Insider’s Cassini: Dr. Michele Dougherty and Cassini’s Magnetometer

Todd J. Barber, Cassini lead propulsion engineer

For the Cassini flight team, 2011 began as busy as ever, including the three-times yearly quarterly Project Science Group (PSG) meeting, this time at JPL in late January. Many Cassini PIs (Principal Investigators) and other instrument team members congregate at PSG meetings, and this particular meeting was no exception. PSG meetings offer a wonderful chance to catch up, present recent science results, and evaluate future plans. They also provide a fertile hunting ground for snagging interviews with prominent PIs for the “Insider’s Cassini” column, so I took this opportunity to interview Dr. Michele Dougherty, the lead scientist for Cassini’s magnetometer (MAG) instrument. We enjoyed the late afternoon Pasadena sunshine (an all-too rare event for Dr. Dougherty) before her return to the United Kingdom, and I now have a new appreciation for MAG and its unique science capabilities.

Dr. Dougherty was born, raised and educated in South Africa before a post-doctoral stint in Germany at the Max Planck Institute in Heidelberg. Her training was exclusively in applied mathematics, and this led her to a second post-doc at Imperial College in the UK, where she remains to this day. She eased into planetary science gradually, working one day per week with David Southwood at Imperial on modeling interplanetary and Jovian magnetic fields using the ESA/NASA Ulysses data. Like many of us, she caught the planetary science “bug” and has not looked back since. Perhaps the seeds for her interest in planetary science were sown in her first view of Saturn at her home in South Africa. Her father built a large homemade telescope, including grinding his own mirror, and she recalls mixing concrete for the base of this large amateur telescope!

Until roughly Saturn Orbit Insertion (SOI) in 2004, Dr. Dougherty worked under Southwood on the Cassini MAG team at Imperial. Upon Southwood's move to ESA, she assumed the MAG PI role just before SOI and the beginning of the “juiciest” science. Since most of the other MAG co-investigators at that time were 30 years her senior, she describes herself as “that young whippersnapper” who took over for Southwood. She has clearly moved beyond this early characterization, given her lead role as the ESA lead scientist in Europe for the Europa Jupiter System Mission (EJSM), potentially the next “flagship” planetary mission to the outer solar system.

MAG itself consists of two parts, a flux gate magnetometer about halfway down the Cassini magnetometer boom (the boom was deployed shortly after the Earth flyby in 1999) and a vector/scalar helium magnetometer at the boom tip. Unfortunately, the latter MAG subassembly first exhibited degraded operation and then stopped working completely, but fortunately through very careful analysis (and onerous but still doable calibrations), most science capabilities have been restored. Since Cassini is a three-axis stabilized spacecraft, it is a bit more difficult getting the complete-sky view afforded spinning spacecraft, but Cassini executes spacecraft rolls to help mitigate this limitation, much to the delight of the MAG team.

It’s very difficult to capture a mission’s worth of MAG science results in a brief column like this, but some highlights before even getting to Saturn include a bounty of excellent heliospheric magnetic field data for Dr. Ed Smith, a flyby through Earth’s magnetic field in 1999 (mostly useful for instrument calibration), and an exciting flyby of Jupiter in late 2000, which, though distant, did “clip” the Jovian magnetosphere.

However, for Dr. Dougherty, the MAG highlight to date was the discovery of what the team called a "localized atmosphere" at Enceladus. She and her team detected the bending of Saturn’s magnetic field lines near this icy moon, where no deflection was expected. Dr. Dougherty successfully lobbied for a closer Enceladus flyby than planned on a future encounter with the moon, and she confessed to me she “didn’t sleep well” before this flyby, given the possibility of her theories being incorrect. Thankfully, this bold plunge closer to Enceladus provided the MAG team with the proof they were seeking—Saturn’s magnetic field lines were definitely modified by Enceladus. Dr. Dougherty reminded me how synergistic this effort was, requiring the assimilation of data from many of the Cassini instruments to understand this perplexing icy satellite. This “cross-fertilization” with other science instruments is probably one reason why Dr. Dougherty has nearly 200 publications to her name from Cassini MAG results, with contributions from team members aged “21 to 83,” in her words. She is particularly gratified that young PhD students and post-docs are “cutting their teeth” with Cassini MAG data and are doing so well. What a rich, multi-generational effort this truly is!

I asked Dr. Dougherty about other MAG science results, and she led me on a whirlwind tour of possible (slight) outgassing of Saturn’s moon Dione, the potential for further reducing Titan-70 flyby data to help understand the subsurface of Titan, observations near the rings to help understand spoke behavior and its possible correlation with Saturn Kilometric Radiation (SKR), and so forth. MAG data and SKR seem to show the same periodicity, but this itself is a mystery, because the tilt of Saturn’s magnetic axis vs. its rotation axis (only 0.06 degrees) is too small to even explain the existence of Saturn’s magnetic field in the first place! Perhaps the proximal orbits in 2017 will finally help solve this decades-old mystery, again thanks to Dr. Dougherty and her very capable team along with other Cassini science instrument teams. In the Cassini Solstice Mission, she is also looking forward to increased spatial and temporal coverage of Titan and icy moon interactions with Saturn’s magnetosphere. She asked me personally to acknowledge the great support and leadership of Bob Mitchell, Cassini Program Manager at JPL, and I would like to say thanks to Dr. Dougherty for her time. Understanding science results from fields and particles instruments like MAG is never easy, but with a little effort, the importance and allure of this science shines through splendidly, like the afternoon sun in Southern California. Best wishes to you and your team upon your return to the United Kingdom, Dr. Dougherty.