Magellan (spacecraft)

The Magellan spacecraft was a robotic space probe launched by NASA on May 4, 1989. Its mission objectives were to map the surface of Venus by using synthetic-aperture radar and to measure the planetary gravitational field.

The Magellan probe was the first interplanetary mission to be launched from the Space Shuttle, the first one to use the Inertial Upper Stage booster, and the first spacecraft to test aerobraking as a method for circularizing its orbit. Magellan was the fifth successful NASA mission to Venus, and it ended an eleven-year gap in U.S. interplanetary probe launches.

History

Beginning in the late 1970s, scientists advocated for a radar mapping mission to Venus. They first sought to construct a spacecraft named the Venus Orbiting Imaging Radar (VOIR), but it became clear that the mission would be beyond the budget constraints during the ensuing years. The VOIR mission was canceled in 1982.

A simplified radar mission proposal was recommended by the Solar System Exploration Committee, and this one was submitted and accepted as the Venus Radar Mapper program in 1983. The proposal included a limited focus and a single primary scientific instrument. In 1985, the mission was renamed Magellan, in honor of the sixteenth-century Portuguese explorer Ferdinand Magellan, known for his exploration, mapping, and circumnavigation of the Earth.

The objectives of the mission included:

Obtain near-global radar images of the Venusian surface with a resolution equivalent to optical imaging of per line pair. (primary)
Obtain a near-global topographic map with spatial and vertical resolution.
Obtain near-global gravity field data with resolution and two to three milligals of accuracy.
Develop an understanding of the geological structure of the planet, including its density distribution and dynamics.

Spacecraft design

thumb|The Voyager probe spacecraft bus that formed the main body of Magellan

The spacecraft was designed and built by the Martin Marietta Company, and the Jet Propulsion Laboratory (JPL) managed the mission for NASA. Elizabeth Beyer served as the program manager and Joseph Boyce served as the lead program scientist for the NASA headquarters. For JPL, Douglas Griffith served as the Magellan project manager and R. Stephen Saunders served as the lead project scientist.

To save costs, most of the Magellan probe was made up of flight spare parts and reused design elements from other spacecraft:

;Reuse Type Legend:

{| class="wikitable sortable"

! Component

! Origin

| Attitude control computer

| Galileo

| Bus

| Voyager program

| Command and data subsystem

| Galileo

| High- and low-gain antenna

| Voyager program

| Medium-gain antenna

| Mariner 9

| Power distribution unit

| Galileo

|- style="background-color:#cccccc;"

| Propellant tank

| Space Shuttle auxiliary power unit

| Pyrotechnic control

| Galileo

| Radio-frequency traveling-wave tube assemblies

| Ulysses

|- style="background-color:#cccccc;"

| Solid rocket motor

|Space Shuttle Payload Assist Module

|- style="background-color:#cccccc;"

| Star scanner

| Inertial Upper Stage

| Thrusters

| Voyager program

The main body of the spacecraft, a spare one from the Voyager missions, was a 10-sided aluminum bus, containing the computers, data recorders, and other subsystems. The spacecraft measured 6.4 meters tall and 4.6 meters in diameter. Overall, the spacecraft weighed 3,445 kilograms.

The actual propulsion system design consisted of a total of 24 monopropellant hydrazine thrusters fed from a single 71cm (28 in) diameter titanium tank. The tank contained 133 kg (293 lb) of purified hydrazine. The design also included a pyrotechnically-isolated external high pressure tank with additional helium that could be connected to the main tank prior to the critical Venus orbit insertion burn to ensure maximum thrust from the 445 N thrusters during the SRM firing. Other hardware regarding orientation of the spacecraft consists of a set of gyroscopes and a star scanner.

Communications

thumb|Positions of the three antennas

For communications, the spacecraft included a lightweight graphite/aluminum, 3.7-meter high-gain antenna left over from the Voyager Program and a medium-gain antenna spare from the Mariner 9 mission. A low-gain antenna attached to the high-gain antenna was also included for contingencies. When communicating with the Deep Space Network, the spacecraft was able to simultaneously receive commands at 1.2 kilobits/second in the S-band and transmit data at 268.8 kilobits/second in the X-band.

The Magellan high-gain parabolic antenna, oriented 28°–78° to the right or left of nadir, emitted thousands of microwave pulses per second that passed through the clouds and to the surface of Venus, illuminating a swath of land. The Radar System then recorded the brightness of each pulse as it reflected back off the side surfaces of rocks, cliffs, volcanoes and other geologic features, as a form of backscatter. To increase the imaging resolution, Magellan recorded a series of data bursts for a particular location during multiple instances called, "looks". Each "look" slightly overlapped the previous, returning slightly different information for the same location, as the spacecraft moved in orbit. After transmitting the data back to Earth, Doppler modeling was used to take the overlapping "looks" and combine them into a continuous, high resolution image of the surface.

