Artist's concept of Galileo at Jupiter.

Artist's concept of Galileo at Jupiter.

Goals: Galileo was designed to make the first study of Jupiter and its moons and magnetosphere from orbit. The orbiter carried 10 science instruments and a atmospheric probe.

Accomplishments: Like the famed astronomer for which it was named, the Galileo spacecraft logged quite a few firsts during its 14-year mission to Jupiter. Among its discoveries: an intense radiation belt above Jupiter's cloud tops, helium in about the same concentration as the Sun, extensive and rapid resurfacing of the moon Io because of volcanism, and evidence for liquid water oceans under the moon Europa's icy surface.

The orbiter carried a small probe that became the first to sample the atmosphere of a gas planet. The probe measured temperature, pressure, chemical composition, cloud characteristics, sunlight and energy internal to the planet, and lightning. During its 58-minute life, the probe penetrated 200 km (124 miles) into Jupiter's violent atmosphere before it was crushed, melted, and/or vaporized by the pressure and temperature of the atmosphere.

Overview

Galileo plunged into Jupiter's crushing atmosphere on Sept. 21, 2003. The spacecraft was deliberately destroyed to protect one of its own discoveries - a possible ocean beneath the icy crust of the moon Europa.

Galileo changed the way we look at our solar system. The spacecraft was the first to fly past an asteroid and the first to discover a moon of an asteroid. It provided the only direct observations of a comet colliding with a planet.

Galileo was the first to measure Jupiter's atmosphere with a descent probe and the first to conduct long-term observations of the Jovian system from orbit. It found evidence of subsurface saltwater on Europa, Ganymede and Callisto and revealed the intensity of volcanic activity on Io.

Facts & Figures

Spacecraft

Dimensions: 5.3 meters (17 feet) high; magnetometer boom extends 11 meters (36 feet) to one side

Weight: 2,223 kilograms (4,902 pounds), including 118 kilograms (260 pounds) of science instruments and 925 kilograms (2,040 pounds) of propellant
Power: 570 watts (at launch) from radioisotope thermoelectric generators

Science Instruments: Solid-state imaging camera, near-infrared mapping spectrometer, ultraviolet spectrometer, photopolarimeter radiometer, magnetometer, energetic particles detector, plasma investigation, plasma wave subsystem, dust detector, heavy ion counter

Atmospheric Probe

Size: 127 centimeters (50 inches) diameter, 91 centimeters (36 inches) high

Weight: 339 kilograms (750 pounds)

Science Instruments: 339 kilograms (750 pounds)

Mission

Launch: Oct. 18, 1989 from Kennedy Space Center, Fla., on space shuttle Atlantis on mission STS-34

Primary Mission: October 1989 to December 1997

Extended Missions: Three, from 1997 to 2003

Venus Flyby: Feb. 10, 1990, at altitude of 16,000 km (10,000 mi)

Earth Flybys: Dec. 8, 1990, at altitude of 960 km (597 mi); Dec. 8, 1992 at altitude of 303 km (188 mi)

Asteroid Gaspra Flyby: Oct. 29, 1991, at 1,601 km (1,000 mi)

Comet Shoemaker-Levy 9: Impacts of comet fragments into Jupiter observed while en route in July 1994

Asteroid Ida Flyby: Aug. 28, 1993, at 2,400 km (1,400 mi)

Atmospheric Probe Release: July 12, 1995

Probe speed into Jupiter's atmosphere: 47.6 km per second (106,000 mi per hour)

Jupiter arrival and orbit insertion: Dec. 7, 1995

Probe atmospheric entry and relay: Dec. 7, 1995

Number of Jupiter orbits during entire mission: 34

Number of flybys of Jupiter moons: Io 7, Callisto 8, Ganymede 8, Europa 11, Amalthea 1

Total distance traveled from launch to final impact: 4,631,778,000 kilometers (approx. 2.8 billion miles)

Speed of spacecraft at time of impact: 48.2 kilometers per second (nearly 108,000 miles per hour)

