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Microwave Anisotropy Probe (MAP)
Unveiling the Early Universe

The Scientific Mission

The Microwave Anisotropy Probe (MAP) mission will determine conditions in the early universe by making a full sky map of the cosmic microwave background temperature.

Analysis of the new information revealed by the MAP observations will help cosmologists to answer several key questions such as: How old is the universe? How did it evolve? How and when did galaxies and clusters of galaxies form?

According to the Big Bang theory, the universe began about 14 billion years ago as an unimaginably hot and dense soup of exotic particles, and has since continuously expanded and cooled. For approximately the first 400,000 years after the Big Bang, the universe was a seething cauldron of matter (electrons, protons, neutrons, and a very small percentage of heavier atomic nuclei), and light (photons). Since photons scatter or bounce off electrons, the universe was opaque. As space expanded, the universe cooled and the electrons combined with the protons (and other atomic nuclei) to create the first atoms, primarily hydrogen. The first light of creation could finally be freed from its pinball-like interactions with the electrons. The universe became transparent.

Since this time, this light has effectively moved through the cosmos unimpeded and brings to us an image of the infant universe. Cosmologists studying the first light from the Big Bang, called the "cosmic microwave background" (CMB) radiation, look back through time and space to about 400,000 years after the Big Bang, when the universe was opaque. A map of the CMB radiation provides the most distant and oldest image of the universe.

Light travels quickly--approximately 186,000 miles per second--over cosmic distances, however, light takes a long time to reach us. As a result, astronomers use observations of distant objects to look back through time. Light takes about eight minutes to travel the 93 million miles (150 million kilometers) from the Sun to the Earth, so we see the Sun as it existed eight minutes ago. Similarly, we see the closest star as it appeared about 4 years ago, because that's how long its light takes to traverse the roughly 24 trillion miles between the star and the Earth. If we choose the right type of light, we can see all the way back to about 400,000 years after the Big Bang.

As the universe expanded, the CMB radiation was stretched to longer wavelengths of light, so that it is now in the low-energy microwave range. The CMB was discovered in 1965 by Arno Penzias and Robert Wilson at the Bell Telephone Laboratories in Murray Hill, New Jersey (who later received the Nobel Prize in physics for this finding). The properties of the radiation contain a wealth of information about physical conditions in the early universe. A great deal of effort has gone into measuring those properties since the CMB discovery, including 35 years of study by Dr. David T. Wilkinson of Princeton University, a member of the MAP Science Team. The radiation (and by extension, the early universe) is remarkably featureless; it has virtually the same temperature in all directions in the sky.

In 1992, NASA's Cosmic Background Explorer (COBE) satellite detected tiny fluctuations, or "anisotropy," in the cosmic microwave background. It found that one part of the sky has a temperature of 2.7251 Kelvins, while another part of the sky has a temperature of 2.7250 Kelvins. (Kelvin, is a unit of temperature: 0 K is the complete absence of heat, called "absolute zero," and 273 Kelvins is the same as 0 degrees Celsius). These fluctuations are related to fluctuations in the density of matter in the early universe and thus carry information about the initial conditions for the formation of cosmic structures such as galaxies, clusters, and voids.

If viewed from afar, we would see the Earth as a uniform sphere. When viewed with improved resolution, we would see blurry images of the continents and oceans. With yet better resolution, the rich features of the Earth become visible: the deserts, mountains and forests. The first observations of the microwave background revealed only a uniform sky. The smallest features that COBE could distinguish were about 7 degrees wide on the sky, so COBE made the equivalent of the first unresolved detection of continents and oceans. Over the past few years, balloon-borne and ground-based experiments have made high-resolution images of small portions of the sky. Thus we know that the CMB is "anisotropic," i.e. it contains structure. By making a high-resolution image of the entire sky, MAP will significantly increase our knowledge of the origin, evolution and content of the universe.

The MAP instrument is comprised of:

• Two primary reflectors- 4.6 x 5.3 feet (1.4 x 1.6) meter - radiation hits here first and is reflected toward the secondary reflectors.

• Two secondary reflectors- reflect radiation toward receivers.

• Differential microwave receivers- receive radiation simultaneously from a dual set of optics at five different frequencies. The receivers retain polarization information.

• Diffraction shielding- shields receiver from stray microwave emission.

• Thermally isolating cylinder- thermally isolates the warm spacecraft from the cold instrument.
• Passive radiators- passively cool the instrument to 95 Kelvin.
• Instrument electronics- provides regulated power to the instrument, controls the instrument, and reports the science information, but is thermally shielded from the instrument.

The spacecraft dimensions are 150 inches (3.8 meters) high by 198 inches (5 meters) wide. MAP weighs 1,850 pounds (840 kilograms). The MAP mission lifetime is 27 months; three months of transit to L2 and 24 months of observing time. The MAP spacecraft is comprised of:

• Solar panels- 6 panels supply 419 Watts of power.
• Battery- 23 Amp-hour nickel hydrogen pressure vessel stores energy.
• Power supply electronics- regulates the use of the battery and solar panels to supply power the all of the spacecraft system.
• Sun shield- protects the instrument from microwave radiation from the Sun, Earth, and Moon.
• Transponders- Prime and redundant transponders send and receive telemetry at data rates ranging from 2 to 666 kilobits per second.
• Two omni antennas- transmit and receive telemetry to and from Earth.

• Two medium gain antennas- transmit data to Earth during normal operations at L2 at a rate of 666 kilobits per second.

• Two star trackers-determine the direction of instrument observation.

• Gyros- determine the spacecraft's rate of angular motion.

• Three reaction wheels- used to maneuver the spacecraft and control MAP's nominal operation spin (0.464 rpm) and precession (1 revolution per hour).

