|Annu. Rev. Astron. Astrophys. 1980. 18:
Copyright © 1980 by Annual Reviews. All rights reserved
It has long been recognized that measurements of the spectrum, especially at high frequency, and the large angular scale intensity distribution of the CBR would benefit greatly by being carried out from a long-lived space platform. Aside from the obvious and important advantages of freedom from atmospheric emission, fluctuations in the emission, and absorption, which limits the spectral range, a satellite platform offers full sky coverage with a single instrument and allows sufficient time to test for systematic errors as well as to integrate the weak CBR signals. Furthermore, a shielded apparatus in space offers a thermally controlled environment in vacuum, which facilitates absolute primary calibration with cryogenic sources.
Since 1974 a group consisting of S. Gulkis (Jet Propulsion Laboratory), M. Hauser (NASA Goddard Space Flight Center), J. Mather (NASA Goddard Space Flight Center), G. Smoot (University of California, Berkeley), R. Weiss (MIT), and D. Wilkinson (Princeton) has been involved in planning the COBE (Cosmic Background Explorer) mission. The advance possible by using a space platform should bring measurements of the CBR to the level where the dominant "noise" will be the contribution of the local astrophysical environment. The complement of instruments chosen and the mission specifications of full sky coverage with ~ 1 year lifetime are designed to discriminate the CBR from more local astrophysical sources by their spectra and anisotropic angular distribution.
The present NASA plan is to fly the COBE mission in early 1985. A schematic diagram of the instrument is shown in Figure 15. The instrument, in sun-synchronous polar orbit, points radially out from the earth and rotates at 1 revolution per minute. A liquid helium cryostat with a 1 year storage time sits in the center of a 5 meter diameter sun and rf shield. The cryostat contains a Fourier transform spectrometer (FIRAS) and an infrared photometer (DIRBE). FIRAS (Far Infrared Absolute Spectrophotometer) points along the axis of rotation while DIRBE (diffuse infrared background experiment) points at 30° to the rotation axis. Four differential microwave radiometers (DMR) are attached to the outside of the cryostat.
Figure 15. Schematic of the proposed COBE satellite.
FIRAS is intended to measure the CBR spectrum. It consists of a rapid-scan polarizing Michelson interferometer divided into two bands 1-20 cm-1 and 20-100 cm-1 with a minimum resolution width of 0.2 cm-1 at long wavelengths and a 5% resolution at short wavelengths. The beam is defined to 7° by a trumpet-shaped horn. The expected sensitivity for each field of view, in a one year mission, is ~ 10-13 W/cm2 sr.
DIRBE is incorporated in the COBE mission to serve two purposes: first, to measure the interstellar dust emission which may contaminate the CBR spectrum at high frequencies and, second, to perform an all-sky survey of the diffuse infrared background from 1 to 250 microns which, although it is not a residue of the primeval radiation, may contain components that are the result of later epochs in cosmic history. The instrument is a multiband filter absolute photometer with a 1° field of view. The low background conditions in space and in particular in the COBE configuration should allow a sensitivity of ~ 10-14 W/cm2 sr per field of view in 1 year, < 160µ, and about a factor of 10 worse > 200µ.
The DMR experiment measures the large angular scale anisotropies of the CBR with a beam width of 7°. The differential radiometers operate at 23, 31, 53, and 90 GHz. Each radiometer compares the radiation of two patches of the sky 60° apart. The expected sensitivity in a one year mission is 0.3 mK in each of 1000 independent elements of the sky. The radiometer frequencies are chosen to pleasure both the CBR as well as the contamination by synchrotron and thermal emission by H II regions at low frequency and the interstellar dust at high frequencies.
The results of the COBE mission will be a set of sky maps from 1 to 10,000 cm-1 which includes all the processes indicated in Figure 3, the galactic synchrotron emission, diffuse H II thermal emission, interstellar and interplanetary dust, as well as the integrated starlight. The residue will be the CBR and whatever other cosmological background might exist.