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Sixty-Five Amazing Galileo Facts
Color illustration of Galileo spacecraft entering Jupiter's atmospehere.
An artist's impression of the Galileo orbiter beginning to burn up in Jupiter's atmosphere. Galileo's 14-year mission to explore the Jovian system ended on 21 Sept. 2003 when the spacecraft was deliberately sent into Jupiter's atmosphere.

Editor's Note: The Galileo mission team produced this list of facts about their mission to Jupiter back in 1996. They are presented here as a historical record of an amazing, historic mission and have not been updated to reflect the latest knowledge about Jupiter, the Galileo mission and our solar system. Enjoy.

The surface gravity on Ganymede is only 15% that of Earth's. A 150 lb. person would only weigh 22.5 lbs. on Ganymede. Interestingly, even though Ganymede is twice our Moon's mass, because its density is much smaller than the Moon's, the Moon's surface gravity is a bit stronger (17% that of Earth's).


Ganymede's surface area is over half as great as Earth's land area.

In certain ways the Galileo spacecraft must be treated as an incredibly capable but occasionally temperamental sports car, with engineers regularly "tuning" the spacecraft for maximum performance. These activities involve exercising certain mechanical devices rather than letting them sit idle for long time periods, regularly flushing propellant through the thrusters to minimize the chance of the filters clogging, and software updates when necessary to allow the spacecraft to know when something major has changed in its own mass or in its environment. All this maintenance must necessarily take place remotely via radio command.

The Galileo spacecraft is made of materials that suggest it is much more like a fighter plane than a car. It uses very lightweight materials such as beryllium to house the subsystems, aluminum for the structure, and carbon composites for the booms.

On its first orbit around Jupiter, the Galileo spacecraft reached a maximum distance from Jupiter of about 20 million kilometers. This is nearly half the distance between the orbits of Earth and Venus, Earth's closest planetary neighbor.

Batteries only get you so far in outer space. The Galileo orbiter carries two radioisotope thermoelectric generators (RTGs), which are used to generate electrical power on board the spacecraft. There are 7.8 kilograms (17.2 pounds) of Plutonium-238 in each RTG.

Galileo's roots date back to an early recommendation for an atmospheric probe that would explore Jupiter's atmosphere down to pressure levels 100 times that of Earth at sea level. This proposal eventually became JOP (for Jupiter Orbiter Probe), which then eventually became Galileo.

When the Galileo Probe entered Jupiter's atmosphere, it was traveling at a speed of 106,000 miles per hour -- the fastest impact speed ever achieved by a man- made object. At that speed, one could drive around the Earth at the equator in 14 minutes (assuming there were bridges across all the oceans) or to the Moon and back in only 5 hours!

Antenna deployment drive motors have been pulsed over 15,000 times as of September 1994 (the last time that they were used) in the effort to open Galileo's High Gain Antenna.
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On its journey from Earth to Jupiter, Galileo traveled 2.4 billion miles. Along the way, about 67 gallons of fuel from the propulsion system were used to control Galileo's flight path and to keep its antenna pointed at Earth. That's equivalent to getting 36 million miles per gallon! With that kind of mileage, one would use up only 4 tablespoons of gasoline to drive to the Moon and back!

The last time Galileo navigators had a chance to control the entry conditions of the atmospheric Probe was approximately 167 days prior to its arrival at Jupiter. To ensure a successful Probe-Orbiter radio link, Probe Entry was required to occur within 8 minutes of the desired time. After 167 days, that allowed the navigators less that a 0.0033% error (that is, an error of 33 parts per million) in the time of arrival. Probe entry actually occurred 14 seconds earlier than the target time, only a 0.0001% error! The last Orbiter trajectory correction maneuver was performed 100 days prior to arrival and delivery occurred 14 seconds later than the target time. An equally impressive 0.0002% error!

After traveling 2.4 billion miles in just over 6 years to reach Jupiter, Galileo missed its target at the Jovian moon Io by only 67 miles. That's like shooting an arrow from Los Angeles at a bull's-eye in New York and missing by only 6 inches!

Since being launched from Earth on October 18, 1989, Galileo has traveled 2.4 billion miles in just over 6 years to reach Jupiter. That's an average speed of 44,000 miles per hour. At that speed, one could drive around the Earth at the equator (assuming there were bridges across all the oceans) in just over half an hour, or to the Moon and back in only 11 hours!

