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Galileo's Fight and Flight to Jupiter

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

Galileo on the final plunge.

Galileo on the final plunge.

We often hear of the numerous space probes that venture out into the solar system. Many of them have been exclusively for a specific planet while others have had to pass by multiple targets. But until 1995, Jupiter never had a dedicated probe exploring it. That all changed with the launch of Galileo, named after the scientist who made so many contributions to our understanding of Jupiter, but even getting the launch was a struggle almost a decade in the making. That Jupiter ever got Galileo ended up being a miracle.

Why Go to Jupiter?

Galileo was born as the Jupiter Orbiter and Probe (JCP) Mission in 1974 by JPL The mission objectives were simple: study the chemistry and physical layout of Jupiter, look for new moons, and learn more about the magnetic field surrounding the system. This was all in line with NASA's planetary exploration program (whose most famous members include the Pioneer and Voyager probes) which sought to find out what is so special about Earth by studying the differences in our solar system. Jupiter is a special piece of that puzzle for several reasons. Its the largest member of the solar system save for the Sun and so is likely in its most original configuration courtesy of its immense gravity and size. This has also allowed it to hold onto many moons which can offer evolutionary hints as to how the solar system grew into what we have today (Yeates 8).


With its goals and parameters established, Galileo was sent to be approved by Congress in 1977. The timing was not good however because the House was not so warm to funding such a mission, which would make use of the Space Shuttle in getting the probe into space. Thanks to the efforts of the Senate however the House was convinced and Galileo moved forward. But then just as that hurdle had been overcome, problems arose with the rocket initially meant to get Galileo to Jupiter once clear of the Shuttle. A 3-stage version of the Internial Upper Stage, or IUS, was designed to take over once the Shuttle got Galileo clear of Earth but a redesign followed. The anticipated 1982 launch was pushed back to 1984 (Kane 78, Yeates 8).

In November of 1981, the President’s Office of Management and Budget was getting ready to pull the plug on Galileo based on the developing problems. Fortunately, just a month later NASA was able to save the project based on how much money had been invested in the program already and how if Galileo did not fly then the US Planetary Project, our effort in exploring the solar system would effectively be dead. But the save came at a cost. The booster rocket initially chosen to launch Galileo would need to be scaled back and another project, the Venus Orbiting Imaging Radar (VOIR) probe would need to sacrifice funds. This effectively killed that program off (Kane 78).


Costs did continue to grow for Galileo. After the work was done on the IUS it was determined that Jupiter was now further away, thus necessitating an additional Centaur booster rocket. This pushed the launch date to April of 1985. The total for this mission had grown from the projected $280 million to $700 million (or from about $660 million to about $1.6 billion in current dollars). Despite this, scientists reassured everyone that the mission was worth it. After all, Voyager had great success and Galileo was a long-term follow-up, not a fly-by (Kane 78-9, Yeates 7).

But VOIR was not the only mission that paid for Galileo’s ticket. The International Solar Polar Mission was cancelled and numerous other projects were delayed. Then the Centaur that Galileo was counting on was out, which left as the only recourse 2 IUS’s and a gravity boost to get Galileo to its destination, adding 2-years to the travel time and also reducing the number of moons it would intercept as it eventually orbited Jupiter. More risk now for something to go wrong and with diminishing potential results. Was it worth it? (Kane 79)


The Probe

Lots of science has to be done with the biggest bang for buck, and Galileo was no exception. With a total mass of 2,223 kilograms and a length of 5.3 meters for the main body with an arm full of magnetic instruments measuring 11 meters long. They were far away from the probe so that the electronics of the probe would not provide false readings. Other instruments included were

- a plasma reader (for low-energy charged particles)

- plasma wave detector (for EM readings of the particles)

- high energy particle detector

- dust detector

- ion counter

- camera composed of CCDs

- near IR mapping spectrometer (for chemical readings)

- UV spectrometer (for gas readings)

- photopolarimeter-radiometer (for energy readings)

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And to ensure that the probe moves, a total of twelve 10-Newton thrusters and 1 400 Newton rocket were installed. The fuel used was a nice mix of monomethyl hydrazine and nitrogen-tetroxide (Savage 14, Yeates 9).

