named, this spacecraft was launched three years ago, and – with a mass over 100 tons – is easily the largest (not to mention most expensive) space probe ever built. One moment it was still transmitting, the next moment it wasn't.

All efforts to re-establish contact with Behemoth have, alas, turned out to be fruitless. However, astronomers and engineers mapping the paths in space of the vessel and comet Siding Spring believe the trajectories may have intersected at a fateful point still far beyond Mars orbit. We may never know – but the laws of probabilities lead us to conclude that Siding Spring's sudden brightening and the equally sudden silence of Behemoth are not unrelated.

However, every cloud has a silver lining, as the saying goes. Astronomers have informed the Mars Express control team that due to the apparent disintegration of comet C/2013 A1 (Siding Spring), its potentially dramatic encounter with Mars on 19 October 2014 is therefore called off and all preparations for the event can cease, as no preventive measures will now be required. In fact, the MEX team have all headed off to the pub for a jolly celebration...

Click on 'Continue reading' to access the full story.

Continue reading

How to hide behind a planet

Remember those raging snowball battles you had as a child in the school yard, during recess? The best strategy, if you were in the thick of it, is to hide behind something (or someone) massive – such as a tree – or another guy on your team. The fatter the guy, the more protection he offered.

Snowball fight on the National Mall, Washington, DC. Credit: CC by-nc-sa/2.0/Joe Newman http://www.cosmicsmudge.com

Snowball fight on the National Mall, Washington, DC. Credit: CC by-nc-sa/2.0/Joe Newman http://www.cosmicsmudge.com

Ah, those were the days... alas, long past. Or are they?

In fact, the situation we are preparing for now, or at least the situation our spacecraft is going to face on 19 October, isn't all that different from the snowball fights of yore, at least in principle. Well, it is true that any cometary dust particles that might have MEX's name on them are likely to be microscopic. On the other hand, they will be travelling at 56 km/s and at that speeds, even microscopic dust can pack a hefty punch.

There are no trees in the immediate vicinity of Mars Express, but there is a rather 'fat guy' to hide behind – one with a diameter of almost 6800 kilometres. That's planet Mars, of course. The orbit of Mars Express around the planet is polar, it is eccentric (during every orbital revolution, which lasts 7 hours, the spacecraft passes as close as 350 km at the lowest point of the orbit (periares, in astronomers' parlance) and as far as 10 500 km at the highest point (apoares) above the Martian surface.

The fundamental laws of celestial mechanics cannot be infringed – not even a tiny little bit and not even in an emergency or when no one is looking – dictate that MEX moves much faster at periares than it does at apoares. Other more complicated laws of physics dictate that a spacecraft orbit is not immutable: its orientation in space and also its shape are subject to variations due to external forces, known as perturbations. Some of these variations in the orbital parameters are periodic, some are not, and you can't do much about them.

You might fight them – for a time – by using the rocket engines aboard the spacecraft and the propellant in its tanks, but that will be effective only if you have lots of propellant – precisely the one thing that Mars Express does not have. Our arrival at Mars and positioning onto the spacecraft's science orbit used up 90% of the fuel that was on board. After all, the spacecraft was designed for a two-year mission and it has been at Mars for more than 10. This gain in mission duration is thanks largely to the skill and ingenuity of the Flight Dynamics team in making the spacecraft perform the observations that the science team required with the least possible fuel expenditure.

By 19 October, the evolution of the orbit will be such that when viewed from the direction from which the comet will be arriving (we know this direction quite accurately and can therefore simulate the encounter with a computer), it will look like in the image below. This shows where the spacecraft would be on its orbit if we did nothing.

Mars Express zipping behind Mars

Mars Express zipping behind Mars

The closest encounter takes place at 18:30. This is when we expect the concentration of mostly microscopic particles of comet dust to be greatest. About an hour and a half later, Mars will pass through the orbital plane of the comet. That's where we foresee the highest risk of being hit by larger particles. Once again, there is a trade-off as the larger particles carry more energy but there are fewer of them. Let’s assume that the danger is greatest at closest approach (although as mentioned in a previous post, more observations are still needed to improve our models of what to expect in the encounter).

