COBE

 1989-089A

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The Cosmic Background Explorer, COBE, was one of NASA-GSFC's most spectacularly successful science satellites, studying the cosmic microwave background. In the 1960s it was discovered that the universe is filled with a uniform faint glow of microwave radiation, with an apparent blackbody temperature of about three degrees above absolute zero. The Big Bang theory predicts just such a glow (or diffuse background), the left over radiation from the moment when the primeval fireball became transparent (the `decoupling' era), less than a million years after the beginning of the universe. It further predicts that the original form of this radiation should have a perfect blackbody spectrum, and that it should be the same in all directions except for small deviations caused by the lumps in the early cosmic gas which would later become galaxies.

COBE was designed to map the microwave and infrared sky with fairly low spatial resolution (scales of 7 degrees) but extremely high accuracy. The DIRBE experiment mapped the infrared radiation coming from our own galaxy, to provide the best measurement of light in the foreground. The FIRAS experiment measured the shape of the cosmic background spectrum, proving that it was indeed a perfect blackbody and measuring its temperature accurately. The DMR experiment compared the intensity in different directions on the sky and showed that there were small fluctuations of a few parts in a hundred thousand which may be the seeds of galaxy formation.

COBE was built at Goddard and redesigned several times to accomodate changes of launch vehicle. A liquid helium dewar was used to cool the FIRAS instrument to near absolute zero. The 2206 kg spacecraft was a hexagonal prism with a conical sunshade and solar panels. Size was 3.90m long 2.2m dia stowed, 4.9m long 8.4m span deployed.

COBE was finally launched by a Delta 5920 from Space Launch Complex 2-West at Vandenberg AFB on 1989 Nov 18 at 1434 UT. The Delta inserted it into a 102.83 min, 888 x 897 km x 99.02 deg orbit and mapping of the sky with the DMR began the next day. On 1989 Nov 21 at 1119 the helium dewar cover was ejected and DIRBE and FIRAS began making observations. First results from the FIRAS experiment were presented at the American Astronomical Society in 1990 Jan (the present author, who was giving a talk in a parallel session in a much smaller room next door, could only hear the thunderous applause through the wall). The first all sky map was complete on 1990 Jun 18, and on 1990 Sep 21 the helium supply was exhausted and FIRAS was shut down. Science operations with the other instruments continued until 1993 Dec 23, and after some end of life tests the end of mission was declared on 1994 Jan 28. The satellite was then transferred to the control of Wallops Island as a test satellite for training of personnel.

COBE
Date	Time	Event	Orbit
1989 Nov 18	1434	Launch by Delta 5920	SLC2W
		T+0:57 SRM 1-6 out
		T+1:00 SRM 7-9 on
		T+1:18 SRM 1-6 sep
		T+1:57 SRM 7-9 out
		T+2:02 SRM 7-9 sep
		T+3:46 MECO
		T+3:54 Stage 1 sep
		T+3:59 SES-1
		T+4:12 Fairing
	1444	T+10:50 SECO-1	180? x 900 x 99?
	1531	T+57:00 SES-2
	1531	T+57:21 SECO-2
	1535	T+1:01:30 Stage 2 sep
	1535	Thermal shield deploy
			102.83 888 x 897 x 99.02
	1557	T+1:23:00 SES-3 evasive
	1557	T+1:23:05 SECO-3 evasive
	1612	T+1:38:00 SES-4 depletion
	1612	T+1:38:06 SECO-4 depletion	100.00 701 x 815 x 97.1
1989 Nov 19		DMR observations begin
1989 Nov 21		Helium dewar cover ejected
1990 Feb 15		Gyro B failed
1990 Jun 18		First all sky map complete
1990 Sep 21		Helium depleted, FIRAS shutdown
1991 Sep		Gyro BX failed
1993		Gyro A and C failed
1993 Dec 23		end of science operations
1993 Dec		support transfer tests
1994 Jan 28		end of mission, transfer to Wallops
1997 May 1		support terminated at Wallops

Payload:

• FIRAS Far IR Absolute Spectrophotometer 100mu-1cm

• DMR Differential Microwave Radiometer 90-23 GHz

• DIRBE Diffuse IR Background Experiment 1.1-240 mu; 0.19m primary mirror