View source for Cosmic Background Explorer

{{Short description|NASA satellite of the Explorer program}}
{{Use American English|date=January 2014}}
{{Use dmy dates|date=November 2021}}
{{Infobox spaceflight
| name              = Cosmic Background Explorer
| names_list        = Explorer 66<br/>COBE
| image             = Cosmic Background Explorer spacecraft model.png
| image_caption     = Artist's concept of the COBE spacecraft
| image_size        = 300px

| mission_type      = [[Cosmic microwave background]] [[Radio astronomy|Astronomy]]
| operator          = [[NASA]]
| COSPAR_ID         = 1989-089A
| SATCAT            = 20322
| website           = {{url|1=http://lambda.gsfc.nasa.gov/product/cobe/|2=lambda.gsfc.nasa.gov/product/cobe}}
| mission_duration  = 6 months (planned)<br />{{time interval|18 November 1989 14:34|23 December 1993|show=ymd}} (achieved)

| spacecraft        = Explorer LXVI
| spacecraft_type   = Cosmic Background Explorer
| spacecraft_bus    = COBE
| manufacturer      = [[Goddard Space Flight Center]]
| launch_mass       = {{cvt|2206|kg}} <ref name="gsfcOverview">{{cite web |url=http://lambda.gsfc.nasa.gov/product/cobe/spacecraft_description.cfm |title=COBE Mission Design, Spacecraft and Orbit |date=18 April 2008 |publisher=Goddard Space Flight Center |access-date=21 July 2015}} {{PD-notice}}</ref>
| dry_mass          = {{cvt|1408|kg}} <ref name="pddnet">{{cite web |url=http://www.pddnet.com/articles/2015/07/spacecraft-week-cosmic-background-explorer |title=Cosmic Background Explorer |first=Megan |last=Crouse |date=16 July 2015 |publisher=Product Design & Development |access-date=21 July 2015}}{{Dead link|date=July 2020|bot=InternetArchiveBot|fix-attempted=yes}}</ref>
| dimensions        = {{cvt|5.49|xx|2.44|m}}
| power             = 750 [[watt]]s

| launch_date       = 18 November 1989, 14:34 [[Coordinated Universal Time|UTC]]
| launch_rocket     = [[Delta 5000|Delta 5920-8]] (Delta 189)
| launch_site       = [[Vandenberg Space Force Base|Vandenberg]], [[Vandenberg Space Launch Complex 2|SLC-2W]] 
| launch_contractor = [[Douglas Aircraft Company]]

| entered_service   = 18 November 1989
| last_contact      = 
| deactivated       = 23 December 1993
| decay_date        =

| orbit_reference   = [[Geocentric orbit]]<ref name="Trajectory">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/displayTrajectory.action?id=1989-089A |title=Trajectory: COBE (1989-089A) |publisher=NASA |date=28 October 2021 |access-date=27 November 2021}} {{PD-notice}}</ref>
| orbit_regime      = [[Sun-synchronous orbit]]
| orbit_periapsis   = {{cvt|900|km}}
| orbit_apoapsis    = {{cvt|900|km}}
| orbit_inclination = 99.00°
| orbit_period      = 103.00 minutes
| apsis             = gee

| instruments       = Differential Microwave Radiometer (DMR)<br />Diffuse Infrared Background Experiment (DIRBE)<br />Far-InfraRed Absolute Spectrophotometer (FIRAS)
 
| insignia          = Cosmic Background Explorer logo.jpg
| insignia_caption  = Cosmic Background Explorer mission patch
| insignia_size     = 200px

| programme         = '''[[Explorers Program|Explorer program]]'''
| previous_mission  = [[AMPTE-CCE]] (Explorer 65)
| next_mission      = [[Extreme Ultraviolet Explorer]] (Explorer 67)
}}

