The Galileo Missions primary objectives were to launch a spacecraft that would travel to Jupiter, achieve orbit as an artificial satellite, study the moons, rings, surface and atmosphere.
The Galileo spacecraft was initially scheduled to be launched by the space shuttle in 1986, but due to the Challenger disaster the launch was delayed. During this delay the design of the was modified to remove the “Centaur”, a portion of the Galileo spacecraft’s propulsion system, that was deemed to be to dangerous to have aboard the space shuttle as part of its payload. The resulting compromise was “The VEEGA Trajectory”.
The Galileo spacecraft was successfully launched in 1989 by the space shuttle Atlantis. It began its voyage to Jupiter and achieved orbit capture on December 7, 1999. On that day the Galileo spacecraft launched its atmospheric probe into Jupiter atmosphere and began sending back information about the composition of its atmosphere.
The Galileo Mission has three basic parts, the initial mission that began in 1995 and concluded in 1997. The extension of the initial mission, "Galileo Europa Mission”, that began in 1997 and concluded in 1999. And finally the "Galileo Millennium Mission" that began where the previous mission ended and is currently ongoing and presumably will continue as long as funding and spacecraft systems allow.
From the beginning this mission was challenged. Once the Galileo spacecraft was deployed it suffered a mechanical failure while trying to deploy the high gain antenna for communication. Numerous attempts were made to free the antenna including turning the spacecraft into the sun hoping that the heating and cooling of the antenna elements would free them. They also tried to free the antenna elements by changing the attitude of the spacecraft and applying maximum thrust from the crafts engines hoping that would force umbrella type antenna to open. None of these attempts were successful. This left the project with only the low gain antenna system to complete the mission with. While this may not seem that tough, the technical challenges placed on the mission were fairly extreme. Lets look at a few of the characteristics of both antenna systems.
· The low gain antenna system was initially capable of 8 bits of data per second (with the implementation of data compression the data rate has been pushed to 160 bits per second with an average of 50 to 80 bits per second for most data).
· The earth bound signal from the low gain antenna requires a 70 meter receiver dish with special low temperature/ noise amplifiers and ultra sensitive receivers.
· The low gain system operates in the 2 Gigahertz range.
· The high gain antenna system would have operated in the 8 Gigahertz range. This would have allowed the signals from the spacecraft to be received on smaller dishes while achieving the same or better sensitivity (the ability to discriminate between the signal and other apparent data that can be created by spurious radio waves).
· The high antenna system would have been capable of transmitting data at about 130,000 bits per second.
Just as a couple of trivial notes:
· Research conducted on the high gain antenna system indicates that the probable cause of its failure was that a dry lubricant that caused the antenna element problems.
· The microprocessors used in this spacecraft are equivalent to the ones used in old Apple II computers (8 bit processors). Obsolete microprocessors like the Intel 486 are approximately 200 times more powerful than the ones on board the Galileo spacecraft.
· Food for thought, how many of us are still driving 1986 vintage automobiles?
The information in this background section came for the links listed above, and is only a miniscule portion of the data that is available on those sites. Additional information about numerous systems can be found at the FAQ section of the old Galileo web page. The two links that are provided in this section will take you to the old and new Galileo web pages. The data of both pages has not been combined yet.
The Galileo spacecraft has recently left the powerful influence of Jupiter's magnetosphere, for the first time since early 1996 when Galileo was last outside Jupiter's magnetic area. The spacecraft has now entered the solar wind, which is a stream of particles emitted continually from the Sun that flows at roughly 400 kilometers per second (about 1 million miles per hour).
This transition from the magnetosphere to the solar wind is the marking of the beginning of a joint data gathering by the Galileo and Cassini spacecraft. Galileo will return close to Jupiter in October of this year, and Cassini is preparing to swing by Jupiter in December 2000 to slingshot toward Saturn. While both spacecrafts are in Jupiter's vicinity, their measurements will be compared so that we have a better understanding of how the solar wind changes as it flows outward near Jupiter's orbit. Later this year the joint observations of the two spacecrafts will allow us to discover more about how the solar wind affects Jupiter's magnetic field and the charged particles trapped within it.
To prepare for its observations of Jupiter, Cassini performed a flight path adjustment last week, on June 14. It is now on the path to fly by Jupiter and will be closest to Galileo towards the end of December. This path adjustment will also allow it to pass by Saturn's outermost moon, Phoebe, at a distance of 2,000 kilometers (about 1,250 miles). Cassini will leave the solar wind and enter Jupiter's magnetosphere shortly after. Until then, Cassini will be studying cosmic dust and making field and particle measurements.
The Galileo mission is continuing its studies under another extension, called the Galileo Millennium Mission. 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. Cassini is a joint mission of NASA, the European Space Agency and Italian Space Agency, and is managed by JPL for NASA's Office of Space Science, Washington, D.C.
NASA Headquarters agreed in principle to extend the Galileo
mission past its planned January 31 finale. Details of funding and itinerary
for the new extended mission, to be called the Galileo Millennium Mission, must
still be resolved. A Europa encounter took place January 3, 2000, and is
technically still part of the current, extended Galileo Europa Mission. Another
Io flyby is planned for February 20, with flybys of Ganymede on May 20 and
December 28, and joint observations of Jupiter with the Cassini spacecraft in
Galileo engineers and scientists like to say that the spacecraft has already lived "well past its warranty", surviving more than twice the radiation exposure levels it was designed to withstand. Although radiation has caused some problems with spacecraft instruments, Galileo is still performing and functioning well. There's no way to forecast how long the spacecraft will remain functioning and reliable, but as long as it remains healthy, it provides opportunities for exploration. In addition, it will serve as an actual test run of how long electronic parts and equipment can survive through high radiation exposure.