Magellan probe
Overview
 
The Magellan spacecraft, also referred to as the Venus Radar Mapper, was a 1,035-kilogram robotic
Robotic spacecraft
A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...

 space probe
Space probe
A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...

 launched by NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 on May 4, 1989, to map the surface of Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 using Synthetic Aperture Radar and measure the planetary gravity. It was the first interplanetary mission to be launched from the Space Shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

, the first to use an inertial upper stage
Inertial Upper Stage
The Inertial Upper Stage , originally known as the Interim Upper Stage, is a two-stage solid-fueled booster rocket developed by the U.S...

 booster and was the first spacecraft to test aerobraking
Aerobraking
Aerobraking is a spaceflight maneuver that reduces the high point of an elliptical orbit by flying the vehicle through the atmosphere at the low point of the orbit . The resulting drag slows the spacecraft...

 as a method for circularizing an orbit.
Unanswered Questions
Encyclopedia
The Magellan spacecraft, also referred to as the Venus Radar Mapper, was a 1,035-kilogram robotic
Robotic spacecraft
A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...

 space probe
Space probe
A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...

 launched by NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 on May 4, 1989, to map the surface of Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 using Synthetic Aperture Radar and measure the planetary gravity. It was the first interplanetary mission to be launched from the Space Shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

, the first to use an inertial upper stage
Inertial Upper Stage
The Inertial Upper Stage , originally known as the Interim Upper Stage, is a two-stage solid-fueled booster rocket developed by the U.S...

 booster and was the first spacecraft to test aerobraking
Aerobraking
Aerobraking is a spaceflight maneuver that reduces the high point of an elliptical orbit by flying the vehicle through the atmosphere at the low point of the orbit . The resulting drag slows the spacecraft...

 as a method for circularizing an orbit. Magellan was the fourth successful, NASA funded mission to Venus and ended an eleven year U.S. interplanetary exploration hiatus.

History

Beginning in the late 1970s, scientists pushed for a radar mapping mission to Venus. First seeking to construct a spacecraft titled, Venus Orbiter Imaging Radar, it became obvious the mission would be outside the limits of the budgetary constraints during the following years and was subsequently canceled in 1982. Recommended by the Solar System Exploration Committee, a stripped down mission proposal was resubmitted and accepted as the Venus Radar Mapper in 1983. The proposal included a limited focus and a single primary scientific instrument. In 1985, the mission was renamed Magellan, after the sixteenth-century Portuguese explorer Ferdinand Magellan
Ferdinand Magellan
Ferdinand Magellan was a Portuguese explorer. He was born in Sabrosa, in northern Portugal, and served King Charles I of Spain in search of a westward route to the "Spice Islands" ....

, for his exploration, mapping and circumnavigation of the Earth, a goal this mission would have for Venus.

The objectives of the mission included:
  • Obtain near-global radar images of Venus' surface with a resolution equivalent to optical imaging of 1 km per line pair. (primary)
  • Obtain a near-global topographic map with 50 km spatial and 100 m vertical resolution.
  • Obtain near-global gravity field data with 700 km resolution and 2–3 milligals accuracy.
  • Develop an understanding of the geological structure of the planet, including its density distribution and dynamics.


The spacecraft was designed and built by Martin Marietta
Martin Marietta
Martin Marietta Corporation was an American company founded in 1961 through the merger of The Martin Company and American-Marietta Corporation. The combined company became a leader in chemicals, aerospace, and electronics. In 1995, it merged with Lockheed Corporation to form Lockheed Martin. The...

 and JPL provided mission management for the NASA division. Elizabeth Beyer served as program manager and Joseph Boyce served as lead program scientist for the NASA headquarters; for operations at JPL, Douglas Griffith served as Magellan project manager and R. Stephen Saunders served as lead project scientist.

Spacecraft design

To save costs, Magellan was made of many spare parts from various missions, including the Voyager Program
Voyager program
The Voyager program is a U.S program that launched two unmanned space missions, scientific probes Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s...