The Original Plan

Galileo's flight into space was delayed because of the Challenger disaster, and the ripple effects were devastating. All the orbital maneuvers and flight plans would have to be scrapped because of the new locations Earth and Jupiter would be in. Here is a brief look at what would have been.

The original orbital insertion. As we will see, this was way simpler than what was needed.

The original orbital insertion. As we will see, this was way simpler than what was needed.

The original orbits of the Jupiter system. This only required minor modifications and essentially is the same as what transpired.

The original orbits of the Jupiter system. This only required minor modifications and essentially is the same as what transpired.

Atlantis launches.

Atlantis launches.

The Mission Begins

Despite all the budget concerns and the loss of Challenger pushing back the original launch of Galileo, it finally happened in October of 1989 aboard space shuttle Atlantis. Galileo, under the direction of William J. O’Neil, was free to fly after a seven year wait and $1.4 billion spent. Modifications to the craft had to be made because the orbital alignment from 1986 no longer existed and so extra thermal protection was added so it could endure its new flight path (which also helped drive down costs). The probe used several gravity assists from Earth and Venus and actually went through the asteroid belt twice because of this! The Venus assist was on February 10, 1990 and two Earth flybys occurred on December 8, 1990 and two years later to the day. But when Galileo finally arrived at Jupiter, a new surprise awaited scientists. As it turns out, all that inactivity may have caused the 4.8 meter diameter high gain antennae to not fully deploy. It was later determined that some of the components which held the structure of the antennae together were stuck from friction. This failure reduced the targeted 50,000 picture goal of the probe for the mission because they would now have to be transmitted back to Earth at a blazing (sarcasm implied) rate of 1000 bits a second using a secondary dish. Still, having something was better than nothing (William 129, 133; Savage 8, 9, Howell, Betz "Inside," STS-34 42-3, Space 1991 119).

Galileo moments before it departs Atlantis.

Galileo moments before it departs Atlantis.

Of course, those flybys were not put to waste. Science was gathered on Venus’ mid-level clouds, a first for any probe, and also data on lightning strikes on the planet. For Earth, Galileo took some readings of the planet and then moved on to the Moon, where the surface was photographed and the area around the north pole was examined (Savage 8).

Galileo heads out.

Galileo heads out.

Asteroid and Comet Encounters

Galileo made history before it even made it to Jupiter when on October 29, 1991 it became the first probe to ever visit an asteroid. Lucky little Gaspra, with dimensions of roughly 20 meters by 12 meters by 11 meters, was passed by Galileo with the closest distance between the two just being 1,601 kilometers. Pictures indicated a dirty surface with much debris about. And if that wasn’t great enough, Galileo became the first probe to visit multiple asteroids when on August 29, 1993 it passed by 243 Ida, which is about 55 kilometers long. Both flybys indicate that the asteroids have magnetic fields and that Ida seems to be older because of the number of craters it possesses. In fact, it could be 2 billion years old, over 10 times the age of Gaspra. This does seem to challenge the idea of Ida being a member of the Koronis family. This means that Ida either fell into its zone from elsewhere or the understanding of the Koronis asteroids. Also, Ida was found to have a moon! Named Dactyl, it became the first known asteroid to have a satellite. Because of Kepler’s Laws, scientists were able to find out Ida’s mass and density based on Dactyl’s orbit, but surface readings indicate separate origins. Ida's surface has mainly olivine and bits of orthopyroxene while Dactyl has equal proportions of olivine, orthopyroxene, and clinopyroxene (Savage 9, Burnhain, Sept. 1994).