Program

Cost: Total from start of planning through end of mission is $1.39 billion. International contribution estimated at an additional $110 million

Partners: More than 100 scientists from United States, Great Britain, Germany, France, Canada and Sweden carried out Galileo's experiments. NASA's Ames Research Center, Mountain View, Calif., responsible for atmosphere probe, built by Hughes Aircraft Company, El Segundo, Calif. Radioisotope thermoelectric generators designed and built by General Electric Co. for the U.S. Department of Energy

Approximate number of people who worked on some portion of the Galileo mission: 800

Discovery Highlights

Jupiter's Storms and Rings

Using data from the Galileo Probe's plunge into the top cloud layers of Jupiter, Galileo has discovered that Jupiter has thunderstorms many times larger than Earth's. These storms result from the vertical circulation of water in the top layers, leaving large areas where air descends and becomes dry like the Sahara desert, and other areas where water rises to form the thunderstorms. Galileo has also found that Jupiter's rings are made of small dust grains blasted off the surface of Jupiter's four innermost satellites by the impacts of meteoroids.

Hot Active Volcanoes on Io

Now considered the solar system's most active body, Io's volcanoes were first discovered by Voyager 1 in 1979 and result from 100 meter (328 ft) tides in its solid surface. By taking Io's temperature with Galileo's instruments, scientists now know that some of Io's volcanoes are hotter than Earth's. From this, scientists surmise that lava made of silicate material rich in magnesium erupts from below Io's surface.

A Possible Ocean on Europa

Possessing more water than the total amount found on earth, Europa appears to have had a salty ocean beneath its icy cracked and frozen surface. Galileo images show ice "rafts" the size of cities that appear to have broken off and drifted apart, a frozen "puddle" smooths over older cracks, warmer material bubbles up from below to blister the surface, evaporative-type salts are exposed. A remarkable lack of craters show the surface to be relatively young.Europa has a thin oxygen atmosphere and an ionosphere.

Ganymede's Own Magnetic Field

Internal tidal friction again causes surprising effects on the solar system's largest moon. Galileo revealed that Ganymede has its own magnetic field. Perhaps from a slightly different orbit in its past, enough heat from tidal friction caused the separation of material inside Ganymede and this stirring of a molten core or iron sulfide is believed to generate Ganymede's magnetic field.

Does An Ocean Hide Beneath Callisto's Surface?

There is evidence to support the existence of a subsurface ocean on Callisto. The ocean would have to be deep enough inside the moon that it does not affect the heavily cratered surface on top. Instead the ocean could be showing itself indirectly, through the magnetic field it generates. This could come from electric flow in a salty ocean generated by Jupiter's strong magnetic field passing through it.

Top 10 Science Images

  1. Thera and Thrace Region of Europa: https://photojournal.jpl.nasa.gov/catalog/PIA02099
  2. Europa Ice Rafting: https://photojournal.jpl.nasa.gov/catalog/PIA01127
  3. Asteroid Ida and its Moon Dactyl: https://photojournal.jpl.nasa.gov/catalog/PIA00069
  4. Ancient Impact Craters in Galileo Regio: https://photojournal.jpl.nasa.gov/catalog/PIA00279
  5. Eruption at Tvashtar Catena on Io: https://photojournal.jpl.nasa.gov/catalog/PIA02584
  6. Volcanic Eruptions Seen on Io While in Eclipse: https://photojournal.jpl.nasa.gov/catalog/PIA00739
  7. Callisto’s Surface: https://photojournal.jpl.nasa.gov/catalog/PIA00561
  8. Ammonia Ice in Jupiter’s Great Red Spot: https://photojournal.jpl.nasa.gov/catalog/PIA02569
  9. Jovian Lightning and Moonlit Clouds: https://photojournal.jpl.nasa.gov/catalog/PIA01096
  10. The Structure of Jupiter's Gossamer Ring: https://photojournal.jpl.nasa.gov/catalog/PIA00659

More images taken by the Galileo Mission

In Depth

Journey to Jupiter: Introduction

Kennedy Space Center, Oct. 18, 1989: A roar shakes the ground as Space Shuttle Atlantis climbs into the sky. The Galileo spacecraft rides in the payload bay, ready to begin a long journey into the realm of the outer planets. Its mission is to study Jupiter and its moons in more detail than any previous spacecraft.