, Galileo, Ulysses and Mariner 9
Mariner 9
Mariner 9 was a NASA space orbiter that helped in the exploration of Mars and was part of the Mariner program. Mariner 9 was launched toward Mars on May 30, 1971 from Cape Canaveral Air Force Station and reached the planet on November 13 of the same year, becoming the first spacecraft to orbit...

. The spacecraft was built by the Martin Marietta Astronautics Group in Denver, Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...

. The main body of the spacecraft, a spare 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 1,035-kilograms and carried 2,414-kilograms of propellant for a total mass of 3449-kilograms.

Attitude control and propulsion

The spacecraft was three-axis stabilized with three reaction wheels and twenty-four thrusters
Reaction engine
A reaction engine is an engine or motor which provides propulsion by expelling reaction mass, in accordance with Newton's third law of motion...

 with 132.5-kilograms of hydrazine
Hydrazine
Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...

 monopropellant
Monopropellant rocket
A monopropellant rocket is a rocket that uses a single chemical as its propellant.-Chemical-reaction monopropellant rockets:...

 onboard. Of the thrusters, eight are aimed aft
Aft
Aft, in naval terminology, is an adjective or adverb meaning, towards the stern of the ship, when the frame of reference is within the ship. Example: "Able Seaman Smith; lay aft!". Or; "What's happening aft?"...

, providing 444.82-N of thrust for course corrections, control of the spacecraft during the Venus orbital insertion maneuver and large orbit corrections during the mission; four along the side of the spacecraft provide 22.24-N for roll; the smallest twelve provide 0.88-N for minor attitude corrections and offsetting, or "desaturating", the reaction wheels. To perform the Venus orbital insertion maneuver, the spacecraft was equipped with a Star 48
Star 48
The Star 48 is a type of solid rocket motor used by many space propulsion and launch vehicle stages, including the Space Shuttle and the New Horizons probe. It is used almost exclusively as an upper stage...

 booster containing 2,014-kilograms of solid-propellant. Information regarding the orientation of the spacecraft was provided by a set of gyroscopes and a star scanner.

Communications

For communications, the spacecraft included a lightweight graphite/aluminum, 3.7-meter high-gain antenna
High-gain antenna
A high-gain antenna is an antenna with a focused, narrow radiowave beam width. This narrow beam width allows more precise targeting of the radio signal - also known as a directional antenna...

 left over from the Voyager Program
Voyager program
The Voyager program is a U.S program that launched two unmanned space missions, scientific probes Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s...

 and a medium-gain antenna spare from the Mariner 9
Mariner 9
Mariner 9 was a NASA space orbiter that helped in the exploration of Mars and was part of the Mariner program. Mariner 9 was launched toward Mars on May 30, 1971 from Cape Canaveral Air Force Station and reached the planet on November 13 of the same year, becoming the first spacecraft to orbit...

 mission; attached to the high-gain antenna, a low-gain antenna was also included for contingency measures. When communicating with the Deep Space Network
Deep Space Network
The Deep Space Network, or DSN, is a world-wide network of large antennas and communication facilities that supports interplanetary spacecraft missions. It also performs radio and radar astronomy observations for the exploration of the solar system and the universe, and supports selected...

, the spacecraft was able to simultaneously receive commands at 1.2-kilobits/second across the S-band and transmit data at 268.8-kilobits/second via X-band.

Power

Magellan was powered by two square solar arrays
Photovoltaic module
A solar panel is a packaged, connected assembly of solar cells, also known as photovoltaic cells...

, each measuring 2.5-meters across. Together, the arrays supplied 1,200-watts of power at the beginning of the mission. However, over the course of the mission the solar arrays gradually degraded due to frequent, extreme temperature changes. To power the spacecraft while occluded from the Sun, two 30-amp hour, 26-cell, Nickel-Cadmium batteries
Nickel-cadmium battery
The nickel–cadmium battery ' is a type of rechargeable battery using nickel oxide hydroxide and metallic cadmium as electrodes....

 were included; the batteries recharged as the spacecraft received direct sun light.