An added surprise was Comet Shoemaker-Levy 9, which was found by scientists on Earth in March of 1993. Shortly thereafter, the comet was broken up by Jupiter’s gravity and was on a collision course. How fortunate that we had a probe which could get valuable intel! And it did, when Levy 9 finally crashed into Jupiter in July of 1994. Galileo’s position afforded it a backside angle to the collision that scientists otherwise wouldn’t have had (Savage 9, Howell).

The probe's descent.

The probe's descent.

Arrival and Findings

On July 13, 1995, Galileo released a probe which would fall into Jupiter at the same time the main probe arrived at Jupiter. That happened on December 7, 1995, when that part of Galileo descended into the clouds of Jupiter at a speed of over 106,000 miles per hour for 57 minutes while the main body of the probe entered Jupiter orbit. As the offshoot was competing its mission, all the instruments were recording data on Jupiter, the first such direct measurements taken of the planet. Preliminary results indicated that the upper atmosphere of the planet was drier than anticipated and that the three-layered structure of the clouds which most models predicted was not correct. Also, the helium levels were just half of what was expected and overall the carbon, oxygen, and sulfur levels were less than expected. This could have implications for scientists decoding the formation of the planets and why levels of certain elements do not match models (O’Donnell, Morse).


Not too shocking but still a fact was a lack of solid structure encountered by the atmospheric probe during its descent. Density levels were higher than expected and this along with a deceleration force up to 230g and the temperature readings seems to indicate an unknown “heating mechanism” present at Jupiter. This was especially true during the portion of the descent with the parachute, where seven different winds with wide temperature differentials were experienced. Other departures from the predicted models included

-no layer of ammonium crystals

-no layer of ammonium hydrosulfide

-no layer of water and other ice compounds

There were some indications that the ammonium compounds were present but not where they would have been expected. No evidence of water ice was found at all despite evidence from Voyager and the Shoemaker-Levy 9 collisions pointing towards it (Morse).

Galileo over Io.

Galileo over Io.

The winds were another surprise. Models pointed to top speeds of 220 mph but the Galileo craft found them to be more like 330 mph and over a larger range of altitude than expected. This may be because of the unknown heating mechanism giving the winds more muscle than expected from sunlight and water condensation action. This would mean a decrease in lightening activity, which the probe found to be true (just 1/10 as many lightning strikes compared to Earth) (Ibid).

Io as imaged by the Galileo probe.

Io as imaged by the Galileo probe.

Of course, Galileo was at Jupiter to learn not just about the planet but its moons also. Measurements of Jupiter’s magnetic field around Io revealed that a hole seems to exist in it. Since readings of the gravity around Io seem to indicate that the moon has a giant iron core over half the diameter of the moon itself, it is possible that Io generates its own field courtesy of the intense gravitational pull of Jupiter. The data used to determine this was achieved during the December flyby when Galileo got to within 559 miles of the Io’s surface. Further analysis of the data pointed to a two layer structure for the moon, with an iron/sulfur core with radius 560 kilometers and a slightly molten mantle/crust ) (Isbell).



The original mission was to conclude after 23-months and a total of 11 orbits around Jupiter with 10 of those coming into close proximity to some of the moons but scientists were able to secure additional funding for a mission extension. In fact, a total of 3 of them were granted which allowed for 35 visits to the major Jovian moons including 11 to Europa, 8 to Callisto, 8 to Ganymede, 7 to Io, and 1 to Amalthea (Savage 8, Howell).

Data from a 1998 flyby of Europa showed interesting "chaos terrain," or circular regions where the surface was rough and jagged. It was years before scientists realized what they were looking at: fresh areas of subsurface material that were on the surface. As pressure from below the surface grew, it pushed upward until the icy surface cracked apart. Subsurface liquid filled the hole then refroze, causing the original edges of the ice to shift and not form a perfect surface again. It also allowed scientists with a possible model for allowing material from the surface to go below, possibly seeding life. Without that extension, results like these would be missed (Kruski).