Jupiter has some truly high velocity winds-- they blow at speed as high as 260 miles per hour at Jupiter's cloud tops!

Magnetic fields can be powerful entities. Jupiter's magnetosphere strips away 1 ton of material from Io a second. Io's orbital motion through Jupiter's magnetosphere generates electricity--an electric current of 3 million amps!

Using Galileo's onboard instruments to observe the asteroid Gaspra was a challenge somewhat akin to attempting to spot the Goodyear Blimp through a soda straw from five miles away, while sitting in a car going 90 mph.

Jupiter's moon Io has some of the most dramatic-appearing volcanoes around. Geysers on Io spew out at speeds as high as 1 km/sec (2,300 mph). On Earth, Mt. Etna's ejecta erupt out at a "mere" 112 mph; terrestrial volcanoes rarely exceed 200 mph. Io's gravity is low (1/6 that of Earth), so the plumes are huge--reaching as much as 162 miles high. If you could move Old Faithful to Io, it would shoot up a plume of water and ice over 21 miles high (note, though, that we have no detection of water on Io and the most likely fluid/gas for the plume geysers is sulfur dioxide).

Io is arguably the most volcanically active body yet known. We've seen at least 200 volcanic caldera bigger than 12 miles in diameter on its surface.

Jupiter's tug on the Galilean moon Io causes tides, just like our own moon raises tides on the Earth's surface. A typical ocean tide on Earth is about one meter (3 feet). Io's surface tides (no water!), though, are truly something to behold: as great as 330 feet high!

How many people have worked on Galileo? Nobody knows for sure, but it's been estimated that roughly 10,000 people have worked directly on Galileo since the Project's start in 1977. That's excluding people associated with the Space Shuttle and the Inertial Upper Stage booster.

Galileo passed about 100 km closer to Io than planned. This meant that the gravity assist from Io slowed Galileo's speed more than was planned, putting the spacecraft into a shorter orbit around Jupiter than expected. Rather than use up over 10 kilograms of propellant to "fix" this, Galileo's navigators realized that they could just let the spacecraft continue on its way...where it would arrive at its first encounter with the moon Ganymede a week early! Amazingly, going slower means we're getting where we're going sooner! (The Ganymede flyby was originally scheduled to occur on July 4, 1996. Since it takes a little over one week for Ganymede to go around its orbit, it would be in about the same location one week earlier on June 27. Since Galileo's orbit was one week shorter than originally planned, the first satellite encounter could simply be moved earlier by one week)

January 7 was the 386th anniversary of Galileo Galilei's discovery of Jupiter's moons Io, Europa, and Ganymede. Galileo spotted the moon Callisto a few days later. The four moons--the largest in the Jovian system--are now called the Galilean satellites, in honor of their discoverer.

Because the asteroid Gaspra is so small (about 19 x 12 x 11 kilometers, or 12 x 7.5 x 7 miles), its surface gravitational force is two thousand times smaller than that of the Earth's, yielding an escape speed of only 10 meters per second (22 miles per hour); an Olympic-caliber sprinter could run himself into orbit! A 200 pound person would weigh 0.1 pounds!

The Gaspra asteroid flyby was yet another example of outstanding navigation: at closest approach, Galileo was just 1.5 seonds and 1 kilometer (0.6 miles) from the aim point. Even so, taking the picture was a dramatic achievement. One of Galileo's scientists said "It was like taking a picture of a large house in San Francisco from Los Angeles."

Most of Galileo's actions are controlled by commands that have been stored on board the spacecraft in advance, but there are still many occasions when the spacecraft's commands have to be sent up in "real time." How many times have we had to do this? As of December 14, 1995, a total of 275,586 real-time commands have been transmitted to Galileo since launch. In the past week alone, 169 real-time commands were transmitted.

When the Galileo Photopolarimeter Radiometer detected the flashes of light caused by Comet Shoemaker-Levy 9 crashing into Jupiter in July 1994, it was using a 4 inch telescope, and it was as far away from Jupiter as Mars is from the Sun.

