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The Magellan space probe to Venus
STS-30/MAGELLAN MISSION OVERVIEW
KSC Release No. 24-89
U.S. planetary exploration resumes with the STS-30 Space Shuttle
mission, which has as its primary objective the deployment of the
Magellan spacecraft on its quest to map the surface of Venus.
The flight will be the fourth for the orbiter Atlantis and the
29th Space Shuttle mission (STS-28 is being launched out of
sequence). Atlantis will be launched from Pad 39B at Kennedy Space
Center into a 184-statute mile circular orbit inclined 28.85 degrees
to the equator.
Commander of the five-member crew is David M. Walker (Capt.,
USN), who piloted STS 51-A in November 1984. Pilot Ronald J. Grabe
(Col., USAF) had the same role on STS 51-J in October 1985. The three
mission specialists are Mary L. Cleave (Ph.D.), Norman E. Thagard,
(M.D.), and Mark C. Lee (Maj., USAF). Cleave was a mission
specialist on STS 61-B, Thagard on STS-7 and STS 51-B; STS-30 is
Lee's first flight.
Magellan marks the first U.S. planetary mission since Pioneer
Venus 12 in 1978. It also kicks off a core program of solar system
exploration involving NASA and organizations from the United States
and the international community. The 31-day launch period, during
which Earth and Venus are properly aligned, extends from April 28 to
May 28.
The Magellan spacecraft arrived at Kennedy Space Center in
October 1988 for pre-launch processing. In February 1989, it was
mated with the Inertial Upper Stage (IUS) booster which will place it
on its trajectory toward Venus.
Three middeck experiments will be conducted during STS-30. All
have flown before. The Fluids Experiment Apparatus, a joint endeavor
agreement between Rockwell International and NASA, is a modular
zero-gravity biology, chemistry and physics laboratory. The Mesoscale
Lightning Experiment is a NASA-sponsored effort involving several
universities. Its objective is to study the visual characteristics of
large scale lightning in the upper atmosphere. Atlantis will act as a
calibration target for a third experiment, involving the Air Force
Maui Optical System Facility in Hawaii.
After a four-day flight, Atlantis and her crew are scheduled to
land at Edwards Air Force Base, Calif. Atlantis' next mission, STS-34
in October, also involves deployment of a planetary explorer.
Galileo, a spacecraft and atmospheric probe, will study Jupiter and
its satellites.
MAGELLAN
Since 1961, the United States and the Soviet Union have sent
more than twenty missions to Venus, making it the most visited of
Earth's fellow planets. But the Magellan Venus Radar Mapper will
yield the most detailed and comprehensive picture to date of the
veiled planet. Magellan will map up to 90% of the planetary surface
with a resolution as sharp as 130 yards. By contrast, the Pioneer
Venus spacecraft launched in 1978 was able to map about the same
amount of surface, but with a resolution of only 60 miles. The
Soviets' Venera spacecraft attained better resolution--about 1.2
miles--but mapped only about 30% of the surface near Venus' north
pole.
The Magellan mission's three specific scientific objectives are
to improve knowledge of: Venus' structure and geologic history; its
geophysics, such as density distribution; and its small-scale surface
physics, such as surface temperature and roughness.
Spacecraft, Instrumentation:
In keeping with the core program goal to keep costs down, much
of Magellan's design and flight hardware came from earlier programs,
primarily Voyager and Galileo. For example, its Synthetic Aperture
Radar (SAR) antenna is a flight-qualified spare from a Voyager
spacecraft.
Because Venus is obscured by a thick cloud cover, radar is the
mapping instrument of choice, rather than an imaging instrument which
relies on optics. Unlike conventional radar, where resolution is
linked to antenna size, SAR uses spacecraft motion to simulate a
large antenna. Computers at the Jet Propulsion Laboratory in
Pasadena, Calif., will take into account this spacecraft motion and
synthesize a much larger antenna than the 12-foot one Magellan
actually has.
Besides radar imaging, the SAR will also collect radiometry data
(measure of surface radiation from which material content can be
inferred) and its antenna will serve as the telecommunications link
with Earth.
Other features of the three axis-stabilized spacecraft are a
low-gain omni-antenna to receive transmissions from Earth and a
horn-shaped altimetry antenna. Its two movable solar panels are
capable of producing up to 1,545 watts of power in Venus orbit; two
nickel cadmium batteries will supply alternative power.
Together with its Star-48 solid rocket motor, Magellan weighs
7,603 pounds. The Jet Propulsion Laboratory, Pasadena, Calif.,
manages the Magellan project for NASA. Prime contractor for the
spacecraft is Martin Marietta Astronautics Group, Denver, Colo., and
for the SAR, Hughes Aircraft Co., Los Angeles, Calif.
Deployment, Journey to Venus:
Magellan will arrive at Venus in August 1990, after a 15-month
journey through space. The trip will include one and a half
revolutions around the sun and two planned mid-course corrections.
