Galileo project
Galileo was an American robotic space program that studied the planet Jupiter and its moons, as well as several other Solar System bodies. Named after the Italian astronomer Galileo Galilei, the Galileo spacecraft consisted of an orbiter and an atmospheric entry probe. It was delivered into Earth orbit on October 18, 1989, by on the STS-34 mission, and arrived at Jupiter on December 7, 1995, after gravity assist flybys of Venus and Earth, and became the first spacecraft to orbit Jupiter. The spacecraft then launched the first probe to directly measure its atmosphere. Despite suffering major antenna problems, Galileo achieved the first asteroid flyby, of 951 Gaspra, and discovered the first asteroid moon, Dactyl, around 243 Ida. In 1994, Galileo observed Comet Shoemaker–Levy 9's collision with Jupiter.
Jupiter's atmospheric composition and ammonia clouds were recorded, as were the volcanism and plasma interactions on Io with Jupiter's atmosphere. The data Galileo collected supported the theory of a liquid ocean under the icy surface of Europa, and there were indications of similar liquid-saltwater layers under the surfaces of Ganymede and Callisto. Ganymede was shown to possess a magnetic field and the spacecraft found new evidence for exospheres around Europa, Ganymede, and Callisto. Galileo also discovered that Jupiter's faint ring system consists of dust from impact events on the four small inner moons. The extent and structure of Jupiter's magnetosphere was also mapped.
The primary mission concluded on December 7, 1997, but the Galileo orbiter commenced an extended mission known as the Galileo Europa Mission, which ran until December 31, 1999. By the time GEM ended, most of the spacecraft was operating well beyond its original design specifications, having absorbed three times the radiation exposure that it had been built to withstand. Many of the instruments were no longer operating at peak performance, but were still functional, so a second extension, the Galileo Millennium Mission was authorized. On September 20, 2003, after 14 years in space and 8 years in the Jovian system, Galileo mission was terminated by sending the orbiter into Jupiter's atmosphere at a speed of over to eliminate the possibility of contaminating the moons with bacteria.
Background
is the largest planet in the Solar System, with more than twice the mass of all the other planets combined. Consideration of sending a probe to Jupiter began as early as 1959, when the National Aeronautics and Space Administration Jet Propulsion Laboratory developed four mission concepts:- Deep space flights would fly through interplanetary space;
- Planetary flyby missions would fly past planets close enough to collect scientific data and could visit multiple planets on a single mission;
- Orbiter missions would place a spacecraft in orbit around a planet for prolonged and detailed study;
- Atmospheric entry and lander missions would explore a planet's atmosphere and surface.
Planning
Initiation
Following the approval of the Voyager missions, NASA's Scientific Advisory Group for Outer Solar System Missions considered the requirements for Jupiter orbiters and atmospheric probes. It noted that the technology to build a heat shield for an atmospheric probe did not yet exist, and indeed facilities to test one under the conditions found on Jupiter would not be available until 1980. There was also concern about the effects of radiation on spacecraft components, which would be better understood after Pioneer 10 and Pioneer 11 had conducted their flybys. Pioneer 10s flyby in December 1973 indicated that the effects were not as severe as had been feared. NASA management designated JPL as the lead center for the Jupiter Orbiter Probe Project. John R. Casani, who had headed the Mariner and Voyager projects, became the first project manager. The JOP would be the fifth spacecraft to visit Jupiter, but the first to orbit it, and the probe the first to enter its atmosphere.Ames and JPL decided to use a Mariner spacecraft for the Jupiter orbiter like the ones used for Voyager rather than a Pioneer spacecraft. Pioneer was stabilized by spinning the spacecraft at 60 rpm, which gave a 360-degree view of the surroundings, and did not require an attitude control system. By contrast, Mariner had an attitude control system with three gyroscopes and two sets of six nitrogen jet thrusters. Attitude was determined with reference to the Sun and Canopus, which were monitored with two primary and four secondary star tracker sensors. There was also an inertial reference unit and an accelerometer. The attitude control system allowed the spacecraft to take high-resolution images, but the functionality came at the cost of increased weight: a Mariner weighed compared to just for a Pioneer.