The '''Cosmic Background Explorer''' ('''COBE''' {{IPAc-en|ˈ|k|oʊ|b|i}} {{respell|KOH|bee}}), also referred to as '''Explorer 66''', was a [[NASA]] satellite dedicated to [[physical cosmology|cosmology]], which operated from 1989 to 1993. Its goals were to investigate the [[Cosmic microwave background|cosmic microwave background radiation]] (CMB or CMBR) 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]] [[Electromagnetic spectrum|spectrum]], and that it has very faint [[Anisotropy|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".<ref>{{cite web |url=https://www.nobelprize.org/nobel_prizes/physics/laureates/2006/ |title=The Nobel Prize in Physics 2006 |publisher=The Royal Swedish Academy of Sciences |date=2006-10-03 |access-date=2011-08-23}}</ref>

COBE was the second cosmic microwave background satellite, following [[RELIKT-1]], and was followed by two more advanced spacecraft: the [[Wilkinson Microwave Anisotropy Probe]] (WMAP) operated from 2001 to 2010 and the [[Planck (spacecraft)|Planck spacecraft]] from 2009 to 2013.

== Mission ==
The purpose of the Cosmic Background Explorer (COBE) mission was to take precise measurements of the diffuse radiation between 1 [[micrometre]] and {{cvt|1|cm}} over the whole celestial sphere. The following quantities were measured: (1) the spectrum of the 3 K radiation over the range 100 micrometres to {{cvt|1|cm}} (2) the anisotropy of this radiation from 3 to {{cvt|10|mm}}; and, (3) the spectrum and angular distribution of diffuse infrared background radiation at wavelengths from 1 to 300 micrometres.<ref name="Display">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1989-089A |title=Display: COBE (1989-089A) |publisher=NASA |date=28 October 2021 |access-date=27 November 2021}} {{PD-notice}}</ref>

== History ==
In 1974, NASA issued an Announcement of Opportunity for astronomical missions that would use a small- or medium-sized [[Explorer program|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 ([[IRAS]]), 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 orbit|polar-orbiting]] satellite called COBE to be launched by either a [[Delta 5000|Delta 5920-8]] [[launch vehicle]] or the [[Space Shuttle]]. It would contain the following instruments:<ref name=leverington>{{cite book |last1=Leverington |first1=David |title=New Cosmic Horizons: Space Astronomy from the V2 to the Hubble Space Telescope |publisher=Cambridge University Press |date=2000 |isbn=0-521-65833-0}}</ref>

{| class="wikitable"
|+Instruments
! Instrument
! Acronym
! Description
! Principal Investigator
|-

| Differential Microwave Radiometer
| DMR
| [[Microwave]] instrument that would map variations (or anisotropies) in the Cosmic microwave background (CMB) 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]]
|-
|}

[[File:COBELaunch.jpg|thumb|upright=1.0|right|Launch of the COBE spacecraft on 18 November 1989.]]

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]] (GSFC) did not begin until 1981. To save costs, the infrared detectors and [[liquid helium]] [[Vacuum flask|dewar]] on COBE would be similar to those used on Infrared Astronomical Satellite (IRAS).

COBE was originally planned to be launched on a [[Space Shuttle]] mission [[Canceled Space Shuttle missions|STS-82-B]] in 1988 from [[Vandenberg Space Force Base|Vandenberg Air Force Base]], but the [[STS-51-L|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 [[American Institute of Physics|APS April Meeting]] in [[Washington, D.C.]] the finding of the "primordial seeds" (CMBE anisotropy) in data from the DMR instrument; until then the other instruments were "unable to see the template."<ref>{{Cite news |last=Schewe |first=Phillip F. |last2=Stein |first2=Ben |date=1992-04-24 |title=COBE PICKS OUT PRIMORDIAL SEEDS AT LAST |volume=77 |work=The AIP Bulletin of Physics News |url=https://www.aip.org/pnu/1992/split/pnu077-1.htm |url-status=dead |access-date=2023-07-23 |archive-url=https://web.archive.org/web/20061023083330/https://www.aip.org/pnu/1992/split/pnu077-1.htm |archive-date=2006-10-23}}</ref> 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."<ref>{{Cite news |last=Wilford |first=John Noble |date=1992-04-24 |title=SCIENTISTS REPORT PROFOUND INSIGHT ON HOW TIME BEGAN |language=en-US |work=The New York Times |url=https://www.nytimes.com/1992/04/24/us/scientists-report-profound-insight-on-how-time-began.html |access-date=2023-07-23 |issn=0362-4331}}</ref>

