Posted: Wed Nov 8, 1989 3:58 PM EST Msg: UJIJ-1622-2716
From: PAO.POST
To: pao
Subj: COBE Press Kit
GENERAL PRESS RELEASE
NASA SPACECRAFT TO LOOK OUT INTO SPACE, BACK IN TIME
NASA will launch a spacecraft on Nov. 17, 1989, to study the
origin and dynamics of the universe, including the theory that
the universe began about 15 billion years ago with a cataclysmic
explosion -- the Big Bang.
The Cosmic Background Explorer (COBE) spacecraft will be
boosted into an Earth polar orbit from Vandenberg Air Force Base,
Calif., aboard the final NASA-owned, NASA-launched Delta vehicle.
By measuring the diffuse infrared radiation (cosmic
background) that bombards Earth from every direction, COBE's
instruments will help clarify such matters as the nature of the
primeval explosion -- which started the expansion of the universe
and made it uniform -- and the processes leading to the formation
of galaxies.
From its orbit 559 miles above Earth, COBE will carry out
its cosmic search using three sophisticated instruments: the
Differential Microwave Radiometer (DMR), Far Infrared Absolute
Spectrophotometer (FIRAS) and Diffuse Infrared Background
Experiment (DIRBE).
DMR will determine whether the primeval explosion was
equally intense in all directions. Patchy brightness in the
cosmic microwave background would unmask the as-yet-unknown
"seeds" that led to the formation of such large bodies as
galaxies, clusters of galaxies, and clusters of clusters of
galaxies. Measurements of equal brightness in all directions
would mean the puzzle of how these systems could have condensed
since the Big Bang will be even more vexing than it is today.
To distinguish the emissions of our own Milky Way galaxy
from the true cosmic background radiation, DMR will measure
radiation from space at wavelengths of 3.3, 5.7 and 9.6
millimeters.
FIRAS, covering wavelengths from 0.1 to 10 millimeters, will
survey the sky twice during the year-long mission to determine
the spectrum (brightness versus wavelength) of the cosmic
background radiation from the Big Bang.
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The spectrum that would result from a simple Big Bang can be
calculated with great accuracy. Such a spectrum would be smooth
and uniform and have no significant releases of energy between
the time of the Big Bang and the formation of galaxies. If
FIRAS' measurements depart from the predicted spectrum,
scientists will know that powerful energy sources existed in the
early universe between these times.
These sources may include annihilation of antimatter, matter
falling into "black holes," decay of new kinds of elementary
particles, explosion of supermassive objects and the turbulent
motions that may have caused the formation of galaxies.
FIRAS' sensitivity will be 100 times greater than that
achieved so far by equivalent ground-based and balloon-borne
instruments. Producing a spectrum for each of 1,000 parts of the
sky, the FIRAS data will allow scientists to measure how much
light was radiated by the Big Bang.
DIRBE will search for the diffuse glow of the universe
beyond our galaxy in the wavelength range from 1 to 300
micrometers. In the final analysis, any uniform infrared
radiation that remains will be very rich in information about the
early universe. One possible source would be light from
primordial galaxies shifted into the far infrared by the
expansion of the universe.
The 5,000-pound spacecraft and its three infrared- and
microwave-measuring instruments were designed and built for the
Office of Space Science and Applications by NASA's Goddard Space
Flight Center, Greenbelt, Md. Goddard also will manage the
launch and analyze the data returned by COBE during its 1-year
nominal mission.
Looking out into space, back in time, the COBE spacecraft
will undertake the esoteric task of providing new insights into
the origin and evolution of the universe.
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COSMIC BACKGROUND EXPLORER SUMMARY
MISSION: During the 2-year mission, COBE will determine the
spectrum of the cosmic background radiation, search for radiation
from the very first stars and galaxies and map the cosmic
background radiation with unprecedented accuracy. COBE will
study the physical conditions in the very early universe and the
onset of organization following the Big Bang.
LAUNCH: No earlier than 11/16/89, aboard a Delta 5920 ELV, from
Space Launch Complex 2 - West, Western Space and Missile Center,
Vandenberg Air Force Base, Calif. Launch window is 1/2 hour
beginning at 6:24 a.m. PST. An Advanced Range Instrumentation
Aircraft will cover the down-range burn of the Delta rocket.