The spacecraft is named in honor of the first modern astronomer --- Galileo Galilei. He made the first observations of the heavens using a telescope in 1610.

What compels us to explore Jupiter? The giant colorful planet holds clues to help us understand how the Sun and planets formed more than 4.5 billion years ago. One of Jupiter's moons has active volcanoes and others have strange icy terrain. How does these strange worlds compare with Earth?

Galileo arrived at Jupiter in December 1995. As fascinating data poured in from the orbiting spacecraft and its atmospheric probe, we knew it was just the beginning.

Getting Off the Ground

Nearly four centuries ago, Galileo Galilei launched the age of modern astronomy by studying the planets and stars with a new invention - the telescope. That tool connected us to the heavens as scientific observers. Humans dreamed of traveling to the planets for a closer look, but only in the 50 years has that idea become reality.

Pioneer 10 and 11 and Voyager 1 and 2 spacecraft scouted Jupiter in the 1970s, but they couldn't stay and that limited what they could tell scientists back on Earth. Scientists proposed sending an orbiting spacecraft that could stick around and study Jupiter's environment in detail. The orbiter also would carry a probe to drop through Jupiter's clouds and collect data. The mission was named Galileo.

In October 1989, Galileo was launched from the cargo bay of the Space Shuttle Atlantis. Astronaut Shannon Lucid performed the delicate maneuvers that started the spacecraft on its journey. The booster rocket that pushed Galileo into interplanetary space was not powerful enough to send the orbiter directly to Jupiter. But engineers devised a way to borrow enough energy to get the spacecraft to its destination.

Launch Date: 10/18/89
Launch Vehicle: Space Shuttle Atlantis
Mission: NASA STS-34
Launch Site: Kennedy Space Center, Pad 39-B
Booster for Galileo: Inertial Upper Stage
Shuttle Crew:
- Donald E. Williams, Commander
- Michael J. McCulley, Pilot
- Franklin R. Chang-Diaz, Mission Specialist 1
- Shannon W. Lucid, Mission Specialist 2
- Ellen S. Baker, Mission Specialist 3

The Cruise - The Winding Road to Jupiter

Galileo didn't have enough fuel to fly directly to Jupiter, but the spacecraft could borrow enough energy from Venus and Earth to make the long journey. Mission planners designed a flight path nicknamed "VEEGA" -- Venus-Earth-Earth Gravity Assist. Galileo would slingshot once by Venus, and twice by Earth, gathering enough momentum to reach distant Jupiter.

First stop: Venus. The Galileo team tried out the spacecraft's instruments and study of the thick, toxic clouds that cloak our sister planet. Flying by our home planet twice, we saw the Earth and Moon together -- as someone from another world might view us.

After the first Earth flyby, Galileo's umbrella-shaped high-gain antenna did not open as planned. But the Galileo team worked hard to reprogram the spacecraft to send back data through a smaller antenna. Engineers at NASA's Deep Space Network upgraded their antennas as well. The result allowed scientists to capture almost all the information originally planned.

On Galileo's first trip through the asteroid belt, the spacecraft took detailed images of an asteroid named Gaspra - the first close approach to an asteroid. On a second pass through the asteroid belt, Galileo discovered a miniature moon orbiting asteroid Ida. This tiny body was named Dactyl.

In 1994, Galileo was perfectly positioned to watch the fragments of comet Shoemaker-Levy 9 crash into Jupiter. The spacecraft made the only direct observations of the impact. Earth-based telescopes had to wait to see the impact sites as they rotated into view.

