Planning Pays Off - Chandra Sails Through the Leonids Unharmed

Leonids
Meteors from the Leonid Shower as seen during the early morning hours of Nov. 18 from Valley Forge National Park, PA, USA (Photo: John Welsh)
Sunday, November 18, 2001 :: At 6:45 a.m. two cars lined up at the security gate to Chandra's Operation Control Center (OCC) parking lot. The drivers waved at each other. They and other members of the Flight Operations and Science Operations teams arrived early on this Sunday morning to keep watch during Chandra's encounter with the 2001 Leonid event.

Actually, these are the "late" arrivals. Some team members had arrived at 5:00 a.m. and still others had been on overnight shifts. Once inside the main OCC control room, staff swapped stories about the meteors seen at viewing sites worldwide earlier in the morning. In addition to scientific curiosity and the beauty of the spectacle, many Chandra staff were motivated to get up extra early to see the Leonids with their own eyes, giving them an idea of what the spacecraft would be encountering in space a few hours later.

Janet Houser, of the OCC Facility Systems Team
Janet Houser, of the OCC Facility Systems Team monitoring critical Chandra functions. (Photo: CXC)
Janet Houser, of the OCC Facility Systems Team, up at 3:00 a.m. and at her desk by 5:00, didn't have time to stop to see the meteors. It was her job to see that all computers, software, and communications systems were functioning during this crucial pass.

A few days before the Leonids, a sequence of planned commands was sent to Chandra from the OCC via the Deep Space Network, the telephone line, so to speak, between the Earth and this orbiting spacecraft. One of these precautionary steps included turning off the high voltage to the High Resolution Camera (HRC) on Thursday, three days before the Leonid storm. Observations with the Advanced CCD Imaging Spectrometer (ACIS) continued until Saturday when it was safely stowed in a secure position aboard the spacecraft.

As the predicted peak of the Leonids approached late Saturday night (EST), Chandra maneuvered to the first of several anti-radiant positions. The term "radiant" refers to the direction from which the Leonid meteors would be coming. Pointing Chandra in the exact opposite direction allows maximum protection for the most vulnerable parts of the spacecraft.

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To allow for continuous monitoring of the spacecraft as it passed through the predicted peaks of the Leonids, the team arranged an unusually long live contact between the control room and two Deep Space Network stations (Canberra, Australia and Goldstone, California) for approximately the next fourteen hours.

Members of the Chandra team
Members of the Chandra team discussing maneuvers of the spacecraft during the Leonids; from L-R: Bill Simmons, Dan Shropshire, Leon McKendrick, Ken Gage. (Photo: CXC)
At 7:10 a.m. EST, November 18, Chandra encountered the first of the two predicted debris peaks in its orbit, the Brown/Cooke Peak. The Brown/Cooke Peak, named for the two scientists who modeled the forecast, predicted that the concentration of meteors would be relatively spread out and of less concern than the other peak that Chandra would pass through several hours later. Shortly after the Chandra spacecraft exited the Brown/Cooke Peak, Flight Director and CXC Manager, Roger Brissenden, announced "there is no indication of any anomaly in the monitoring data" and concluded that Chandra was not hit by any of the debris.

Chandra's orbit for the 2001 Leonid event posed some difficulties that called for an innovative approach to get Chandra into the anti-radiant relative to the second area of predicted intensity. The Asher/McNaught Peak was located on the part of Chandra's orbit just after the exit from Earth occultation. During occultation, the position of the Earth would block the target guide stars, making it impossible to maintain the correct orientation through the usual interaction of aspect camera and gyros. Additionally, Chandra's aspect camera, the instrument that detects and locks on to guide stars to provide information to orient the satellite, is typically not operated within 20 degrees of the Earth. This is because the Earth reflects sufficient Sunlight to saturate the camera's detectors. If the guide stars are lost or cannot be acquired, Chandra could go into a "bright star hold" or "normal sun mode" and require a time-consuming series of steps to recover operations. This was not a chance the Chandra team wanted to take during a meteor event.

Two Deep Space Network stations
A screen from the Satellite Tool Kit that was used by the Flight Ops team throughout the Leonid passage. This image shows Chandra's exact position and orientation between the two DSN stations, Canberra, Australia (lower left), and Goldstone, California (upper right) (Photo: CXC)
The answer that the planning team developed was to place Chandra in its second anti-radiant position before it entered occultation of the Earth. (The trick was to keep it there when the guide stars were blocked by the Earth, but more on that later!) The maneuver was executed (again by pre-loaded command) at 7:46 a.m., about 45 minutes before Chandra reached perigee, the point in its orbit closest to Earth. At this place in the orbit, Chandra was still "on com" with the Deep Space Network (DSN) station in Canberra, Australia. As Canberra disappeared over the horizon, the Goldstone DSN station in California was beginning to rise.