Radar System (RDRS)

200px|left200px|left

The Radar System (RDRS) functioned in three modes: synthetic aperture radar (SAR), altimetry (ALT), and radiometry (RAD). The instrument cycled through the three modes while observing the surface geology, topography, and temperature of Venus using the 3.7-meter parabolic, high-gain antenna and a small fan-beam antenna, located just to the side.

– In the Synthetic Aperture Radar mode, the instrument transmitted several thousand long-wave, 12.6-centimeter microwave pulses every second through the high-gain antenna, while measuring the doppler shift of each hitting the surface.

– In Altimetry mode, the instrument interleaved pulses with SAR, and operating similarly with the altimetric antenna, recording information regarding the elevation of the surface on Venus.

– In Radiometry mode, the high-gain antenna was used to record microwave radiothermal emissions from Venus. This data was used to characterize the surface temperature.

The data was collected at 750 kilobits/second to the tape recorder and later transmitted to Earth (10 Bit per second * 365 *4 * 24 * 60 = 21 Mbit (maximum) = 85 Foto (maximum) ) to be processed into usable images, by the Radar Data Processing Subsystem (RDPS), a collection of ground computers operated by JPL. The principal investigator for this instrument was Gordon Pettengill from MIT.

Synthetic Aperture Radar (SAR), already covered above while discussion the RDRS instrument;
Gravimetry, consisting on detailed measurements of the Venus gravitational field, with the principal investigator being Georges Balmino from Centre National d'Etudes Spatiales;
Magellan Radio Science Occultation Experiment (ROCC), consisting on measurements of the atmospheric density and radio occultation data on the atmospheric profile. The principal investigator was Jon M. Jenkins from NASA Ames Research Center.

Gallery

File:Magellan diagramm.png|alt=Annotated diagram of Magellan|Annotated diagram of Magellan

File:Magellan - Magellan Spacecraft in Preflight Checkout at Kennedy Space Center.png|alt=Magellan during pre-flight checkout|Magellan during pre-flight checkout

File:Magellan Preparations.jpg|alt=Magellan being fixed into position inside the payload bay of Atlantis prior to launch|Magellan being fixed into position inside the payload bay of Atlantis prior to launch

</gallery>

Mission profile

{| class="wikitable floatright" width="auto"

! Date

! Event

| Space Shuttle vehicle launched at 18:46:59 UTC.

| Spacecraft deployed from Atlantis at 01:06:00 UTC.

| Begin Venus primary mission operations

| Venus orbital insertion maneuver

| Begin mapping cycle 1

| Phase stop

| Begin Venus extended mission operations

| Begin mapping cycle 2

| Begin mapping cycle 3

| Begin mapping cycle 4

| Begin testing aerobraking maneuver to place Magellan into an almost circular orbit.

| Begin mapping cycle 5

| Begin mapping cycle 6

| Begin "Windmill" experiment

| Phase stop

| End of mission. Deorbited into Venusian atmosphere. Loss of contact at 10:05:00 UTC.

x300px|thumb|center|Mosaic of the "left-looking" data collected during cycle 1

Mission extension

Mapping cycle 2

Goal: Image the south pole region and gaps from Cycle 1.
May 15, 1991 – January 14, 1992

Beginning immediately after the end of cycle 1, cycle 2 was intended to provide data for the existing gaps in the map collected during first cycle, including a large portion of the southern hemisphere. To do this, Magellan had to be reoriented, changing the gathering method to "right-looking". Upon completion during mid-January 1992, cycle 2 provided data for 54.5% of the surface, and combined with the previous cycle, a map containing 96% of the surface could be constructed. The method has since been used extensively on later interplanetary missions.

September 6, 1994 – September 14, 1994

In September 1994, the orbit of Magellan was lowered to begin the "Windmill experiment". During the experiment, the spacecraft was oriented with the solar arrays broadly perpendicular to the orbital path, where they could act as paddles as they impacted molecules of the upper-Venusian atmosphere. Countering this force, the thrusters fired to keep the spacecraft from spinning. This provided data on the basic oxygen gas-surface interaction. This was useful for understanding the impact of upper-atmospheric forces which aided in designing future Earth-orbiting satellites, and methods for aerobraking during future planetary spacecraft missions.

