On 30 April, in a carefully planned data-gathering exercise, ESA's Mars Express team used the passage of Phobos between Earth and the spacecraft to record precisely when a radio signal was blocked. The data may now lead to the best-ever determination of Phobos's orbit, knowledge that will help scientists better understand our planetary neighbour and its double-moon system.
You'll recall our earlier posting, when we described MEX team plans to take advantage of a rare alignment of Earth - Phobos - Mars Express to measure precisely when the bulk of Phobos would block radio communications between the spacecraft and ESA's 35m Cebreros station in Spain.
The prediction was that the 'occultation' would last around 12 seconds, from 21:21:22.9 to 21:21:34.1 UTC (spacecraft time). The predicted ground receive time was between 21:29:13.0 and 21:29:24.2 UTC (i.e. 7:50 mins later on Earth).
The activity took place as planned on Monday, 30 April.
Based on a reconstruction of the MEX orbit by ESA's Flight Dynamics team at ESOC, results indicate that the expected start time of the occultation by Phobos was 0.5 seconds earlier than that predicted. Similarly, the end time is 0.4 seconds earlier than predicted.
The plot below indicates the actual situation as measured very precisely from Cebreros station.
It shows the view from Earth of the 'expected' occultation of the MEX signal by Phobos. The large, grey-green circle corresponds to the moon's disk; it is shown (at this scale) as a circle having a radius of 13.4 km.
So why was the actual recorded occultation times a few seconds shorter than expected?
ESA's Trevor Morley, lead flight dynamics specialist for Mars Express, says there are two possible reasons:
- The best fit ellipsoid for the shape of Phobos has semi-axes of 13.4 x 11.2 x 9.2 km. The dashed circle shown in the plot has a radius of 9.2 km. The occultation duration depended upon the length of the chord across the real face of Phobos between the entry and exit points. The actual profile of Phobos needs to be applied to calculate this length (which is why photos/observations of Phobos are so important - Ed).
- A shorter than expected occultation duration would also occur if the present error in our prediction of Phobos's orbit is such that the moon's true position at the time was further towards the south.
Trevor says that the recorded timing data will now be shared with the MEX science team as well as with colleagues at NASA JPL; it is expected that they will be able to use these results to improve models of the Phobos orbit:
“The Phobos orbit model that we are presently using was generated by Bob Jacobson of JPL and is almost certainly the most accurate one available. We will provide Bob with all the signal data plus our determined orbit of MEX,” he added.
“Of course we can provide the data to any scientists working on this topic, including those who have been using directional data derived from Phobos images made by the MEX camera for the same purpose.”
Mars Express Spacecraft Operations Manager Michel Denis says:
It's part of the scientific method to make use of every opportunity to investigate our natural world.
Also, it's second nature and ‘pride of craft’ for the mission operators to figure out how to achieve more with the same resources– whether that’s a spacecraft or a ground station. In this case, we could also achieve ‘extra science’ through tight cooperation between several teams here at ESOC – flight dynamics, ground segment, flight controllers – and we were able to fine-tune, second-by-second, an already packed science and communication plan.
The team here have really done a nice job!