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February 7, 2012

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Geoff Brown
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It's All in the Swing: RBSP Undergoes Magnetic Swing Test

With NASA’s Radiation Belt Storm Probes scheduled to launch in fewer than 200 days, the pace of spacecraft testing has continued to ramp up. In mid-January, RBSP – built for NASA by the Johns Hopkins University Applied Physics Lab (APL) in Laurel, Md. – was subjected to a science test that ensures the spacecraft is magnetically clean when it begins its mission. This is done so that one of RBSP’s five instruments, the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS), can properly gather data. EMFISIS will help scientists understand the important role played by magnetic fields and plasma waves in the processes of radiation belt particle acceleration and loss.

EMFISIS uses two types of magnetometers; each is fixed to the end of a three meter long boom that deploys from two of the RBSP’s solar panels. One type of magnetometer is used by RBSP’s science team to study distortions in Earth’s magnetic field that affect the Van Allen radiation belts. It also uses the planet’s magnetic field to estimate the spacecrafts’ attitude (the orientation in space of the probe); it’s therefore very important that the magnetic field generated by spacecraft is so small that it will not affect these crucial measurements.

“EMFISIS has very sensitive magnetometers,” explains APL’s Nelli Mosavi, the lead engineer for the RBSP magnetic swing test. “We need to make sure that RBSP is very magnetically clean. In order to meet the tight magnetic requirement for RBSP, from the start of the project, we had a very strict commitment to the material and component selection. All components, subsystems and instruments were magnetically ‘sniffed’ [evaluated] prior to spacecraft integration.” RBSP must have a static magnetic field of less than five nanotesla (a nanotesla is a billionth of a tesla, the measurement of magnetic field strength) at the end of the three-meter-long boom where EMFISIS will be mounted; for comparison, a typical refrigerator magnet generates about 5,000,000 nanotesla at the point it adheres to your refrigerator.

Mosavi explains the January 23 swing test of spacecraft A, shown in the video at top:

“The one tripod to the left of the spacecraft has two APL magnetometers, which use a laser pointer as a range finder,” Mosavi says. “On the right are two tripods, each with two magnetometers from NASA’s Goddard Space Flight Center. Those use ultrasonic range finders that communicate with the magnetometers via Bluetooth. Prior to starting the spacecraft swinging, it is necessary to create a timing synchronization for all the magnetometers. In order to do that, a circular coil of wound wire is placed underneath the spacecraft. A current is briefly sent through the windings, which generates a magnetic field pulse that is recorded by all the magnetometers, providing the necessary timing synchronization for the analysis.”

Mosavi explains why rangefinders are needed, and why it was good to have different magnetometers in the setups: “It’s important to have the real time distance measurements on the motion of the spacecraft with respect to the test magnetometers,” she says, “and having test magnetometers in different orientations and positions ensures complete coverage of the spacecraft magnetic field.”

To accurately characterize the magnetic field of the RBSP spacecraft, it must be in motion. That’s why the probes were carefully rigged and suspended from a crane hook, and manually coaxed into a pendular motion with a push from an RBSP engineer. “A total of 12 pendulum oscillation sets of measurements were taken. For each set, the first four or five swings are the most important, because the spacecraft gets closest to the test magnetometers,” says Mosavi. Each swing is about one-half inch lower than the last, and data is taken until the spacecraft comes to a nearly complete stop.

Magnetometer swing testing on both spacecraft was completed in late January, and “initial measurements indicated that the spacecraft are indeed below the threshold,” Mosavi says.

RBSP is scheduled for launch no earlier than Aug. 15, 2012, from the Kennedy Space Center, Fla. APL built the RBSP spacecraft for NASA and manages the mission. The RBSP mission is part of NASA's Living With a Star program, guided by the Heliophysics Division of the NASA Headquarters Science Mission Directorate in Washington.

Read about EMFISIS

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