Results

thumb|150px|Rendered animation of Venus rotating using data gathered by Magellan

thumbnail|Five global views of [[Venus by Magellan]]

Study of the Magellan high-resolution global images is providing evidence to better understand Venusian geology and the role of impacts, volcanism, and tectonics in the formation of Venusian surface structures.
The surface of Venus is mostly covered by volcanic materials. Volcanic surface features, such as vast lava plains, fields of small lava domes, and large shield volcanoes are common.
There are few impact craters on Venus, suggesting that the surface is, in general, geologically young – less than 800 million years old.
The presence of lava channels over 6,000 kilometers long suggests river-like flows of extremely low-viscosity lava that probably erupted at a high rate.
Large pancake-shaped volcanic domes suggest the presence of a type of lava produced by extensive evolution of crustal rocks.
The typical signs of terrestrial plate tectonics - continental drift and basin floor spreading - are not evident on Venus. The planet's tectonics is dominated by a system of global rift zones and numerous broad, low domical structures called coronae, produced by the upwelling and subsidence of magma from the mantle.
Although Venus has a dense atmosphere, the surface reveals no evidence of substantial wind erosion, and only evidence of limited wind transport of dust and sand. This contrasts with Mars, where there is a thin atmosphere, but substantial evidence of wind erosion and transport of dust and sand.

Magellan created the first (and currently the best) near-photographic quality, high resolution radar mapping of the planet's surface features. Prior Venus missions had created low resolution radar globes of general, continent-sized formations. Magellan, however, finally allowed detailed imaging and analysis of craters, hills, ridges, and other geologic formations, to a degree comparable to the visible-light photographic mapping of other planets.

File:Maxwell Montes of planet Venus.jpg|alt=Maxwell Montes, highest point on Venus|Maxwell Montes, highest point on Venus

File:Bahet and Onatah Coronae PIA00461.jpg|alt=Coronae as seen in the Fortuna region of Venus|Volcanoes as seen in the Fortuna region of Venus

File:Aphrodite Terra on Venus.jpg|alt=Aphrodite Terra, a rugged landscape|Aphrodite Terra, a rugged landscape

File:Addams crater on Venus.jpg|alt=Addams crater|Addams crater

File:Alpha Regio.jpg|alt=Pancake domes visible in Alpha Regio|Pancake domes visible in Alpha Regio

File:Mgn f45n019 1.gif|alt=A meandering lava channel from Fortuna Tessera to Sedna Planitia|A meandering lava channel from Fortuna Tessera to Sedna Planitia

File:Venusvulkan Tick-Typ.jpg|alt=An unusual volcanic edifice in the Eistla region|An unusual volcanic edifice in the Eistla region

File:Isabella Crater PIA00480.jpg|alt=175-kilometer Isabella crater|175-kilometer Isabella crater

</gallery>

Scientists

The Magellan project was set up so that the initial images and data from the Magellan probe were only for use and study by a team of principal investigators from a variety of universities and institutions, and by the Magellan Project Science Team. These scientists were responsible for validating the data, contributing input for spacecraft acquisition of data, and interpreting the data results for their release to the public. Data was shared with three visiting Soviet scientists (Alexander Basilevsky, Effaim Akim and Alexander Zacharov), a first, and sensitive issue, for NASA at the time considering the Cold War was just coming to a close.

The Magellan Project Science room became notorious for its hanging of long thermal print strips of image data (FBIDRs) along the walls of a spacious room. This was the first form in which the imagery of the surface of Venus was seen due to the long, narrow swathes acquired by the spacecraft. Significant guests during the mission's operation included Margaret Thatcher.

After the initial investigation stage Magellan's full data set was released for public consumption.

Project Science Team

The Magellan Project Science Team consisted of Dr. R. Stephen Saunders, the Project Scientist; Dr. Ellen Stofan, the Deputy Project Scientist; research assistants Tim Parker, Dr. Jeff Plaut, and Annette deCharon; and Project Science Aide, Gregory Michaels.

Other Magellan scientists were involved with the mission's science including principal investigators and three visiting Soviet scientists.

End of mission

thumb|alt=A poster designed for the Magellan end of mission|A poster designed for the Magellan end of mission

On September 9, 1994, a press release outlined the termination of the Magellan mission. Due to the degradation of the power output from the solar arrays and onboard components, and having completed all objectives successfully, the mission was to end in mid-October. The termination sequence began in late August 1994, with a series of orbital trim maneuvers which lowered the spacecraft into the outermost layers of the Venusian atmosphere to allow the Windmill experiment to begin on September 6, 1994. The experiment lasted for two weeks and was followed by subsequent orbital trim maneuvers, further lowering the altitude of the spacecraft for the final termination phase.

On October 13, 1994 at 10:05:00 UTC, communication was lost when the spacecraft entered radio occultation behind Venus. The team continued to listen for another signal from the spacecraft until 18:00:00 UTC, when the mission was determined to have concluded. Although much of Magellan was expected to vaporize due to atmospheric stresses, some amount of wreckage is thought have hit the surface by 20:00:00 UTC.