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".<ref>{{cite web |url=http://nobelprize.org/nobel_prizes/physics/laureates/2006/index.html |title=The Nobel Prize in Physics 2006 |publisher=Nobel Foundation |access-date=2008-10-09}}</ref>

== Spacecraft ==
{{Cosmology|experiments}}
[[File:COBEDiagram.jpg|thumb|upright=1.0|left|Diagram of COBE 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]].<ref name=boggess>{{cite journal |author=Boggess, N. W. |author-link1=Nancy Boggess |author2=J. C. Mather |author3=R. Weiss |author4=C. L. Bennett |author5=E. S. Cheng |author6=E. Dwek |author7=S. Gulkis |author8=M. G. Hauser |author9=M. A. Janssen |author10=T. Kelsall |author11=S. S. Meyer |author12=S. H. Moseley |author13=T. L. Murdock |author14=R. A. Shafer |author15=R. F. Silverberg |author16=G. F. Smoot |author17=D. T. Wilkinson |author18=E. L. Wright |name-list-style=amp |date=1992 |title=The COBE Mission: Its Design and Performance Two Years after the launch |journal=Astrophysical Journal |volume=397 |issue=2 |page=420 |doi=10.1086/171797 |bibcode=1992ApJ...397..420B |doi-access=free}}</ref> 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 [[Revolutions per minute|rpm]] spin rate.<ref name=boggess/> 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 wheel]]s were employed with their axis oriented along the spin axis .<ref name=boggess/> 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.<ref name=boggess/> A circular [[Sun-synchronous orbit]] satisfied all these requirements. A {{cvt|900|km}} 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 [[Van Allen radiation belt|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 {{cvt|650|L}} 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.<ref name=boggess/>

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.<ref name="Display"/>

=== Instruments ===
==== Differential Microwave Radiometers (DMR) ====
The Differential Microwave Radiometer (DMR) investigation uses three differential [[radiometer]]s to map the sky at 31.4, 53, and 90 [[Hertz|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 {{cvt|120|kg}}, uses 114 watts, and has a data rate of 500 [[Bit rate|bit/s]].<ref name="Experiment3">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1989-089A-03 |title=Experiment: Differential Microwave Radiometers (DMR) |publisher=NASA |date=28 October 2021 |access-date=27 November 2021}} {{PD-notice}}</ref>

==== 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 ([[Photoconductivity|photoconductors]]) and filters for the 8 to 100 micrometre channels are the same as for the [[IRAS]] mission. [[Bolometer]]s 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 {{cvt|34|kg}}, uses 100 W and has a data rate of 1700 bit/s.<ref name="Experiment2">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1989-089A-02 |title=Experiment: Diffuse Infrared Background Experiment (DIRBE) |publisher=NASA |date=28 October 2021 |access-date=27 November 2021}} {{PD-notice}}</ref>

==== Far Infrared Absolute Spectrophotometer (FIRAS) ====
The Far Infrared Absolute Spectrophotometer (FIRAS) is a cryogenically cooled polarizing [[Michelson interferometer]] used as a [[Fourier transform]] spectrometer. The instrument points along the spin axis and has a 7° field of view. This device measures the spectrum to a precision of 1/1000 of the peak flux at {{cvt|1.7|mm}} for each 7° field of view on the sky (over the range 0.1 to {{cvt|10|mm}}). The FIRAS uses a special flared trumpet horn flux collector having very low sidelobe levels and an external calibrator covering the entire beam; precise temperature regulation and calibration are required. The instrument has a differential input to compare the sky with an internal reference at 3 K. This feature provides immunity from systematic errors in the spectrometer and contributes significantly to the ability to detect small deviations from a blackbody spectrum. The instrument weighs {{cvt|60|kg}}, uses 84 watts and has a data rate of 1200 bit/s.<ref name="Experiment1">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1989-089A-01 |title=Experiment: Far Infrared Absolute Spectrophotometer (FIRAS) |publisher=NASA |date=28 October 2021 |access-date=27 November 2021}} {{PD-notice}}</ref>

== Scientific findings ==
[[File:COBE cmb fluctuations.png|thumb|upright=1.0|right|The map of the CMB anisotropy formed from data taken by the COBE spacecraft.]]

