The Cosmic Background Explorer (COBE), also referred to as Explorer 66, was the first satellite built dedicated to cosmology. Its goals were to investigate the cosmic microwave background radiation (CMB) of the universe and provide measurements that would help shape our understanding of the cosmos. The argument of perigee is the angle subtending from the ascending node to the point of perigee in an Earth-centered orbit. ... In physics, spectrophotometry is the quantitative study of spectra. ... A Radiometer is a device used to measure the radiant flux or power in Electromagnetic radiation. ... An Earth observation satellite, ERS 2 For other uses, see Satellite (disambiguation). ... Physical cosmology, as a branch of astrophysics, is the study of the large-scale structure of the universe and is concerned with fundamental questions about its formation and evolution. ... In cosmology, the cosmic microwave background radiation (most often abbreviated CMB but occasionally CMBR, CBR or MBR, also referred as relic radiation) is a form of electromagnetic radiation discovered in 1965 that fills the entire universe. ... Universe is a word derived from the Old French univers, which in turn comes from the Latin roots unus (one) and versus (a form of vertere, to turn). Physicists concept of the Universe is motivated[] by the attempt to describe the whole of space-time, including all matter and energy...

This work helped cement the big-bang theory of the universe. According to the Nobel Prize committee, "the COBE-project can also be regarded as the starting point for cosmology as a precision science". ^[1] Two of COBE's principal investigators, George Smoot and John Mather, received the Nobel Prize in Physics in 2006. According to the Big Bang, the universe emerged from an extremely dense and hot state (bottom). ... // Cosmology, from the Greek: κοσμολογία (cosmologia, κόσμος (cosmos) order + λογια (logia) discourse) is the study of the Universe in its totality, and by extension, humanitys place in it. ... George Smoot celebrating his Nobel Prize on October 3, 2006 at Lawrence Berkeley National Laboratory. ... John C. Mather at NASA John Cromwell Mather (b. ... Hannes Alfvén (1908–1995) accepting the Nobel Prize for his work on magnetohydrodynamics [1]. List of Nobel Prize laureates in Physics from 1901 to the present day. ...

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 ultimately these proposals lost out to the Infrared Astronomical Satellite (IRAS), the strength of the three proposals sent a clear message to NASA that this was a matter to look into. In 1976, NASA had selected members from each of the three proposal teams of 1974 to get together and propose a joint conceptual satellite. A year later, this team came up with a polar orbiting satellite that could be launched by either a Delta rocket or the Shuttle, called COBE. It would contain the following instruments^[2]: 1974 (MCMLXXIV) was a common year starting on Tuesday. ... For other uses, see NASA (disambiguation). ... The Explorer program was the United Statess first successful attempt to launch an artificial satellite . ... The Infrared Astronomical Satellite (IRAS) was a space-based observatory that performed a survey of the entire sky at infrared wavelengths. ... 1976 (MCMLXXVI) was a leap year starting on Thursday. ... Delta EELV family of launch vehicles (US Govt) Delta rocket (sometimes retroactively called Delta I) Delta II rocket Delta III rocket Delta IV rocket The Delta family of expendable launch vehicles has been a mainstay of the United States space launch capability since 1960. ...

• Differential Microwave Radiometer (DMR) - a microwave instrument that would map variations (or anisotropies) in the CMB (Principal Investigator: George Smoot)
• Far-InfraRed Absolute Spectrophotometer (FIRAS) - a spectrophotometer used to measure the spectrum of the CMB (Principal Investigator: John Mather)
• Diffuse InfraRed Background Experiment (DIRBE) - a multiwavelength infrared detector used to map dust emission (Principal Investigator: Mike Hauser)

Launch of the COBE spacecraft November 18, 1989.

NASA accepted the proposal provided that the costs be kept under $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 (GSFC) did not begin until 1981. To save costs, COBE would use similar infrared detectors and liquid helium dewar to those used on the IRAS. Microwaves are electromagnetic waves with wavelengths longer than those of terahertz (THz) frequencies, but relatively short for radio waves. ... In cosmology, the cosmic microwave background radiation (most often abbreviated CMB but occasionally CMBR, CBR or MBR, also referred as relic radiation) is a form of electromagnetic radiation discovered in 1965 that fills the entire universe. ... George Smoot celebrating his Nobel Prize on October 3, 2006 at Lawrence Berkeley National Laboratory. ... John C. Mather at NASA John Cromwell Mather (b. ... Image File history File links Download high resolution version (625x667, 35 KB) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Download high resolution version (625x667, 35 KB) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... November 18 is the 322nd day of the year (323rd in leap years) in the Gregorian Calendar. ... 1989 (MCMLXXXIX) was a common year starting on Sunday of the Gregorian calendar. ... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Atomic mass 4. ... A Dewar flask is a vessel designed to provide very good thermal insulation. ... The Infrared Astronomical Satellite (IRAS) was a space-based observatory that performed a survey of the entire sky at infrared wavelengths. ...

