INFO 19-1996: Three spacecraft observe Jupiter's glowing polar regions
30 September 1996The International Ultraviolet Explorer has completed a campaign of special observations of Jupiter in concert with the Hubble Space Telescope and with NASA's Galileo spacecraft now in orbit around the giant planet. IUE provided an unrepeatable opportunity for sustained observations by ultraviolet light, over 40 days, as its contribution to the programme called the International Jupiter Watch. Important targets were the aurorae, activated by charged particles hitting Jupiter's atmosphere, which IUE discovered around the planet's magnetic poles in 1980.
The aurorae on Jupiter are like the Aurorae Borealis and Australis on the Earth, although visible only by ultraviolet light. They flicker in a similar way in response to variations in the solar wind of charged particles blowing from the Sun. While Galileo monitored the changing environment of particles and magnetism in Jupiter's vicinity, IUE recorded surprisingly large and rapid variations in the overall strength of the auroral activity.
IUE's main 45-centimetre telescope did not supply images, but broke up the ultraviolet rays into spectra, like invisible rainbows, from which astrophysicists could deduce chemical compositions, motions and temperatures in the cosmic objects under examination. In the case of Jupiter's aurorae, the strongest emission came from activated hydrogen atoms at a wavelength of 1216 angstroms. The Hubble Space Telescope's contributions to the International Jupiter Watch included images showing variations in the form of the aurorae, and "close-up" spectra of parts of the auroral ovals.
Astronomers will compare the flickering aurorae on Jupiter with concurrent monitoring of the Sun and the solar wind by the ESA-NASA SOHO spacecraft and several satellites of the Interagency Solar-Terrestrial Programme. It is notable that changes in auroral intensity by a factor of two or three occurred during the 1996 observational period, even though the Sun was in an exceptionally quiet phase, with very few sunspots. In principle, a watch on Jupiter's aurorae could become a valuable means of checking the long-range effects of solar activity, which also has important consequences for the Earth. The situation at Jupiter is quite different from the Earth's, with the moons strongly influencing the planet's space environment. But with Hubble busy with other work, any such Jupiter-monitoring programme will have to await a new ultraviolet space observatory.
IUE observed Jupiter intensively in 1979-80 in conjunction with the visits of NASA's Voyager spacecraft, and again in 1994, when the fragments of Comet Shoemaker-Levy 9 hit Jupiter in a spectacular series of events. The explosive impacts appeared to repress the auroral activity at the time, suggesting a remarkable effect of comet dust on the charged particles creating the aurorae in Jupiter's atmosphere. The new results on variability due to other causes will help astronomers to assess that effect more confidently. They will also compare the 1994 and 1996 IUE data to see how the atmosphere of Jupiter has recovered from the impacts.
In Jupiter's vicinity IUE registered ultraviolet emissions from oxygen and sulphur atoms littering the orbit of Io, and probably released by volcanic emissions from that peculiar moon. This Io Torus is highly variable too. The record of its ultraviolet emissions, both within the 1996 campaign and in comparison with earlier observations, will help the astronomers to understand the reasons for the variations.
A Remarkable History
The close scrutiny of Jupiter and its moons was the final astronomical task of IUE, before the termination of space operations on 30 September 1996. Over the past few months the IUE science team and collaborating astronomers in Europe have fulfilled a wish-list of important observations precluded by the intense demands on their ultraviolet space observatory throughout its life of nearly nineteen years. The observations in the final science programme confirmed and extended IUE's record, as the most reliable and productive astronomical satellite that ever flew.
In March of this year the spacecraft was ailing, with only one of its six gyros still functioning, which severely limited the scope of its original mission. By skillful control and spacecraft engineering it went on harvesting new data, including prolonged observations of Comet Hyakutake. The concluding campaigns that began in April targeted the gamma-ray emitting "blazar" Markarian 421, various other active galaxies, and stellar winds, as well as Jupiter.
"I am sad but also privileged to be the last observer with IUE," says Rene Prang of Orsay, France, who was in charge of the Jupiter programme, "At the end it provided us wit 800 observations of Jupiter, so it was still doing important work at the leading edge of planetary astronomy and space research." Created jointly by NASA, the UK government and ESA, IUE was supposed to last for three years, when it was launched on 26 January 1978. Instead, the 700-kilogram spacecraft went on supplying astronomers with ultraviolet spectroscopic information available from no other spacecraft until the launch of the Hubble Space Telescope in 1990.
As the only space observatory offering them a hands-on mode of operation, at ESA's ground station at Villafranca near Madrid, IUE was a favourite with astronomers. An astounding total of 114,000 individual observations of planets, stars, galaxies and quasars assures the spacecraft a cherished place in the history of astronomy.
IUE supplied the bedrock ultraviolet data on top events during its lifetime. These included the apparition of Halley's Comet in 1986. At the comet's approach in September 1985, IUE detected the ultraviolet signature of water molecules, and regular observations thereafter showed that the comet shed 300 million tonnes of water during its visit to the Sun's vicinity. With the explosion of a star in the Large Magellan Cloud, as Supernova 1987A, IUE was trained instantly on the scene.
Comparisons with previous IUE observations of the same region revealed exactly which star had blown up. The characteristic emissions of chemical elements flung into space by the explosion were also identified, IUE's detection of a delayed light echo, from a ring of dust surrounding the defunct star, later enabled the Hubble Space Telescope to measure the distance to Supernova 1987A precisely.
Eruptions in the nuclei of active galaxies were a prominent theme in IUE's work throughout its lifetime. Intensive studies of selected galaxies, sometimes in concert with X ray observations by other spacecraft, have built up unprecedented albums of data from which astronomers can puzzle out the behaviour of these violent objects.
After the termination of space operations, the IUE mission continues on the ground with the task of reprocessing all the raw data ever transmitted from the spacecraft, using the latest computational techniques. This will create the IUE archive of ultraviolet spectra, from which future generations of astronomers will continue to cull unique information on nearly 10 000 objects in the sky.
Over the years, frequent international symposia have digested the results from IUE. At a special meeting of the European Astronomical Society next year November in Sevilla (Spain), astronomers will have the opportunity to put the latest findings in perspective, in relation to all the other observations since 1978.
"When NASA decided last year to terminate its operations as the leading partner in IUE, we expanded our European operations at Villafranca," comments Roger Bonnet, ESA's Science Director, "Although we were unable to prolong the life of the spacecraft indefinitely, it is gratifying to see what excellent use the astronomers have made of this final phase of IUE's long career."