The science mission was conducted by the three instruments detailed previously: DIRBE, FIRAS and 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.<ref name=boggess/>

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

=== Black-body curve of CMB ===
[[File:Cmbr.svg|thumb|upright=1.0|left|Data from COBE showed a perfect fit between the black body curve predicted by big bang theory and that observed in the microwave background. The error bars are smaller than the width of the blue curve. The red crosses merely locate the data points.]]
[[File:PIA16874-CobeWmapPlanckComparison-20130321.jpg|thumb|upright=1.0|right|Comparison of [[CMB]] results from COBE, [[Wilkinson Microwave Anisotropy Probe|WMAP]] and [[Planck (spacecraft)|Planck]] - 21 March 2013.]]

During the 15-year-long period between the proposal and launch of COBE, there were two significant astronomical developments:
* First, in 1981, two teams of astronomers, one led by David Wilkinson of [[Princeton University]] 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. In particular, the Florence group claimed a detection of intermediate angular scale anisotropies at the level 100 [[Orders of magnitude (temperature)|microkelvins]]<ref>{{cite journal |url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1981ApJ...250L...1M&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf |first1=Francesco |last1=Melchiorri |first2=Bianca O. |last2=Melchiorri |last3=Pietranera |first3=Luca |title=Fluctuations in the microwave background at intermediate angular scales |journal=The Astrophysical Journal |date=November 1981 |bibcode=1981ApJ...250L...1M |access-date=2011-08-23 |doi=10.1086/183662 |last4=Melchiorri |first4=B. O. |volume=250 |page=L1}}</ref> in agreement with later measurements made by the [[BOOMERanG experiment]]. However, a number of other experiments attempted to duplicate their results and were unable to do so.<ref name=leverington/>
* Second, in 1987 a Japanese-American team led by [[Andrew E. Lange]] and Paul Richards of [[University of California, Berkeley]] and Toshio Matsumoto of [[Nagoya University]] made an announcement that CMB was not that of a true black body.<ref>{{cite journal |title=Cosmological implication of a new measurement of the submillimeter background radiation |author=Hayakawa, S. |author2=Matsumoto, T. |author3=Matsuo, H. |author4=Murakami, Hiroshi |author5=Sato, S. |author6=Lange A. E. |author7=Richards, P. |name-list-style=amp |journal=Publications of the Astronomical Society of Japan |issn=0004-6264 |volume=39 |issue=6 |date=1987 |pages=941–948 |bibcode=1987PASJ...39..941H |url=http://articles.adsabs.harvard.edu//full/1987PASJ...39..941H/0000941.000.html |access-date=17 May 2012}}</ref> In a [[sounding rocket]] experiment, they detected an excess brightness at 0.5 and {{cvt|0.7|mm}} wavelengths.

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, in contrast to the Berkeley-Nagoya results.

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 were collected over a ten-month period.<ref name=fixsen>{{cite journal |author1=Fixsen, D. J. |author2=Cheng, E. S. |author3=Cottingham, D. A. |author4=Eplee, R. E. Jr. |author5=Isaacman, R. B. |author6=Mather, J. C. |author7=Meyer, S. S. |author8=Noerdlinger, P. D. |author9=Shafer, R. A. |author10=Weiss, R. |author11=Wright, E. L. |author12=Bennett, C. L. |author13=Boggess, N. W. |author14=Kelsall, T. |author15=Moseley, S. H. |author16=Silverberg, R. F. |author17=Smoot, G. F. |author18=Wilkinson, D. T. |date=1994 |title=Cosmic microwave background dipole spectrum measured by the COBE FIRAS instrument |bibcode=1994ApJ...420..445F |journal=Astrophysical Journal |volume=420 |issue=2 |pages=445–449 |doi=10.1086/173575}}</ref>

=== Intrinsic anisotropy of CMB ===
[[File:COBE DMR Image.PNG|thumb|upright=1.0|right|Data obtained at each of the three DMR frequencies — 31.5, 53, and 90 [[Hertz|GHz]] — following dipole subtraction.]]

