Galileo Spacecraft: First Orbiter of Jupiter

Artist rendering of Galileo arriving at Jupiter

Artist rendering of Galileo arriving at Jupiter – Credit: NASA

Space Shuttle Atlantis carried a special payload during its STS-34 mission. Commander Don Williams and crew transported the Galileo spacecraft into Earth orbit, from which point it was launched on a years-long voyage to Jupiter. Galileo would become the first spacecraft to orbit an outer planet and would go on to reveal fascinating views of the gas giant and its moons, as well as make monumental discoveries about the nature of the Jovian system.


Quick Facts:

  • Launch Date: October 18, 1989, Shuttle Atlantis STS-34
  • Primary Mission: October ’89 to December ’97.
  • Extended Missions: 3, from ’97 to ’03.
  • Number of Jupiter orbits: 34
  • Total distance traveled during mission: 4,631,778,000 km (approx 2.8 billion miles)
  • Mission End: September 21, 2003

Getting Galileo to Jupiter

Work on the Galileo craft began in 1977, after the exploration of Jupiter was listed as the number one priority in the Planetary Science Decadal Survey published in 1968. Fly-bys of the massive planet were conducted by the twin Pioneer 10 and 11 and Voyager 1 and 2 spacecrafts, but Galileo was set to do more than just perform a fly-by. It would launch an instrument-laden probe into Jupiter’s atmosphere, and then continue to orbit the planet for years. This mission would provide knowledge of the Jupiter system that could hardly even be imagined.

Galileo deploying from Shuttle Atlantis

Galileo deploying from Shuttle Atlantis – Credit: NASA

Galileo suffered a number of postponements. The first planned launch was to be from Space Shuttle Columbia in 1982, but development delays in the Space Shuttle program made that early of a launch unfeasible. The upside is that this gave the Galileo developers more time to work on the probe. Further planned launches and postponements occurred in 1984, 1985, and 1986.

As we all know, 1986 was the year of the Challenger disaster. Galileo would be put on hold during the 32-month hiatus that followed the tragedy, as every detail of the Shuttle program was examined and made safer. Galileo was originally planned to be attached to a liquid hydrogen-fueled Centaur-G booster; however, new safety protocols following Challenger prohibited the booster from being carried in the Space Shuttle’s payload bay. Mission designers had to reconsider how they would get Galileo from the Shuttle’s low Earth orbit to Jupiter. They decided on employing a solid-fuel Inertial Upper Stage booster (IUS). Whereas the Centaur-G would have propelled Galileo on a short and direct trajectory to Jupiter, the IUS would take longer and also require some technical gravitational slingshot maneuvers to make it to the gas giant.

Galileo was finally launched from Space Shuttle Atlantis, during mission STS-34 on October 18, 1989. From there, its IUS booster was started and it began its unique “VEEGA”, or Venus Earth Earth Gravity Assist, maneuvers.

Galileo spacecraft trajectory

Galileo spacecraft trajectory – Source: NASA

  • Galileo flew by Venus on February 10, 1990 at an altitude of 16,000 km (10,000 miles).
  • It then flew by Earth on December 8, 1990 at an altitude 960 km (597 miles).
  • Its trajectory took it near Asteroid Gaspra on October 29, 1991, coming within 1,601 km (1,000 miles).
  • Then it was back to another Earth fly-by on December 8, 1992, this time at an altitude of only 303 km (188 miles).
  • On its way back towards the outer solar system it flew by Asteroid Ida on August 28, 1993, coming within 2,400 km (1,400 miles) of the asteroid.

On its way to Jupiter, Galileo was positioned perfectly to observe the doomed Comet Shoemaker-Levy 9 as it impacted the planet. Pieces of the comet, having been torn into fragments by Jupiter’s immense tidal forces, impacted Jupiter from July 16 – 22, 1994, on the side facing away from Earth. Fortunately, Galileo had a prime view and was able to record the impact. Earth-based telescopes could only observe the impact sites as they rotated into view a few minutes afterwards.

In July, 1995, Galileo released its atmospheric probe component. For the next five months, the probe and orbiter continued their cruise to Jupiter. On December 7, 1995, Galileo had arrived. The orbiter and probe diverged onto their separate missions.

Atmospheric Probe

On December 7, 1995 Galileo’s atmospheric probe sliced into Jupiter’s atmosphere at 47.6 kilometers per second (106,000 miles per hour). As the atmosphere began to slow the probe, it deployed its drogue and main parachutes and dropped its heat shield to expose its scientific instruments. The probe began recording data and transmitting it up to the main Galileo spacecraft orbiting high above, which then re-transmitted the data to Earth. The probe recorded 58 minutes of data on Jupiter’s weather and atmosphere. Towards the end of its descent, the probe measured wind speeds of 724 kilometers per hour (450 miles per hour). The intense heat and pressure of Jupiter’s atmosphere melted and vaporized the probe less than an hour into its journey through Jupiter’s atmosphere.