COBE was originally planned to be launched on a Space Shuttle in 1988, but the Challenger explosion delayed this plan when the Shuttles were grounded. NASA kept COBE's engineers from going to other space agencies to launch COBE, but eventually, a redesigned COBE was placed into sun-synchronous orbit on November 18, 1989 aboard a Delta rocket. A team of American scientists announced, on April 23, 1992, that they had found the primordial "seeds" (CMBE anisotropy) in data from COBE. The announcement was reported worldwide as a fundamental scientific discovery and ran on the front page of the New York Times. NASAs Space Shuttle, officially called Space Transportation System (STS), is the United States governments current manned launch vehicle. ... 1988 (MCMLXXXVIII) was a leap year starting on Friday of the Gregorian calendar. ... The launch of the Space Shuttle Challenger on mission 51L/STS-33, the 25th of the STS (Space Transportation System) program, began at an estimated time of 16:38:00. ... By analogy with the geosynchronous orbit, a heliosynchronous orbit is a heliocentric orbit of radius 24. ... November 18 is the 322nd day of the year (323rd in leap years) in the Gregorian Calendar. ... 1989 (MCMLXXXIX) was a common year starting on Sunday of the Gregorian calendar. ... April 23 is the 113th day of the year in the Gregorian Calendar (114th in leap years). ... Media:rofl. ... The New York Times is an internationally known daily newspaper published in New York City and distributed in the United States and many other nations worldwide. ...

The Nobel Prize in Physics for 2006 was jointly awarded to John C. Mather, NASA Goddard Space Flight Center, Greenbelt, MD, USA, and George F. Smoot, University of California, Berkeley, CA, USA "for their discovery of the blackbody form and anisotropy of the cosmic microwave background radiation" Hannes Alfvén (1908–1995) accepting the Nobel Prize for his work on magnetohydrodynamics [1]. List of Nobel Prize laureates in Physics from 1901 to the present day. ...

The spacecraft

It 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. ^[3] 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. Adjectives: Terrestrial, Terran, Telluric, Tellurian, Earthly Atmosphere Surface pressure: 101. ... The Sun is the star at the center of the Solar System. ... Apparent magnitude: up to -12. ...

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.^[3] 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 of that would result from fast neutral particles hitting its surfaces at supersonic speeds. Look up anisotropy in Wiktionary, the free dictionary. ...

In order to meet the demands of the slow rotation and the attitude three-axis controls, a sophisticated pair of yaw angular momentum wheels were employed with their axis oriented along the spin axis .^[3] 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. Image File history File links Download high resolution version (740x601, 129 KB) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Gyroscope of the International Space Station A momentum wheel or reaction wheel is a type of flywheel used primarily by spacecraft to change their angular momentum without using fuel for rockets or other reaction devices. ...

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 .^[3] A circular Sun-synchronous orbit satisfied all these requirements. A 900 km altitude orbit with a 99° inclination was chosen as it fit within the capabilities of either a Shuttle (with an auxiliary propulsion on COBE) or a Delta rocket. This altitude was a good compromise between Earth's radiation and the charged particle in Earth's radiation belts at higher altitudes. An ascending node at 6 p.m. was chosen to allow COBE to follow the boundary between sunlight and darkness on Earth throughout the year. A shuttle, in general, is something which travels back and forth between places in a regular and relatively frequent manner. ... Van Allen radiation belts The Van Allen Radiation Belt is a torus of energetic charged particles (plasma) around Earth, trapped by Earths magnetic field. ...

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 liter 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.^[3]

The "famous" map of the CMB anisotropy formed from data taken by the COBE spacecraft.

Data obtained at each of the three DMR frequencies — 31.5, 53, and 90 GHz — following dipole subtraction.

Download high resolution version (1024x512, 380 KB)Cosmic microwave background temperature data were extracted from the released FITS files and then combined into two linear combinations. ... Download high resolution version (1024x512, 380 KB)Cosmic microwave background temperature data were extracted from the released FITS files and then combined into two linear combinations. ... Image File history File links Data obtained by COBE at each of the three DMR frequencies - 31. ... Image File history File links Data obtained by COBE at each of the three DMR frequencies - 31. ...

Scientific findings

The science mission was conducted by the three instruments detailed previously: DIRBE, FIRAS and the DMR. The instruments overlapped in wavelength coverage, providing consistency check on measurements in the regions of spectral overlap and assistance in discriminating signals from our galaxy, solar system and CMB.^[3]

COBE's instruments would fulfill each of their objectives as well as making observations that would have implications outside of COBE’s initial scope.