ORBIT: 559-mile, sun-synchronous, near polar orbit, will circle
the globe 14 times a day.
SCIENCE DATA: Once a day, data are transmitted to Goddard Space
Flight Center's Wallops Processing Flight Facility then forwarded
to the COBE Science Data Center at GSFC.
SPACECRAFT: With 3 solar arrays deployed, 16 feet long, 28 feet
in diameter, weighing 5,000 lbs.
INSTRUMENTS: Differential Microwave Radiometer, Diffuse Infrared
Background Experiment and the Far Infrared Absolute
Spectrophotometer.
NOTE: a) Explorers are relatively small, free-flying scientific
spacecraft. b) COBE is the 65th Explorer mission. c) COBE has
the most sensitive detectors ever flown in a space mission. d)
COBE will use the 184th and last NASA-owned Delta.
THE COSMIC BACKGROUND EXPLORER MISSION
NASA's COBE mission will produce the most comprehensive
observations to date of the early universe.
The wavelength band to be studied by COBE includes the
cosmic background radiation or so-called "remnant radiation,"
believed to be the signature of the primeval cosmic explosion,
the "Big Bang." Current theory also holds that this band
contains radiation characteristic of the formation of the first
galaxies and stars. It also might provide evidence of other
exotic and energetic events occurring in the epochs between the
Big Bang and the formation of galaxies.
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COBE will carry three sophisticated, state-of-the-art
instruments to study the background radiation: the Differential
Microwave Radiometer (DMR), the Far Infrared Absolute
Spectrophotometer (FIRAS) and the Diffuse Infrared Background
Experiment (DIRBE).
Because the diffuse cosmic background radiation itself is
extremely faint, the COBE spacecraft and its three experiments
have been designed to allow observations at unprecedented
sensitivities. To that end, the spacecraft will carry the
instruments high above the Earth's atmosphere, protect them from
the light and heat of the sun and the Earth, supply them with
electrical power and commands and transmit the data they
accumulate to the ground.
Two of the three science instruments aboard the spacecraft,
FIRAS and DIRBE, reside in a Dewar -- a giant "thermos bottle" --
filled with liquid helium to provide a stable, low-temperature
environment within 2 degrees Celsius of absolute zero.
The COBE spacecraft weighs 5,000 pounds, is 16 feet long and
is 28 feet in diameter with its three solar panels deployed. The
upper half of the observatory is the instrument module,
consisting of the three instruments, the liquid helium Dewar and
a shield that is deployed when COBE reaches its orbit to protect
the instruments from radiation from the sun and the Earth.
Directly under the instrument module is the spacecraft
module which includes the mechanical support structure, the
attitude control system and the spacecraft and instrument
electronics. To allow its instruments to scan the sky, COBE will
spin on its axis at a rate of 0.8 rpm.
COBE's attitude control system will keep the spin axis
pointed almost directly away from the Earth and 94 degrees away
from the sun. The sophisticated attitude control system is
comprised of sun and Earth sensors, reaction wheels to provide
control torque from the Earth's magnetic field, a pair of large
rotating momentum wheels, electromagnets to transfer excess
angular momentum from the spacecraft to the Earth's magnetic
field and a complex set of control electronics.
Monitoring of the status of the spacecraft and operational
commands from the ground will go through the Tracking and Data
Relay Satellite System (TDRSS). The science data from the
instruments will be recorded on two onboard tape recorders and
played back to a ground receiving station at Wallops Island, Va.,
once a day. These data then will be forwarded to the science
team at the COBE Science Data Center, Goddard Space Flight
Center, Greenbelt, Md.
COBE will be launched by a two-stage Delta 5920 launch
vehicle from Space Launch Complex 2 West at the Western Space and
Missile Center, Vandenberg Air Force Base, Calif.
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COBE will be placed into a circular, near polar orbit 559
miles above the surface of the Earth. Because the plane of the
orbit will be inclined 99 degrees to the Equator, the orbital
plane will precess (turn) approximately 1 degree per day, thus
maintaining a constant orientation of the spacecraft and its
orbit with respect to the sun.