Orbiter

While the atmospheric probe’s job was complete, the Galileo orbiter still had years of work left to do. The orbiter received its electric power from two radioisotope thermoelectric generators (RTGs). That may sound complicated, but it’s really quite simple. These RTGs carry the radioactive element plutonium-238. As the plutonium decays, it releases energy in the form of heat. That heat can then be easily turned into electricity through the Seebeck effect. This type of energy generation is long-lasting and reliable, as well as impervious to the cold temperatures and strong radiation fields of the Jupiter system. Galileo carried two of these RTGs, with a combined total of approximately 22.7 kilograms (50 pounds) of plutonium-238. While these radioactive components had been used on previous space missions, Galileo drew extra concern due to it being both carried by the Shuttle as well as the multiple Earth fly-bys. Anti-nuclear activists protested Galileo’s launch, fearing a malfunction could cause radiation poisoning for many thousands of people on Earth. NASA, however, argued that the probability of risk was extremely low.

Jupiter's ring system, as observed by Galileo

Jupiter’s ring system, as observed by Galileo – Credit: NASA/JPL/Cornell University

Galileo conducted slow orbits of Jupiter, approximately 2 months long each. The orbits were elongated, and designed to bring the spacecraft within different distances to Jupiter, which allowed it to sample different areas of the planet’s magnetosphere. These orbits were also designed to bring Galileo and its instruments into close fly-bys of Jupiter’s largest moons. Galileo completed its primary mission on December 7, 1997; however, the craft was still functioning extremely well and was able to continue taking measurements and sending valuable data back to Earth. Its mission was extended three times, operating until 2003.

Volcanic activity on Io, as observed by Galileo

Volcanic activity on Io, as observed by Galileo – Credit: NASA/JPL

The orbiter made several discoveries during its mission:

  • It discovered a possible ocean under Europa’s icy crust
  • Revealed Ganymede’s very own magnetic field, the only moon known to have this feature
  • Made the first observations of ammonia clouds in another planet’s atmosphere
  • It created hundreds of images of Jupiter’s large ‘Galilean moons’: Io, Callisto, Europa, and Ganymede
  • It measured the high levels of volcanic activity on Io

Sagan Criteria for Life

The late astronomer Carl Sagan devised a set of experiments to be conducted by Galileo during its first fly-by of Earth. The purpose of the experiments was to see if life could be easily detected from a spacecraft. The results of the experiments were published by Sagan in 1993, in the scientific journal Nature. The experiments were a success, as Galileo was easily able to detect what are referred to as the ‘Sagan requirements for life’. These include strong absorption of light at the red end of the spectrum (indicative of plant photosynthesis), absorption bands of molecular oxygen (again, indicative of plant life), the detection of methane in the atmosphere (a gas created by either volcanic or biological activity), and the detection of narrowband radio wave transmissions (could indicate a technologically advanced civilization).


By the end of its mission, Galileo had conducted 34 orbits of Jupiter and had made multiple fly-bys of Jupiter’s moons: Io 7 times, Callisto 8 times , Ganymede 8 times, Europa 11 times, and one fly-by of Amalthea.

Due in part to Galileo’s discovery of potential oceans on Europa (and possibly other Jovian moons), the decision was made to end the orbiter’s mission by sending it to the same fate as the atmospheric probe eight years prior. Rather than risk contaminating (with either Earth bacteria or radiation from the RTGs) one of Jupiter’s potentially life-harboring moons, Galileo would be ordered to impact Jupiter. On September 21, 2003, Galileo entered Jupiter’s atmosphere at 48.2 kilometers per second (108,000 mph).

The Galilean Moons: Jupiter's four largest satellites

The Galilean Moons: Jupiter’s four largest satellites – Credit: NASA/JPL/DLR

The total mission cost was approximately $1.4 billion USD, had more than 100 scientist partners from many different countries, and involved the work of more than 800 individuals.

In spite of postponements, an antenna that failed to fully deploy, and a tape recorder malfunction, Galileo performed magnificently. It was a mission that brought us up close and personal with our Solar system’s largest planet and provided us with a much more detailed understanding of the Jovian system. Galileo paved the way for future studies of Jupiter and its moons. Its successor, the Juno orbiter, is currently en route and arriving in July of 2016, and plans are being considered to investigate Europa’s oceans. Like the astronomer that the spacecraft took its name from, Galileo Galilei, this mission revealed new worlds that we previously could only distantly wonder about.

 

The Pioneer Plaque: Our Calling Card to the Cosmos

In 1972 and 1973, Pioneer 10 and 11, respectively, left planet Earth with one-way tickets out of the Solar System. These two pioneers (heh) explored Jupiter, Saturn, and their associated moons before heading out into the great unknown on an uncharted interstellar voyage. Each of them carried a plaque, dubbed the Pioneer Plaques, and that’s what this story is about.

Eric Burgess, science correspondent for the Christian Science Monitor, recognized that by being the first spacecraft designed to leave our Solar System, it too would be planet Earth’s emissary to the stars. He believed the Pioneers should contain a message from its creators, one that could serve as an introduction and greeting from any being that might make contact with the Pioneers thousands or millions or more years from now. This thought spawned the idea for what became the Pioneer plaques. Burgess approached Carl Sagan, who was at NASA’s Jet Propulsion Laboratory in Pasadena, CA, working in connection with the Mariner 9 program. Sagan was thrilled with the idea and agreed to promote the idea with NASA officials.

