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