July-August 1982:the Galileo mission

Introduction

In mid-1985 NASA will launch its only outer planets mission of this decade, the Galileo mission to Jupiter. The two-component spacecraft, consisting of a Jupiter orbiter and an atmospheric entry probe, is designed to expand our understanding of the Jovian system gained from the Voyager project. Galileo should allow scientists to determine the chemical composition and physical state of Jupiter's atmosphere and satellites and to characterise the planet's magnetic and charged particle environment.

The quality of Galileo’s scientific investigations will be enhanced by four major advances over previous missions: extended observations during the two year orbiter mission, closer flybys of the major satellites, direct sampling of Jupiter’s atmosphere, and the utilization of advanced instrumentation.

History

At its inception in 1977 Galileo was scheduled to be launched in 1982 by a Space Shuttle plus a 3 stage planetary version of the Inertial Upper Stage. The trajectory would have included a close (275 km) flyby of Mars to gain heliocentric energy, thus reducing the required launch energy. Unfortunately, development problems with the Shuttle necessitated delaying the mission to 1984, and the resulting poorer planetary alignment required additional propulsive capability during the Mars flyby. This increased the spacecraft mass and forced the planners to divide the mission into two parts, launching Orbiter and Probe separately about 1 month apart. Furthermore, this proposal required the development of a Probe Carrier spacecraft to deliver the probe to Jupiter and to relay its data to Earth.

In January 1981, with continuing problems in the development of the three stage IUS, NASA decided to replace the IUS with a modified Centaur upper stage and move the Galileo launch to 1985. The higher performance of the Centaur stage allowed the Orbiter and Probe to once again be combined in a single launch. In addition, a direct trajectory was again possible, dispensing with a Mars flyby.

Spacecraft Description

The Galileo Orbiter is a dual-spin spacecraft. Part of the craft will be three-axis stabilised to provide a steady base for those instruments which must be accurately pointed. The main body of the Orbiter will spin at 3 rpm to provide stability and to allow the. fields and particles instruments to scan the sky.

The spun section contains both the high and low gain antennae, the retropropulsive module (RPM), the Radio Isotopic Thermoelectric Generators, most of the electronics, command and data equipment and the fields and particle experiments.

Galileo will use a 4.8 metre diameter high gain antenna to communicate with Earth. This antenna is similar to that under development for NASA’s Tracking and Data Relay Satellites. A small (1 m) antenna on the despun portion of the Orbiter will be used to receive data from the Probe for relay to Earth.

The RPM will be used for all propulsive and attitude control manoeuvres and consists of a 400 Newton engine plus two clusters of 10 Newton thrusters. Because of minimum burn size and total wetted lifetime constraints, the 400 Newton engine will be used only for the three major trajectory changes, i.e. deflection after probe release, orbit insertion and the perijove raise manoeuvre. All course corrections will be achieved by the use of the small 10 N thrusters. The RPM is being provided by the German company MBB, under the management of the DFVLR.

The fields and particles instruments are carried either on the body of spinning section or, in the case of the plasma-wave and magnetometer sensors, on a long boom to remove them from the influence of the spacecraft structure. The despun section is a relatively small platform carrying the Probe data relay antenna and the four remote sensing instruments which require accurate pointing.

The atmosphere entry Probe is designed to protect its cargo of seven scientific instruments as they penetrate the Jovian atmosphere and to decelerate them until they can be lowered gently through the atmosphere by parachute. It consists of a nearly spherical descent module surrounded by a two-piece aeroshell and heatshield, similar to the Pioneer Venus probes in external appearance. The heatshield makes up nearly half of the total probe mass and is jettisoned after entry. The Probe is powered by internal batteries and remains inert throughout the cruise phase, being activated only a short time before entry into Jupiter’s atmosphere. No attitude control or propulsion is provided and the Probe must be accurately aimed prior to separation from the Orbiter. It will be spun at 10 rpm to provide stability during its coast and atmospheric entry.

Mission Profile

The launch of Galileo in its modified form is now expected to take place in May or August 1985, with the Shuttle Orbiter taking the craft and its 2-stage Inertial Upper Stage into low Earth orbit. The IUS will then ignite and accelerate Galileo to just -below Earth-escape velocity. Since the IUS is a much less powerful stage than Centaur, Galileo must now carry an extra kick stage to provide adequate energy. The three-axis controlled Spacecraft Injection Module (SIM), using a hydrazine attitude control system and a Star 48 solid motor, will be attached to the bottom of the spacecraft stack.

