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