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Compton Gamma-Ray Observatory Science Support Center

Successful Reboost of Compton Gamma Ray Observatory

The reboost of the Compton Gamma Ray Observatory

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The reboost of the Compton Gamma Ray Observatory (CGRO) was succesfully completed at 1:40 pm EDT on June 3, 1997. The pre reboost orbital altitude was 440 km. The altitude of the reboosted orbit is 515 km. The orbital inclination with respect to Earth's equator remains at 28.5 degrees. The uppermost point of the orbit occurs at 28.5 degrees north latitude, and the lowermost point at 28.5 degrees south. The rotation of the Earth beneath CGRO means that these extreme points continually occur at different longitudes. The reboost will keep CGRO in orbit until perhaps 2007, after which a controlled reentry into Earth's atmosphere will ensure that the 17-ton spacecraft falls harmlessly into an ocean. All four scientific experiments on board the spacecraft remain operational.

About the Compton Gamma Ray Observatory (CGRO)

  • NASA's CGRO was launched into orbit on April 5, 1991 on board the Space Shuttle Atlantis (Space Shuttle mission 37), and was successfully deployed on April 7, 1991 at an orbital altitude of 450 km (distance above Earth's surface).
  • It is named after the American Nobel-prize-winning physicist, Arthur Holly Compton.
  • At 17 tons, CGRO is the most massive scientific satellite ever launched by the space shuttle. It is also the most massive unmanned civilian spacecraft of all time (some unmanned classified satellites may be more massive).
  • The heavy material on CGRO is needed to stop cosmic gamma radiation, or gamma rays, which are the most energetic type of electromagnetic radiation (more energetic than x-rays). Earth's atmosphere shields us from this lethal radiation.
  • After six years in orbit, all four scientific instruments are still operational. These observe cosmic gamma rays with energies ranging from 20,000 to 30,000,000,000 electron volts. For comparison, the energy of a photon of visible light is only two electron volts.


Some Facts About the Reboost of CGRO

  • Before the reboost, CGRO's altitude was 440 km above Earth's surface.
  • After the successful reboost, CGRO's altitude is 515 km.
  • This will keep CGRO in orbit until perhaps 2007 AD. Until then, CGRO has the potential to continue its scientific observations.
  • The reboost required several rocket burns per week over two months.
  • The final burn was completed at 1:40 p.m. EDT on June 3, 1997.
  • CGRO's orbital inclination with respect to the equator has always been and will continue to be 28.5 degrees. This results from a due east launch from Kennedy Space Center.
  • Collisions with particles in the Earth's sparse upper atmosphere, even at an altitude of several hundreds of km, will eventually drag the spacecraft back down to Earth.
  • When exactly CGRO will reenter Earth's atmosphere in the next decade is uncertain, as it will depend on the level of solar activity, which cannot be precisely predicted. The active sun causes the Earth's atmosphere to expand, which in turn increases the drag on the spacecraft.
  • Sufficient fuel is being kept in reserve to be able to control CGRO's final descent, so that it will safely fall into an ocean (unlike Skylab).
  • CGRO was reboosted once before, in the final quarter of 1993, from 340 km to 450 km in altitude.


Some Scientific Accomplishments of CGRO

  • The cosmic gamma-ray bursts are brief (tenths to tens of seconds) bursts of gamma rays that greatly outshine the rest of the gamma-ray sky. These occur roughly once per day. CGRO established that these mysterious bursts appear at random locations on the sky, and discovered that their distribution in space has an "edge", beyond which there are no burst sources. This is commonly interpreted as meaning that the gamma-ray burst sources lie billions of light years away, at truly cosmological distances. If true, then the bursts would be due to extraordinarily energetic events indeed, such as mergers of pairs of neutron stars and/or black holes.
  • CGRO discovered that a class of active galaxies, called blazars, are strong emitters of gamma rays. CGRO has detected more than 50 blazars, including 3C 279 and PKS 1622-297. All of these galaxies lie at great distances (hundreds of millions of light years) from our own galaxy. An active galaxy is seen as a blazar when one of the two "jets" of high-energy particles spewing from the poles of its central supermassive black hole just happens to be pointed towards Earth (supermassive means millions to billions times our sun's mass). CGRO also discovered that these blazars occasionally undergo factor-of-ten increases in gamma-ray brightness.
  • In late April of this year, it was announced that CGRO has detected a gigantic cloud of antimatter lying 3000 light years above the center of our own galaxy (and at quite a safe distance from Earth). This may add to the evidence that a supermassive black hole lurks at the center of our galaxy.
  • An energetic burst of neutrons released from the sun during a solar flare was detected by CGRO on June 15, 1991. This was the first image of a star based on energetic particles (not gamma rays in this case). Gamma rays have also been detected by CGRO from strong solar flares, including nuclear emission lines (of carbon, oxygen, nitrogen, iron, magnesium, neon and silicon), and the electron-positron annihilation line (traces of antimatter from the sun).


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This page was last modified on Monday, 01-Aug-2005 13:41:47 EDT.

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