Galileo's second Earth flyby brought the spacecraft within 303 kilometers (182 miles) of the Earth's surface. The gravity assist added 3.7 kilometers per second (8,300 miles per hour) to the spacecraft's speed in its solar orbit. As always, Galileo's navigation was impeccable: the spacecraft was within a kilometer of its intended path, and was just 0.1 second early.

Galileo Galilei didn't always want to study mathematics. As a young boy, he wanted to be a monk.

Galileo's first earth flyby, delta v was 5.2 kilometers per second, or 11,600 mph, with closest approach at 960 km, or 597 miles, above the Earth's surface.

Galileo's cameras will capture pictures that can detect objects as small as 12 meters (39 feet). That's an improvement on Galileo Galilei's original telescopic observations by factors up to 100,000 to 1,000,000.

Although Galileo Galilei was a college dropout, he went on to become a respected professor.

The gravity assists from Venus and Earth (two times) that Galileo received on its way to Jupiter caused 11.1 km/sec of velocity change. It would have taken an extra 10,900 kilograms of propellant to get that same boost--about twelve times more than what was on board at launch. Close flybys and gravity assists will also be used to enable Galileo to make a complex tour of Jupiter's system. An additional 3,600 kilograms of propellant (roughly four times the total on board at launch) would be needed to fly the tour without the billiards-like gravity assist technique.

Galileo became the first spacecraft to see the surface of Venus without the use of radar. On February 10, 1989, it observed the oven-hot surface with its near- infrared camera, observing numerous mountain rag ranges and valleys through Venus'thick atmosphere and clouds.

Galileo Galilei was an Italian astronomer and physicist (1564-1642) who was the first to use the telescope for serious astronomical study. Using his homemade telescopes, he discovered the four large moons of Jupiter, demonstrating for the first time that bodies in motion could themselves be centers of motion. This contradicted common teaching of the time that all celestial bodies went around the Earth.

Galileo's first picture of Jupiter was taken in 1989, just two months after launch! To check out the camera before the Venus encounter, we snapped a bunch of images, including one of Jupiter. Even from a half-billion miles away, the Solid State Imaging camera could clearly see atmospheric banding and the Jovian satellites.

The Great Red Spot has been seen since the 17th century. It is thought to be a large storm system and is wider than two Earths.

Galileo's 400-Newton engine looks powerful, but it actually gives a pretty gentle ride. The "nudge" it gives the spacecraft is like sitting in a car going from 0 - 60 miles per hour in just under 2 _minutes_! (an 8 year old Toyota can beat that by almost a factor of 10.) One the other hand, unlike that car, Galileo can keep accelerating as long as the engine fires. At the end of the 45 minute JOI burn, Galileo's speed has changed by about 1400 mph.

The Galileo mission to Jupiter was approved by Congress in 1977. It is an international cooperation involving scientists and engineers from the USA, Germany, Canada, Great Britain, France, Sweden, Spain and Australia. Germany's contribution has included development and operations support of the propulsion system.

Galileo's planetary flybys have allowed the spacecraft to speed up, at the expense of the planet slowing down slightly. How much were Venus and the Earth slowed by Galileo's gravity assists? In a billion years, Venus will be 1.6 inches behind where it would have been in its orbital path if Galileo hadn't interfered. The Earth will be 5.2 inches behind (a 2.9 inch lag from the first Earth gravity assist, and the other 2.3 inches from the second assist).

If you could see Jupiter's magnetosphere (the region of space where Jupiter's magnetic field predominates over the solar magnetic field) from Earth with your eyes, it would appear larger than our full moon. It is the largest single structure inside the Solar System.

Most people imagine that the pictures taken by interplanetary spacecraft are pretty much what the naked eye might see if an observer could ride along. It isn't until one looks at actual fields of view and slew rates (how fast things seem to move through the field of view) that one appreciates how big a job pointing the cameras can be, especially with a limited picture budget. For example: the view through Galileo's Solid State Imaging camera is about half a degree square, roughly the size of a full moon. Looking through a soda straw gives a much larger view than that! To see the world as SSI does, you need to join _three_ soda straws end-to-end and peer through it. Now remember that you're moving relative to the target. Taking photos during Galileo's flyby of Ida was like trying to look through those soda straws and trying to target a building at JPL while driving down the 210 freeway (when you're about three miles away) at 55 miles per hour.

