1. TAGSAM Testing Complete: OSIRIS-REx Prepared to TAG an Asteroid

    By Christine Hoekenga

    November 16, 2018 -

    On Nov. 14, NASA’s OSIRIS-REx spacecraft stretched out its robotic sampling arm for the first time in space. The arm, more formally known as the Touch-and-Go Sample Acquisition Mechanism (TAGSAM), is key to the spacecraft achieving the primary goal of the mission: returning a sample from asteroid Bennu in 2023.

    OSIRIS-REx's TAGSAM Head as Imaged by SamCam

    This image, showing the OSIRIS-REx Touch-and-Go Sample Acquisition Mechanism (TAGSAM) sampling head extended from the spacecraft at the end of the TAGSAM arm, was taken by the SamCam camera on Nov. 14, 2018 during a visual checkout of the spacecraft’s sampling system. A similar observation will be taken after TAG to help document the asteroid material collected in the TAGSAM head. Credit: NASA/Goddard/University of Arizona

    As planned, engineers at Lockheed Martin commanded the spacecraft to move the arm through its full range of motion – flexing its shoulder, elbow, and wrist “joints.” This long-awaited stretch, which was confirmed by telemetry data and imagery captured by the spacecraft’s SamCam camera, demonstrates that the TAGSAM head is ready to collect a sample of loose dirt and rock (called regolith) from Bennu’s surface.

    “The TAGSAM exercise is an important milestone, as the prime objective of the OSIRIS-REx mission is to return a sample of Bennu to Earth,” said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. “This successful test shows that, when the time comes, TAGSAM is ready to reach out and tag the asteroid.”

    Years of innovation

    Lockheed Martin engineers spent more than a decade designing, building, and testing TAGSAM, which includes an 11-foot (3.35-meter) arm with three articulating joints, a round sampler head at the end of the arm that resembles the air filter in a car, and three bottles of high-pressure nitrogen gas.

    This test deployment was a rehearsal for a date in mid-2020 when the spacecraft will unfold the TAGSAM arm again, slowly descend to Bennu’s surface, and briefly touch the asteroid with the sampler head. A burst of nitrogen gas will stir up regolith on the asteroid’s surface, which will be caught in the TAGSAM head. The TAG sequence will take about five seconds, after which the spacecraft will execute small maneuvers to carefully back away from Bennu. Afterward, SamCam will image the sampler head, as it did during the test deployment, to help confirm that TAGSAM collected at least 2.1 ounces (60 grams) of regolith.

    The TAGSAM mechanism was designed for the key challenge unique to the OSIRIS-REx mission: collecting a sample from the smallest planetary body ever to be orbited by a spacecraft. “First-of-its-kind innovations like this one serve as the precursor for future missions to small bodies,” said Sandy Freund, systems engineer manager and Lockheed Martin OSIRIS-REx MSA manager. “By proving out these technologies and techniques, we are going to be able to return the largest sample from space in half a century and pave the way for other missions.”

    A month of testing

    The unfolding of the TAGSAM arm was the latest and most significant step in a series of tests and check-outs of the spacecraft’s sampling system, which began in October when OSIRIS-REx jettisoned the cover that protected the TAGSAM head during launch and the mission’s outbound cruise phase. Shortly before the cover ejection, and again the day after, OSIRIS-REx performed two spins called Sample Mass Measurements. By comparing the spacecraft’s inertial properties during these before-and-after spins, the team confirmed that the 2.67-pound (1.21-kilogram) cover was successfully ejected on Oct. 17.

    A week later, on Oct. 25, the Frangibolts holding the TAGSAM arm in place fired successfully, releasing the arm and allowing the team to move it into a parked position just outside its protective housing. After resting in this position for a few weeks, the arm was fully deployed into its sampling position, its joints were tested, and images were captured with SamCam. The spacecraft will execute two additional Sample Mass Measurements over the next two days. The mission team will use these spins as a baseline to compare with the results of similar spins that will be conducted after TAG in 2020 in order to confirm the mass of the sample collected.

    Although the sampling system was rigorously tested on Earth, this rehearsal marked the first time that the team has deployed TAGSAM in the micro-gravity environment of space.

    “The team is very pleased that TAGSAM has been released, deployed, and is operating as commanded through its full range of motion.” said Rich Burns, OSIRIS-REx project manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “It has been restrained for over two years since launch, so it is gratifying to see it out of its shackles and performing well.”

    OSIRIS-REx is scheduled to arrive at Bennu on Dec. 3. It will spend nearly one year surveying the asteroid with five scientific instruments so that the mission team can select a location that is safe and scientifically interesting to collect the sample.

    “Now that we have put TAGSAM through its paces in space and know it is ready to perform at Bennu, we can focus on the challenges of navigating around the asteroid and seeking out the best possible sample site,” said Lauretta.

  2. NASA’s OSIRIS-REx Executes Fourth Asteroid Approach Maneuver

    November 13, 2018 -

    Artist’s conception of NASA’s OSIRIS-REx spacecraft during a burn of its Attitude Control System (ACS) thrusters.

    NASA’s OSIRIS-REx spacecraft executed its fourth Asteroid Approach Maneuver (AAM-4) yesterday. The spacecraft fired its Attitude Control System (ACS) thrusters to slow the spacecraft from approximately 0.31 mph (0.14 m/sec) to 0.10 mph (0.04 m/sec). The ACS thrusters are capable of velocity changes as small as 0.02 mph (0.01 m/sec). The mission team will continue to examine telemetry and tracking data over the next week to verify the new trajectory. The maneuver targeted the spacecraft to fly through a corridor designed for the collection of high-resolution images that will be used to build a shape model of Bennu.

    With the execution of AAM-4, the OSIRIS-REx spacecraft concludes a six-week series of Bennu approach maneuvers. AAM-1 and AAM-2, which executed on Oct. 1 and Oct. 15 respectively, slowed the spacecraft by a total of approximately 1,088 mph (486 m/sec). AAM-3 and AAM-3A, which executed on Oct. 29 and Nov. 5 respectively, further refined the spacecraft’s trajectory and speed to set the conditions for a successful AAM-4 maneuver. After a final correction maneuver scheduled for Nov. 30, the spacecraft will be on track to arrive at a position 12 miles (20 km) from Bennu on Dec. 3.

  3. Behind the Scenes of REXIS: Uncovering an Instrument in Flight

    By Rebecca Masterson, REXIS Instrument Program Manager

    November 13, 2018 -

    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.

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