Engineers closely monitor Chandra
The Flight Director, Flight Team Manager, Flight Engineers, and PCAD (Pointing Control and Attitude Determination) Engineers closely monitor Chandra's status during Earth Occultation. (Photo: CXC)
While Canberra continued as the official "com" (point of communication), Goldstone acquired and began tracking Chandra. About half an hour later, the official handover was made and Goldstone took over the com for the rest of the Leonid passage.

At this point, the crucial command to place Chandra in "gyro hold" was successfully executed. "Gyro hold" is a standard mode (the same one used for maneuvers) which stops using the information coming from the star camera and relies solely on the gyro data to hold position. Thus, in this key part of the orbit where the guide stars could either be obscured by reflected Earth light or blocked by the Earth's disk, Chandra maintained position based on the last information that the gyros had from the guide stars. That information held Chandra in its second anti-radiant position.

Transit from a position 20 degrees on one side of Earth to one 20 degrees on the other side of Earth takes about 3 hours. During this time, the assembled team closely monitored Chandra's orbit for "gyro drift" - any indication that the position is changing. If gyro drift had occurred, Chandra could have had difficulty reacquiring the guide stars needed to orient it away from the Asher/McNaught peak when Earth occulation ended. Contingency plans were made for recovery from a "bright star hold". During this time, Dan Shropshire, head of the Flight Operations engineering team, and engineer Bill Simmons, provided continual updates from real-time engineering data. All indicators showed that Chandra's orientation was holding steady.

Rob Cameron
Rob Cameron of the Science Operations Team at his console during the aspect camera calibration test.
At the same time, the team had planned an important experiment. Because Chandra was in gyro hold mode, the aspect camera was freed up for a calibration test. Chandra has been operating on the conservative assumption that guide star acquisition must take place 20 degrees away from the bright limb on either side of the Earth's disk. In the weeks prior to the Leonid shower, the Chandra team saw a chance to test this assumption in real time. Roger Brissenden and Dan Shropshire began communicating with Rob Cameron, a member of the Science Operations team who was sitting on another console tracking the now independent operations of the aspect camera.

There was much speculation on when the last guide star would be lost as Chandra headed for Earth Occultation. As a prize for the best estimate, Bill Simmons offered up the lone remaining bagel in the "break room." Amazingly, the aspect camera was able to maintain its lock on the guide stars to within 1 degree of occultation, probably because night was rising on that side of Earth. (There was no official word on who ate the bagel.)

Confirmation that Chandra's aspect camera has acquired 8 guide stars
Confirmation that Chandra's aspect camera has acquired 8 guide stars. The row of small windows in the upper right of the screen contains actual images of the 8 stars taken by the aspect camera. (Photo: CXC)
The real test was the reacquisition after Earth transit. Rob Cameron had 8 small squares running along a window at the top of his console. One by one the little windows were filled in with pixellated images of the 8 stars that the camera was told to find (these are the same guide stars that Chandra needed to acquire later in order to leave gyro hold). Despite the bright Earth light, Chandra's very sensitive aspect camera was able to identify and hold the stars at 4.7 degrees past Earth. Ken Gage, head of the Flight Operations mission planning team, was excited to start examining this data to understand its implications for future operations.

At 2:34 pm Chandra left gyro hold and successfully acquired the guide stars that continued to keep it oriented to the Leonids' anti-radiant. About an hour and a half later, Chandra passed through the Asher/McNaught peak of the debris stream without incident. Smiles of relief and satisfaction were seen around the control room as that second area of danger was safely traversed. After the excitement, people retreated to various offices to make notes and file reports, and gradually, around 5PM, the control center pretty much emptied out.

Roger Brissenden
Checking the HRC status - John Chappell of the HRC instrument team, and Roger Brissenden. (Photo: CXC)
At 5:45 p.m., many hours after her day at the Control Center started, Janet Houser is still at the OCC, as are Darell Wicker, the Operations Controller, and Roger Bachiere, the command controller. They await the arrival of the night team. The second shift of the Ground Operations team monitors computers down the hall. Chandra is still in the anti-radiant position as it travels through the tail of the debris stream. The ACIS instrument has been moved back into the focal plane. Around 9:00 p.m. EST Chandra will start taking science observations again.

Read Part I: Chandra Prepares for the Leonid Meteor Shower

More images from Chandra's Operation Control Center during the 2001 Leonids

Where's Chandra Where's Chandra now? See a representation of the current position of Chandra in relation to the surface of the Earth.



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Updated: March 27, 2008