The increase in weight had implications. The Voyager spacecraft had been launched by Titan IIIE rockets with a Centaur upper stage, but Titan was retired afterwards. In the late 1970s, NASA was focused on the development of the reusable Space Shuttle, which was expected to make expendable rockets obsolete. In late 1975, NASA decreed that all future planetary missions would be launched by the Space Shuttle. The JOP would be the first to do so. The Space Shuttle was supposed to have the services of a space tug to launch payloads requiring something more than a low Earth orbit, but this was never approved. The United States Air Force instead developed the solid-fueled Interim Upper Stage, later renamed the Inertial Upper Stage, for the purpose.
The IUS was constructed in a modular fashion, with two stages, a large one with of propellant, and a smaller one with. This was sufficient for most satellites. It could also be configured with two large stages to launch multiple satellites. A configuration with three stages, two large and one small, would be enough for a planetary mission, so NASA contracted with Boeing for the development of a three-stage IUS. A two-stage IUS was not powerful enough to launch a payload to Jupiter without resorting to using a series of gravity-assist maneuvers around planets to garner additional speed. Most engineers regarded this solution as inelegant and planetary scientists at JPL disliked it because it meant that the mission would take months or even years longer to reach Jupiter. Longer travel times meant that the spacecraft's components would age and possibly fail, and the onboard power supply and propellant would be depleted. Some of the gravity assist options also involved flying closer to the Sun, which would induce thermal stresses that also might cause failures.
It was estimated that the JOP would cost $634 million, and it had to compete for fiscal year 1978 funding with the Space Shuttle and the Hubble Space Telescope. A successful lobbying campaign secured funding for both JOP and Hubble over the objections of Senator William Proxmire, the chairman of the Independent Agencies Appropriations Subcommittee. The United States Congress approved funding for the Jupiter Orbiter Probe on July 19, 1977, and JOP officially commenced on October 1, 1977, the start of the fiscal year. Project manager Casani solicited suggestions for a more inspirational name for the project from people associated with it. The most votes went to "Galileo", after Galileo Galilei, the first person to view Jupiter through a telescope, and the discoverer of what are now known as the Galilean moons in 1610. It was noted at the time that the name was also that of a spacecraft in the Star Trek television show. In February 1978, Casani officially announced the choice of the name "Galileo".
Preparation
To enhance reliability and reduce costs, the project engineers decided to switch from a pressurized atmospheric probe to a vented one, so the pressure inside the probe would be the same as that outside, thus extending its lifetime in Jupiter's atmosphere, but this added to its weight. Another was added in structural changes to improve reliability. This required additional fuel in the IUS, but the three-stage IUS was itself overweight with respect to its design specifications, by about. Lifting Galileo and the three-stage IUS required a special lightweight version of the Space Shuttle external tank, the Space Shuttle orbiter stripped of all non-essential equipment, and the Space Shuttle main engines running at full power level—109 percent of their rated power level. Running at this power level necessitated the development of a more elaborate engine cooling system. Concerns were raised over whether the engines could be run at 109 percent by the launch date, so a gravity-assist maneuver using Mars was substituted for a direct flight.Plans called for the to launch Galileo on the STS-23 mission, tentatively scheduled for sometime between January 2 and 12, 1982, this being the launch window when Earth, Mars and Jupiter were aligned to permit Mars to be used for the gravity-assist maneuver. By 1980, delays in the Space Shuttle program pushed the launch date for Galileo back to 1984. While a Mars slingshot was still possible in 1984, it would no longer be sufficient.
NASA decided to launch Galileo on two separate missions, launching the orbiter in February 1984 with the probe following a month later. The orbiter would be in orbit around Jupiter when the probe arrived, allowing the orbiter to perform its role as a relay. This configuration required a second Space Shuttle mission and a second carrier spacecraft to be built for the probe to take it to Jupiter, and was estimated to cost an additional $50 million, but NASA hoped to be able to recoup some of this through competitive bidding. The problem was that while the atmospheric probe was light enough to launch with the two-stage IUS, the Jupiter orbiter was too heavy to do so, even with a gravity assist from Mars, so the three-stage IUS was still required.
By late 1980, the price tag for the IUS had risen to $506 million. The USAF could absorb this cost overrun on the development of the two-stage IUS, but NASA was faced with a quote of $179 million for the development of the three-stage version, which was $100 million more than it had budgeted for. At a press conference on January 15, 1981, Robert A. Frosch, the NASA Administrator, announced that NASA was withdrawing support for the three-stage IUS, and going with a Centaur G Prime upper stage because "no other alternative upper stage is available on a reasonable schedule or with comparable costs."