After SIM burnout, the spacecraft booms will deploy and the SIM control system will begin a spin rate of 2-3 rpm. The SIM will then be jettisoned and Galileo will revert to its pre- Centaur cancellation configuration. But, instead of moving towards a direct encounter with Jupiter, Galileo will move in a 2-year period orbit that will bring it back to Earth in mid-1987 for a 200 km flyby in order to pick up enough energy for the trip to Jupiter. This “AVEGA” (Earth Gravity Assist) trajectory means that Jupiter encounter will not occur until 1990, and the less-favourable launch conditions dictate a more restricted mission once the craft is established in orbit about the giant planet.

About 5 months before Jupiter encounter the Probe and Orbiter will be separated. The RPM will then be used to deflect the path of the Orbiter onto its required trajectory. Each spacecraft will then proceed independently to Jupiter.

Four hours before the Probe begins its entry into Jupiter’s atmosphere, the Orbiter will make its only close approach to the innermost satellite, Io. The Orbiter will pass within about 500 km of this volcanically active satellite. As well as detailed investigations of lo this encounter will provide an additional bonus for mission planners. The gravitation attraction of Io will slow the Orbiter, reducing the size of the Jovian Orbit Insertion burn, required from the RPM. The Orbiter will not return to Io since repeated exposure to the intense radiation environment close to Jupiter could disable the spacecraft.

As the probe enters the atmosphere the Orbiter will be about 200,000 km above Jupiter’s cloud tops. All of the data gathered from the probe will be transmitted to the Orbiter in real time for onward transmission to Earth. As with Apollo re-entries the probe must accurately enter the Jovian atmosphere via a narrow corridor. Too steep an entry will produce intense deceleration and frictional heating which will destroy the Probe, too shallow an entry angle will cause it to skip back into space. This entry will, however, differ from Apollo in two very significant ways; the Probe will be travelling much faster than a returning Apollo and entering an atmosphere whose detailed structure is unknown. It is intended that the probe will enter the atmosphere within 5.5 degrees of the equator, penetrating the high level equitorial zone. This will allow measurements of all Jupiter’s significant cloud levels.

During entry the probe will decelerate at nearly 400g and the intense frictional heating will cause most the craft’s pro¬ tective heat shield to ablate away. The Probe will briefly appear as a bright red meteor flashing through the ammonia clouds. Shortly afterwards, the rear cover of the Probe will be jettisoned to allow deployment of the drogue parachute. This parachute will both stabilise the->falling Probe and provide additional deceleration. A few seconds later the main parachute will be opened and the forward portion of the heat shield will be jettisoned to fall away into the depths of Jupiter’s atmosphere. The entire entry operation will have lasted about two minutes and the Probe will then be suspended below its single parachute, and gently drifting down into the atmosphere. The battery of scientific instruments will begin their work of analysing the Jovian atmosphere, returning the results to Earth via the Orbiter as it hurtles overhead.

The operating life of the Probe'will be short, probably only about an hour. As it descends, the pressure around it will increase and the temperature will rise. After about 40 minutes the pressure will have increased to ten Earth atmospheres and it will be below the lowest layer of water clouds. After an additional 20 minutes the pressure will have risen to nearly 20 Earth atmospheres and the probe will be reaching the end of its life. Eventually, it will be silenced by a combination of pressure, temperature and weakened radio signals.

Shortly after the Probe mission is completed the Orbiter will fire its 400 N engine in the RPM for the hour long insertion burn. The initial orbit will be highly elliptical, ranging from 200,000 to 15 million km.

For the next 20 months the Orbiter will carry out an intensive survey of the Jovian system. The basic mission before Centaur was dropped called for 11 orbits of the planet, includ¬ ing at least one close encounter of an inner satellite on each orbit. The new mission may now be able to achieve only six encounters. This will be achieved by using the gravitational field of the satellites to bend the Orbiter’s trajectory each time a close encounter occurs. This allows a reduction in the amount of propellant required to achieve these multiple satellite encounters. The precise orbital tour to be used is not yet decided, studies will continue until shortly before Jupiter arrival when the final plan will be chosen. If all goes well, there is a possibility that an extended mission could be flown until the spacecraft’s propellants are finally exhausted.