COBE

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 background radiation of the universe and provide measurements that would help shape our understanding of the cosmos.

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 large number of 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 experiments (Leverington, 2000):

1. Differential microwave radiometer (DMR) that would map the CMB to detect the intrinsic anisotropy in the microwave background, with George Smoot as Principal Investigator (PI).
2. Far-infrared absolute spectrophotometer (FIRAS) to measure the spectrum of the CMB to see if it was a blackbody curve, with John Mather as PI.
3. Diffuse Infrared background experiment (DIRBE) to detect early infrared galaxies with Mike Hauser as PI.

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.

After several delays, 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 spacecraft

As noted before, it was an Explorer class satellite with technology borrowed heavily from IRAS. There are several characteristics about COBE that made it a unique spacecraft.

The need to control and measure all the sources of systematic errors required a rigorous and integrated design and operation concept. To capture the science data needed, 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 (Boggess, 1992). The instruments themselves 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 (Boggess, 1992). 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.

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 (Boggess, 1992). 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 (Boggess, 1992). 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 PM 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 (Boggess, 1992).

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

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 (Boggess, 1992).

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

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).

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 quadripole 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 (Leverington, 2000).

Second, in 1987 a Japanese-American team led by Andrew Lange and Paul Richardson of UC Berkeley and Toshio Matsumoto of Nagoyo 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 of the Universe (Leverington, 2000).

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-Nagoyo 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 (Fixsen, 1994).

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 (Sodroski, 1994).

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

Other contributions of COBE

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

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 (Spiesman, 1995).

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 (Freudenreich, 1996).

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 as zodiacal light, is 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 (Leverington, 2000).

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 (Dwek, 1998).

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 that 5-15% of the baryonic mass density implied by big bang nucleosynthesis analysis has been processed in stars to He and heavier elements (Dwek, 1998).

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 (Dwek, 1998). The exact star formation history cannot unambiguously be resolved by COBE and further observations must be made in the future.

Sources

• Template:Book reference
• 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. (1992). "The COBE Mission: Its Design and Performance Two Years after the launch," Astrophysical Journal, 397, 420, Preprint No. 92-02
• 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. (1998). "The COBE Diffuse Infrared Background Experiment search for the cosmic infrared background: IV. Cosmological Implications", Astrophysical Journal, 508, 106
• 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.. (1994). "Cosmic microwave background dipole spectrum measured by the COBE FIRAS instrument", Astrophysical Journal, 420, 445
• Freudenreich, H.T.. (1996). "The shape and color of the galactic disk", Astrophysical Journal, 468, 663
• Template:Book reference
• 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
• Sodroski, T.J., C. Bennett, N. Boggess, E. Dwek, B. Franz, M.G. Hauser, T. Kelsall, S.H. Moseley, N. Odegard, R.F. Silverberg, and J.L. Weiland. (1994). "Large-scale characteristics of interstellar dust from COBE DIRBE observations", Astrophysical Journal, 428, 638, Preprint No. 93-13
• 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. (1995). "Near and far infrared observations of interplanetary dust bands from the COBE Diffuse Infrared Background Experiment", Astrophysical Journal, 442, 662, Preprint No. 94-12

External link

• NASA's website on COBE (http://lambda.gsfc.nasa.gov/product/cobe/)de:COBE

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