The DMR was able to spend four years mapping the detectable 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 sky 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 K 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.<ref>{{Cite web |last=Dick |first=Steven J. |date=16 October 2006 |title=Voyages to the Beginning of Time |url=https://www.nasa.gov/exploration/whyweexplore/Why_We_24.html |access-date=2022-06-16 |website=nasa.gov |publisher=NASA |language=en |archive-date=16 June 2022 |archive-url=https://web.archive.org/web/20220616205611/https://www.nasa.gov/exploration/whyweexplore/Why_We_24.html |url-status=dead}} {{PD-notice}}</ref>

=== 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]]. 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 these data were 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 [[H I region]] 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 [[H II region]]s.<ref name=sodroski>{{cite journal |author=T. J. Sodroski |title=Large-Scale Characteristics of Interstellar Dust from COBE DIRBE Observations |bibcode=1994ApJ...428..638S |volume=428 |issue=2 |pages=638–646 |date=1994 |doi=10.1086/174274 |journal=The Astrophysical Journal |doi-access=free}}</ref>

=== DIRBE ===
{{Main|Diffuse Infrared Background Experiment}}
[[File:COBE galactic disk.PNG|thumb|upright=1.0|right|Model of the Galactic disk as seen edge-on from Earth's position.]]

On top of the findings DIRBE had on galaxies, it also made two other significant contributions to science.<ref name=sodroski/> 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 were able to conclude that grains of [[asteroid]]al origin populate the IPD bands and the smooth IPD cloud.<ref name=spiesman>{{cite journal |author1=Spiesman, W. J. |author2=M. G. Hauser |author3=T. Kelsall |author4=C. M. Lisse |author5=S. H. Moseley Jr. |author6=W. T. Reach |author7=R. F. Silverberg |author8=S. W. Stemwedel |author9=J. L. Weiland |name-list-style=amp |date=1995 |title=Near and far-infrared observations of interplanetary dust bands from the COBE Diffuse Infrared Background Experiment |bibcode=1995ApJ...442..662S |journal=Astrophysical Journal |volume=442 |issue=2 |page=662 |doi=10.1086/175470 |doi-access=free}}</ref>

The second contribution DIRBE made was a model of the [[Galactic disk]] as seen edge-on from our position. According to the model, if the Sun is 8.6 [[Parsec|kpc]] from the [[Galactic Center]], then it is 15.6% 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.<ref name=freudenreich>{{cite journal |author=Freudenreich, H. T. |date=1996 |title=The shape and color of the galactic disk |bibcode=1996ApJ...468..663F |journal=Astrophysical Journal |volume=468 |pages=663–678 |doi=10.1086/177724 |doi-access=free}} See also {{cite journal |author=Freudenreich, H. T. |date=1997 |title=The shape and color of the galactic disk: Erratum |bibcode=1997ApJ...485..920F |journal=Astrophysical Journal |volume=485 |issue=2 |page=920 |doi=10.1086/304478 |doi-access=free}}</ref>

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]].<ref name=leverington/>

=== 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.<ref name=dwek>{{cite journal |author=Dwek, E. |author2=R. G. Arendt |author3=M. G. Hauser |author4=D. Fixsen |author5=T. Kelsall |author6=D. Leisawitz |author7=Y. C. Pei |author8=E. L. Wright |author9=J. C. Mather |author10=S. H. Moseley |author11=N. Odegard |author12=R. Shafer |author13=R. F. Silverberg |author14=J. L. Weiland |name-list-style=amp |date=1998 |title=The COBE Diffuse Infrared Background Experiment search for the cosmic infrared background: IV. Cosmological Implications |bibcode=1998ApJ...508..106D |journal=Astrophysical Journal |volume=508 |issue=1 |pages=106–122 |doi=10.1086/306382 |arxiv=astro-ph/9806129 |s2cid=14706133}}</ref>

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 [[Watt|nW]]/(m<sup>2</sup>·sr). This is consistent with the energy released during [[nucleosynthesis]] and constitutes about 20–50% of the total energy released in the formation of [[helium]] 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 helium and heavier elements.<ref name=dwek/>

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.<ref name=dwek/> The exact star formation history cannot unambiguously be resolved by COBE and further observations must be made in the future.