File:Model of Centaur G with Galileo probe.jpg|thumb|right|Model of Galileo atop a Centaur G Prime upper stage in the San Diego Air and Space Museum |alt=refer to caption
Centaur provided many advantages over the IUS. The main one was that it was far more powerful. The probe and orbiter could be recombined, and the probe could be delivered directly to Jupiter in two years' flight time. The second was that, despite this, it was gentler than the IUS, because it had lower thrust. This reduced the chance of damage to the payload. Thirdly, unlike solid-fuel rockets which burned to completion once ignited, a Centaur could be switched off and on again. This gave it flexibility, which increased the chances of a successful mission, and permitted options like asteroid flybys. Centaur was proven and reliable, whereas the IUS had not yet flown. The only concern was about safety; solid-fuel rockets were considered safer than liquid-fuel ones, especially ones containing liquid hydrogen. NASA engineers estimated that additional safety features might take up to five years to develop and cost up to $100 million.
In February 1981, JPL learned that the Office of Management and Budget was planning major cuts to NASA's budget, and was considering cancelling Galileo. The USAF intervened to save Galileo from cancellation. JPL had considerable experience with autonomous spacecraft that could make their own decisions. This was a necessity for deep space probes, since a signal from Earth takes from 35 to 52 minutes to reach Jupiter, depending on the relative position of the planets in their orbits. The USAF was interested in providing this capability for its satellites, so that they would be able to determine their attitude using onboard systems rather than relying on ground stations, which were not "hardened" against nuclear weapons, and could take independent evasive action against anti-satellite weapons. It was also interested in the manner in which JPL was designing Galileo to withstand the intense radiation of the magnetosphere of Jupiter, as this could be used to harden satellites against the electromagnetic pulse of nuclear explosions. On February 6, 1981 Strom Thurmond, the President pro tem of the Senate, wrote directly to David Stockman, the director of the OMB, arguing that Galileo was vital to the nation's defense.
File:Astronauts John Fabian and Dave Walker pose in front of a model of the Shuttle-Centaur.jpg|thumb|left|Astronauts John M. Fabian and David M. Walker pose in front of a model of the Shuttle-Centaur with Galileo in mid-1985 |alt=refer to caption
In December 1984, Casani proposed adding a flyby of asteroid 29 Amphitrite to the Galileo mission. In plotting a course to Jupiter, the engineers wanted to avoid asteroids. Little was known about them at the time, and it was suspected that they could be surrounded by dust particles. Flying through a dust cloud could damage the spacecraft's optics and possibly other parts of the spacecraft as well. To be safe, JPL wanted to avoid asteroids by at least. Most of the asteroids in the vicinity of the flight path like 1219 Britta and 1972 Yi Xing were only a few kilometers in diameter and promised little scientific value when observed from a safe distance, but 29 Amphitrite was one of the largest, and a flyby at even could have great value. The flyby would delay the spacecraft's arrival in Jupiter orbit from August 29 to December 10, 1988, and the expenditure of propellant would reduce the number of orbits of Jupiter from eleven to ten. This was expected to add $20 to $25 million to the cost of the Galileo project. The 29 Amphitrite flyby was approved by NASA Administrator James M. Beggs on December 6, 1984.
During testing, contamination was discovered in the system of metal slip rings and brushes used to transmit electrical signals around the spacecraft, and they were returned to be refabricated. The problem was traced back to a chlorofluorocarbon used to clean parts after soldering. It had been absorbed, and was then released in a vacuum environment. It mixed with debris generated as the brushes wore down, and caused intermittent problems with electrical signal transmission. Problems were also detected in the performance of memory devices in an electromagnetic radiation environment. The components were replaced, but then a read disturb problem arose, in which reads from one memory location disturbed the contents of adjacent locations. This was found to have been caused by the changes made to make the components less sensitive to electromagnetic radiation. Each component had to be removed, retested, and replaced. All of the spacecraft components and spare parts received a minimum of 2,000 hours of testing. The spacecraft was expected to last for at least five years—long enough to reach Jupiter and perform its mission. On December 19, 1985, it departed JPL in Pasadena, California, on the first leg of its journey, a road trip to the Kennedy Space Center in Florida. The Galileo mission was scheduled for STS-61-G on May 20, 1986, using.