Just had the following message through from Mars Express Project Scientist Olivier Witasse. -- Stuart
“The Mars Express Radio-Science team, led by Martin Pätzold (Cologne University), has performed a preliminary analysis of the radiometric data recorded during the evening of closest approach, 3 March 2010.
The NASA ground-station DSS-63 near Madrid recorded the frequency of the transmitted signal, at about 8.4 GHz and 2.3 GHz, which contains the signature of Phobos’ gravity field. To be able to decipher this weak signature, the team has subtracted all known variations, which would have been measured even in the absence of Phobos. What remains is produced by the gravity of Phobos pulling Mars Express.
Credit: ESA/ Department of Planetary Research at the University of Cologne (M. Pätzold).
The grey line in the image shows the frequency change due to Phobos during a 20-minute window, centred on the closest approach. Before closest approach, the effect of Phobos on the spacecraft is negligible. Then there is a clear jump in frequency at closest approach. This is Phobos slightly changing the orbit of Mars Express.
The blue line is the expected frequency change assuming the mass of Phobos, as measured during a previous flyby, is evenly distributed throughout the moon’s interior. There are clearly small differences between the blue and grey lines. The challenge now for the Radio-Science team is to dig into these small differences to prise out information on the mass distribution. “The real work starts right here,” says Pätzold.
“It may take a few weeks for the extraction of precise information on the interior of Phobos,” says Tom Andert, from Munich University.
The Mars Express close encounter with Phobos was also observed by three European VLBI network stations: the 20 m Wettzell radio telescope (Bundesamt für Kartographie und Geodäsie, Forschungseinrichtung Satellitengeodäsie, Germany), the 14 m Metsähovi (Aalto University - School of Science and Technology, Finland), and 40 m Yebes (Observatorio Astronómico Nacional, Instituto Geográfico Nacional, Spain). Data processing was performed at the Metsähovi Radio Observatory, and analysis at the Joint Institute for VLBI in Europe (The Netherlands).
The residual Doppler frequency pattern, as detected by the Wettzell radio telescope is shown on the plot. The sharp swing of the carrier line frequency at 21:03 UT corresponds to the closest proximity of MEx spacecraft to Phobos. The people who contributed to this project are Guifre Molera Calves and Jan Wagner (MRO, Finland), Gerhard Kronschnabl (BKG, Germany), Pablo de Vicente (OAN-IGN, Spain), and Sergei Pogrebenko (JIVE, The Netherlands).