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:

1. 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).
2. 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!

On Monday, 30 April, Mars Express mission controllers at ESOC will take advantage of a rare alignment of the spacecraft with Phobos and Earth to use radio signals to determine the Martian moon's orbit.

A (very) rough sketch of the alignment

If the team's careful planning and preparation pay off, the occultation of Mars Express by Phobos (as seen from Earth) will provide highly accurate data that can be used to determine Phobos's orbit with unprecedented accuracy, possibly improving it by a factor of 2.

Essentially, here's what's going to happen:

• On 30 April, Phobos will, for a few seconds, pass between MEX and Earth, blocking the direct line-of-sight path for radio signals transmitted from MEX to ESA's 35m deep-space station at Cebreros, Spain.
  
• In a pointing specially extended for this event, Mars Express will already be turned toward Earth — a mere pale blue dot some 140 mn km away — and will already be transmitting a steady signal to Cebreros.
  
• Special radiometric recording equipment installed at the Cebreros ESTRACK station will record — with sub-second precision — the start/finish time of the break in the transmission.
  
• The occultation is predicted to last just 12 seconds, from 21:21:22.9 to 21:21:34.1 UTC (spacecraft time). The predicted ground receive time is between 21:29:13.0 and 21:29:24.2 UTC (i.e. 7:50 mins later on Earth).
  
• Once the occultation ends, MEX will perform a 'high-speed' slew to re-orient itself so that the HRSC camera and other instruments are pointing toward Mars to conduct a regular observation pass over the Red Planet starting at 21:22 UTC.

Results from Cebreros station will be passed to ESA's flight dynamics team and to our colleagues at NASA's JPL, who will use these times in calculations that, once confirmed, could yield the best-ever determination of the moon's orbit.

"The occultation tracking promises to provide a very nice boost in our understanding of Phobos, an enigmatic body if ever there was one, without affecting routine science," says Mars Express Spacecraft Operations Manager Michel Denis.

We'll keep in touch with the Mars Express mission team at ESOC throughout the weekend and Monday/Tuesday to post any updates; you can also follow news via Twitter (@esaoperations).

The animation below shows what will happen immediately after the occultation tracking, when Mars Express re-orients itself for the Mars observation pass.

-- Daniel