Cosmic Background Explorer (COBE)
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{{Infobox Space telescope
|name = Cosmic Background Explorer (COBE)
|image = [[Image:020597COBE OV.jpg|250px]]
|organization = [[NASA]]
|major_contractors = [[Goddard Space Flight Center]]
|alt_names =
|nssdc_id = 1989-089A
|location = [[Earth orbit]]
|orbit_type =
|height = 900.2 km
|period = 103 minutes
|velocity =
|accel_gravity =
|launch_date = [[November 18]], [[1989]]
|launch_location =
|launch_vehicle = [[Delta rocket]]
|mission_length = ≈4 years
|deorbit_date =
|wavelength =
|mass = 2270 kg
|website = [http://lambda.gsfc.nasa.gov/product/cobe/ LAMBDA - Cosmic Background Explorer]
|style =
|diameter =
|area =
|focal_length =
|instrument_1_name = DIRBE
|instrument_1_characteristics = Diffuse Infrared Background Experiment
|instrument_2_name = FIRAS
|instrument_2_characteristics = Far-InfraRed Absolute [[Spectrophotometer]]
|instrument_3_name = DMR
|instrument_3_characteristics = Differential Microwave [[Radiometer]]
|as_of =
|stats_ref =
}}
The '''Cosmic Background Explorer''' ('''COBE'''), also referred to as '''Explorer 66''', was the [[satellite]] built dedicated to [[physical cosmology|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.
This work helped cement the [[big-bang]] theory of the universe. Two of COBE's principal investigators, [[George F. Smoot|George Smoot]] and [[John C. Mather|John 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=http://nobelprize.org/nobel_prizes/physics/laureates/2006/info.html
|title=Information for the public
|publisher=[[The Royal Swedish Academy of Sciences]]
|format=[[PDF]]
|date=2006-10-03
|accessdate=2006-10-05
}}
</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 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<ref name=leverington> {{cite book | author=Leverington, David | title=New Cosmic Horizons: Space Astronomy from the V2 to the Hubble Space Telescope | location=Cambridge | publisher=Cambridge University Press | year=2000 | id=ISBN 0-521-65833-0}}</ref>:
{| class="wikitable"
|+Instruments
!Instrument
!Acronym
!Description
!Principal Investigator
|-
|Differential Microwave Radiometer
|DMR
|a [[microwave]] instrument that would map variations (or anisotropies) in the [[Cosmic microwave background radiation|CMB]]
|[[George F. Smoot|George Smoot]]
|-
|Far-InfraRed Absolute Spectrophotometer
|FIRAS
|a spectrophotometer used to measure the spectrum of the CMB
|[[John C. Mather|John Mather]]
|-
|Diffuse InfraRed Background Experiment
|DIRBE
|a multiwavelength infrared detector used to map dust emission
|[[Mike Hauser]]
|}
[[Image:COBELaunch.jpg|thumb|right|230px|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 flask|dewar]] to those used on the [[Infrared Astronomical Satellite|IRAS]].
COBE was originally planned to be launched on a [[Space Shuttle]] in [[1988]], but the [[STS-51-L|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]].
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."
== 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., J.C. Mather, R. Weiss, C.L. Bennett, E.S. Cheng, E. Dwek, S. Gulkis, M.G. Hauser, M.A. Janssen, T. Kelsall, S.S. Meyer, S.H. Moseley, T.L. Murdock, R.A. Shafer, R.F. Silverberg, G.F. Smoot, D.T. Wilkinson, and E.L. Wright |year=1992 |title=The COBE Mission: Its Design and Performance Two Years after the launch |journal=Astrophysical Journal |volume=397 |issue=2 |pages=420, Preprint No. 92–02 |doi=10.1086/171797}}</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 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 of that would result from fast neutral particles hitting its surfaces at extremely high speed.
[[Image:COBEDiagram.jpg|250px|left]]
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 900 km altitude orbit with a 99° inclination was chosen as it fit within the capabilities of either a [[Space Shuttle|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 [[Van Allen radiation belt|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.
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.<ref name=boggess/>
== Scientific findings ==
[[Image:COBE cmb fluctuations.gif|thumb|left|250px|The "famous" 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 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.<ref name=boggess/>
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 ===
[[Image:cobeslide36.jpg|thumb|300px|Data from COBE showed a perfect fit between the black body curve predicted by big bang theory and that observed in the microwave background.]]
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<ref name=leverington/>.