Black-body curve of CMB

During the long gestation period of COBE, there were two significant astronomical developments. First, in 1981, two teams of astronomers, one led by David Wilkinson of Princeton and the other by Francesco Melchiorri of the University of Florence, simultaneously announced that they detected a quadrupole distribution of CMB using balloon-borne instruments. This finding would have been the detection of the black-body distribution of CMB that FIRAS on COBE was to measure. However, a number of other experiments attempted to duplicate their results and were unable to do so^[2]. Schematic quadrupole magnet(four-pole) used to focus particle beams in a particle accelerator. ... As the temperature decreases, the peak of the black body radiation curve moves to lower intensities and longer wavelengths. ...

Second, in 1987 a Japanese-American team led by Andrew Lange and Paul Richardson of UC Berkeley and Toshio Matsumoto of Nagoya University made an announcement that CMB was not that of a true black body. In a sounding rocket experiment, they detected an excess brightness at 0.5 and 0.7 mm wavelengths. These results cast doubt on the validity of the Big Bang theory in general and help support the Steady State theory.^[2] A sounding rocket, sometimes called an elevator research rocket, is an instrument-carrying suborbital rocket designed to take measurements and perform scientific experiments during its flight. ...

With these developments serving as a backdrop to COBE’s mission, scientists eagerly awaited results from FIRAS. The results of FIRAS were startling in that they showed a perfect fit of the CMB and the theoretical curve for a black body at a temperature of 2.7 K, thus proving the Berkeley-Nagoya results erroneous.

FIRAS measurements were made by measuring the spectral difference between a 7° patch of the sky against an internal black body. The interferometer in FIRAS covered between 2 and 95 cm^-1 in two bands separated at 20 cm^-1. There are two scan lengths (short and long) and two scan speeds (fast and slow) for a total of four different scan modes. The data was collected over a ten month period.^[4]

Intrinsic anisotropy of CMB

The DMR was able to spend four years mapping the anisotropy of cosmic background radiation as it was the only instrument not dependent on the dewar’s supply of helium to keep it cooled. This operation was able to create full maps of the CMB by subtracting out galactic emissions and dipole at various frequencies. The cosmic microwave background fluctuations are extremely faint, only one part in 100,000 compared to the 2.73 kelvin average temperature of the radiation field. The cosmic microwave background radiation is a remnant of the Big Bang and the fluctuations are the imprint of density contrast in the early universe. The density ripples are believed to have produced structure formation as observed in the universe today: clusters of galaxies and vast regions devoid of galaxies (NASA). Look up anisotropy in Wiktionary, the free dictionary. ... The Kelvin scale is a thermodynamic (absolute) temperature scale where absolute zero—the lowest possible temperature where nothing could be colder and no heat energy remains in a substance—is defined as zero kelvin (0 K). ... According to the Big Bang, the universe emerged from an extremely dense and hot state (bottom). ... It has been suggested that this article or section be merged into Large-scale structure of the cosmos. ...

Detecting early galaxies

DIRBE also detected 10 new far-IR emitting galaxies in the region not surveyed by IRAS as well as nine other candidates in the weak far-IR that may be spiral galaxies. A spiral galaxy is a type of galaxy in the Hubble sequence which is characterized by the following physical properties: Spiral Galaxy M74 presents a face-on view of its spiral arms. ...

Galaxies that were detected at the 140 and 240 μm were also able to provide information on very cold dust (VCD). At these wavelengths, the mass and temperature of VCD can be derived.

When this data was joined with 60 and 100 μm data taken from IRAS, it was found that the far-infrared luminosity arises from cold (~17-22 K) dust associated with diffuse HI cirrus clouds, 15-30% from cold (~19 K) dust associated with molecular gas, and less than 10% from warm (~29 K) dust in the extended low-density HII regions.^[5]

Model of the Galactic disk as seen edge on from our position

Image File history File links Image of the Galactic disk seen edge on by COBE (NASA Image) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Image of the Galactic disk seen edge on by COBE (NASA Image) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...

Other contributions of COBE

On top of the findings DIRBE had on galaxies, it also made two other significant contributions to science.^[5]

The DIRBE instrument was able to conduct studies on interplanetary dust (IPD) and determine if its origin was from asteroid or cometary particles. The DIRBE data collected at 12, 25, 50 and 100 μm was able to conclude that grains of asteroidal origin populate the IPD bands and the smooth IPD cloud. ^[6] It has been suggested that Space dust be merged into this article or section. ... 253 Mathilde, a C-type asteroid. ...

