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.
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.
Mars Express components
This diagram shows the major components in the spacecraft body.
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.
Mars Express edge-on view
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 face-on view
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 (ensur