Comet Siding Spring Mars encounter: Ya gotta have a little ‘tude

This article was originally posted on the Mars Express blog and is reposted here with permission.

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.

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.

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), thereby presenting the smallest target.

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?

(See the bottom of this page for answers – Ed.)

We're also working on another plan to avoid nasty bits of comet. 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, was to hide behind something (or someone) massive – such as a tree – or another guy on your team. The bigger the guy, the more protection he offered.

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 Mars Express'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 'big 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. They 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.

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 Mars Express 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, Mars Express 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 Mars Express' 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 Mars Express 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.

Answers:

What do you think the problem would be if a safe mode were to happen on 19 October?

The answer is that, in safe mode, the HGA is pointed toward Earth and not towards the flux. The right and top sides of Mars Express would then be exposed to the flux, potentially harming the instruments. The solar arrays may be exposed for a short time but would end up edge-on again.

The SAS is needed in the safe mode, as are the thrusters. Without these Mars Express cannot find the Sun or control its attitude. The trouble is that these bits have to stick out to do their job and so there will always be a SAS in the firing line and the only side where the thrusters are not visible is from the top.