The second contribution DIRBE made was a model of the Galactic disk as seen edge on from our position. According to the model, if our Sun is 8.6 kpc from the Galactic center, then the sun is 15.6 pc above the midplane of the disk, which has a radial and vertical scale lengths of 2.64 and 0.333 kpc, respectively, and is warped in a way consistent with the HI layer. There is also no indication of a thick disk.^[7] It has been proposed below that Disc (galaxy) be renamed and moved to galactic disc. ...

To create this model, the IPD had to be subtracted out of the DIRBE data. It was found that this cloud, which as seen from Earth is Zodiacal light, was not centered on the Sun, as previously thought, but on a place in space a few million kilometers away. This is due to the gravitation influence of Saturn and Jupiter.^[2] The zodiacal light in the eastern sky before the beginning of morning twilight. ... Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ...

Cosmological implications

In addition to the science results detailed in the last section, there are numerous cosmological questions left unanswered by COBE’s results. A direct measurement of the extragalactic background light (EBL) can also provide important constraints on the integrated cosmological history of star formation, metal and dust production, and the conversion of starlight into infrared emissions by dust.^[8] The Extragalactic background light (EBL) is the faint diffuse light of the night sky, consisting of the combined flux of all extragalactic sources. ...

By looking at the results from DIRBE and FIRAS in the 140 to 5000 μm we can detect that the integrated EBL intensity is ~16 nW/(m²·sr). This is consistent with the energy released during nucleosynthesis and constitutes about 20%-50% of the total energy released in the formation of He and metals throughout the history of the universe. Attributed only to nuclear sources, this intensity implies that more than 5-15% of the baryonic mass density implied by big bang nucleosynthesis analysis has been processed in stars to He and heavier elements.^[8]

There were also significant implications into star formation. COBE observations provide important constraints on the cosmic star formation rate, and help us calculate the EBL spectrum for various star formation histories. Observation made by COBE require that star formation rate at redshifts of z ≈ 1.5 to be larger than that inferred from UV-optical observations by a factor of 2. This excess stellar energy must be mainly generated by massive stars in yet-undetected dust enshrouded galaxies or extremely dusty star forming regions in observed galaxies.^[8] The exact star formation history cannot unambiguously be resolved by COBE and further observations must be made in the future. Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. ...

On June 30, 2001, NASA launched a follow-up mission to COBE led by DMR Deputy Principal Investigator Charles L. Bennett. The Wilkinson Microwave Anisotropy Probe has clarified and expanded upon COBE's accomplishments. Charles L. Bennett Dr. Charles L. Bennett (born November 1956) is an American observational astrophysicist and a Professor of Physics and Astronomy at the Johns Hopkins University. ... Artist depiction of the WMAP satellite at the L2 point The Wilkinson Microwave Anisotropy Probe (WMAP) is a NASA satellite whose mission is to survey the sky to measure the temperature of the radiant heat left over from the Big Bang. ...

See also

• RELIKT-1 - Soviet cosmic microwave background anisotropy experiment of 1983 year

RELIKT-1 (sometimes RELICT-1 from Russian РЕЛИКТ-1) - a Soviet cosmic microwave background anisotropy experiment onboard the Prognoz 9 satellite (launched 1 July 1983) first gave only upper limits on the large-scale anisotropy, but reanalysis of the data in 1992 claimed a signal roughly compatible with the later experiments. ...

Sources

• Arny, Thomas T. (2002). Explorations: an Introduction to Astronomy (Third Edition). New York: McGraw-Hill Higher Education. ISBN 0-07-246570-0. 
• Odenwald, S., J. Newmark, and G. Smoot (1998). "A study of external galaxies detected by the COBE Diffuse Infrared Background Experiment". Astrophysical Journal 500: 554-568. 
• A. R. Liddle, D. H. Lyth (1993). "The Cold Dark Matter Density Perturbation". Physics Report-Review Section of Physics Letters 231: 1-105. 

PDF is an abbreviation with several meanings: Portable Document Format Post-doctoral fellowship Probability density function There also is an electronic design automation company named PDF Solutions. ... The Royal Swedish Academy of Sciences or , founded in 1739 by King Frederick I, is one of the Royal Academies in Sweden. ... -1... October 5 is the 278th day of the year in the Gregorian Calendar (279th in Leap years). ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ...

Text written for the general public

• Wrinkles in Time by George Smoot and Keay Davidson, Harper Perennial, Reprint edition (October 1, 1994) ISBN 0-380-72044-2
• The Very First Light: The True Inside Story of the Scientific Journey Back to the Dawn of the Universe, by John C. Mather and John Boslough, Basic Books edition (November 1998) ISBN 0-465-01576-X

External links

• NASA's website on COBE
• XKCD comic reflecting the COBE mission