The approaching comet would see MEX pass behind Mars just after 16:30, and it would reappear about half an hour later. It so happens that the time when the spacecraft is behind Mars (and therefore protected from the dust that accompanies the comet) coincides with periares. If you recall, that is when it is also moving fastest on its orbit. The fact that the spacecraft is behind Mars for only half an hour, while the passage through the dust tail is likely to last a few hours, means that this alone is not a (full) answer to our problem of how to endure passing through a comet coma. It does, however, provide a short respite from exposure to potentially damaging particles.

There is one thing we can change, however. Note that if we do nothing, then at 18:30, at the closest encounter, MEX would be in the open, far from any protection. We can at least change the time of periares such that it occurs at 18:30, not 16:41. Though we can change neither the shape nor the orientation of the orbit, we can change this one parameter. And it will cost only very little propellant.

In essence, what we need to do is to delay the spacecraft passage behind Mars by 109 minutes. That sounds like a major task, and it is, if you try to do it just shortly before the comet is there. But we won't wait until then.

We plan to do a manoeuvre using the MEX engine already several months in advance. We can cut this delay to, for example, 109 slices of 60 seconds each. If we increase the orbital period by this 60 seconds, then 109 orbits later we have accumulated exactly 109 minutes of delay and MEX will be behind Mars when the snowballs hail down – or rather, when the onslaught of cometary dust is expected to peak.

We could also split the delay into 218 slices of 30 seconds, or 327 slices of 20 seconds, and so on. The important part is that the earlier the change is made, the more revolutions are available and so the smaller the change needed and hence the smaller the fuel consumption.

How do we delay the arrival of a spacecraft at a given position?

You won't believe this – it's done by increasing the orbital velocity. Nobody ever said that celestial mechanics was intuitive, right? You know that MEX’s orbit is an ellipse. Draw a line between the closest and the farthest point from Mars and you have the major axis; divide that by two and you have the semi-major axis. Every orbit represents a large amount of energy (that's why it takes a big rocket to launch something into orbit).

The orbital period (the time it takes to complete one revolution) depends on the orbital energy around a given planet (here Mars), and on nothing else. The orbital energy in turn depends on the size of the semi-major axis, and on nothing else. The larger the semi-major axis, the higher the orbital energy, the longer the orbital period.

Now, if you speed up a spacecraft, you add orbital energy, and therefore, inevitably, you increase its orbital period. This is a simple physical fact, and we are going to make use of it to maximize the chances MEX has of weathering the encounter with Comet Siding Spring.

We can't break the laws of physics but we can, and will, use physics to our advantage.

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

How to determine the orbit of a comet?

In movies about the impending end of the world due to a comet impact, one thing is certain: Detecting the comet and computing its orbit are dead easy. The scene starts with a computer screen showing a telescope image of a star field, where we can make out a faint, fuzzy little object that doesn't quite look as if it belonged there.

Control room at ESA's Optical Ground Station Credit: ESA

Control room at ESA's Optical Ground Station Credit: ESA

A scientist will throw a brief, disinterested glance at the screen and turn away. Then he stops. His eyes go wide and he snaps around to stare at the screen. He'll call in his colleagues. Computer programs are started, and people frantically hack away at keyboards. In no time at all, they will have identified the fuzzy blob as a comet that is hurtling in from the frozen recesses of space.

Bruce Willis at the 2010 Comic Con in San Diego. Credit: Gage Skidmore

Better call this guy!

What's more, in no time at all, they will have determined the comet's trajectory and they can categorically state that it will hit Earth. A few more frantic calculations and they also know the date and time of impact – Quick, call Bruce Willis!

Neat. But does it really work like that?

In actual fact, one single picture of a comet is just that: a single picture of a comet. All this picture tells you is that there is a comet which, at a certain date and time – a certain epoch (astronomers like to use common words with meanings that are just different enough from what you think the word means to create confusion, so they say 'epoch') – appeared to be near stars A, B, and C in constellation X. No more.

From one picture, you can't tell where it's heading; you don't know how close it will get to the Sun, nor if or when a close encounter with any other planet is due. To find out these things, you need more observations – many more of images that were taken at different dates, ideally spanning a long time frame. The longer the better.