COBE's nominal mission lifetime is 1 year, allowing its
instruments to scan the entire sky at least twice. The actual
operational lifetime of the FIRAS and DIRBE instruments may be
somewhat longer and will be determined by the rate at which the
liquid helium boils away as heat flows into the dewar. It is
anticipated that the spacecraft will be operated for a second
year to allow the DMR to repeat its scans of the sky and achieve
even greater sensitivity.
The Delta 5920 is approximately 116 feet long and a maximum
of 8 feet in diameter. The first stage is a modified Thor
booster incorporating nine Castor 4A strap-on, solid-fuel rocket
motors. The first stage main engine is gimbal-mounted and uses
liquid oxygen and kerosene. The second stage has a gimbal-
mounted, pressure-fed restartable engine fueled with liquid
nitrogen tetroxide and aerozene 50.
Injection into the final mission orbit is accomplished at
completion of the second burn of the Delta second stage,
approximately 1 hour after lift-off. An 8-foot diameter fairing
protects the spacecraft from aerodynamic heating during the boost
and is jettisoned as soon as the vehicle leaves the sensible
atmosphere (shortly after second stage ignition). The fairing
separation initiates signals to the spacecraft to properly
configure the dewar vent valves in the observatory cryogenic
cooler.
MAJOR MISSION EVENTS
Once the final mission orbit is reached, the Delta reorients
to the required separation attitude and the Delta inertial
guidance computer sends a signal to the spacecraft signal
conditioning unit to start deployment. That sequence begins with
the RF/thermal shield deployment prior to spacecraft separation
from the second stage.
The COBE spacecraft is attached to the second stage Delta by
a 6019 payload attach fitting. Because the spacecraft requires a
near-zero tip-off rate at separation, a two-step release system,
consisting of three explosive nuts and a secondary latch system
will be used. At spacecraft separation, the Delta vehicle second
stage will use cold gas to back away from the COBE spacecraft.
The signal conditioning unit then initiates momentum wheel
spin-up, solar array deployment, transmitter turn-on and antenna
deployment. The dewar cover is deployed by ground command
approximately 4 days after separation.
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Three solar arrays provide 712 watts of power to the 5,000-
lb. spacecraft. During solar eclipses, batteries will be used to
support the power loads and will be recharged during the sunlit
portion of the orbit.
COBE MISSION PHASES
LAUNCH
Location: Space Launch Complex 2-West (SLC-2W), Western Space
and Missile Center, Vandenberg Air Force Base, Calif.
Time/Date: 6:24 a.m. (PST),Thurs., Nov. 17, 1989 with a launch
window of 30 minutes.
Launch vehicle: Delta expendable launch vehicle (ELV) model
5920.
EARLY ORBIT
Liftoff (LO) +57 min., 21 sec.: The COBE spacecraft will be
placed into its operational orbit of 559 miles by the second
stage of the Delta 5920.
LO+60 min., 28 sec.: The Delta ELV sends discrete signals to
start COBE's signal conditioning unit (SCU) -- a sophisticated
electronic timer -- as the Delta is reoriented to the attitude
required for the spacecraft to separate from the Delta.
LO+60 min., 29 sec.: The COBE SCU turns on the telemetry
transmitter.
LO+60 min., 30 sec.: The SCU initiates thermal/radio frequency
shield deployment.
LO+61 min., 30-45 sec.: The Delta second stage releases and
backs away from the COBE.
LO+61 min., 49 sec. to 62 min., 7 sec.: The SCU initiates
momentum wheel spin-up, solar array deployment and antenna
deployment.
OBSERVATORY/INSTRUMENT CHECK-OUT
There will be a 14-day checkout phase, followed by an
additional 16-day instrument characterization and calibration
phase. During this phase, transition to normal survey operations
will occur. After initial ground contact at separation,
communications between COBE and the Earth will be via the
Tracking and Data Relay Satellite System (TDRSS). During
observatory checkout, TDRSS support on an every orbit basis will
be requested, to be gradually reduced over a transition period.