Two identical plaques were made–one for Pioneer 10 and one for Pioneer 11. They are 9 inches by 6 inches, .05 inches thick, and constructed of gold-anodized aluminum. They were constructed and engraved by Precision Engravers of California, a company that is still in business today and sells replica plaques. The design itself was created by Carl Sagan and Frank Drake, with the artistic help of Sagan’s then-wife Linda Salzman Sagan. NASA accepted the idea and their design, and received approval to have them flown aboard Pioneer 10 and 11. They would be attached to the craft’s antenna supports, positioned such that they would be protected from erosion caused by interstellar dust.

The design consists of a few different elements symbolizing humanity’s place within the galaxy, and information about our species.

The Pioneer Plaque

Beginning in the top-left is a schematic representing the hyperfine transition of  neutral hydrogen.Hyperfine transition of neutral hydrogen extracted from the Pioneer plaque

Wait! Don’t go! Give me a chance to try and unpack that gobbledygook for you. 

This piece of the plaque is actually kind of important, because it serves as a reference for the other elements of the plaque. For this explanation, consider that the electrons in atoms exist in one of two states: spin up and spin down. Hydrogen was chosen for the diagram due to it being the most abundant element in the Universe as well as one of the simplest, containing a single electron. Basically, the magnetic field of an electron can either be oriented parallel to the magnetic field of the atom’s nucleus, or it can be oriented in the opposite direction. These are the two states I referred to. The diagram shows both of these phases connected by a line that represents the transition–a hyperfine transition I might add–between these two states. When this occurs, a photon is emitted with a specific wavelength of about 21 centimeters and a frequency of 1420 MHz. A being that might one day come into contact with the plaque would hopefully understand the distance and frequency represented, for if they could they would then be able to use it as a reference for the other diagrams on the plaque.

Like, for example, the diagram of us.

Depiction of humans on the Pioneer plaque

 

Here, the plaque depicts a nude male and female human. To the right of the woman figure are hash marks indicating the top and bottom of her height. Between those marks is the symbol “| – – -“, which is the binary symbol for 8. The woman is 8 tall. 8 what, you’re asking? 8 feet? 8 inches? Remember when we created our scale using the hydrogen transition thingamajig, and came up with 21 centimeters? That’s right, the woman is 8 x 21 cm, which equals 168 cm (just a skosh over 5′ 6”). Make sense?

There have been claims made that the original drawing had the man and woman holding hands, but that a conscious decision was made to separate the two out of concern that an alien gazing upon the plaque would think of the two humans as a single being. There are also rumors that the original design included a more anatomically-correct woman body, but that single extra line needed to be erased to garner top NASA official authorization.

What a wonderful time to have been around JPL for those discussions. There’s a lot we can learn about ourselves within a debate on how to present ourselves to alien beings thousands or millions of years into the future.

Moving on…

Silhouette of the Pioneer spacecraft relative to the size of the humans.Behind us (the humans), there’s a silhouette of the Pioneer spacecraft, showing the relative size of humans to the craft. I guess this is there in case the aliens are too lazy to do the hydrogen transition conversion thing we just talked about.

At the bottom of the plaque, we have a depiction of our solar system and where Pioneer came from. Also, more hash marks. I hope the aliens realize that this time they’re supposed to be multiplying by 1/10th of the distance of Mercury’s orbit from the Sun, and not 21 cm like they were to do with the human models. If not, they’ll have a hard time finding us if they’re looking for tiny planets that have orbits mere hundreds of centimeters from their star. I really hope aliens enjoy puzzles.

 

The Solar System with the trajectory of the Pioneer spacecraft.

 

I also hope that by the time they see this part of the plaque that word hasn’t gotten to them about Pluto being downgraded to dwarf planet….

But ours is only one of millions of solar systems within our corner of the galaxy. Providing a map of our solar system won’t help them if they have no way to find it to begin with. That brings us to the next part of the plaque:

800px-Pioneer_plaque_sun

This schematic shows the location of Sol (our sun) relative to the center of the Milky Way and 14 pulsars. I’m going to spare you the technical details and give you the bare bones version. The length of the lines indicate the relative distance between the Sun and the various pulsars. The long binary numbers give the periods of the pulsars, basically their signature. One thing worth noting about the periods of the pulsars, is that their frequency will change over time. Knowing this, a being deciphering this part of the plaque would be able to not only figure out where in the galaxy the Pioneers originated from, but also when they left Earth. Depending on where the plaque is encountered, only some of the pulsars might be visible thus the redundancy of including 14. This should be enough to allow for triangulation back to us. There’s a 15th line coming out of the center of the figure (which, if you haven’t guessed already is where the Sun is located); it’s the long one pointing to the right. It shows the relative distance from the Sun to the center of the Milky Way galaxy.

So there you have it. The Pioneer Plaque: a representation of humans and their size, a celestial map to the place and time the craft and its plaque originated from, and a tool to use as a standard unit of measure to decode all of the details.

If only we put so much effort into the selfies we post of ourselves on Facebook.