Comet C/2013 A1 Siding Spring

There it is!

And even then, computing the orbit is not really a straightforward matter. In fact, it's a drudgery and you have to go about it in a roundabout way. The pictures don't tell you the comet's path. All they do is to tell you where the comet appeared to be seen at the stated epochs from wherever the observatory was located. But that is not what you want to know.

So you have to make an educated guess at the parameters that describe the comet's trajectory, also known – unsurprisingly – as its 'orbital parameters'. This initial guess (as even the mathematicians rather candidly refer to it) in all likelihood will be quite far off. That's OK, it's just a guess. But at least, you have something to work with.
Euklid-von-Alexandria 1

A famous mathematician

Once you have the initial guess, you have a set of orbital parameters that apply to a given reference epoch that you are free to choose. The known laws of celestial mechanics allow you, if you have a full set of orbital parameters at one epoch, to compute where the object will be at other epochs in the past or in the future. So you can compute where it would have been at the epochs when the comet images were taken. Because the Earth's orbit is well known, you can then compute in which constellation and next to which stars the comet would have appeared at these epochs, assuming that your initial guess holds.

For the initial guess, you will probably find out that the locations in the sky that you compute for the image epochs are quite different from what the images show. Hardly surprising, given that that was just the initial guess. But then you can make small changes to each of the orbital parameters for the initial guess to see which change makes the computed locations at the image epochs move closer to the actually observed locations. At all times, you can use the very accurately known positions of the background stars to get your bearings.

This procedure is known as 'orbit determination'.  It is very time-consuming and involves a lot of complicated and repetitive mathematical calculations, which is why nowadays we let a computer handle most of it. The entire process is known as 'parametric optimisation' and each step is referred to as an 'iteration'. As the optimisation process goes on and many iterations have been performed, you will see that for the epochs at which the images were taken, the computed locations, based on the current estimate of the orbital parameters, will move quite close to what you can see in the actual images.

This diagram is indicative of how the orbit determination accuracy for comet C/2013 A1 (Siding Spring) evolved over time.

This diagram is indicative of how the orbit determination accuracy for comet C/2013 A1 (Siding Spring) evolved over time. It shows a derived quantity: the expected encounter distance from Mars. Source of data: NASA/JPL

At some point, there will be no further improvement. The process will have 'converged' in mathematicians' parlance. But wait: this does not mean that you're done. It just means that you can do no better with the available information, in this case, the available set of photographic images. There is always some uncertainty in measuring just where an object is with respect to the background stars. Comets appear fuzzy and blurred. Who's to say what their exact position in the celestial vault is on that image you're gazing at? Might it not be a few hundredths of a degree further up? But, on a cosmic scale, such minute differences matter – a lot!

Furthermore, if the time span between the earliest and the latest available image is short (just a few weeks or even a few days) the information they contain may be ambiguous. Comets on a wide variety of orbits might still appear to be in that location over a short period of time. But you will see that in the results of the orbit determination, because the mathematical process involved will not only give you a best estimate of the orbital parameters, it will also tell you how accurate (or inaccurate) that best estimate is.

The problem is that if you know the orbit only approximately at that reference epoch you selected at the start of the process, and you want to propagate ( i.e. compute the time change in the orbit, subject to the laws of celestial mechanics and any effects that may change the orbit) this orbit to another epoch, you will find that any uncertainty in the initial parameters will balloon, the further you propagate.

In the diagram above, it took almost 200 days to find out that comet Siding Spring would not hit Mars. At that time, the uncertainty in the predicted encounter distance still ran into hundreds of thousands of kilometres. Though the most probable encounter distance was established fairly early, the uncertainty was still significant after more than a year of observation. It took 44 days of observation to achieve even a semblance of an orbit determination – one that was still all over the place, with a predicted mean Mars distance at flyby 900,000 km, with a high guess of 3.6 million!

Sincere thanks to Michael Khan for today's post – Ed.

 

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

Why orienting our spacecraft is the heart of the challenge

Today's post continues where we started last week with an update from the Mars Express Flight Control Team at ESOC on their preparations for the 19 October Comet Siding Springs flyby. Today: defining the challenge!

