Waking up with a little help from our friends – Part 2

Not only is NASA helping Rosetta exit hibernation: ESA's very own Mars Express has been standing in for Rosetta in a series of crucial tests to ensure the NASA ground stations are ready to track the comet chaser. Andy Johnstone, from the Mars Express team here at ESOC, sent in this report.

Although all the attention for Rosetta wake up is mainly on the spacecraft itself, the other half of the equation is the ground stations that will be used to listen for the signal, NASA's DSS-14 in Canberra and DSS-63 in Goldstone.

If, by chance, no signal were to be detected on 20 January, this could mean that either (a) Rosetta has a problem, or that (b) possibly there is something wrong at the ground station.

Mars Express Credit: ESA/Alex Lutkus

Mars Express Credit: ESA/Alex Lutkus

Therefore, to reduce the possibility that there are any problems on ground, and since the radio systems on our two spacecraft are very similar, a test campaign was carried out using Mars Express; MEX 'pretended' to be Rosetta transmitting to the ground stations to ensure they are in perfect working condition.

The testing involved us, the MEX team, setting Mars Express to use its S-band transponders (which are normally only used for radio science or for emergency communications) to transmit at a very low bit rate, as Rosetta will on Monday.

This involved a lot of behind-the-scenes work from both ESA's Mars Express team and our colleagues at NASA DSN (including having them come in to work on weekends and on US Thanksgiving). But it paid off: a series of five test passes demonstrated to us that the 70m antennas and the teams manning them do a great job and are ready for Rosetta's wake up.

Best of luck to the Rosetta team and we're looking forward to the event on Monday!

When will we see the results?

ESA's Olivier Witasse provided this update on Phobos flyby results.

The flyby data analysis should give 2 information:

  1. The mass of Phobos and
  2. A gravity coefficient called "J2" (sometimes called C20), that will tell us something about the interior structure of Phobos

The mass of Phobos is already known with a great accuracy. From this flyby, we might expect a better accuracy on the mass determination, but that will not change the big picture.

Same for the density: The density is calculated from the mass and the volume, therefore the new density estimated from the flyby won't be that different compared to the known Phobos density. We can expect a number before the end of the month for sure.

In contrast, the most interesting and new result would be about the J2 gravity field coefficient. The data analysis to get the value will be very complex, and the first result may take a few months.

And the crater is…

On Christmas Eve we posted a new image of a small portion of Phobos taken by Mars Express only a few days before, on 22 December, and gave you the challenge of working out where on Phobos you thought it was located.

As correctly commented by JSch, the crater in the bottom left corner is indeed Wendell crater!

Here’s part of the image again, rotated and with coordinates overlaid:

Wendell crater on Phobos

Wendell crater on Phobos. Credits: ESA/DLR/FU Berlin (G. Neukum)

Thanks to our DLR colleagues Marita Waehlisch and Juergen Oberst for confirming the location!

And congratulations to JSch for the quick and correct call!

Why no radar science?

During Phobos flyby on 29 December, @AsteroidEnergy asked a question via Twitter:

It took some time (due to holidays – Happy New Year!!!), but we now have a reply from ESA's Olivier Witasse, MEX Project Scientist at ESTEC.

Mars Express in orbit around Mars with the MARSIS antenna unfurled. Credit: ESA

Mars Express in orbit around Mars with the MARSIS antenna unfurled. Credit: ESA

The radar was originally designed solely for the observation of Mars. For safety reasons, the radar software blocks operations when the target is closer than 240 km. In the case of past Phobos flybys, because the distance is sometimes lower than that, the radar was therefore re-configured to operate at close distance by bypassing protections preventing the opening of the receiver before a certain time from transmission had elapsed.

Although not without risk, this procedure was thoroughly tested and successfully used throughout several Phobos flybys. The operational distance is now *** 175 km ***. Therefore, we usually switch on MARSIS when the minimum flyby distance is around 175-200 km.

See you in 2014!

– Olivier