Key Dates
Venus Flyby: 2/10/1990 (16,000 km distance)
Earth-1 Flyby: 10/8/1990 (960 km distance)
Gaspra Flyby: 10/29/1991 (1600 km distance)
Earth-2 Flyby: 12/8/1992 (305 km distance)
Ida Flyby: 8/28/1993 (2400 km distance)
Discovery: Dactyl, first known moon of an asteroid
Comet S/L-9 Jupiter Impact: July 16-22, 1994

Arrival at Jupiter and the Probe Mission

The Galileo spacecraft and probe traveled as one for almost six years. In July 1995, the probe was released to begin a solo flight into Jupiter.

Five months later, the probe sliced into Jupiter's atmosphere at one-hundred-six-thousand miles per hour. It slowed, released its parachute, and dropped its heat shield. As the probe descended through ninety-five miles of the top layers of the atmosphere, it collected fifty-eight minutes of data on the local weather. The data were sent to the spacecraft overhead, then transmitted back to Earth.

It appeared that Jupiter's atmosphere is drier than we thought. Measurements from the probe showed few clouds, and lightning only in the distance. It was only later that we discovered that the probe had entered an area called a "hot spot."

Towards the end of the 58 minute descent, the probe measured winds of four-hundred-and-fifty miles per hour - stronger than anything on Earth. The probe was finally melted and vaporized by the intense heat of the atmosphere.

To get into orbit around Jupiter, the spacecraft had to use its main engine. An error could send Galileo sailing past the planet. There was just one chance to get it right. After hours of anxious waiting, mission controllers confirmed that the spacecraft was safely in orbit. Galileo was alive and well and had begun its primary mission. The maneuver was precisely carried out, and Galileo entered orbit around Jupiter.

Quick Facts about Probe Mission:

  • Release Date: 7/13/95

  • Penetration into Jupiter's Atmosphere: 12/7/95

  • Entry Speed: 106,000 mph

  • Probe Data Return: 59 min, 3.5 megabits

  • Penetration depth: 200 km (124 mi)

The Orbital Tour of Jupiter - Galileo's Prime Mission

On December 7, 1995 Galileo began its prime mission: a two-year study of the Jovian system.

Galileo travels around Jupiter in elongated ovals --- each orbit lasts about two months. By traveling at different distances from Jupiter, Galileo can sample different parts of the planet's extensive magnetosphere. The orbits are designed for close-up flybys of Jupiter's largest moons.

To keep track of Galileo's journey, each orbit is numbered, and named for the moon that the spacecraft encountered at closest range. During orbit "C-3" for example --- the third orbit around Jupiter --- Galileo flew near the moon Callisto.

The data collected on Jupiter and the moons are stored on the on-board tape recorder. During the rest of the orbit, the data are sent to Earth using the low-gain antenna. At the same time, measurements are made of Jupiter's magnetosphere and transmitted back to Earth.

The intriguing data gathered during the eleven orbits of the prime mission left many questions to be answered. Since Galileo obviously was capable of much more, its mission was extended.

Extended Tours - GEM and the Millennium Mission

Galileo's prime mission ended on December 7, 1997. With more to learn, and the spacecraft in good health, NASA approved a two-year study called "GEM" -- the Galileo Europa Mission. For fourteen more orbits, the spacecraft focused on ice, water, and fire: the icy moon Europa, which might have an ocean; Jupiter's majestic thunderstorms; and the fiery volcanoes of Io.

The spacecraft came so close to Europa that if there were something there the size of a school bus, Galileo would have detected it. The additional observations of Europa supported the theory that an ocean of water currently exists below the surface. NASA began considering plans for future missions to orbit Europa, and perhaps to send a lander.

Approaching Io --- Jupiter's innermost moon --- meant surviving Jupiter's intense radiation, so these encounters were saved until last. When radiation upset the spcecraft's computer, engineers worked all night to get them back on line. But Galileo came through again, and even discovered a lava fountain erupting on Io.

These successful flybys led to another exciting mission --- the Galileo Millennium Mission, extending into 2001. The data are collected on Io and Europa, and studies made of the effects of radiation on a spacecraft close in to Jupiter. The Cassini spacecraft, on its way to Saturn, swings by Jupiter in late 2000 and for a few weeks, both spacecraft observe the giant of our Solar System.

