06601030301800 6L....T....T....T....T....T....T....T....T....T....T....T....T....R €THE GALILEO PROJECT NASA's Project Galileo is being enhanced with inner-planet scientific observations and the first asteriod flybys, which the spacecraft will make on its way to the planet Jupiter. Galileo is a planetary exploration mission that will provide the first direct sampling of the atmosphere of Jupiter and the first extended observations of the planet, its Galilean satellites and intense magnetospheric environment. The Jet Propulsion Laboratory is preparing Galileo for an October l989 launch. Three general and equal scientific objectives have guided development of Galileo: Investigation of (1) the structure and physical dynamics of Jupiter's complex magnetosphere, (2) the chemical composition and physical state of the Jovian satellites, and (3) the chemical composition and physical state of Jupiter's atmosphere. To accomplish these objectives requires three sets of scientific instruments and a three-part instrument platform. The main spacecraft will be placed in an adjustable orbit around Jupiter in December 1995. There, instruments fixed to the spinning spacecraft will measure fields and particles directly while remote sensors on an independently stabilized platform will observe the planet and its major satellites in a series of fly-by encounters as Galileo's orbit is adjusted. Very early in operations at Jupiter, an atmospheric probe released 5 months earlier during the spacecraft's approach will fly into the planet's atmosphere to analyze its composition and properties directly. The Galileo spacecraft system will be sent to Jupiter by the combination of Space Shuttle Discovery (to Earth orbit), a two- step Inertial Upper Stage (escape from Earth's gravity), and a triple gravity-assist from two planets (from the inner solar system to Jupiter). The new trajectory involves a flight to Venus, gravity assist by that planet, and then two gravity-assist flybys of Earth before it takes up its final flight path to Jupiter. Between planets, the spacecraft will maneuver to fine- tune its approach to the next gravity assist. The new trajectory has been dubbed VEEGA, for Venus-Earth-Earth Gravity Assist. This new flight path to Jupiter will permit the Galileo spacecraft to fly close by two main belt asteroids, Gaspra and Ida, for our first visit to such bodies. Both are of spectral type S, representative of most asteroids and believed similar to stony meteorites; Gaspra exhibits different spectral characteristics within the type. Gaspra is about 10 mi (16 km) in diameter, Ida about 20 mi (32 km). Galileo scientists will be able to do, for the first time, the kind of analysis of asteroids planned for the satellites of Jupiter (making allowance for the extremely small size and swift passage of the asteroids) as well as similar observations of Venus and the Earth-Moon system. Galileo was originally planned for a direct boost to Jupiter by a Centaur upper stage from Earth orbit, after launch aboard a Space Shuttle. After destruction of Challenger on Jan. 28, 1986, the Galileo mission had to be not only delayed but redesigned, because the Shuttle/Centaur combination was cancelled. Mission designers examined many alternatives, including different shuttle/upper-stage combinations, and an expendable launch vehicle, all in conjunction with Earth gravity assist. Ultimately, using the computer program designed to work out Galileo's tour of the Jovian system via repeated satellite gravity-assist encounters, designers discovered the Venus-Earth- Earth trajectory. This makes up for the reduced launch energy available, but adds years to the flight time to Jupiter. Launch is planned for the launch period beginning Oct. 8, l989. Galileo will fly over the morning terminator of Venus at an altitude of about 9,000 miles (15,000 km), on about Feb. 9, 1990. A search will be made for evidence of lightning storms in the atmosphere, to confirm indications found previously by Pioneer 12. In addition, the atmospheric composition and its distribution will be mapped through infrared spectroscopy. Because Galileo's high-gain antenna will not be deployed so close to the Sun, these data will be recorded for playback when the spacecraft approaches Earth several months later. In the ten- month flight between Venus and the Earth, ultraviolet instruments will search for Lyman-alpha radiation from neutral hydrogen atoms which, it is believed, might indicate a shell or ring of gaseous relics of comets, similar to the dust residues which lead to meteor showers. On about Dec. 8, 1990, Galileo will loop back to Earth, approaching from the night-side and flying past the terminator at an altitude of about 600 miles (l,000 km). Exactly two years later, the spacecraft will return to earth for a 200-mile (300 km) fly-by to send it out to Jupiter. During the Earth-Moon encounters, ultraviolet spectroscopy and other techniques will help examine the geocorona and the magnetic tail which streams out opposite the Sun. The far side of the moon will be mapped in the near-infrared for the first time to discover variations in composition across the surface, and, on the near side, Galileo will augment Earth-based observations. Between the first and second Earth gravity-assist passes, Galileo will fly by the asteroid Gaspra at a range of less than 620 miles (1,000 km) and a relative velocity of about five miles per second. During the approach, it will take hundreds of pictures, the early ones for optical navigation to correct the flight path and point the camera as precisely as possible for the last few pictures. One picture might show the tiny