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After just over seventeen years in space the Ulysses space probe looks as if its mission is drawing to a close. According to the European Space Agency Ulysses is beginning to show the signs of age, after nearly two decades touring the Solar System in orbit around its main interest – the Sun. The spacecraft is in a somewhat peculiar six year orbit around the Sun which takes it at one extreme flying right out to Jupiter and at the other extreme whirling over the Sun’s polar regions.

Ulysses has scrutinized the Sun in great detail over its seventeen or so years, telling us about the solar wind (the stream of charged particles emanating from the Sun), the Sun’s powerful magnetic field as well as how the Sun’s chromosphere and corona interact. One of Ulysses’ most interesting finds was that the solar wind at the poles of the Sun is emerging much faster than at the equator (with speeds of 750km/s and 350km/s respectively); and that the polar winds dominate about 2/3 of the heliosphere (the vast ’shield’ around the Sun created by the solar wind). The probe, which was launched from the Space Shuttle’s cargo bay in 1990 has also studied Jupiter’s magnetic fields and the plasma environment around the giant planet.

So why is Ulysses dying after all these years? Well the probe itself is powered by the radioactive decay of a special isotope of plutonium. This decay slowly releases heat which the probe’s on-board technology converts into electricity. This in turn powers heaters, the science instruments and the communication antennae that are needed to send data back to Earth. However over time the radioactive isotope decay levels drop meaning that so to do the amounts of electricity produced.

This poses a problem for the craft because as it ventures out into space it needs its on-board heaters to keep the spacecraft at the right temperature. If they don’t and the craft’s temperature drops below 2°C then the probe’s fuel hydrazine (where have you heard that before) will freeze solid. It reminds me a bit of the lizards on David Attenborough’s incredible new BBC series ‘Life in Cold Blood’. They either need the Sun to warm them or find some heat of their own, otherwise they will freeze and die.

The scientists working with the Ulysses probe realised that if they didn’t do something the power levels would drop so much that the heaters and the science instruments would stop working – freezing the all important fuel. So in January they turned off the main transmitter that was sending data back to Earth in the hope that the re-routed power would go to the heaters and the science packages; keeping the hydrazine liquid. Unfortunately when they wanted to turn the transmitter back on to see if it had worked nothing happened. Now it seems Ulysses’ fuel will freeze as the probe cools in the vacuum of space to that crucial 2°C, bringing with it the end of an illustrious mission. Thankfully though there are many missions which are taking up Ulysses’ baton like SOHO, Stereo, Trace and Hinode. So solar science will be continuing into solar cycle 24, long after little Ulysses ends.

Above: An artist’s impression of the Ulysses probe
Credit: ESA