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Galileo Energetic Particles Detector


Frequently Asked Questions



What was the Energetic Particles Detector?


The Energetic Particle Detector, or EPD, was a metal and fiberglass box containing sensors and other electronic components that was attached to the Galileo orbiter spacecraft.


How big was the EPD?


The Galileo orbiter had a height of 6.15 meters and a mass of 2,223 kilograms--about the size of a bus. The EPD instrument measured about 30 centimeters by 30 centimeters by 51 centimeters and had a mass of about 10.6 kilograms.


What did the EPD do?


The Energetic Particle Detector counted radioactive particles.


What are radioactive particles?


When matter is heated far beyond the temperature needed to vaporize, atoms and molecules in the matter travel with enough speed and collide with enough force that electrons are sometimes knocked loose from the outer orbits of the atoms. These electrons carry negative electrical charges, and the charged atoms they leave behind, called ions, carry positive electrical charges. Since electrons and ions have charges, they experience strong electrical forces from other charged particles. These forces can cause particles to accelerate. Uranium and other chemical elements sometimes emit radioactive particles as they spontaneously change or decay.


Why is it important to count radioactive particles in space?


When radioactivity is found in nature, it shows that matter has been severely overheated or has become unstable and has decayed. Some electrons and ions are found to contain a larger average amount of energy than would be predicted from their temperature, according to the laws of thermodynamics. By measuring the energies of these superfast particles, we can begin to figure out the conditions under which these particles were born: how hot, how dense, and what kinds of matter and electrical forces were present. For places too far away or too difficult to visit by humans or with robots, measuring radioactivity may be the only way to determine what happens there. Much of what we know about the Sun, planets, supernovas, and other objects in the galaxy has been learned by measuring radioactivity.


How did the EPD work?


The EPD had silicon wafers which were exposed to space. Radioactive particles would strike these wafers and cause electrical signals, which were then amplified, and, if the signals were of the desired size, a counter would "click". The number of "clicks" recorded measured the intensity of the radioactivity.


Who built it, and what did it cost?


The EPD was built by engineers, technicians, and scientists at the Johns Hopkins University Applied Physics Laboratory (JHU/APL) and the Max Planck Institute for Aeronomy in Lindau, Germany, at a cost of about $8.5 million. The overall cost for the entire Galileo mission, from the start of planning through the end of the mission in December 1997, was $1.354 billion, including $892 million in spacecraft development costs.


How was information sent back to Earth?


Information stored in Galileo's on-board computer was transmitted to NASA's Deep Space Network, using tracking stations in California, Spain, and Australia.


How long have these communications been received, and how long will they continue?


Since the launch of Galileo in October 1989, information was received from Earth and Venus flybys, encounters with asteroids Gaspra and Ida, comet Shoemaker-Levy's impact with Jupiter, and the probe of Jupiter's atmosphere. During the probe's 11 orbits around Jupiter, information and images were gathered from the Jovian system until December, 1997. Next, the Galileo Europa Mission conducted a detailed 14-month study of Europa comprised of 8 close encounters, with additional orbits also focused on Io and on Jupiter's thunderstorms. As the spacecraft ran out of the propellant needed to keep the antenna pointed toward Earth, we would have eventually lost control of and contact with the spacecraft. Following the Galileo Millenium Mission, the Galileo team chose a planned impact in December 2003 with Jupiter, to insure that Galileo did not hit Europa.


Were there any problems with the mission?


Despite several setbacks, innovative engineers and scientists devised several techniques to enable the spacecraft to achieve the majority of its scientific objectives. (1) The Challenger Space Shuttle disaster led to changes in the shuttle launch system, which meant that Galileo's trajectory was changed from a direct flight of about two-and-a-half years to an interplanetary flight path using gravity-assisted flybys of Earth and Venus. (2) Galileo's main antenna, which was to open like an umbrella shortly after launch, could not be made to open even after repeated attempts. Thus, communications were received only from a smaller, low-gain antenna on the spacecraft. (3) The tape recorder used particularly to store imaging data for transmission by the slower low-gain antenna malfunctioned just before arrival at Jupiter. The recorder failed to cease re-winding after recording an image of Jupiter. It was then operational but unreliable, so scientists have not used the portion of the tape near the end of the reel and reduced the number of images recorded.


Were scientists and engineers able to use technology developed since launch in 1989 to upgrade the mission?


Upgraded computer software programs were transmitted to Galileo's on-board computer system. On Earth, new technology was used to greatly sharpen the hearing of the telecommunications equipment that was receiving Galileo's "whisper" of a signal. Scientists located at various laboratories, linked by computer, were involved in developing the sequences affecting their experiments. In some cases, they helped to change preplanned sequences loaded into Galileo's computers to follow up on unexpected discoveries with second looks.


Why was the EPD on the spinning part of the orbiter?


The Galileo Orbiter actually had two sections--one, called the "despun," not spinning and the other section spinning at 3.25 or 10.5 rpm. Instruments such as the camera system that depended on steady pointing capability were located on the despun section. The EPD and other instruments such as the main antenna, high-energy particle detector, and plasma-wave detector were mounted on the spinning section to allow maximum scan of space.


Contact for questions and further information on the EPD project:
Dr. Thomas P. Armstrong (785) 840-0800



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Updated 8/23/19, Cameron Crane


Manufacturer: The Galileo Spacecraft was manufactured by the Jet Propulsion Laboratory, Messerschmitt-Bölkow-Blohm, General Electric, and the Hughes Aircraft Company.

Mission Duration: Galileo was planned to have a mission duration of around 8 years, but was kept in operation for 13 years, 11 months, and 3 days, until it was destroyed in a controlled impact with Jupiter on September 21, 2003.

Destination: Galileo's destination was Jupiter and its moons, which it orbitted for 7 years, 9 months, and 13 days.