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.
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.
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).
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).
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.
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.
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.