Spacecraft

Introduction
Some scientific instruments on the Galileo spacecraft observed from a distance while others measured fields and particles directly. The Galileo probe was dropped right into Jupiter's cloudtops to collect data about the atmosphere.

At launch, the spacecraft and probe together had a mass of almost six-thousand pounds, about as much as two sport utility vehicles. Galileo is over twenty feet tall.

The spacecraft is a "dual-spin" design --- a controlled spin keeps Galileo stable. One section of the spacecraft rotates at 3rpm. On this section, six instruments rapidly gather data from many different directions. The other section of the spacecraft holds steady for the four instruments that must point accurately while Galileo is flying through space.

The 700-pound probe measured about four feet across. Inside the heat shield, the scientific instruments were protected from ferocious heat during entry. The probe had to withstand extreme heat and pressure on its high-speed journey at one-hundred-six-thousand miles per hour.

The Jet Propulsion Laboratory built the Galileo Spacecraft and manages the Galileo mission for NASA. Germany supplied the propulsion module. NASA's Ames Research Center managed the probe, which was built by Hughes Aircraft Company.

Spacecraft Basics

Sending humans into deep space isn't practical -- yet. But we can send a robot -- a spacecraft that observes, listens, and processes information and commands. A spacecraft listens to our commands through its antenna and radio receiver, and sends images and science data back to Earth.

The spacecraft's brain is a computer, which controls the scientific instruments. On its journey, the spacecraft needs fuel for its engines to burn -- a large engine is used for big changes such as entering orbit; and small thrusters fine-tune the direction. The probe did not need fuel --- it was released and captured by gravity as it plunged into Jupiter's atmosphere

Planets and moons emit or reflect radiation. We use Galileo's scientific instruments to see different kinds of radiation --- and even to look through clouds or beneath the solid surface of a moon.

A camera sees the way we do, at wavelengths of light we've called "visible." But we are surrounded by other wavelengths of radiated energy that we cannot see. We talk on cellular phones -- they use radio waves -- cook in ovens that use microwaves, and change TV channels with remote-control units that use infrared. We avoid ultraviolet by wearing dark glasses and sun block.

There are 10 scientific instruments aboard the spacecraft. Some of the instruments are mounted on long arms so they don't pick up unwanted signals from the spacecraft. The probe used six instruments plus its radio to investigate Jupiter's atmosphere. In addition, we use Galileo's radio system for experiments.

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Observing Atmospheres

We studied Jupiter's atmosphere at the very top-most cloud layers -- about ninety-five miles deep. As the probe descended, the temperature and pressure rapidly became too intense for the probe to survive.

One big question was ---- What drives Jupiter's colossal weather? Galileo studied the atmosphere from the inside with the probe, and from the outside with the spacecraft's instruments. The probe's instruments told us about the local weather -- temperature, winds, lightning -- and the type of clouds and gases there. Near the equator, the probe's instruments looked at the energy coming from inside and outside the planet. The spacecraft's instruments gave a view of things over a wider area, and looked at cloud patterns and how they changed over time across the planet.

Much of the information comes from observations in visible and infrared light. Galileo's camera tracks the shape and movements of clouds and storms. This camera also measures near-infrared light -- to tell us how high the cloud decks are. The near-infrared mapping spectrometer, known as NIMS, identifies molecules in the clouds. An instrument called the Photopolarimeter Radiometer -- the PPR -- can determine the size and shape of droplets or particles in the clouds.

NIMS and the PPR work together to measure the temperature of gas and clouds -- important to know because temperature differences drive atmospheric storms and circulation. NIMS measures energy coming from Jupiter's "hot spots" --- cloudless areas of hot updrafts from the interior.

