Cosmic Background Explorer

The Cosmic Background Explorer (COBE), also designated Explorer 66, was a NASA satellite mission dedicated to physical cosmology that operated between 1989 and 1993. Its primary scientific objective was the detailed study of the cosmic microwave background radiation (CMB), the relic radiation from the early universe. By making precise, full-sky measurements of this faint radiation, COBE provided transformative evidence supporting the Big Bang model and marked the beginning of cosmology as a precision observational science.

Scientific background and objectives

The cosmic microwave background is a nearly uniform background of microwave radiation filling the universe, corresponding to a temperature of approximately 2.7 kelvin. Prior to COBE, the existence of the CMB had been established, but its detailed spectral properties and spatial variations were poorly constrained. COBE was designed to address three fundamental questions in cosmology:

• Whether the CMB spectrum follows that of a perfect blackbody
• Whether the CMB contains small anisotropies, or temperature variations, across the sky
• How much diffuse infrared radiation exists from early galaxies and cosmic structures

Answering these questions was essential for testing theoretical models of the early universe and the formation of galaxies.

Mission overview

The COBE mission aimed to measure diffuse radiation across the entire celestial sphere over wavelengths ranging from 1 micrometre to 300 micrometres. Specifically, it focused on:

• The precise spectrum of the 3 K cosmic background radiation
• Large-scale anisotropies in the CMB at angular scales of several degrees
• The spectrum and angular distribution of the diffuse infrared background

The mission successfully demonstrated that the CMB possesses an almost perfect blackbody electromagnetic spectrum, a key prediction of the Big Bang theory, and that it exhibits extremely faint anisotropies at the level of one part in 100,000. These anisotropies represent the primordial density fluctuations from which galaxies and large-scale cosmic structures later formed.

Historical development

In 1974, NASA issued an Announcement of Opportunity inviting proposals for small and medium-sized Explorer missions. Among the 121 proposals submitted, three independently proposed missions to study cosmological background radiation. Although these proposals were not initially selected, their scientific merit prompted NASA to further investigate the concept.
In 1976, a committee formed from the original proposal teams recommended a dedicated satellite mission, later named COBE. The proposed spacecraft would operate in polar orbit and carry multiple instruments designed for high-precision background measurements. NASA approved the mission under the condition that costs remain below a strict budget cap, excluding launch and data analysis.
Development was delayed by cost overruns associated with the Infrared Astronomical Satellite (IRAS), leading to construction beginning in 1981 at NASA’s Goddard Space Flight Center. To reduce expenses, COBE reused several technologies developed for IRAS, including infrared detectors and cryogenic systems.
Originally intended for launch aboard the Space Shuttle from Vandenberg Air Force Base, the mission was delayed following the Space Shuttle Challenger disaster. COBE was eventually redesigned for launch aboard a Delta rocket, achieving orbit on 18 November 1989.

Spacecraft design and orbit

COBE was an Explorer-class satellite engineered with exceptional attention to thermal stability, cleanliness, and systematic error control. These requirements were essential for detecting minute variations in the CMB against a background dominated by local sources of radiation.
The spacecraft was placed into a sun-synchronous polar orbit at an altitude of approximately 900 kilometres and an inclination of about 99 degrees. This orbit allowed COBE to remain near the boundary between day and night on Earth, minimising thermal fluctuations and stray radiation from the Sun and Earth.
COBE rotated at a slow rate of approximately 0.8 revolutions per minute, enabling its instruments to scan large swathes of the sky while maintaining stable observational conditions. The spin axis was carefully oriented to reduce contamination from atmospheric particles and infrared emission.
A sophisticated attitude control system using angular momentum wheels ensured precise pointing, while a large Sun–Earth shield protected the instruments from unwanted radiation and radio interference.

Instrumentation

COBE carried three major scientific instruments, each addressing a specific component of the mission’s objectives.

Differential Microwave Radiometers (DMR)

The Differential Microwave Radiometers were designed to map temperature anisotropies in the CMB. The DMR system consisted of three radiometers operating at frequencies of 31.5 GHz, 53 GHz, and 90 GHz. Each radiometer compared the temperature difference between two regions of the sky separated by 60 degrees.
This differential measurement technique was crucial for suppressing systematic errors and instrumental drift. Over the course of the mission, the DMR achieved sensitivity to temperature variations of approximately 30 microkelvin, sufficient to detect the primordial anisotropies predicted by cosmological theory.

Diffuse Infrared Background Experiment (DIRBE)

The Diffuse Infrared Background Experiment investigated infrared radiation from both galactic and extragalactic sources across wavelengths from 1 to 300 micrometres. DIRBE measured absolute flux levels in ten wavelength bands, enabling the separation of foreground emissions, such as zodiacal dust and interstellar matter, from the cosmic infrared background.
DIRBE data provided insights into the cumulative emission from early star formation and galaxy evolution, complementing the CMB observations.

Far Infrared Absolute Spectrophotometer (FIRAS)

The Far Infrared Absolute Spectrophotometer was the most critical instrument for testing the blackbody nature of the CMB. FIRAS was a cryogenically cooled Michelson interferometer that compared the sky signal directly with an internal reference blackbody.
FIRAS demonstrated that the CMB spectrum matches a perfect blackbody to an extraordinary precision of one part in 10,000, establishing one of the most accurate confirmations of any physical theory in astronomy.

Key discoveries and scientific impact

On 23 April 1992, COBE scientists announced the detection of CMB anisotropies using data from the DMR instrument. This discovery revealed the tiny primordial irregularities that later evolved into galaxies, clusters, and large-scale cosmic structures. The announcement received widespread attention and was hailed as a landmark moment in modern cosmology.
Together, the FIRAS and DMR results provided compelling empirical support for the Big Bang model, ruling out alternative steady-state theories. According to the Nobel Prize committee, COBE transformed cosmology into a discipline grounded in precise measurement rather than speculative theory.

Legacy and subsequent missions

COBE was the second space mission dedicated to CMB studies, following the Soviet RELIKT-1 satellite. Its success paved the way for more advanced missions, notably the Wilkinson Microwave Anisotropy Probe (WMAP), which operated from 2001 to 2010, and the Planck spacecraft, which collected data between 2009 and 2013.
These later missions built upon COBE’s foundations, achieving higher angular resolution and sensitivity, but the fundamental discoveries of COBE remain central to modern cosmology.