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Once the observatory and instruments have been fully
checked, characterized and calibrated (approximately 30 days
after launch), an S-band, single access forward and return link
will be required for up to 2 hours per day. The 2-hour total
time will be scheduled over a 24-hour period on an every-other-
orbit basis (15 orbits per day).
The observatory engineering checkout extends from day 1
through day 3; the instrument engineering checkout goes from day
3 through day 14; and the instrument characterization and
calibration phase lasts from day 15 through day 30. In addition,
the day-to-day schedule will plan the following.
Day 1: RF acquisition, attitude stabilization and
spacecraft subsystem initialization.
Day 2: Differential Microwave Radiometer (DMR) instrument
power up and calibration, and spacecraft subsystem checkout
(including attitude maneuvers).
Day 3: Far Infrared Absolute Spectrophotometer (FIRAS) and
Diffuse Infrared Background Explorer (DIRBE) instrument power up.
Day 4: Attitude maneuver and dewar cover ejection (by
ground command from the Payload Operations Control Center at
GSFC).
Day 5: FIRAS instrument mechanism unlatching and additional
instrument engineering checkout.
Day 6: Spacecraft spin-up to operational spin rate (0.815
rpm).
Day 7: Attitude pitch maneuver checkout.
Day 8: Attitude roll maneuver checkout and additional
instrument checkout.
Day 9-11: Instrument checkout aided by attitude roll and
pitch maneuvers.
Day 12-14: Instrument checkout and survey mode parameters
adjustments.
During the characterization and calibration phase, the
instruments collect science data, are calibrated and are further
characterized as orbital and astronomical events occur.
By day 30 the instruments have been calibrated,
characterized and adjusted to proceed with normal survey
operations.
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MISSION OPERATIONS
The COBE flight operations team will control the COBE
spacecraft from the Payload Operations Control Center, Goddard
Space Flight Center, Greenbelt, Md., 24 hours a day, 7 days a
week following launch. During this time, the following data
events are programmed daily:
o Real-time contact by the flight operations team through
TDRSS every other COBE orbit. This contact will allow for up-
link of stored commands once a day; monitoring of subsystems for
health and safety; collection of tracking data and updating of
the COBE clock drift. This will maintain clock accuracy within
10 milliseconds of Universal Time.
o One onboard tape recorder playback transmitted each day to
Wallops Flight Facility (WFF), Va., for data relay to the COBE
Science Data Room at Goddard Space Flight Center. At the 655.4
kilobits per second data dump rate, 24 hours of recorded data can
be transmitted to Wallops in about 9 minutes.
There will be a minimum of three passes within range of the
WFF ground station each day. These passes will be a minimum of
10 minutes long and will occur at nearly the same time each
day. This regularity will be used to routinely schedule the data
acquisitions.
MISSION LIFETIME
COBE is planned to operate for 24 months following launch.
The nominal mission lifetime is 12 months. Minimum mission
lifetimes to complete an all-sky survey are 6 months for FIRAS
and DIRBE and 12 months for DMR. FIRAS and DIRBE are planned to
operate until the liquid cryogen is exhausted, while the short
wavelength dectors on DIRBE can operate somewhat longer, current
estimate is 14 months. DMR is planned to operate for the full 24
months.
COBE SCIENCE
Cosmology, the study of the earliest beginnings and the
largest structures in the universe, has been the subject of
speculation for thousands of years. Early in the twentieth
century a remarkable combination of technology and new physical
theory led scientists to put forward the Big Bang theory of the
origin and evolution of the universe.
Some 25 years ago that theory received its strongest
observational support to date with the discovery of the cosmic
background radiation. COBE's mission is to investigate the
cosmic background radiation in sufficient detail to uncover the
nature of the fundamental processes which have shaped the
universe as seen today.
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The first step in the evolution of modern cosmology was
development of the general theory of relativity by Albert
Einstein. Subsequently, in 1917, Willem de Sitter applied
Einstein's equations to the universe as a whole with the
startling result that the universe was not required to be static,
but instead that the universe was likely in a state of expansion
or collapse.
In the 1920's, Edwin Hubble provided the first observational
confirmation of this picture through his pioneering work on faint
nebulae. Hubble proved that many of the nebulae were galaxies,
huge collections of stars similar to the Milky Way galaxy, and
also showed that these distant galaxies were receding from the
Earth. The nature of the recession was that the farther a galaxy
lies from the Earth, the higher is its recessional velocity.