asteroid filling half the picture width, at a surface resolution of a few tens of meters (or yards) per pixel, several times better than Voyager's 1986 pictures of Miranda, one of the moons of Uranus. Other satellite-type investigations will look at surface composition and roughness, optical and thermal properties, rotation rate, and the mass of Gaspra. On about Aug. 28, 1993, Galileo will fly by the asteroid Ida at about the same distance, but at about eight miles per second. Because this asteroid is twice as large (in diameter) as Gaspra, the observations may be easier, in spite of the faster passage. The spacecraft will make the same kinds of observations and measurements on both bodies, so scientists can begin to characterize asteroids from two different specimens. Almost two years later, in early July 1995, the probe will separate from the orbiter and travel unattended toward entry of the planet. A trajectory correction maneuver will alter the orbiter's flight path to overfly the probe and go into orbit around Jupiter. On Dec. 7, 1995, a few hours before probe entry (and the orbiter's closest approach to Jupiter), the orbiter will fly within 620 miles (1,000 km) of the Galilean satellite Io to make scientific observations. Io's gravity will help to slow the orbiter and thus reduce the propulsive maneuver needed to achieve Jupiter orbit. Near the time of the orbiter's closest approach to Jupiter, and before insertion into Jupiter orbit, the probe will penetrate the atmosphere. Current targeting is for the about 6 degrees north latitude. The probe will strike the upper layers of the atmosphere at about 100,000 miles per hour (48 km/sec) and for about four seconds will be subjected to up to 400 g deceleration (one g, a measure of acceleration, is the force of gravity at Earth's surface). About one minute after the peak deceleration forces have passed, the probe will deploy a parachute and the descent module's instruments will begin to take measurements. Data from the probe's instruments will be relayed to Earth through the orbiter, targeted to pass about 125,000 miles (200,000 km) above the cloud tops. The Galileo orbiter will track the probe for up to 75 minutes of operation in Jupiter's atmosphere, while the probe is descending to an atmospheric pressure level of about 20 times the atmospheric pressure at Earth's surface, some 90 miles (l50 km) below the one-atmosphere altitude. The orbiter's primary mission is planned to last 22 months at Jupiter. The new trajectory to Jupiter allows the spacecraft to complete the planned, full 10 orbits of the planet. The orbiter will make a near encounter (as close as 125 miles or 200 km) with one Galilean satellite on each orbit. The close encounters will provide images of the satellites 20 to 100 times better than any obtained before. The orbits have also been designed so Galileo can explore Jupiter's magnetosphere in detail. Previous planetary spacecraft have used one of two methods for stabilization -- spin-stabilized or three-axis stabilized. Galileo combines both methods in a new, dual-spin design that will give all the orbiter instruments the best conditions for observations. The main Galileo spacecraft, to which the fields-and- particles sensors are fixed, will be spun up to nearly 3 rpm. The aft portion of the spacecraft, called the despun section, will be stabilized so the camera and other instruments on the scan platform can be pointed at their targets. For communication with Earth, Galileo will use a furlable antenna 16 feet (4.8 meters) in diameter, similar to those on the Tracking and Data Relay Satellites. Jupiter is too far from the Sun to use solar cells to generate electric power, so Galileo will use radioisotope thermoelectric generators (RTGs). Through thermocouples, the RTGs convert heat from the natural radioactive decay of plutonium dioxide directly into electric power to run the spacecraft and its instruments. The spacecraft has been modified because of the new VEEGA trajectory, which extends flight duration and makes new requirements for heat protection and telecommunications. Some changes include installation of new Sun shades for temperature control: A large one to protect the bus that houses the electronics compartments, and others to protect the science boom, the magnetometer boom, RTG booms, and the probe-relay antenna. The spacecraft's high-gain and low-gain antennas will point away from Earth for as long as several hundred days after launch, so a third, aft-pointed antenna has been attached to one of the RTG booms. Other alterations relate to power allocations, software changes, and launch vehicle integration. Galileo's scientific instruments will conduct a more intensive and comprehensive investigation of the Jovian system than during all the missions that preceded it. One-hundred-fourteen science investigators from six nations have formed teams to interpret the data returned to Earth by Galileo's instruments. The Galileo orbiter carries a science payload of 10 instruments and the probe contains six. The instruments were chosen to enable scientists to make an in-depth study of the Jovian system. The orbiter will carry these instruments: *A camera system that uses a charge-coupled device instead of the vidicon tubes flown on all previous planetary missions. *A near-infrared mapping spectrometer that uses an advanced IR focal plane array to take multispectral images and reflected- light spectra of the satellites and atmosphere of Jupiter, and of Venus, Earth, the Moon, and the two asteroids on the way to Jupiter. *An ultraviolet spectrometer to study composition and structure of the upper atmosphere of