Comet C/2013 A1 Siding Spring

NASA's NEOWISE mission captured images of comet C/2013 A1 Siding Spring, which is slated to make a close pass by Mars on Oct. 19, 2014. The infrared pictures reveal a comet that is active and very dusty even though it was about 355 million miles (571 million kilometers) away from the sun on Jan. 16, 2014, when this picture was taken. Credit: NASA/JPL

Before we look at Mars Express in more detail and decide what we can do to try and protect it from the speeding particles in the comet's coma (the cloud of dust and gas surrounding the nucleus), we should take a moment to briefly describe the spacecraft and the encounter period.

The shape and structure of spacecraft are normally described using a coordinate reference frame. For Mars Express, we on the team often use a more informal description where the high-gain antenna is referred to as the 'front', the thrusters are on the 'bottom' and the instruments face out from the 'top'.

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

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

Nice view of MEX – Click image for a 3D model

As these directions are given from the Mars Express point of view, the MARSIS (Subsurface Sounding Radar / Altimeter) booms are therefore mounted on the right of the spacecraft.

Further, the left and right side each have a solar array extending away from the main spacecraft body that can rotate through 360°.

Hacked-up version of the nice view showing spacecraft directions (some of you may prefer to assemble your own MEX paper model – Ed.)

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

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

Constraints, constraints...

The spacecraft is, in principle, able to turn in any direction, however the left, right and rear sides have radiators for shedding heat from the platform and payload systems and should not be illuminated by the Sun.

The top should also not be pointed toward the Sun as some of the instruments require cooling to operate effectively and optics may be damaged by direct sunlight.

During scientific observations, the instruments are pointed toward a target to collect data, and – for communication – the antenna must point toward Earth.

These two tasks, as you may have guessed, do not happen at the same time and science data is recorded and downlinked to Earth later.

Also, for the majority of observations, the attitude of a science observation is in no way compatible with communications pointing.

Finally, the solar arrays should be pointed towards the Sun whenever possible to generate electricity (although power can be stored in batteries for short periods).

The orientation of things

Siding Spring flyby of Mars - Mars orbit plane. Credit: ESA/M. Khan

Siding Spring flyby of Mars - Mars orbit plane. Credit: ESA/M. Khan

This image illustrates the relative orientations of Mars, the comet, Earth and the Sun on 19 October.

The particles in the coma are ejected away from the comet with a speed of a few metres per second (m/second) but as the overall speed is so high we are treating them as arriving along a line parallel to the path of the comet.

In other words, we are treating them as a stream of hyper-velocity particles washing past, over and around MEX.

It is worth noting the relative direction of Earth and Sun; if we want to stay in touch with the spacecraft during the flyby, the antenna must point toward Earth.

So, in summary, the direction in which we orientate the spacecraft and the solar arrays has a big impact on how Mars Express communicates with Earth, generates power, controls its temperature and conducts science observations.

Siding Spring - trajectory in 2014 Credit: ESA/M. Khan

Siding Spring - trajectory in 2014 Credit: ESA/M. Khan

Now we have additional factors, as we have an interesting target passing by that our science teams really wish to observe as directly as possible – but with it comes a stream of potentially damaging particles!

The threat...

These particles might not only physically abrade the outer surface of the spacecraft (which can damage insulation, radiators and instrument optics), but also – if large enough – can penetrate parts of the spacecraft structure.

Additionally, at the impact speed expected here, even minute specks of dust will be converted into an electrically charged plasma, which can lead to a current and might short out and damage some of the electronics.

The challenge...

So the challenge we face is simple: how do we orient the spacecraft to maximise the science possibilities, best protect the most vulnerable and critical areas of the spacecraft body, respect the always-present pointing restrictions, maintain communication and minimise the possibility of any damage from hyper-velocity impacts?

The answer, which we are developing now, will undoubtedly lie in trade-offs: to reduce risks and maximise science and survivability.

We do know one thing for certain: there is no perfect answer!

More news next week!

Andy, Michel, Kees, Simon and Luke

 

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