• Thrusters- eight one pound (4.45 Newton) thrusters are used to place and maintain the spacecraft in its final orbit.

• Coarse Sun sensors- six sensors are used to determine the spacecraft attitude.
• Digital Sun sensors- two sensors are used to lock in with precision the spacecraft attitude. Helps maintain the spacecraft in the right orientation
• Attitude control electronics- control and read information from the attitude components.

• Propellant- hydrazine fuel.

• Propulsion tank- holds 72 kg of propellant.
• Command and data handling system- the flight computer controls the hardware and packages the data for transmission to the ground.

MAP is scheduled to launch June 30, 2001 aboard a Boeing Delta II rocket from Cape Canaveral, FL. The launch window is June 30 – July 5, 2001, with a launch opportunity ranging from 5-20 minutes, depending on the date. Weights on strings are unwound and released (a "yo-yo" de-spin mechanism) to slow the spin rate of the rapidly spinning rocket before MAP separates from the rocket. After separation, a restraining cable is cut to deploy the solar arrays, which unfold outwards like a huge vegetable steamer to form a single flat Sun shield across the bottom of the spacecraft.

While two other spacecraft, GEOTAIL and ISEE-3, have flown through the vicinity of L2, MAP is the first spacecraft to use an orbit about the L2 point as its observing station. MAP's own propulsion system, and a lunar gravity-assist, will carry the spacecraft to its final destination in orbit around the second Lagrange (L2) point of the Sun-Earth system. L2 is four times farther from the Earth than the Moon in the direction opposite the Sun, or about one million miles (1.5 million kilometers) from Earth. The Lagrange points in the Sun-Earth system are very stable and require little fuel to maintain position. Lagrange points were named after Joseph Louis Lagrange, a French mathematician and astronomer, who made a number of contributions to the study of celestial mechanics.

MAP will perform a series of "phasing loops" in the Earth-Moon system to place it in the proper position for the lunar flyby, which will occur three to six weeks after launch. MAP will reach L2 approximately three months after launch. This trajectory is designed to minimize the use of fuel. From L2, MAP will have an unobstructed view of the sky, and will be free from near- Earth disturbances such as magnetic fields and microwave emission.

MAP does not measure the absolute sky temperature, but rather the difference in temperature between two points in the sky approximately 140 degrees apart. MAP spins every two minutes and its spin axis maintains a fixed angle of 22.5 degrees to the Sun-Earth line. The spin axis moves around the Sun-Earth line, allowing the instrument to view 30 percent of the sky every hour. In addition, MAP rotates annually with the Earth around the Sun so MAP can see all points in the sky from many different viewpoints. It will take six months at L2 for MAP to see the entire sky. To determine the validity of the signals received, MAP will cover the sky multiple times and at multiple frequencies.

During the phasing loops and until MAP is past the Moon, MAP communicates with Earth with the use of its transponders and two omni antennas located at the top and bottom of the spacecraft. On the way to L2, MAP will switch over to the Medium Gain Antennas located at the bottom of the spacecraft. Data is transmitted to Earth once per day from L2.

To realize the full value of the MAP measurements, sources of error must be controlled to an extraordinary level. This was the most important factor driving the MAP design, and led to the following design choices:

• Differential: MAP measures temperature differences on the sky using symmetric microwave receivers coupled to back-to-back telescopes. By measuring temperature differences, rather than absolute temperatures, most spurious signals will cancel. This is analogous to measuring the relative height of bumps on a high plateau rather than each bump's elevation above sea level.

• Sky scan pattern: MAP spins and precesses like a top. This observing pattern covers a large fraction of the sky (approximately 30 percent) during each one hour precession.

• Multifrequency: Five frequency bands from 22 GHz to 90 GHz will allow emission from the galaxy and environmental disturbances to be modeled and removed based on their frequency dependence.

• Stability: The L2 Lagrange point offers an exceptionally stable environment and an unobstructed view of deep space, with the Sun, Earth, and Moon always behind MAP's protective shield. MAP's great distance from Earth protects it from near-Earth emission and other disturbances. At L2, MAP maintains a fixed orientation with respect to the Sun for thermal and power stability.

Goddard built the MAP spacecraft "in-house" in partnership with Princeton University, which built key components of the instrument flight hardware. Goddard built the spacecraft and instrument support/thermal structure, attitude control electronics, instrument electronics, power systems electronics, flight software, and command and data handling. Princeton University built the radiometers and was a close partner with Goddard in the design, specification, and validation of many flight systems, including the flight optical system.

The National Radio Astronomy Observatory in Charlottesville, VA, provided the critical low-noise microwave amplifiers used in the MAP radiometers. A list of companies involved in supplying components of the MAP spacecraft is available at the following website: http://map.gsfc.nasa.gov/html/institutions.html

The MAP science team is comprised of scientists from Goddard, Princeton University, University of California at Los Angeles, the University of Chicago, Brown University, and the University of British Columbia, Canada. All institutions contribute to the scientific conduct of the mission and the creation of its data products.

The purpose of the Explorers Program is to allow for frequent, high quality space science investigations. MAP is the second satellite in the series of Medium Class Explorer (MIDEX) missions. The Imager for Magnetopause-to-Aurora Global Exploration mission (IMAGE), launched March 2000, was the first. The MIDEX Program is managed by Goddard for NASA’s Office of Space Science, Washington, DC.

MAP is a Principal Investigator-class mission. The MAP management team is as follows:

Dr. Charles L. Bennett, Principal Investigator, Goddard Space Flight Center

Elizabeth A. Citrin, Project Manager, Goddard Space Flight Center

Clifton E. Jackson Jr., Mission Systems Engineer, Goddard Space Flight Center