Jupiter and its satellites, and the torus of charged particles injected into the magnetosphere by Io; other studies will be made during the transit phase. *A photopolarimeter radiometer that will measure the temperature profiles and energy balance of Jupiter's atmosphere and the cloud characteristics and composition. *A magnetometer to measure magnetic fields and their dynamics, and effects of the satellites' interaction with the planet's magnetic field. *A plasma instrument to provide information on low-energy charged particles and clouds of ionized gases in the magnetosphere. *An energetic-particle detector that will measure composition, distribution and energy spectra of high-energy particles trapped in Jupiter's magnetic field. *A plasma-wave instrument to investigate plasma waves generated in Jupiter's magnetosphere and plasma waves generated by lightning discharges in Jupiter's atmosphere, plus the studies of Venus. *A dust detection instrument that will determine the size, speed and charge of small particles near Jupiter and its satellites, and throughout interplanetary space. *Radio science experiments that will use tracking data to measure gravity fields of Jupiter and its satellites, and the two asteroids, and atmospheric measurements during radio occultations. The probe will carry these instruments: *An atmospheric structure instrument that will provide information about temperature, density and pressure to determine atmospheric structure. *A neutral mass spectrometer to measure the composition of the gases in Jupiter's atmosphere and how they vary with height. *A helium abundance interferometer to measure the ratio of hydrogen to helium in the atmosphere. *A nephelometer that will determine location of cloud layers and characteristics of cloud particles. *A net flux radiometer that will measure difference in energy flux being radiated inward and outward at each level in the atmosphere. *A lightning and radio emission instrument that will measure electromagnetic waves generated by lightning in the atmosphere and detect light and radio emissions from the flashes. The lightning instrument will also contain an energetic-particle detector to measure electrons and protons in the inner region of the radiation belts. Jupiter is of great interest to scientists because, unlike Earth and the other planets, which lost many of their light constituents, it retains much of its original chemical nature. Scientists will, therefore, by studying Jupiter, gain greater understanding of the solar system's beginnings. Current models of Jupiter have a deep and dense atmosphere that is primarily hydrogen and helium in about the same concentrations as are found in the Sun. A small, rocky core with the mass of a few Earths may exist at its center. Jupiter is 317.9 times more massive than Earth. It is accompanied in its slow march about the Sun by at least 16 satellites. The four largest satellites are of great interest. The satellites, named Io, Europa, Ganymede and Callisto, were discovered in 1610 by Galileo Galilei, for whom the mission is named. The Galilean satellites range in size between the Moon and Mercury. Io, innermost of the four, is the most geologically active body in the solar system. When Voyager 1 flew past Io in early 1979, it photographed eight volcanoes in the act of erupting. Europa, next distant from Jupiter, appears to be crisscrossed by long cracks. The cracks, however, are as smooth as if they had been painted on the satellite with a felt pen. Like Io, Europa is primarily a rocky body. Ganymede and Callisto, most distant of the four Galilean satellites, appear to be about half rock and half ice. Ganymede is the largest satellite yet measured in the solar system. Differences in the compositions of the satellites were probably caused by heat from Jupiter early in the formation process. The heat probably boiled away most of the volatiles from Io and Europa, while Ganymede and Callisto did not receive enough heat to cause loss of many of their volatile elements. Jupiter still radiates about twice as much energy at it receives from the Sun. The Galileo project is managed for NASA's Office of Space Science and Applications by the Jet Propulsion Laboratory, California Institute of Technology. JPL designed and built the orbiter. NASA's Ames Research center, Moffett Field, Calif., has management responsibility for the probe, which was built by Hughes Aircraft Co., El Segundo, Calif. John Casani is Galileo project manager, William J. O'Neil is science and mission design manager, Dr. Torrence V. Johnson is project scientist, and Dr. Clayne M. Yeates is deputy science and mission design manager. GALILEO VEEGA MISSION 1989 __________________ Orbiter Probe Spacecraft Mass 5870 pounds 737 pounds (includes 227 (includes 62 pounds of science pounds of instruments and science 2057 pounds of instruments) propellant) Science Experiments 11 6 ------------------------------------------------------------ ______________ STS/IUS Discovery STS-34 _______________ Launch Oct. 8, l989 Venus Gravity Assist (9300 miles alt.) Feb. 9, l990 Earth l Gravity Assist (620 miles alt.) Dec. 8, l990 Asteroid Gaspra Observation (620 miles alt.) Oct. 29,l99l Earth 2 Gravity Assist (200 miles alt.) Dec. 8, l992 Asteroid Ida Observation (620 miles alt) Aug. 28 1993 Probe Release July l995 Jupiter Arrival Dec. 7, l995 Io Closest Approach (620 miles) Dec. 7, l995 Probe Entry and Relay (appx. 75 min. duration) Dec. 7, l995 Jupiter Orbit Insertion Dec. 7, l995 Galilean Satellite Tour Dec. l995 - Oct. l997 First Ganymede Encounter June l996 _________ Gaspra: Type S l0 miles diameter Ida: Type S 20 miles diameter