The Galileo's six-year voyage to Jupiter is nearly over, but it still has hurdles to overcome.
Tim Furniss/London
NASA's $1.4 BILLION Galileo interplanetary spacecraft ploughed through the most intense dust storm ever measured as it homed in on the planet Jupiter, aiming for a 7 December rendezvous. A small valve in part of the craft's propulsion system is stuck open and, earlier in the mission, its X-band, high-gain antenna did not fully deploy. The Galileo seems to be flying by the seat of its pants, but NASA remains confident.
The dust storm is the latest and greatest of several encountered by the Galileo since December 1994, when it was still 175 million kilometres from the giant planet. During the most recent storm, up to 20,000 dust particles a day were counted, compared with the normal interplanetary rate of about one particle every three days, says Dr Eberhard Grun, principal investigator for the spacecraft's dust-detector experiment.
The particles are emanating from somewhere in the Jovian system and may be the product of volcanoes on the planet's moon, Io, or could be coming from Jupiter's faint two-ring system. They may also be material left over from the impact of the comet Shoemaker-Levy 9 with Jupiter in July 1994. The particles are electrically charged and then accelerated by Jupiter's powerful magnetic field to a maximum speed of 720,000km/h.
They seem not to have harmed the Galileo, and the problem with its stuck valve seems unrelated. "There's no reason that the stuck valve has to have any impact on the mission, given that we manage it properly." says a cautious Bill O'Neil, the Galileo project manager, at the Jet Propulsion Laboratory, Pasadena, California.
The Galileo's 12, 10N (1lb) thrusters - six each mounted on two 2m-long booms - and its 400N main engine are driven by hypergolic monomethyle hydrazine propellant and nitrogen-tetroxide oxidiser, in two tanks each which are together fed by pressurised helium from two tanks. Valves are used to prevent vapour in the oxidiser tank from flowing backwards and mixing with vapour from the fuel tank.
One of these valves is stuck open. A worst-case scenario would be a small explosion, but, by controlling tank temperatures, scientists can minimise the movement of vapours, says O'Neil, who thought there was little danger of combustion.
The Galileo's main engine was successfully fired on 28 July, placing it on course for its rendezvous with Jupiter on 7 December, when the craft's engine will be fired again to enable it to become the first man-made object to go into orbit around the planet. Another firing to alter the orbit is planned later.
Even if the Galileo reaches its Jovian target, its mission would have been a fiasco had it not been for the unheralded action by computer and communications engineers on the ground, who have ensured that the craft can return the maximum amount of scientific data, despite the loss of the high-gain antenna.
The spacecraft has to act as the data-relay satellite for an instrumented capsule which will plunge into the bowels of Jupiter on the same day. The capsule was deployed successfully from the Galileo on 12 July (Flight International, 26 July-1 August).
The Galileo's 4.8m-diameter high-gain antenna did not unfurl correctly on 11 April, 1991 - after the craft's deployment from the Space Shuttle Atlantis in October 1989. Three of the 18 ribs of the 4.8m wide, umbrella-like, wire-mesh antenna remained in the stowed position. Scientists will have to rely on the less powerful primary low-gain antenna to gather data from the planet.
The high-gain antenna is thought to have lost lubricant during numerous cross-country truck trips after the Galileo's launch had been delayed by three years after the Challenger disaster in 1986. Further attempts to vibrate the stuck ribs loose failed In December 1992 and January 1993.
There is just the possibility that the balky high-gain antenna may spring open when the Galileo fires its main engine for an orbit-change manoeuvre in March 1996, which will provide the greatest acceleration forces on the craft since launch.
NASA is assuming the worst, however, and has released details of its new Galileo telecommunications strategy, to retrieve as much data as possible from the mission. Because of interrelated upgrades to the Galileo's on-board computer software and its ground- based communications hardware, the low-gain antenna can be used more effectively.
Without any enhancements, there would have been a "profound loss" of science data from the orbiter, says NASA. The low-gain antenna's transmission rate at Jupiter would have been 8-16 bits/s, compared with the high-gain antenna's 134,000 bits/s. The improvements will increase the low-gain data rate to 160 bits/s.
New software has been loaded into Galileo's computer for the capsule plunge portion of the mission and a second set of new software will be transmitted to the craft in March 1996 for the main Jovian science mission. Two data-compression methods will be used and NASA Deep Space Network antennae will be arrayed to allow more of the weak signals to be captured.
As a result, says NASA, 100% of the science data from the atmospheric capsule will be provided. A near continuous, real-time survey of the Jovian magnetosphere can be made for two years; and about 1,500 images of the four Galilean satellites - Io, Ganymede, Callisto and Europa - four minor satellites and Jupiter and its rings will be returned.
There will be a total of 11 close encounters with the Galilean satellites - five at a distance of less than 80,000km - which will provide science on the same level as the Voyager 1 and 2 spacecraft which were flown past that planet in 1979, says NASA.
Source: Flight International