Observing Surface of Moons

You might think that the moons of a planet are all alike -- but the opposite is true. We want to study features on the surface, like mountains, valleys, lava flows, and craters. Is the surface rock or ice -- and what kind? The information helps scientists write the history of each moon, and figure out why a particular moon looks the way it does today. Galileo's camera gives us stunning visual images of these strange worlds, with their volcanoes, craters, or icy terrain. The NIMS identifies the minerals that make up the surface.

What is the texture of rocks and ice on the surface? Is the surface hard ... or loose.... sandy... fine-grained.... clumpy? The PPR determines the size of crystals on the surface by looking at light that has been polarized by these crystals. Polarization can reveal information about the nature of the object that is reflecting the light. We take advantage of it on Earth by wearing sunglasses with polarized lenses that give a "cool" view of brightly lit objects.

What is the environment on the surface of a moon like? Is it hot, or cold? And how hot or cold? By measuring the infrared light emitted from the surface, NIMS and the PPR can take a moon's temperature.

Observing Magnetic Fields

Jupiter's huge, intense magnetic field behaves as though there were a giant bar magnet inside the planet. The magnetic field acts as a deflector shield against the stream of particles in the solar wind coming from our Sun. Instead of striking Jupiter, some of these charged particles get caught inside the magnetosphere, where they are accelerated to enormous speeds and zip along the planet's magnetic field lines.

Galileo measures Jupiter's magnetic field while passing through it. To avoid picking up interference, certain instruments are held away from the spacecraft. The Magnetometer measures the strength and direction of the field. The Energetic Particle Detector and the Plasma Instrument determine the number, energy, and direction of the particles inside the magnetosphere and in the solar wind. A Plasma Wave detector senses waves in the streams of particles.

Galileo's camera records a bright aurora, as fast-moving particles from the Sun follow magnetic field lines into Jupiter's atmosphere. The beautiful glow results as the charged particles strike the upper atmosphere. The aurora also glows in ultraviolet light, which is measured by Galileo's Ultraviolet Spectrometer and Extreme Ultraviolet Spectrometer.

Observing Interiors

How can we tell what's beneath the surface of a moon or planet? A moon might have layers of different material, or be the same throughout. There might be an ocean underneath. Even though we can't make a direct measurement, we can measure effects and draw conclusions.

Scientists use the spacecraft's communications system to measure how the frequency of a radio signal changes by the time it is received on Earth. When Galileo passes close by Jupiter or one of the large moons, gravity tugs on the spacecraft, changing its speed. By precisely measuring the change in the radio frequency received on Earth, we can estimate the mass and internal structure of Jupiter or its moons.

Close to a moon, if Galileo's Magnetometer detects a magnetic field, it is a likely reflection of what is inside. Magnetic fields have unique signatures that can be read by the Magnetometer. One kind of field is created when there is a flowing, liquid metallic substance inside the moon. But a different sort of field results when Jupiter's huge magnetic field sweeps through a layer of salty water, generating electric currents. It appears that this is the case on the moon Europa, and perhaps on Callisto as well.

Mission Operations

Introduction

Flying a spacecraft to a far planet, millions of miles away, takes many talented people working with very special equipment.

In planning the mission, engineers and scientists decide what kinds of instruments ride on board the spacecraft and what kind of information is gathered. They plan the spacecraft's journey by making precise calculations of its path through space.

The spacecraft itself is a complex machine that must operate perfectly in alien environments -- under conditions of intense radiation, and extreme cold and heat. The spacecraft receives commands from mission controllers and sends scientific data back to Earth. A computer on board the spacecraft manages the two-way communications equipment and controls the scientific instruments and the other activities of the spacecraft.

NASA tracks missions using a world-wide communications system called the Deep Space Network. Huge antennas --- some nearly as big across as a football field --- capture the faint signals from spacecraft. The signals carry the science data, which must be decoded into information or images. The Deep Space Network also has powerful transmitters to send commands to distant spacecraft.

Mission Planning
A space mission depends on scientists and engineers working together --- teamwork is critical. Everyone has a special job to do to achieve the mission goals.

Once the scientists and engineers agree just on what the spacecraft will do, the mission engineers program the operating instructions into a series of commands that the spacecraft can understand.

