Notes on the ISEE-3 Vector Helium Magnetometer From the Original Principal Investigator

Ed Smith, Original Original Principal Investigator on ISEE-3 Vector Helium Magnetometer: The effort to recapture the ISEE-/ ICE spacecraft has just achieved a notable scientific success. Data recovered from the spacecraft very recently show that the magnetometer is not only operating well but has observed a large rapid change in the Interplanetary Magnetic Field/IMF.

What makes this accomplishment so remarkable is that it is the first science data returned by the spacecraft after its long 29 -year voyage around the Sun (traveling ahead of and slightly faster than the Earth). That trip began shortly after ISEE carried out the first encounter with a comet, Giacobini- Zinner, in September 1985. Shortly afterward, the ISEE experiments continued to operate but were disconnected from the radio telemetry so that only a beacon was being transmitted. In the intervening years, no scientific data were received.

A large magnetic pulse is not unusual in space so the science objective now is to compare the event just observed in the IMF with simultaneous magnetic field observations by another spacecraft in a "halo orbit" where it travels around the Sun at the same speed as Earth. ISEE was the first spacecraft placed in a Halo Orbit in 1978 and demonstrated the scientific advantages of continuous measurements upstream of Earth in the approaching solar wind, the ionized gas escaping from the Sun's corona (visible only at eclipses) and the cause of geomagnetic storms and aurora. The comparison of interplanetary phenomena at two or more locations is still an on-going scientific interest. More such comparative observations of this kind are likely now that ISEE is returning to the Earth-Moon system.

In the near future, the operational plan is to acquire data from the other ISEE experiments, have the scientists assess their performance and participate once more in making scientific measurements. The ISEE magnetometer operates differently than another commonly- used magnetometer developed during World War II to detect submarines. The ISEE instrument measures the three components, or magnetic field vector, equivalent to the magnitude and direction of the magnetic field.

It is an adaption of physics principles underlying magnetic resonance, a diagnostic technique now commonly used in medicine in the form of Magnetic Resonance Imaging/MRI. It uses helium gas as the active element and accordingly is called the Vector Helium Magnetometer. The magnetometer was developed by the Jet Propulsion Laboratory in Pasadena, California and used by JPL scientists on numerous space missions to the other planets and investigations of interplanetary magnetic fields. It has been the preferred magnetometer because of its sensitivity (measurement of ultra-weak fields) and stability (accuracy of measurement over long time periods).

The Vector Helium Magnetometer/VHM on ISEE has an interesting history. It was a prototype, the first flight-like model, of the VHM flown on the pioneer 10,11 missions to Jupiter and Saturn launched in the 1971-1972. Those spacecraft carried the next two VHM flight models. The remaining Pioneer prototype was subsequently proposed for ISEE-3 as a cost saving and chosen to be one of the experiments on the spacecraft.

The recent contact with ISEE shows that the magnetometer, built before 1970 or 44 years ago, is still working after being in space for the last 36 years, a remarkable achievement in reliability. The Principal Investigator of the Magnetic Field Investigation is Edward Smith of JPL who also continues to be active in space research.

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