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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Cosmic Background Explorer | 2/4 | https://en.wikipedia.org/wiki/Cosmic_Background_Explorer | reference | science, encyclopedia | 2026-05-05T12:47:58.679749+00:00 | kb-cron |
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 900 km (560 mi) altitude orbit with a 99° inclination was chosen as it fit within the capabilities of either a Space Shuttle (with an auxiliary propulsion on COBE) 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 650 L (140 imp gal; 170 US gal) 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 (WFF) for use as a test satellite.
=== Instruments ===
==== Differential Microwave Radiometers (DMR) ==== The Differential Microwave Radiometer (DMR) investigation uses three differential radiometers to map the sky at 31.4, 53, and 90 GHz. The radiometers are distributed around the outer surface of the cryostat. Each radiometer employs a pair of horn antennas viewing at 30° from the spin axis of the spacecraft, measuring the differential temperature between points in the sky separated by 60°. At each frequency, there are two channels for dual-polarization measurements for improved sensitivity and for reliability. Each radiometer is a microwave receiver whose input is switched rapidly between the two horn antennas, obtaining the difference in brightness of two fields of view 7° in diameter located 60° apart and 30° from the axis of the spacecraft. High sensitivity is achieved by temperature stabilization (at 300 K for 31.4 GHz and at 140 K for 53 and 90 GHz), by spacecraft spin, and by the ability to integrate over the entire year. Sensitivity to large-scale anisotropies is about 3E-5 K. The instrument weighs 120 kg (260 lb), uses 114 watts, and has a data rate of 500 bit/s.
==== Diffuse Infrared Background Experiment (DIRBE) ==== The Diffuse Infrared Background Experiment (DIRBE) consists of a cryogenically cooled (to 2 K) multiband radiometer used to investigate diffuse infrared radiation from 1 to 300 micrometres. The instrument measures the absolute flux in 10 wavelength bands with a 1° field of view pointed 30° off the spin axis. Detectors (photoconductors) and filters for the 8 to 100 micrometre channels are the same as for the IRAS mission. Bolometers are used for the longest wavelength channel (120 to 300 micrometres). The telescope is a well baffled, off-axis, Gregorian flux collector with re-imaging. The instrument weighs approximately 34 kg (75 lb), uses 100 W and has a data rate of 1700 bit/s.