Second, in 1987 a Japanese-American team led by Andrew Lange and Paul Richards 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]].<ref name=leverington/>
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<sup>−1</sup> in two bands separated at 20 cm<sup>−1</sup>. 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.<ref name=fixsen> {{cite journal |author=Fixsen, D. J.; Cheng, E. S.; Cottingham, D. A.; Eplee, R. E., Jr.; Isaacman, R. B.; Mather, J. C.; Meyer, S. S.; Noerdlinger, P. D.; Shafer, R. A.; Weiss, R.; Wright, E. L.; Bennett, C. L.; Boggess, N. W.; Kelsall, T.; Moseley, S. H.; Silverberg, R. F.; Smoot, G. F.; Wilkinson, D. T. |year=1994 |title=Cosmic microwave background dipole spectrum measured by the COBE FIRAS instrument |url=http://adsabs.harvard.edu/abs/1994ApJ...420..445F |journal=Astrophysical Journal |volume=420 |issue=2 |pages=445–449 |doi=10.1086/173575}}</ref>
=== Intrinsic anisotropy of CMB ===
[[Image:COBE DMR Image.PNG|thumb|Data obtained at each of the three DMR frequencies—31.5, 53, and 90 GHz—following dipole subtraction.]]
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).
=== 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 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.<ref name=sodroski>{{cite journal |author= T. J. Sodroski et al.<!--, C. Bennett, N. Boggess, E. Dwek, B. A. Franz, M. G. Hauser, T. Kelsall, S. H. Moseley, N. Odegard, R. F. Silverberg, and J. L. Weiland--> |title= Large-Scale Characteristics of Interstellar Dust from COBE DIRBE Observations |url=http://adsabs.harvard.edu/abs/1994ApJ...428..638S |volume=428| issue=2 |pages=638–646 |year=1994 |doi=10.1086/174274|journal= The Astrophysical Journal}}</ref>
=== Other contributions of COBE ===
[[Image:COBE galactic disk.PNG|thumb|Model of the Galactic disk as seen edge on from our 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 was able to conclude that grains of [[asteroid]]al origin populate the IPD bands and the smooth IPD cloud. <ref name=spiesman>{{cite journal|author=Spiesman, W.J., M.G. Hauser, T. Kelsall, C.M. Lisse, S.H. Moseley, Jr., W.T. Reach, R.F. Silverberg, S.W. Stemwedel, and J.L. Weiland |year=1995 |title=Near and far infrared observations of interplanetary dust bands from the COBE Diffuse Infrared Background Experiment |url=http://adsabs.harvard.edu/abs/1995ApJ...442..662S |journal=Astrophysical Journal |volume=442 |issue=2 |pages=662–667, Preprint No. 94–12 |doi=10.1086/175470}}</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 our Sun is 8.6 [[Parsec|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.<ref name=freudenreich>{{cite journal|author=Freudenreich, H.T. |year=1996 |title=The shape and color of the galactic disk |url=http://adsabs.harvard.edu/abs/1996ApJ...468..663F |journal=Astrophysical Journal |volume=468 |pages=663–678 |doi=10.1086/177724}} See also {{cite journal|author=Freudenreich, H.T. |year=1997 |title=The shape and color of the galactic disk: Erratum |url=http://adsabs.harvard.edu/abs/1997ApJ...485..920F |journal=Astrophysical Journal |volume=485 |pages=920 |doi=10.1086/304478}}</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 (planet)|Saturn]] and [[Jupiter (planet)|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., R. G. Arendt, M. G. Hauser, D. Fixsen, T. Kelsall, D. Leisawitz, Y. C. Pei, E. L. Wright, J. C. Mather, S. H. Moseley, N. Odegard, R. Shafer, R. F. Silverberg, and J. L. Weiland |year=1998 |title=The COBE Diffuse Infrared Background Experiment search for the cosmic infrared background: IV. Cosmological Implications |url=http://adsabs.harvard.edu/abs/1998ApJ...508..106D |journal=Astrophysical Journal |volume=508 |issue=1 |pages=106–122 |doi=10.1086/306382}}</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 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.<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 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.
== See also ==
* [[RELIKT-1]] — Soviet cosmic microwave background anisotropy experiment of 1983.
* [[9997 COBE]] — A [[minor planet]] named after the experiment.
== Sources ==
{{reflist}}
* {{cite book | author=Arny, Thomas T. | title=Explorations: an Introduction to Astronomy (Third Edition) | location=New York | publisher=McGraw-Hill Higher Education | year=2002 | id=ISBN 0-07-246570-0}}
* {{cite journal|author=Odenwald, S., J. Newmark, and G. Smoot|year=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 |id={{arxiv|astro-ph|9610238}} |doi=10.1086/305737}}
* {{cite journal|author=A. R. Liddle, D. H. Lyth |title=The Cold Dark Matter Density Perturbation |journal=Physics Report—Review Section of Physics Letters |volume=231 |issue=1–2 |pages=1–105 |year=1993 |id={{arxiv|astro-ph|9303019}}}}
== 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 ==
{{commonscat|COBE}}
* [http://lambda.gsfc.nasa.gov/product/cobe/ NASA's website on COBE]
* [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 kps motion of the Earth relative to the cosmic background radiation
* [http://www.xkcd.com/54/ A webcomic from XKCD] that celebrates the COBE Mission
{{CMB experiments}}
{{Explorer program}}
{{Space telescopes}}
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[[Category:Space telescopes]]
[[Category:Explorer program]]
[[Category:Artificial satellites currently orbiting Earth]]
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