Computer

The computing system on the spacecraft, derived from the Galileo mission, included two ATAC-16 computers to control attitude, and four RCA 1802
RCA 1802
The RCA CDP1802, also known as the COSMAC , is an 8-bit CMOS microprocessor introduced by RCA in early 1976. It is being by Intersil Corporation as a high-reliability microprocessor...

 microprocessors to control the Command and Data Subsystem (CDS). The CDS was able to store commands for up to three days, and to autonomously control the spacecraft if problems were to arise while mission operators weren't in contact with the spacecraft. For storing the commands and recorded data, the spacecraft also included two multitrack digital tape recorders, able to store up to 225-Megabytes until contact with Earth was restored and the tapes were played back.

Scientific instruments

Radar System (RDRS)


The Radar System 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
High-gain antenna
A high-gain antenna is an antenna with a focused, narrow radiowave beam width. This narrow beam width allows more precise targeting of the radio signal - also known as a directional antenna...

 and a small fan-beam antenna
Fan-beam antenna
A fan-beam antenna is a directional antenna producing a main beam having a narrow beamwidth in one dimension and a wider beamwidth in the other dimension. This pattern will be achieved by a truncated paraboloid reflector or a circular paraboloid reflector...

, located just to the side.
- In Synthetic Aperture Radar mode, the instrument transmitted several thousand long-wave, 12.6-centimeter microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

 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 to be processed into usable images, by the Radar Data Processing Subsystem (RDPS), a collection of ground computers operated by JPL.


Aperture synthesis

Thick and opaque, the atmosphere of Venus
Atmosphere of Venus
The atmosphere of Venus is much denser and hotter than that of Earth. The temperature at the surface is 740 K , while the pressure is 93 bar. The Venusian atmosphere supports opaque clouds made of sulfuric acid, making optical Earth-based and orbital observation of the surface impossible...

 required a method beyond optical survey, to map the surface of the planet. The resolution of conventional radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 depends entirely on the size of the antenna, which is greatly restricted by costs, physical constraints by launch vehicles and the complexity of maneuvering a large apparatus to provide high resolution data. The Magellan spacecraft avoided this problem by using a method known as aperture synthesis
Aperture synthesis
Aperture synthesis or synthesis imaging is a type of interferometry that mixes signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection...

, where a large antenna is imitated by processing the information gathered, by ground computers.

The Magellan high-gain antenna, oriented 28°–78° to the right or left of nadir
Nadir
The nadir is the direction pointing directly below a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface there. Since the concept of being below is itself somewhat vague, scientists define the nadir in more rigorous...

, emitted thousands of microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

 pulses 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
Backscatter
In physics, backscatter is the reflection of waves, particles, or signals back to the direction they came from. It is a diffuse reflection due to scattering, as opposed to specular reflection like a mirror...

. 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
Doppler
-Doppler effect and its applications:* Doppler effect* Doppler beaming* Doppler broadening* Doppler cooling** Doppler cooling limit* Doppler echocardiography** Doppler ultrasound, also called Doppler sonography** Transcranial doppler* Doppler fetal monitor...

 modeling was used to take the overlapping "looks" and combine them into a continuous, high resolution image of the surface.



Mission profile

Timeline of travel

----
Date Event

----
Space shuttle vehicle launched at 18:46:59 UTC.
Spacecraft deployed from shuttle at 01:06:00 UTC.
End of mission. Deorbited into Venusian atmosphere. Loss of contact at 10:05:00 UTC. >

Launch and trajectory

Magellan was launched on May 4, 1989, at 18:46:59 UTC by the National Aeronautics and Space Administration from KSC Launch Complex 39B
Kennedy Space Center Launch Complex 39
Launch Complex 39 is a rocket launch site at the John F. Kennedy Space Center on Merritt Island in Florida, USA. The site and its collection of facilities were originally built for the Apollo program, and later modified to support Space Shuttle operations. NASA began modifying LC-39 in 2007 to...

