On Sept. 14, OSIRIS-REx’s REXIS instrument opened its radiation cover, as scheduled, after two years in space. The cover had been in place to protect REXIS’s charge-coupled devices (CCDs) from degradation due to radiation exposure during the spacecraft’s cruise to Bennu. The REXIS student team designed and built the cover with input from MIT and NASA Goddard mentors and members of the OSIRIS-REx review board.
Figure 1: CAD model of the REXIS cover in the closed position, showing the Frangibolt housing and cover heater. The flight instrument also includes a heater on the Frangibolt housing that is not shown here.
Since its installation, the radiation cover had been held closed with a titanium bolt that was threaded through a TiNi Aerospace FD04 Frangibolt and a switch washer (see Figure 1). When it was time to release the cover, the team sent REXIS a command to heat the Frangibolt until the shape memory alloy expanded. This put the bolt under tension, causing structural failure, so that the cover was free to swing open. The switch washer sensed the change in pre-load at the joint and sent a signal to the REXIS electronics board to cut power to the Frangibolt, stopping the activity.
Figure 2: Photo of the REXIS flight instrument (before spacecraft integration) with radiation cover open. This configuration is the most-likely current configuration of the instrument on-board OSIRIS-REx after firing the Frangibolt.
In the event that the switch washer did not work as expected, the team had also set a software timer to end the actuation activity in order to ensure that the Frangibolt did not overheat. The exact time needed to actuate the Frangibolt was unknown, as it depended heavily on the temperatures of the actuator and the cover as well as the pre-load in the joint. Therefore, the REXIS team tested both the flight instrument and spare covers in advance to try to set this timer correctly. Three attempts to open the instrument were planned, each with a slightly longer timer setting.
For the first attempt, the team set the REXIS firing timer to 57 seconds. If the switch washer didn’t show actuation in that time, the bolt was programmed to turn off. The firing command was sent to the instrument at about 16:32 UTC. Fifty-five seconds later, the switch washer indicated that the Frangibolt had actuated … with only two seconds to spare. Kudos to the team that tested the spare cover over various temperature ranges in order to guide us to such a perfect timer setting.
We also saw evidence of the firing in the onboard instrument temperature sensors: the housing temperature and the CCD temperature both decreased after the firing, which indicated that the cover was no longer conductively connected to the instrument tower (see Figure 3).
Figure 3: The REXIS housekeeping temperatures before and after firing also show changes expected with the cover open. Both the CCD temperatures (top plot) and the Frangibolt housing temperature (middle plot, “Frangibolt”) decrease after the firing event indicating that the cover and its heater are no longer thermally coupled to the rest of the instrument.
The duty cycle of the housing heater and the cover heater also changed as expected. The REXIS Frangibolt housing heater stayed on after the cover opening since the housing is no longer getting heat from the cover heater. The cover heater duty cycle has also lengthened since the thermal mass has decreased (see Figure 4).
Figure 4: REXIS cover heater current values during the cover opening event. Before the cover was open both the housing heater and the cover heater were cycling with a short duty cycle. The changes in the data after the firing attempt indicate that the housing heater is now always on and the cover heater is cycling less frequently. This behavior is expected in the cover open state.
REXIS took 30 minutes of science data before and after the Frangibolt firing so we could compare the spectra. With the cover open, we were expecting to see an increase in the rate of events detected by the CCDs, dominantly in the low energy portion of the x-ray spectrum due to the Cosmic X-ray Background (CXB). The cover included an Fe-55 calibration source that shone on the CCDs and had been used to monitor the instrument status for its two years in flight. Opening the cover removed that source from the field of view of the CCDs, so we also expected to see a decrease in the Fe-55 energy detected. As expected, the rate of events detected by the CCDs increased, as shown in Figure 4. The x-ray spectrum from one of the instrument’s highest performing CCD nodes is shown in Figure 5.
Figure 5: Rate of X-ray events detected by the CCDs during the cover opening activity. The detected event rate has a discontinuous step in the 30 minutes before and after the cover opened. The increase in the event rate is due to diffuse x-ray emission of the cosmic x-ray background as expected with an open cover.
The x-ray spectrum measured in the low energy range increased, just as expected for the CXB ,while the calibration source signal from Fe-55 became less pronounced. The data in Figure 5 and Figure 6 are consistent with a fully open REXIS cover.
Figure 6: The x-ray spectrum is a measure of the number of events detected by the CCDs as a function of energy. The red line is the x-ray spectra detected by one of the highest-performing nodes in the REXIS detector array before the cover opened. There is a peak 5.9 keV (the blue shaded region) from the cover-mounted internal Fe-55 x-ray calibration source. The low energy spectrum is due to only internal noise. The spectrum after the cover opened for the same node (black line) shows detection of a low-energy continuum consistent with predictions of the cosmic x-ray background (blue dashed line) and a weakening of the Fe-55 line indicating that the instrument is now looking at cosmic x-rays from space instead of the underside of the cover.
Given both this science and engineering data, we are confident that our radiation cover is open and out of the field of view of the REXIS CCDs. Now the REXIS team can start doing real external calibrations. REXIS will be looking at some Cosmic X-Ray Background (CXB) in early October and then will turn to check out the Crab Nebula in November.