Why is Galileo is going to Jupiter when the two Voyagers have already been there? The Voyagers were like a quick car trip past the Grand Canyon -- drive by, snap a few pictures, check off that you've been there and move on. Galileo is like stopping to explore the canyon, walking the trails (satellite tour), riding the rapids (Probe mission), and generally taking in the full majesty of the place (fields and particles).

Jupiter has 16 known moons. Of the four largest, Europa is just slightly smaller than our Moon, while Io, Ganymede, and Callisto are larger than our Moon. In fact, Ganymede is larger than the planet Mercury.

The amount of power being transmitted out of the spacecraft radio is about the same as that from a refrigerator lightbulb--about 20 watts.

The Galileo probe will hit the atmosphere of Jupiter at over 47 km per second, the highest impact speed achieved by a human-made object. During its entry phase, the probe will slow down so fast (within two minutes) that it will feel 250 times heavier than its original weight.

Many people have a mental image of Galileo coming up behind Jupiter and overtaking it. However, if we look at the spacecraft's trajectory in a Sun- centered coordinate system, the orbiter is actually AHEAD of Jupiter in the days before arrival; it's Jupiter that is coming up from "behind" and "descending upon" the orbiter. Even the engineers and scientists working on Galileo are often surprised by this!

Roughly 60 percent of Galileo's radiation dose will be received within an hour of Jupiter closest approach. Unshielded, 15 minutes would be fatal to a human being. Your average PC probably wouldn't do a lot better. We've had to use some special computer chips and a lot of shielding to protect our computers. (The attitude control computer has so much shielding around it that the compartment it goes into has been nicknamed "the Brinks Bay" after the people who build safes for banks.)

Jupiter has no solid surface; it is composed almost entirely of hydrogen and helium and is hot enough inside to vaporize all elements. After the Galileo probes's mission is completed, it will continue to sink into Jupiter until the temperatures reach points that, in turn, cause various parts of the probe to melt and then vaporize.

Think it would be easy to use a spacecraft to find life on other planets? As an exercise, scientists used observations from Galileo's Earth flybys to look for life on Earth. Galileo detected several signs of life as it passed by the Earth for gravity assists in December 1990 and December 1992. The existence of oxygen and methane in the air and the strong infrared signature from the land areas were indications that something biological might be taking place. The only indication of intelligent life, however, were strong radio emissions that were difficult to attribute to natural causes.

Galileo's radio signal is monitored by the Deep Space Network, whose antennas are located near Canberra, Australia, at Goldstone, near Barstow, California, and near Madrid, Spain. Having three stations spaced roughly equally around the world ensures that at least one station can communicate with the spacecraft at all times as the Earth rotates.

More than one-third of Galileo's mass is propellant (fuel and oxidizer), as compared to about 5% for gasoline (fuel) for the typical automobile. Remember that automobiles burn the gasoline using oxygen from the air while spacecraft have to carry both fuel and oxidizer because there is no air in space.

A day on Jupiter is only 9 hours and 48 minutes long. Such fast rotation causes Jupiter to be somewhat squashed due to centrifugal force: its polar radius is 4000 km less than its equatorial radius (the latter being 71,392 km). That means you'd weigh almost 25% more at Jupiter's poles than at its equator, that is, if you could find a place to stand! So a 100 pound person (on Earth) would weigh 230 pounds at Jupiter's equator but 285 pounds at the pole!


It's much more difficult to go into orbit around a planet than to just fly by it because you have to slow the spacecraft down somehow for it to be captured by the planet's gravity. Five spacecraft have previously flown by Jupiter: Pioneer 10 and 11, Voyager 1 and 2, and Ulysses. Galileo will be the first to go into orbit around Jupiter, or for that matter, any outer planet.

The Galileo probe weighs 339 kilograms (750 pounds) and will enter the atmosphere at a top speed of 170,000 kilometers per hour (106,000 mph), or about 50 times faster than a bullet shot out of a rifle. The probe will experience deceleration forces as high as 230 times Earth's gravity. In about two minutes, the probe's speed will be slowed to about 1,600 kilometers per hour (1,000 mph) as it begins its 75-minute mission to measure the planet's atmosphere and clouds, while descending into the dense atmosphere under its parachute. It's possible that the probe will also encounter lightning and rain.