The spacecraft and IUS will be deployed from the orbiter payload
bay nominally about six hours into the mission. Magellan's solar
panels will then be deployed. Less than an hour later, the IUS first
stage motor will fire and then separate. Only minutes later, the IUS
second stage motor fires, placing Magellan on its interplanetary
trajectory toward Venus, and then separates. This rapid sequential
firing of the IUS motors means the spacecraft is not parked in an
intermediate orbit, as occurs when the IUS is used to place a
spacecraft in geosynchronous Earth orbit. In the latter instance, an
interval of several hours separates the first and second stage motor
firings.
Once Magellan arrives at Venus, the Star-48 motor will fire to
place the spacecraft in an elliptical orbit around the planet.
Magellan will be in a fixed polar orbit and will pass nearly, but not
quite, over the planet's north and south poles, coming as close as
155 miles when near the equator and moving as far away as 4,977
miles.
After an approximately two-week checkout process is completed,
the spacecraft will begin its mapping operation. Since it takes Venus
243 Earth days to complete a single rotation, it will take the same
amount of time for nearly every point on the planet to pass under
Magellan's radar; hence, the mapping mission is meticulously timed to
last exactly 243 days.
Mission:
Magellan's highly elliptical orbit will be divided into two
phases, mapping and playback. When closer to the planet--about 35
minutes of each three and a quarter hour orbit--Magellan's high gain
antenna will be pointed toward the planetary surface for mapping, and
the data saved on an onboard tape recorder.
As the spacecraft swings away from Venus, the same antenna will
be turned toward the Earth. Magellan's tape recorder will play back
to Deep Space Network (DSN) ground station antennas the raw data it
has just collected. Monitor, command and control of the spacecraft
will be from the Jet Propulsion Laboratory, which also manages the
DSN for NASA. Magellan's Star Scanner, a navigation instrument, will
also be used in conjunction with reference stars to reset the
spacecraft attitude control system's pointing knowledge during the
playback phase.
During the mapping phase, the SAR will image a swath of the
Venusian surface between 10 and 17 miles wide and 9,942 miles long,
starting at or near the north pole and continuing into the southern
hemisphere. Altogether, 1,852 of these immense imaging swaths will be
produced. They eventually will be compressed into mosaics which will
then be made into maps of the planet.
Magellan's altimeter will measure with up to better than 50-yard
accuracy the height of surface features. When combined with radar
imaging, researchers will be able to catalogue the volcanic,
tectonic, cratering and erosional processes shaping Venus.
Gravity data will be obtained through radio measurements of the
minute deviations in the spacecraft's orbital path, caused by
variations in the planet's density.
This primary mission is scheduled for completion in 1991.
Should there be adequate fuel remaining, Magellan will map areas
previously missed and perform gravity experiments.
ATLANTIS
The Shuttle orbiter Atlantis (OV-104) joined NASA's fleet of
reusable winged spaceships in April 1985, when it was delivered to
Kennedy Space Center for flight processing. It was ordered under a
January 1979 contract with Rockwell International.
On Oct. 3, 1985, Atlantis roared off Pad 39A on its maiden
flight, STS 51-J, the second Shuttle mission totally dedicated to the
Department of Defense. On Atlantis' second mission, STS 61-B in
November 1985, three communications satellites were deployed. The
orbiter flew its second classified Department of Defense mission,
STS-27, in December 1988.
Like its two sister orbiters, Discovery and Columbia, Atlantis
is named for a famous sailing ship. The Woods Hole Oceanographic
Institute, a research facility, operated a two-masted ketch named
Atlantis that traversed more than half a million miles of the Earth's
surface between 1930 and 1966.
As part of the Shuttle return-to-flight effort, Atlantis
underwent more than 200 modifications. These included various vehicle
upgrades and hardware changes to enhance performance and provide
added safety margins. Post-STS-27 modifications included extensive
repairs to the orbiter's outer Thermal Protection System tiles.
The delta-winged spaceship looks a lot like an airplane and is
about the size of a DC-9. It is launched into space like a
conventional rocket, bolted to an external propellant tank and two
solid rocket boosters.
Kennedy Space Center engineers and technicians prepare the
orbiter for flight by servicing its systems and loading cargo into
its bus-sized payload bay. They attach the orbiter to the tank and
boosters on a mobile launcher platform and the entire vehicle is
transported out to the launch pad.
After liftoff, the boosters burn for a little more than two
minutes. They are jettisoned, and parachutes slow their descent to
the Atlantic Ocean, where recovery ships are waiting to retrieve the
spent casings and return them to port. The orbiter's three main
engines burn for about six more minutes following booster separation.
After the engines shut down, the external tank is jettisoned to break
up upon reentry into the Earth's atmosphere.
The orbiter then carries out its mission in space and returns to
Earth like a glider. Planned end-of-mission landing site is Edwards
Air Force Base, Calif. Atlantis will then be towed to NASA's Dryden
Flight Research Facility and prepared by the Kennedy Space Center
recovery team for the ferry flight back to KSC and turnaround for its
next mission.
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