Galileo Joins Jupiter, Literally

Summary (Sep 21, 2003): The spectacular journey of the Jupiter-orbiting spacecraft, Galileo, ended by crashing into the giant planet's atmosphere. Partly because of its successful mapping of Europa's probable frozen ocean, Galileo couldn't be left unattended to, or it might risk contamination of one of our solar system's most intriguing prospects for habitability.

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Galileo Joins Jupiter, Literally

The flight team for NASA's Jupiter-orbiting Galileo spacecraft ended the probe's long, image-rich journey.

The team had written commands for the onboard computer to manage the spacecraft for its short remaining lifetime earlier this year. Galileo then coasted for the last seven months before transmitting a few hours of science measurements in real time, leading up to its Sept. 21 plunge into Jupiter's atmosphere. The spacecraft team ceased operations on Friday, Feb. 28 after a final playback of scientific data from the robotic explorer's tape recorder.

Observations of Io by Galileo spacecraft. Click image for higher resolution black and white. The image has a resolution of 335 meters (1,100 feet) per picture element. North is to the top of the image. Credit: Arizona/ Galileo images NASA/JPL. False color. BW Image produced by: Zibi Turtle, Planetary Image Research Lab. (PIRL), Lunar and Planetary Lab. (LPL), University of Arizona

"This mission has exemplified successful team efforts to overcome obstacles to make outstanding discoveries," said Dr. Eilene Theilig, Galileo project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "While the team is sad to see it come to an end, there is great pride in Galileo's remarkable accomplishments."

In the years since astronauts deployed Galileo from the cargo bay of Space Shuttle Atlantis in 1989, the mission has produced a string of discoveries about asteroids, a fragmented comet, Jupiter's atmosphere, Jupiter's magnetic environment, and especially about the geologic diversity of Jupiter's four largest moons.

The prime mission ended six years ago, after two years of orbiting Jupiter. NASA extended the mission three times to continue taking advantage of Galileo's unique capabilities for accomplishing valuable science.

Now, the onboard supply of propellant is nearly depleted. Without propellant, the spacecraft would not be able to point its antenna toward Earth nor adjust its trajectory, so controlling the spacecraft would no longer be possible. Before that could happen, the flight team last year put Galileo on course for disposal by a dive into the crushing pressure of Jupiter's atmosphere.

This strategy eliminates any possibility of an unwanted impact between the spacecraft and the moon Europa. Galileo's own discovery of a likely subsurface ocean on Europa has raised interest in the possibility of life there and concern about protecting it.

On Nov. 5, 2002, the orbiter passed closer to Jupiter than it had ever ventured before, flying near an inner moon named Amalthea and through part of Jupiter's gossamer ring to begin its 35th and last orbit around the giant planet. That elongated farewell loop took Galileo farther from Jupiter than it had been since before it entered orbit in 1995, to a point more than 26 million kilometers (16 million miles) away on April 14 before heading back in for impact on September 21.

"After this month, we have no further activities planned until the day of impact," Theilig said last February.

The specifications on the Galileo spacecraft and its science milestones are:

On-orbit mass: 2380 Kg
Power System: Radioisotope Thermal Generators (RTGs) of 570 W
Dimensions: The length of the spacecraft is 9 m and, with the high-gain antenna (HGA) deployed, is 4.6 m in diameter.
Gravity Assist: The trajectory which the spacecraft followed was called a VEEGA (Venus-Earth-Earth Gravity Assist), traveling first in toward the Sun for a gravity assist from Venus before encountering the Earth two times (spaced two years apart). These encounters with Venus and the Earth allowed Galileo to gain enough velocity to get it out to Jupiter.
Launch Date: 18 October 1989 at 22:23 UTC
Launch Vehicle: Shuttle/Inertial Upper Stage

Milestone Science:

First mission to make a close flyby of an asteroid (Gaspra).
First mission to discover a satellite of an asteroid (Ida's satellite Dactyl).
First multispectral study of the Moon.
First atmospheric probe to enter Jupiter's atmosphere.
First spacecraft to go into orbit around Jupiter.
First direct observations of a comet impacting a planet (Shoemaker-Levy 9).
The discovery of a satellite (Dactyl) of an asteroid (Ida).
Confirmation of the existence of a huge ancient impact basin in the southern part of the Moon's far side (inferred from Apollo data but never before mapped).
Evidence of more extensive lunar volcanism than previously thought.
Discovery of an intense interplanetary dust storm (the most intense ever observed).
Discovery of an intense new radiation belt approximately 50,000 km (31,000 miles) above Jupiter's cloud tops.
Jovian wind speeds in excess of 600 kilometers per hour (> 400 mph) were detected.
Far less water was detected in Jupiter's atmosphere than estimated from earlier Voyager observations and from models of the Comet Shoemaker-Levy 9 impact.
Far less lightning activity (about 10% of that found in an equal area on Earth) than anticipated. The individual lightning events, however, are about ten times stronger on Jupiter than the Earth.
Helium abundance in Jupiter is very nearly the same as its abundance in the Sun (24% compared to 25%).
Extensive resurfacing of Io's surface due to continuing volcanic activity since the Voyagers flew by in 1979.
Preliminary data support the tentative identification of intrinsic magnetic fields for both Io and Ganymede.
Evidence for liquid water ocean under Europa's surface.

The Galileo flight team numbered about 300 people at its peak during the prime mission, but has run much leaner in recent years, with about 30 since the Amalthea flyby. That smaller team is now disbanding, mostly to work on other JPL-managed NASA missions that are in development or already flying.

JPL, a division of the California Institute of Technology in Pasadena, manages the Galileo mission for NASA's Office of Space Science, Washington, D.C. Additional information about the mission and its discoveries is available online at http://galileo.jpl.nasa.gov.

Related Web Pages:

Life and Giant Planets series
Galileo Home Page
Io Image Library- JPL
Evidence of bacteria on Jupter's moon?
The Galileo Mission to Jupiter and Its Moons (Scientific American)
Scientific Assessment for Galileo Disposal
Almathea Viking Images

Note: Life and Giant Planets: [2003-09-21]

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Sunday, September 21, 2003