Engineers have taken several steps to boost the performance of the ground antennas that will be receiving Galileo's signals. One trick is to use arrays of antennas instead of single antennas. How big (in diameter) would a single antenna be to provide the same performance as, say, the combination of the Canberra Deep Space Network site, the Parkes Radio Telescope, and the largest Goldstone antenna? 125 meters (410 feet)! That's about the distance from home plate to the wall in center field in Shea Stadium!

Galileo was somewhat slowed by having to plough through a dust storm originating from Jupiter--after three weeks, Galileo's speed dropped by a whopping 25 angstroms per second (1 angstrom is one ten-billionth of a meter, or about 4 billionths of an inch)!

How big are those dust motes that were slowing Galileo? The dust motes are .01 to .1 micrometers in diameter, which is 1/100th to 1/10th the size of airborne dust that you might see floating in sunlight in front of a window, or, several hundred times smaller than a human hair.

There's usually not that much dust for Galileo to plough through: a 1300 square foot house has maybe 290 cubic meters of space inside (about 10,000 cubic feet). At a usual interplanetary dust density of .00006 particles per cubic meter, you'd find one particle of dust in every 57 houses! Equivalently, that's the volume that you'd get if you covered a football field with a 4 meter high roof-- and you'd only find one dust particle in that entire volume!

The Galileo spacecraft is designed to survive extreme environmental hardship. The temperatures on various parts of Galileo can range from -220 to +220 degrees celsius. The average radiation dose per minute absorbed by Galileo during its orbital mission is equivalent to what the average person receives in a whole year on Earth. On 7 December, as it makes its closest approach to Jupiter, the radiation dose per minute to Galileo will exceed by several times what a person on Earth would receive in their entire lifetime!

Jupiter's volume is about 1,400 times that of the Earth. In fact, its volume is half again bigger than all of the Solar System's other planets, moons, asteroids, and comets combined.

The radio signals from Galileo will be incredibly weak (about a billion times fainter than the sound of a transistor radio in New York as heard from Los Angeles). Radio science tries to measure effects that are very minute (about the same as measuring the distance between Los Angeles and New York to the accuracy of a human hair).

It's not necessarily true that the shortest distance between two points is a straight line: as of November 15, 1995, Galileo still has 9,329,160 kilometers to travel along the curved arc that will take it to Jupiter. However, if you drew a straight line between the spacecraft and Jupiter on November 15, they'd be separated by 16,141,701 kilometers. What's going on? If Galileo headed out for where Jupiter is right now, it would arrive to find Jupiter long gone--the planet's orbital motion would have carried it further along its path around the Sun. Instead, the spacecraft is heading towards the point where its trajectory and Jupiter's orbit will intersect. Since Jupiter's speed going around the Sun is much faster than Galileo's current speed with respect to the Sun, Jupiter's orbital motion will eat up much of the distance between spacecraft and planet. That means that Galileo itself doesn't have to travel as far.

Galileo has already bagged a number of scientific firsts, including: the first close-up look at an asteroid (Gaspra), the first discovery of an asteroid moon (Ida's moon Dactyl), the first and only documented direct observation of a collision between solar system bodies (the impact of Comet Shoemaker-Levy 9 with Jupiter), and the first global scale multi-spectral (i.e. using several different wavelengths of light) imaging of the Moon's far side.

Typical home insulation has a rating R=19, and is 4" thick. Galileo's insulating blankets are 3 times as effective, and 1/20th the thickness (2/10ths of an inch thick). The spacecraft insulation is 65 times "better" than the fiberglass insulation that you'll find in your home!

Since Galileo is going into orbit around Jupiter (unlike the two Voyager spacecraft, which flew by the planet), it can fly by Jupiter and its moons at far closer distances than did Voyager, which means that Galileo's pictures will be a dramatic improvement over those from Voyager. Comparing Voyager images with those to be sent back from Galileo is like viewing a book at the base of the Empire State Building from the top story, as opposed to holding that book in your hands.

Galileo's power sources, radioisotope thermo-electric generators, are putting out only about 520 watts on 7 December. That's not even enough to run a kitchen toaster!

Last Updated: 24 January 2011

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