Since the universe was observed to be in a state of
expansion, it was natural to deduce that the universe was smaller
in the past. In fact, the evidence has led to the astounding
conclusion that the galaxies were crowded together into a small,
extremely dense volume, whose explosive expansion began some 15
billion years ago and has been dubbed The Big Bang.
In the 1940's, George Gamow, Ralph Alpher and Robert Herman
theorized that the early universe was not only extraordinarily
dense, but also was extremely hot. This led them to suggest that
the nuclear reactions taking place in such a hot, dense
environment accounted for the abundances of hydrogen and helium
seen in the universe today, together with a small fraction of
heavier elements.
Alpher and Herman showed that another consequence of the hot
Big Bang theory is that the universe should be filled with the
radiation emitted by the hot matter. That is, if scientists can
look out in space, back in time to that distant early epoch, then
they should see the glow of the initial fireball.
In 1964, Arno Penzias and Robert Wilson of the Bell
Telephone Laboratories, using a new and very sensitive microwave
receiver and antenna, found an unexplained source of noise or
static which came to their antenna equally from all parts of the
sky. Their discovery sparked a number of independent
observations and theoretical analyses to characterize the
background radiation which they had found. Today the evidence is
overwhelming that Penzias and Wilson provided the first glimpse
back to the primeval fireball which emerged from the Big Bang.
Since the initial measurements, study of the cosmic
background radiation has been the subject of hundreds of
experiments throughout the world, using ground-based, balloon-
and rocket-borne telescopes. Because the radiation is faint and
easily distorted by the Earth's atmosphere, the investigation of
the relic radiation from such sites is limited confirmation of
the general shape of the spectrum and its overall uniformity.
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However, hidden in the details of the spectral shape and
spatial distribution of the background radiation are essential
clues to the nature of the fundamental processes which shaped the
early universe and produced the universe as it appears today.
COBE's instruments are designed to make full use of the
vantage point of space to examine the cosmic background radiation
with unprecedented sensitivity across a broad range of
wavelengths. COBE will scan the sky to look for spatial non-
uniformities at a sensitivity level many times what has been
possible to date. It will search the spectrum of the relic
radiation for deviations from the simplest predicted shape, and
it will carefully dissect the radiation at shorter wavelengths to
look for evidence of the first stars and galaxies.
COBE's search for variations in the brightness of the cosmic
background radiation across the sky is designed to probe the
mystery surrounding the formation of galaxies and clusters of
galaxies in the universe.
To the present level of measurement accuracy, the background
radiation appears smooth, characteristic of an early universe
with an extraordinary degree of uniformity in its density and
temperature. Yet examination of the present day universe reveals
a great deal of non-uniformity: stars are collected into
galaxies, galaxies are gathered into clusters and even these
gigantic clusters of galaxies may themselves be clustered into
even more immense structures. Enormous voids, regions of space
with almost no galaxies, exist between the clusters.
Theory indicates that the seeds of this universal structure
must have been present in the early universe and the imprint of
these seeds must be found as brightness variations in the relic
radiation. COBE has the sensitivity to search for the smallest
conceivable brightness differences which are consistent with
modern theory.
COBE's investigation of the detailed spectral shape of the
remnant radiation is motivated by the suggestion that enormously
powerful and energetic processes may have taken place in the
interval of time after the Big Bang and before the formation of
galaxies. For example, if massive black holes existed and
swallowed large quantities of matter, the resulting energy
release would have been sufficient to distort the spectrum of the
fireball radiation to a degree measureable by COBE.
Exotic processes, some of which have been suggested on the
basis of modern theories of high energy particle physics, also
have the potential of releasing immense quantities of radiative
energy into the early universe and distorting the spectrum of the
cosmic background radiation. COBE will characterize the shape of
the spectrum of the relic radiation at such a level of precision
as to allow detailed study of the nature of these postulated
energetic events.
