Cosmic Background Explorer
The Cosmic Background Explorer, also referred to as Explorer 66, was a NASA satellite dedicated to cosmology, which operated from 1989 to 1993. Its goals were to investigate the cosmic microwave background radiation of the universe and provide measurements that would help shape the understanding of the cosmos.
COBE's measurements provided two key pieces of evidence that supported the Big Bang theory of the universe: that the CMB has a near-perfect black-body spectrum, and that it has very faint anisotropies. Two of COBE's principal investigators, George F. Smoot III and John C. Mather, received the Nobel Prize in Physics in 2006 for their work on the project. According to the Nobel Prize committee, "the COBE project can also be regarded as the starting point for cosmology as a precision science".
COBE was the second cosmic microwave background satellite, following RELIKT-1, and was followed by two more advanced spacecraft: the Wilkinson Microwave Anisotropy Probe operated from 2001 to 2010 and the Planck spacecraft from 2009 to 2013.
Mission
The purpose of the Cosmic Background Explorer mission was to take precise measurements of the diffuse radiation between 1 micrometre and over the whole celestial sphere. The following quantities were measured: the spectrum of the 3 K radiation over the range 100 micrometres to the anisotropy of this radiation from 3 to ; and, the spectrum and angular distribution of diffuse infrared background radiation at wavelengths from 1 to 300 micrometres.History
In 1974, NASA issued an Announcement of Opportunity for astronomical missions that would use a small- or medium-sized Explorer spacecraft. Out of the 121 proposals received, three dealt with studying the cosmological background radiation. Though these proposals lost out to the Infrared Astronomical Satellite, their strength made NASA further explore the idea. In 1976, NASA formed a committee of members from each of 1974's three proposal teams to put together their ideas for such a satellite. A year later, this committee suggested a polar-orbiting satellite called COBE to be launched by either a Delta 5920-8 launch vehicle or the Space Shuttle. It would contain the following instruments:| Instrument | Acronym | Description | Principal Investigator |
| Differential Microwave Radiometer | DMR | Microwave instrument that would map variations in the Cosmic microwave background radiation | George F. Smoot III |
| Diffuse Infrared Background Experiment | DIRBE | Multiwavelength infrared detector used to map dust emission | Michael G. Hauser |
| Far-InfraRed Absolute Spectrophotometer | FIRAS | Spectrophotometer used to measure the spectrum of the CMB | John C. Mather |
NASA accepted the proposal provided that the costs be kept under US$30 million, excluding launcher and data analysis. Due to cost overruns in the Explorer program due to IRAS, work on constructing the satellite at Goddard Space Flight Center did not begin until 1981. To save costs, the infrared detectors and liquid helium dewar on COBE would be similar to those used on Infrared Astronomical Satellite.
COBE was originally planned to be launched on a Space Shuttle mission STS-82-B in 1988 from Vandenberg Air Force Base, but the Challenger explosion delayed this plan when the Shuttles were grounded. NASA prevented COBE's engineers from going to other space companies to launch COBE, and eventually a redesigned COBE was placed into Sun-synchronous orbit on 18 November 1989 aboard a Delta launch vehicle.
On 23 April 1992, COBE scientists announced at the APS April Meeting in Washington, D.C. the finding of the "primordial seeds" in data from the DMR instrument; until then the other instruments were "unable to see the template." The following day The New York Times ran the story on the front page, explaining the finding as "the first evidence revealing how an initially smooth cosmos evolved into today's panorama of stars, galaxies and gigantic clusters of galaxies."
The Nobel Prize in Physics for 2006 was jointly awarded to John C. Mather, NASA Goddard Space Flight Center, and George F. Smoot III, University of California, Berkeley, "for their discovery of the blackbody form and anisotropy of the cosmic microwave background radiation".
Spacecraft
COBE was an Explorer class satellite, with technology borrowed heavily from IRAS, but with some unique characteristics.The need to control and measure all the sources of systematic errors required a rigorous and integrated design. COBE would have to operate for a minimum of 6 months and constrain the amount of radio interference from the ground, COBE and other satellites as well as radiative interference from the Earth, Sun and Moon. The instruments required temperature stability and to maintain gain, and a high level of cleanliness to reduce entry of stray light and thermal emission from particulates.
The need to control systematic error in the measurement of the CMB anisotropy and measuring the zodiacal cloud at different elongation angles for subsequent modeling required that the satellite rotate at a 0.8 rpm spin rate. The spin axis is also tilted back from the orbital velocity vector as a precaution against possible deposits of residual atmospheric gas on the optics as well against the infrared glow that would result from fast neutral particles hitting its surfaces at extremely high speed.
In order to meet the twin demands of slow rotation and three-axis attitude control, a sophisticated pair of yaw angular momentum wheels were employed with their axis oriented along the spin axis. These wheels were used to carry an angular momentum opposite that of the entire spacecraft in order to create a zero net angular momentum system.
The orbit would prove to be determined based on the specifics of the spacecraft's mission. The overriding considerations were the need for full sky coverage, the need to eliminate stray radiation from the instruments and the need to maintain thermal stability of the dewar and the instruments. A circular Sun-synchronous orbit satisfied all these requirements. A altitude orbit with a 99° inclination was chosen as it fit within the capabilities of either a Space Shuttle or a Delta launch vehicle. This altitude was a good compromise between Earth's radiation and the charged particles in Earth's radiation belts at higher altitudes. An ascending node at 18:00 was chosen to allow COBE to follow the boundary between sunlight and darkness on Earth throughout the year.
The orbit combined with the spin axis made it possible to keep the Earth and the Sun continually below the plane of the shield, allowing a full sky scan every six months.
The last two important parts pertaining to the COBE mission were the dewar and Sun-Earth shield. The dewar was a superfluid helium cryostat designed to keep the FIRAS and DIRBE instruments cooled during the duration of the mission. It was based on the same design as one used on IRAS and was able to vent helium along the spin axis near the communication arrays. The conical Sun-Earth shield protected the instruments from direct solar and Earth-based radiation as well as radio interference from Earth and the COBE's transmitting antenna. Its multilayer insulating blankets provided thermal isolation for the dewar.
In January 1994, engineering operations concluded and the operation of the spacecraft was transferred to Wallops Flight Facility for use as a test satellite.