 at the Kennedy Space Center
Kennedy Space Center
The John F. Kennedy Space Center is the NASA installation that has been the launch site for every United States human space flight since 1968. Although such flights are currently on hiatus, KSC continues to manage and operate unmanned rocket launch facilities for America's civilian space program...

 in Florida, aboard Space Shuttle Atlantis
Space Shuttle Atlantis
The Space Shuttle Atlantis is a retired Space Shuttle orbiter in the Space Shuttle fleet belonging to the National Aeronautics and Space Administration , the spaceflight and space exploration agency of the United States...

 during mission STS-30
STS-30
STS-30 was a NASA Space Shuttle mission, during which Space Shuttle Atlantis deployed the Venus-bound Magellan probe into orbit. The mission launched from Kennedy Space Center, Florida, on 4 May 1989, and landed four days later...

. Once in orbit, an Inertial Upper Stage
Inertial Upper Stage
The Inertial Upper Stage , originally known as the Interim Upper Stage, is a two-stage solid-fueled booster rocket developed by the U.S...

 booster, deployed from the shuttle and launched on May 5, 1989 01:06:00 UTC, sending the spacecraft into a Type IV, heliocentric orbit
Heliocentric orbit
A heliocentric orbit is an orbit around the Sun. All planets, comets, and asteroids in our Solar System are in such orbits, as are many artificial probes and pieces of debris. The moons of planets in the Solar System, by contrast, are not in heliocentric orbits as they orbit their respective planet...

 where it would circle the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 1.5 times, before reaching Venus 15 months later on August 10, 1990.

Originally, Magellan had been scheduled for launch in 1988 with a trajectory lasting six months. However, due to the Space Shuttle Challenger disaster
Space Shuttle Challenger disaster
The Space Shuttle Challenger disaster occurred on January 28, 1986, when Space Shuttle Challenger broke apart 73 seconds into its flight, leading to the deaths of its seven crew members. The spacecraft disintegrated over the Atlantic Ocean, off the coast of central Florida at 11:38 am EST...

 in 1986, several missions, including Galileo and Magellan, were deferred until the shuttle flights resumed September 1988. Intended to be launched with a new, liquid fueled, Centaur-G
Centaur
In Greek mythology, a centaur or hippocentaur is a member of a composite race of creatures, part human and part horse...

 shuttle deploy-able upper-stage booster, subsequently canceled after the Challenger disaster, Magellan had to be modified to attach to a less powerful solid-fueled, Inertial Upper Stage
Inertial Upper Stage
The Inertial Upper Stage , originally known as the Interim Upper Stage, is a two-stage solid-fueled booster rocket developed by the U.S...

. The next best opportunity for launch would occur in October 1989.

Further complicating the launch however, was the upcoming Galileo mission to Jupiter, which included a flyby of Venus. Intended for launch in 1986, the pressures to ensure a launch for Galileo in 1989, mixed with a short launch-window necessitating a mid-October launch, resulted in replanning the Magellan mission. Weary of rapid shuttle launches, the decision was made to launch Magellan in May, and into an orbit that would require 1 year and 3 months before encountering Venus.

Orbital encounter of Venus


On August 1, 1990, Magellan encountered Venus and began the orbital insertion maneuver which placed the spacecraft into a 3 hour and 9 minute, elliptical orbit which brought the spacecraft 295-kilometers from the surface at approximately 10° North during apoapsis and out to 7762-kilometers during periapsis.

During each orbit, the spacecraft would capture radar data while the spacecraft was nearest to the surface and then transmit it back to Earth as it moved away from Venus. This maneuver required extensive use of the reaction wheels to continuously rotate the spacecraft as it imaged the surface for 37-minutes and as it pointed toward Earth for 2 hours. The primary mission intended for the spacecraft to return images of at least 70% of the surface during one Venusian day, which lasts 243 Earth days as the planet slowly spins. To avoid overly redundant data at the highest and lowest latitudes Magellan alternated between a Northern-swath, a region designated as 90° north latitude to 54° south latitude, and a Southern-swath, designated as 76° north latitude to 68° south latitude. However, due to apoapsis being 10° north of the equatorial line, imaging the South Pole region was unlikely to be possible.