Months before launch, hundreds of things are decided -- how much fuel to carry, how the spacecraft can be protected during its journey, and what the scientific instruments will do.

Mission scientists study different things about a planet or moon -- geology, weather, chemistry, and magnetic fields, to name a few. They identify which instruments can best collect the information they need. The scientists and engineers determine how to balance the mission goals against the limited resources on the spacecraft.

When an instrument makes a measurement or observation, it uses power. If the spacecraft needs to maneuver to get a better look at something -- this consumes fuel.

On-board power and fuel are limited. And for Galileo, space on the tape recorder to store scientific data is the most precious resource of all. Everything a spacecraft does is planned in advance for the best use of limited resources.

Guiding the Spacecraft

Spacecraft navigators do the same thing that ocean-going ship navigators do -- except that the ocean of space is much, much bigger and more dangerous. How do the navigators know where the spacecraft is once it leaves Earth?

The path of the spacecraft -- the trajectory -- is planned well before launch. But once the spacecraft leaves Earth, knowing where it is, and predicting where it will be at a certain time, is a complicated process.

We can't see the spacecraft, even with a telescope, so we use an indirect method to find it. If we know the velocity -- the speed and direction -- of the spacecraft, we can figure out where it is. As the spacecraft travels away from Earth, the radio signals it sends to us appear to change frequency. By monitoring this change, we can calculate speed and direction -- and thus, the exact location of the spacecraft. This shift in frequency, called the Doppler shift after the physicist who first described it, is caused by the motion of the spacecraft in relation to Earth. You hear the Doppler affect in a police siren when it changes to a higher pitch as it approaches you, then lowers after it passes by.

To track the spacecraft, NASA located the antennas of the Deep Space Network equally around Earth. The huge antennas are located in California, Spain, and Australia. As Earth turns, an antenna at one location hands over the spacecraft signal to the next antenna.

Commanding the Spacecraft

None of NASA's exciting missions to other planets would be possible without the Deep Space Network, which receives scientific data and images sent from the spacecraft and also transmits coded instructions, or commands. NASA uses the Deep Space Network to control the spacecraft, load or reprogram its computer, and to make sure it stays on the right path to its destination.

The farther away the spacecraft travels, the longer it takes for the commands from Earth to get there. It can be as long as 50 minutes before commands from Earth reach Galileo at Jupiter.

The Mission Operations Team plans which commands to send. The commands are collected into packages called sequences several days before they are scheduled to be transmitted.

The Deep Space Network's antennas have a very narrow view of the sky --- something like looking up through a soda straw. To send the commands, mission controllers have to point the antennas -- very precisely -- to the location of the spacecraft. One of the three Deep Space Network stations tracks the spacecraft and transmits the sequence of commands.

Receiving Data and Images

The Galileo spacecraft sends us amazing images of other worlds -- erupting volcanoes, giant icebergs, and furious storms. The images of Jupiter and its moons -- as well as other scientific information -- are sent out from the spacecraft as digital code.

The onboard computer converts information from the scientific instruments into binary code --- zero's and one's --- called bits. A flow of digital data -- a bitstream -- is sent from the spacecraft's radio transmitter to the Deep Space Network antennas.

But the signals carrying the information to Earth are very faint, and the farther they travel, the weaker they get. Millions of miles away, the giant antennas of the Deep Space Network capture the faint whisper of the bitstream. There is a background of natural radio noise that threatens to drown out the spacecraft signal, so the Deep Space Network uses sensitive receivers and powerful amplifiers to separate the radio signals from the noise.

The signals are converted into data for image-processing experts to reassemble them into pictures of faraway, mysterious worlds.

Additional Resources

National Space Science Data Center Master Catalog: Galileo Orbiter

National Space Science Data Center Master Catalog: Galileo Probe

Selected References

Siddiqi, Asif A. Deep Space Chronicle: A Chronology of Deep Space and Planetary Probes 1958-2000, NASA, 2002.

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