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COBE's measurement of the diffuse background at wavelengths
shorter than those characteristic of the remnant radiation from
the initial fireball is intended to look for the radiation from
the earliest stages of galaxy and star formation. This faint
signature must be detected against the foreground radiation from
the solar system, the Milky Way galaxy and other nearby
galaxies.
Detection of this signature requires the observational
sensitivity and stability that has been carefully engineered into
the COBE system. Study of the radiation from the protogalaxies
and protostars will aid scientists to probe into the nature of
galaxy and star formation.
COBE SCIENCE QUESTIONS
COBE will produce a complete map of the sky at each of 100
different wavelengths to answer three primary questions:
1. What is the variation in brightness of the cosmic background
radiation across the sky?
2. Does the cosmic background radiation have the spectrum
predicted by contemporary cosmological theory?
3. Can we detect the accumulated light from the first stars and
galaxies?
COBE INSTRUMENTS
COBE's three instruments -- the Differential Microwave
Radiometer, the Far Infrared Absolute Spectrophotometer and the
Diffuse Infrared Background Experiment -- will be able to observe
the entire sky at least twice during the nominal mission lifetime
of one year.
Differential Microwave Radiometer (DMR)
This instrument will search for minute differences in the
brightness of background radiation between different parts of the
sky. The DMR is capable of detecting brightness variations that
are many times fainter than limits set by current observations
and may reveal previously undiscovered physical phenomena.
To distinguish the radiation of our galaxy from the true
cosmic background radiation, the DMR will map the sky at three
wavelengths: 3.3, 5.7, and 9.6 millimeters. To accomplish this,
it will have six receivers, two for each wavelength, mounted so
that neither the sun nor Earth will shine directly on them. Each
receiver will sensitively measure the difference in microwave
power entering two antennae looking at different parts of the
sky.
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Far Infrared Absolute Spectrophotometer (FIRAS)
This instrument will survey the sky to search for deviations
in the spectrum of the cosmic background radiation from spectrum
predicted on the basis of the simple Big Bang model. FIRAS, as
well as the DMR, can resolve the sky into 1,000 separate picture
elements and will produce a spectrum for each element.
Scientists will be able to compare the spectrum produced by COBE
against predicted spectra with at least 100 times better accuracy
than ever before.
FIRAS looks out along the spin axis of the spacecraft. It
does not scan the sky as rapidly as the other two instruments
onboard COBE but will nevertheless scan the entire sky twice
during the nominal mission.
FIRAS will detect radiation by using a trumpet-shaped cone
antenna. Four detectors, each a tiny silicon resistance
thermometer glued to a piece of blackened diamond only one
thousandth of an inch thick, are used to detect the radiation
collected by the cone antenna. The diamond absorbs the
infinitesimal heat from the cosmic background radiation and
conducts this heat to the thermometer where the temperature is
measured electrically.
The data collected by FIRAS will be carefully analyzed to
determine any deviations from the theoretically predicted
spectrum. Even the slightest discrepancy between measurement and
theory will have great significance for cosmology.
Diffuse Infrared Background Experiment (DIRBE)
This instrument will search for the light from the earliest
stars and galaxies, luminous energy that is thought to have been
produced some 200 million years after the Big Bang. DIRBE
operates in the infrared part of the spectrum, covering a
wavelength range of 1 to 300 micrometers in 10 discrete bands.
It is an off-axis Gregorian telescope with baffles, stops,
and super-polished mirrors, which will minimize response to
unwanted "stray" light coming from outside its field-of-view.
This design allows DIRBE to achieve the measurement accuracy
necessary to distinguish between nearby objects and those at
cosmological distances.
DIRBE will not focus on a single object, but will instead
measure the collective glow of millions of objects. It will
measure emission from warm dust in the Solar System and the Milky
Way galaxy so precisely that scientists should be able to detect
the uniform glow from the first stars and galaxies even if it is
only 1 percent as bright as our local celestial environment.
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Analysis of DIRBE data is complicated by the many kinds of
known celestial objects as well as by the motion of the Earth
within the interplanetary dust cloud. When analysis is complete,
a faint and uniform residual signal may remain after all known
sources have been understood and subtracted. The small residue
would be the long-sought light of first, primordial objects.
DELTA/COBE LAUNCH VEHICLE PREPARATIONS
The COBE