And after scientists looked at Galileo images (despite just being 6 meters per pixel because of the aforementioned antennae problem), they realized that Europa's surface rotates at a different rate than the moon! This amazing result makes sense only after looking at the complete picture of Europa. Gravity pulls on the moon and heats it up, and with both Jupiter and Ganymede pulling at different directions, it caused the shell to extend as much as 10 feet. With a 3.55 day orbit, different places are constantly being tugged and at different rates depending on when perihelion and aphelion are achieved, causing a 12 mile deep shell with a 60 mile deep ocean to be slowed down at perihelion. In fact, the data from Galileo shows that it will take about 12,000 years before the shell and the main body of the moon hit a brief sync before again going at different rates (Hond, Betz "Inside").

Europa as imaged by the Galileo probe.

Europa as imaged by the Galileo probe.

The End

And as the saying goes, all good things must come to an end. In this case, Galileo completed its mission when it fell into Jupiter on September 21, 2003. This was a necessity when scientists figured out that Europa likely has liquid water and thus possibly life. To have Galileo possibly crash into that moon and contaminate it was unacceptable, so the only recourse was to allow it to fall into the gas giant. For 58 minutes it lasted in the extreme conditions of high pressure and 400 mile per hour winds but finally succumbed. But the science we gathered from it was trend setting and helped pave the path for future missions like Cassini and Juno (Howell, William 132).

Works Cited

Burnhain, Robert. "Heres Looking at Ida." Astronomy Apr. 1994: 39. Print.

"Galileo en Route to Jupiter." Space 1991. Motorbooks International Publishers & Wholesalers. Osceola, WI. 1990. Print. 118-9.

Hond, Kenn Peter. "Does Europa's Shell Rotate at a Different Rate from the Moon?" Astronomy Aug. 2015: 34. Print.

Howell, Elizabeth. “Spacecraft Galileo: To Jupiter and Its Moons.” Purch, 26 Nov. 2012. Web. 22 Oct. 2015.

Isbell, Douglas and Mary Beth Murrill. “Galileo Finds Giant Iron Core in Jupiter’s Moon Io.” 03 May 1996. Web. 20 Oct. 2015.

Kane, Va. “Galileo’s Mission Saved – Just Barely.” Astronomy Apr. 1982: 78-9. Print.

Kruski, Liz. "Europa May Harbour Subsurface Lakes." Astronomy Mar. 2012: 20. Print.

Morse, David. “Galileo Probe Suggests Planetary Science Reappraisal.” 22 Jan. 1996. Web. 14 Oct. 2015.

O’Donnell. Franklin. “Galileo Crosses Boundary Into Jupiter’s Environment.” 01 Dec. 1995. Web. 14 Oct. 2015.

Savage, Donald and Carlina Martinex, D.C. Agle. “Galileo End of Mission Press Kit.” NASA Press 15 Sept. 2003: 8, 9, 14, 15. Print.

"STS-34 Atlantis." Space 1991. Motorbooks International Publishers & Wholesalers. Osceola, WI. 1990. Print. 42-4.

Unknown. "Similar But Not The Same." Astronomy Sept. 1994. Print. 26.

William, Newcott. “In the Court of King Jupiter.” National Geographic Sept. 1999: 129, 132-3. Print.

Yeates, Clayne M. and Theodore C. Clarke. "Galileo: Mission to Jupiter." Astronomy. Feb. 1982. Print. 7-9.

© 2015 Leonard Kelley


Leonard Kelley (author) on November 13, 2015:

Reynold Jay, thank you for your kind words. I know it personally makes me feel gratified and accomplished to hear such praise from a reader. It means a lot, so thank you!

Reynold Jay from Saginaw, Michigan on November 13, 2015:

This is an impressive piece of research. If I had written it I would have been sooooo very proud! In that I do not circulate in science circles it could very well be that this is all known information to those in the field. I suspect it is. Nevertheless it would be worth sending to science journals as is so succinctly sums up the history of the entire project. I learned a ton! I found the trajectory chart fascinating. ( I knew nothing about the entire project5!! ) Well done!

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