FACT SHEET: THE MAGELLAN PROJECT

A mission to send a spacecraft to map the surface of Venus with imaging radar
is underway at NASA's Jet Propulsion Laboratory. The project is called
Magellan.

The spacecraft's basic science instrument is its synthetic aperture radar, or
SAR, which can look through the thick clouds that perpetually shield the
surface of Venus. Magellan will orbit the planet for 243 Earth days, one Venus
day, on its primary mission. During that time it will map from 70 to 90 percent
of the Venus surface.

Magellan was launched aboard the space shuttle Atlantis on May 4, 1989.
Atlantis carried the spacecraft into low Earth orbit and an inertial upper
stage, or IUS, sent Magellan on its way to Venus. The cruise will take 15
months because the Magellan spacecraft will make more than one and one-half
revolutions around the Sun before it arrives at Venus.

Once at Venus, Magellan will be placed in an elliptical orbit that will take it
as close as 250 kilometers (155 miles) and as far as 8,029 kilometers (4,889
MORE !s
miles) from the planet. The spacecraft will complete one orbit every three
hours and nine minutes.

At the part of its orbit close to the planet, the SAR will image a swath
between 17 and 28 kilometers (10 and 17 miles) beginning at or near the north
pole and continuing into the southern hemisphere.

Subsequent swaths will slightly overlap. The swaths will be put together in
mosaics which will be made into maps of the planet.

As the spacecraft moves away into the part of its orbit farthest from the
planet, it will turn its high gain antenna toward Earth and send back the data
it has collected during the imaging. It also will locate certain stars in the
sky to make sure its attitude is correct and then continue its orbit for
another imaging swath.

During its primary mission it will map most of the planet and if a further
mission cycle is approved, the spacecraft will map areas it will have missed
and perform gravity experiments.

The surface of Venus is obscured by thick clouds of carbon dioxide. It is not
visible to optical instruments. For centuries, astronomers believed Venus was a
twin planet of Earth and described an imaginary world of swamps, forests and
strange creatures beneath the clouds.

Then spacecraft began exploring the planets. The first U.S. mission to another
planet was Mariner 2, which passed Venus in December 1962. Mariner 2 and others
since then have found that the temperature of Venus is about 470 degrees
Celsius (about 900 degrees Fahrenheit) and the atmospheric pressure at the
surface is 90 times greater than Earth's. Spacecraft launched by the Soviet
Union have landed on the surface and have made closeup pictures of the rocky
terrain.

Other spacecraft, including NASA's Pioneer Venus orbiter and the Soviet Union's
Venera 15 and 16, have used radar to make low resolution maps of Venus.

While different in many ways, Venus and Earth do share some similarities. They
are about the same size. (Earth is 12,756 kilometers or 7,926 miles in
diameter. Venus is 12,103 kilometers or 7,520 miles). Their densities are
similar. (Earth is 5.52 grams per cubic centimeter. Venus is 5.24 grams per
cc). Earth is 149,600,100 kilometers or 92,957,200 miles from the Sun. Venus is
108,150,900 kilometers or 67,200,000 miles from the Sun.

Despite the similarities, the differences are striking. In addition to the
searing heat and crushing pressure, Venus has an atmosphere almost devoid of
water. Its atmosphere is 97 percent carbon dioxide and its upper clouds contain
sulfuric acid. Venus has no moons and no magnetic field has been detected.

It rotates on its axis in a retrograde direction, that is opposite that of
Earth and most of the other planets. It rotates very slowly, once in 243 Earth
days.

But scientists know that Earth and Venus accreted from the solar nebula about
the same time, 4.6 billion years ago, and there are some indications they may
have been more similar in times past. Both planets are volcanic. Earth is still
very much so, and it is not known for sure if Venus still has active volcanoes.
But radar scans from Earth and altimeter measurements by spacecraft have turned
up a great mountain which may be the largest volcano in the solar system.

Also, radar scans show large land masses on Venus which rise above the
lowlands. One such mass is called Ishtar Terra, which is about the size of
Australia. Another large high altitude area is called Aphrodite.

Venus is the victim of a runaway greenhouse effect. The heat from the Sun is
captured in the carbon dioxide atmosphere and is not radiated back into space.
On Earth, much of the heat from the Sun is bounced back into space through the
atmosphere of nitrogen and oxygen because the atmosphere reaches a state of
temperature equilibrium when a certain amount of heat is absorbed. Venus's
atmosphere, however, acts like a giant greenhouse.

Scientists are looking to Magellan to answer some questions of long standing
about our sister planet. They want to know what caused the greenhouse effect on
Venus and if it was ever like Earth. On Earth, continents ride on great plates
moving above the mantle. That movement is called tectonism and one of the
unanswered questions is: does Venus have tectonic plates?

Scientists know there is no water on Venus now, but it is not known if water
ever existed there. Magellan will search for signs that Venus once may have had
oceans similar to, but smaller than, Earth's. If Venus did once have seas,
their ancient shorelines would probably still be detectable in the radar
images.

Synthetic aperture radar was first used by NASA on the Seasat oceanographic
satellite in 1978. Seasat was a JPL project. Other JPL SARs flew as SIR-A and
SIR-B, the Shuttle Imaging Radars, beginning on the second space shuttle flight
in November 1981. Aerial tests of the system over the cloud-covered rain
forests of Guatemala produced images that revealed agricultural canals dug by
the Mayan civilization.

SAR sends out millions of radio energy pulses each second at an angle across
its target swath. The signals bounce off the target and are detected by the
spacecraft's high gain antenna. Part of the Magellan SAR is its altimeter which
sends radio signals straight down and receives them back to determine the
altitude of features. A separate, smaller, antenna attached to the high-gain
antenna is used for this function.

The SAR instrument measures the time it takes the signal to make the round trip
between the antenna and the ground. It also measures the Doppler shift, a
measurement of relative motion that is akin to a change in pitch, as the radar
and the target pass each other. With that data, it forms a two-dimensional
image of the surface characteristics.

Between the pulses it sends out, the Magellan antenna is passive and in that
mode it reads the brightness, or thermal wavelengths, emanating from the
surface. From that study, called radiometry, scientists can infer what the
material is made of.

The gravity measurements are made by radio. Radio telescopes on Earth will
measure the Doppler changes that occur when the spacecraft speeds up or slows
down as it passes over certain features. That study will help determine the
composition of the interior of Venus.

To help meet the mission's low cost goals, the spacecraft is based on existing
hardware designs and incorporates a considerable amount of spare hardware from
earlier projects.

Its high-gain antenna is a 3.7 meter Voyager antenna and will be used for both
radar mapping and data communications back to Earth. The primary structure and
the small thrusters also are Voyager spares. Its command data system, attitude
control computer and power-distribution units are spare parts from the Galileo
project. Its medium gain antenna is from the Mariner 9 project.

Martin Marietta Corp. is the prime contractor for the spacecraft. Hughes
Aircraft Co. is the prime contractor for the radar system.

The Magellan project is managed by JPL for NASA's Office of Space Science and
Applications. Anthony J. Spear is the JPL project manager and Dr. R. Stephen
Saunders is the project scientist.

4-11-90 JJD