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Chandra Stars In Double Feature Weekend

August 9, 1999 ::

Chandra X-ray Observatory
An artist's illustration of the Chandra X-ray Observatory in space.
Illustration: NASA/NGST
Chandra orbit path
View orbit animations.
Illustration: NGST
While many Americans were trying to beat the summer heat, the Chandra team spent an intense weekend firing rocket engines and heating wax. As a result of their efforts, Chandra is in its operating orbit, and an important scientific instrument has passed a critical test.

The double feature started when engines on Chandra's Integral Propulsion System (IPS) were ignited at 1:43 a.m. EDT on August 7 for the fifth and final time. The seven-minute, 44 second firing put Chandra in its operating orbit, which ranges from approximately 6,000 miles (9,700 kilometers) at the low point (perigee) to approximately 86,500 miles (139,200 kilometers) at its high point (apogee).

CXC's orbit - artist illustration
Illustration of Chandra's Integral Propulsion System (IPS) making two burns at perigee and three at apogee to raise and finalize the orbit. Intermediate orbits after the IPS-3 and -4 burns are not shown.
Illustration: NASA/NGST
After a careful review of the orbit, and a brief 'wiggle' of the observatory to check for the effects of sloshing fuel, the flight operations team decided to permanently shut down the propulsion system and seal it off from the rest of the spacecraft.

Sunday afternoon's feature event was an outer space bake-off. This wasn't something sponsored by Pillsbury, but the result was even more delicious to the Chandra team than a double layered devil's food cake.
Diagram of Chandra CCD Imaging Spectrometer (ACIS).
The Action Rooms were once again filled with top management from NASA's Marshall Space Flight Center, the Smithsonian Chandra Center, MIT, Penn State, and TRW (anyone who says the taxpayers don't get their money's worth out of government officials should take a look at the overtime these NASA officials put in, at no charge). The goal was to warm up the housing of the Advanced CCD Imaging Spectrometer (ACIS), and then open the door of this X-ray camera which will be used to make the majority of the Chandra images during its first year of operation.

The plan was simple: warm up the housing to evaporate any contaminants, then open the door to the camera. Yet, one troubling historical fact hovered about the room like an ominous summer afternoon thundercloud -- last year, in a routine test, the ACIS door had failed to open. It had stuck and the opening mechanism had self destructed, causing a delay in Chandra's launch. Although the door mechanism was now repaired and tested, it had never been determined exactly why the door had stuck in the first place.

OCC Action Room
Working hard in the Action Room, at the OCC.
Photo: CXC
"I'm more nervous than I was at the first IPS burn," Martin Weisskopf, Chandra Project Scientist from Marshall Space Flight Center said, as Paul Plucinsky from the Chandra Center announced that the commands to open the door were being sent to the spacecraft.

At 5:12 p.m. the first heating cycle began, and the Action Room fell completely silent. The heater's job was to melt wax in an tiny piston on the ACIS door mechanism. The hydraulic pressure in the piston would then rotate a shaft that would open the door. One line of the busy figure-filled computer monitors would tell the story: the angle of rotation of the door shaft should show 70 degrees or more.

The first heating cycle produced no rotation. The second one, 16 minutes later, a 13 degree rotation. "Needs to be at least 18 degrees to break the seal," someone said anxiously. The third cycle produced a rotation of 19.5 degrees. "Now, I know it's going to work," Weisskopf said, and Roger Brissenden, Chandra Center manager, gave a thumbs up from the Control Room. Everyone agreed, but no one moved away from the monitors. The fourth heat pulse opened the door to 36 degrees. Then at 6:46 p.m, on the fifth pulse the shaft rotated to 71.5 degrees. Spontaneous clapping and cheering broke out. The door was open!

"Yes, I'm a little relieved," said Gordon Garmire, the Penn State University scientist who is the ACIS principal investigator. "Actually, I feel really good!"

Labelled sketch of the CXO

Chandra schematic

(Illustration: CXC/TRW)

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