Mapping cycle 1

  • Goal: Complete primary objective.
  • September 15, 1990 - May 15, 1991


The primary mission began on September 15, 1990, with the intention to provide a "left-looking" map of 70% of the Venusian surface at a minimum resolution of 1-kilometer/pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....

. During cycle 1, the altitude of the spacecraft varied from 2000-kilometers at the north pole, to 290-kilometers near apoapsis. Upon completion during May 15, 1991, having made 1,792 orbits, Magellan had mapped approximately 83.7% of the surface with a resolution between 101 to 250-meters/pixel.

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 with 180°, 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.

Mapping cycle 3

  • Goal: Fill remaining gaps and collect stereo imagery.
  • January 15, 1992 - September 13, 1992


Immediately after cycle 2, cycle 3 began collecting data for stereo imagery on the surface that would later allow the ground team to construct, clear, three-dimensional renderings of the surface. Approximately 21.3% of the surface was imaged in stereo by the end of the cycle on September 13, 1992, increasing the overall coverage of the surface to 98%.

Mapping cycle 4

  • Goal: Measure Venus' gravitational field.
  • September 14, 1992 - May 23, 1993


Upon completing cycle 3, Magellan ceased imaging the surface. Instead, beginning mid-September 1992, the Magellan maintained pointing of the high-gain antenna toward Earth where the Deep Space Network
Deep Space Network
The Deep Space Network, or DSN, is a world-wide network of large antennas and communication facilities that supports interplanetary spacecraft missions. It also performs radio and radar astronomy observations for the exploration of the solar system and the universe, and supports selected...

 began recording a constant stream of telemetry. This constant signal allowed the DSN to collect information on the gravitational field of Venus by monitoring the velocity of the spacecraft. Areas of higher gravitation would slightly increase the velocity of the spacecraft, registering as a Doppler shift in the signal. The space craft completed 1,878 orbits until completion of the cycle on May 23, 1993; a loss of data at the beginning of the cycle necessitated an additional 10 days of gravitational study.

Mapping cycle 5

  • Goal: Aerobraking to circular orbit and global gravity measurements.
  • May 24, 1993 - August 29, 1994


At the end of the fourth cycle in May 1993, the orbit of Magellan was circularized using an unproven technique known as aerobraking
Aerobraking
Aerobraking is a spaceflight maneuver that reduces the high point of an elliptical orbit by flying the vehicle through the atmosphere at the low point of the orbit . The resulting drag slows the spacecraft...

. The circularized orbit allowed a much higher resolution of gravimetric data to be acquired when cycle 5 began on August 3, 1993. The spacecraft performed 2,855 orbits and provided high-resolution gravimetric data for 94% of the planet, before the end of the cycle on August 29, 1994.
Aerobraking
  • Goal: To enter a circular orbit
  • May 24, 1993 - August 2, 1993

Aerobraking had long been sought as a method for slowing the orbit of interplanetary spacecraft. Previous suggestions included the need for aeroshell
Aeroshell
An aeroshell is a rigid heat-shielded shell that protects a vehicle from pressure and heat created by drag during atmospheric entry , slows it down during entry, and may protect it from debris during spaceflight...

s that proved too complicated and expensive for most missions. Testing a new approach to the method, a plan was devised to drop the orbit of
Magellan into the outer-most region of the Venusian atmosphere
Atmosphere of Venus
The atmosphere of Venus is much denser and hotter than that of Earth. The temperature at the surface is 740 K , while the pressure is 93 bar. The Venusian atmosphere supports opaque clouds made of sulfuric acid, making optical Earth-based and orbital observation of the surface impossible...

. Slight friction on the spacecraft slowed the velocity over a period, slightly longer than two months, bringing the spacecraft into an approximate, circular orbit from 180-kilometers at apoapsis to 540-kilometers at periapsis. The method has since been used extensively on subsequent interplanetary missions.

