The Earth Swing-By and Boom Deployment
In August 1999 the magnetometer boom was deployed placing the two MAG sensors respectively 5.5m and 11m away from the spacecraft. The boom had been retracted for thermal reasons since launch in October 1997 and now for the first time we could observe the local magnetic field relatively undisturbed by the magnetic field created by the spacecraft itself. The boom (and the MAG sensors which are attached to it) rotates as it deploys, thus the magnetic field observed by the sensors during deployment appears to rotate (as well as reduce in strength as the distance from the spacecraft increases). Short animations of the deployment have been made using the detected field of the sensors, these can be viewed by clicking on the following links:
Boom deployment movies by Justin Ashmall: FGM and VHM.
A few days after boom deployment, Cassini made its second planetary flyby - as before to gain a gravity assist for its trajectory. This time the planet was the Earth and many instruments, including MAG, were switched on both for calibration in the relatively well known Earth's environment and also to make new scientific observations.
Our main calibration goal was to cross-calibrate our Scalar Helium Magnetometer (SHM) with our Fluxgate Magnetometer (FGM), a technique that will be used during Saturn flybys. This was the only time that the spacecraft would be in a magnetic field strong enough to operate the SHM properly before arrival at Saturn. Both sensors worked correctly and initial results showed excellent agreement between them. Our main science goals were to make detailed, and high time-resolution measurements of the Earth's magnetosphere, particularly in the tail and boundary regions. Cassini's high speed took it through the magnetosphere in about 7 hours, providing us with a unique snapshot of the environment. We were fortunate to find that both the solar wind and the magnetosphere were in highly disturbed states making our measurements even more valuable.
Plot of Magnetic field magnitude during Earth flyby.
The plot shows the magnitude of the magnetic field measured by the FGM during the closest approach period. To complement these measurements, our Leicester University Co-I's had arranged for simultaneous observations to be made by a whole variety of ground-based and Earth orbiting instruments - ACE, WIND, IMP8, Interball, Fast, Geotail, Polar, Samnnet, Greenland and Canadian magnetometers, Oersted, Canopus, Image, Sonderstorm, Cutlass and Superdarn. This overall dataset is expected to provide a rich harvest for the MAG science team.
MAG was then powered off again, having been operated for more than 3 weeks after Earth flyby. During this time an extensive set of solar wind magnetic field data was gathered and towards the end, the spacecraft crossed the very distant Earth magnetotail region providing more unique measurements for the MAG science team to investigate.