On 30 June 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. Following WMAP, the European Space Agency's probe, [[Planck (spacecraft)|Planck]] has continued to increase the resolution at which the background has been mapped.<ref>{{cite web |last=Thomas |first=Christopher |title=Planck's Probe Map - A picture of the Universe |date=3 April 2013 |url=http://dawn.com/2013/04/03/plank-probes-map-a-picture-of-our-universe/ |publisher=Spider Magazine |access-date=28 May 2013}}</ref><ref>{{cite web |title=Planck's HFI Completes Its Survey of the Early Universe |url=http://www.esa.int/Our_Activities/Space_Science/Planck/Planck_s_HFI_completes_its_survey_of_early_Universe |publisher=ESA |access-date=28 May 2013}}</ref>

== See also ==
{{Portal|Spaceflight}}
* [[9997 COBE]], a [[minor planet]] named after the experiment.
* [[S150 Galactic X-Ray Mapping]]
* [[Wilkinson Microwave Anisotropy Probe]]

== References ==
{{Reflist}}

== Sources ==
* {{cite book |author=Arny, Thomas T. |title=Explorations: An Introduction to Astronomy |edition=3rd |location=Dubuque, Iowa |publisher=McGraw-Hill |date=2002 |isbn=978-0-07-241593-3}}
* {{cite journal |author1=Liddle, A. R. |author2=Lyth, D. H. |title=The Cold Dark Matter Density Perturbation |journal=Physics Reports  |volume=231 |issue=1–2 |pages=1–105 |date=1993 |arxiv=astro-ph/9303019|bibcode=1993PhR...231....1L |doi=10.1016/0370-1573(93)90114-S |s2cid=119084975}}
* {{cite journal |author1=Odenwald, S. |author2=J. Newmark |author3=G. Smoot |name-list-style=amp |date=1998 |title=A study of external galaxies detected by the COBE Diffuse Infrared Background Experiment |journal=Astrophysical Journal |volume=500 |issue=2 |pages=554–568 |doi=10.1086/305737 |arxiv=astro-ph/9610238 |bibcode=1998ApJ...500..554O |s2cid=18799050}}

== Further reading ==
* {{cite book |last1=Mather |first1=John C. |title=The Very First Light: The True Inside Story of the Scientific Journey Back to the Dawn of the Universe |date=1996 |publisher=BasicBooks |location=New York |isbn=0-465-01575-1 |last2=Boslough |first2=John |url=https://archive.org/details/veryfirstlight00john}}
* {{cite book |last=Smoot |first=George |title=Wrinkles in Time |date=1993 |publisher=W. Morrow |location=New York |author2=Davidson, Keay |isbn=0-688-12330-9}}
* [[WMAP]]

== External links ==
{{Commons category|COBE}}
* [http://lambda.gsfc.nasa.gov/product/cobe/ NASA's website on COBE]
* [https://web.archive.org/web/20110811140214/http://anon.nasa-global.edgesuite.net/anon.nasa-global/ccvideos/GSFC_20091117_COBE20th.asx NASA informational video prior to COBE launch]
* [https://web.archive.org/web/20091202151659/http://solarsystem.nasa.gov/missions/profile.cfm?MCode=COBE COBE Mission Profile] by [http://solarsystem.nasa.gov/ NASA's Solar System Exploration]
* [http://antwrp.gsfc.nasa.gov/apod/ap030209.html APOD picture of the COBE dipole], showing the 600&nbsp;km/s motion of the Earth relative to the cosmic background radiation
* [http://www.scholarpedia.org/article/Cosmic_background_explorer_(COBE) Cosmic Background Explorer] article from [[Scholarpedia]]

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