Mapping cycle 6

  • Goal: Collect high-resolution gravity data and conduct radio science experiments.
  • April 16, 1994 - October 13, 1994


The sixth and final orbiting cycle was another extension to the two previous gravimetric studies. Toward the end of the cycle, a final experiment was conducted, known as the "windmill" experiment to provide data on the composition of the upper atmosphere of Venus. Magellan performed 1,783 orbits before the end of the cycle on October 13, 1994, when the spacecraft entered the atmosphere and disintegrated.
Windmill experiment
  • Goal: Collect data on atmospheric dynamics.
  • 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 would be 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.

Discoveries

  • Study of the Magellan high-resolution global images is providing evidence to understand the role of impacts, volcanism, and tectonism 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
    Plate tectonics
    Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...

     - 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.


Study of the Magellan high-resolution global images is providing evidence to better understand Venusian geology and the role of impacts, volcanism
Volcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....

, and tectonism in the formation of Venusian surface structures.

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. Magellan's global radar map will remain the most detailed Venus map in existence for the foreseeable future, although the planned Russian Venera-D
Venera-D
The Venera-D probe is a proposed Russian space probe to Venus, to be launched around 2016. Venera-D's prime purpose is to make remote-sensing observations around the planet Venus in a manner similar to that of the U.S. Magellan spacecraft in the 1990s, but with the use of more powerful radar....

 may carry a radar that can achieve the same, if not better resolution as the radar used by 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
Atmosphere of Venus
The atmosphere of Venus is much denser and hotter than that of Earth. The temperature at the surface is 740 K , while the pressure is 93 bar. The Venusian atmosphere supports opaque clouds made of sulfuric acid, making optical Earth-based and orbital observation of the surface impossible...

 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 11, 1994, moving at a velocity of 7-kilometers/second, the final orbital trim maneuver was performed, placing the spacecraft 139.7-kilometers above the surface, well within the atmosphere. At this altitude the spacecraft encountered tremendous friction, raising temperatures on the solar arrays to 126-degrees Celsius.

On October 13, 1994 at 10:05:00 UTC, communication was lost when the spacecraft entered radio occultation
Occultation
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...

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.

Quoted from Status Report - October 13, 1994
"Communication with the Magellan spacecraft was lost early Wednesday morning, following an aggressive series of five Orbit Trim Maneuvers (OTMs) on Tuesday, October 11, which took the orbit down into the upper atmosphere of Venus. The Termination experiment (extension of September "Windmill" experiment) design was expected to result in final loss of the spacecraft due to a negative power margin. This was not a problem since spacecraft power would have been too low to sustain operations in the next few weeks due to continuing solar cell loss.

Thus, a final controlled experiment was designed to maximize mission return. This final, low altitude was necessary to study the effects of a carbon dioxide atmosphere.

The final OTM took the periapsis to 139.7 km (86.8 mi) where the sensible drag on the spacecraft was very evident. The solar panel temperatures rose to 126 deg. C. and the attitude control system fired all available Y-axis thrusters to counteract the torques. However, attitude control was maintained to the end.

The main bus voltage dropped to 24.7 volts after five orbits, and it was predicted that attitude control would be lost if the power dropped below 24 volts. It was decided to enhance the windmill experiment by changing the panel angles for the remaining orbits. This was also a preplanned experiment option.

At this point, the spacecraft was expected to survive only two orbits.

Magellan continued to maintain communication for three more orbits, even though the power continued to drop below 23 volts and eventually reached 20.4 volts. At this time, one battery went off-line, and the spacecraft was defined as power starved.

Communication was lost at 3:02 AM PDT just as Magellan was about to enter an Earth occultation on orbit 15032. Contact was not re-established. Tracking operations were continued to 11:00 AM but no signal was seen, and none was expected. The spacecraft should land on Venus by 1:00 PM PDT Thursday, October 13, 1994."

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