On 2 March 1972 an Atlas rocket lifted off with Pioneer 10. After the Centaur stage had boosted it away from the Earth, a small solid-rocket accelerated the spacecraft further to 50,000 kilometres per hour, faster than any previous probe, in order to fly a billion-kilometre 'fast track' route to Jupiter. It soon crossed Mars's orbit, and entered uncharted territory. In July 1972 it officially entered the asteroid belt, but the boundary was rather arbitrarily defined. This was a very nervous time for the people who expected the spacecraft to fall silent at any moment, but it emerged unscathed from the other side in February 1973. During the interplanetary cruise, the particles and fields instruments mapped the magnetic field and reported how the strength of the solar wind varied in response to solar storms and with increasing distance from the Sun; monitored the interactions between the plasma and the electric and magnetic fields; measured the size, mass and velocity of dust, particularly while traversing the asteroid belt; and monitored the flux of cosmic rays which were passing through the Solar System from their mysterious sources far beyond.
On 26 November 1973 the instruments noted an abrupt change in the state of the electromagnetic environment. This was the 'bow shock' where the solar wind struck the sunward side of the Jovian magnetosphere. Although this had been predicted, the fact that it was 8.5 million kilometres from the planet was a considerable surprise. A day later, Pioneer 10 crossed the magnetopause and entered the magnetosphere. It became evident that the magnetosphere spans at least 100 planetary radii, making it the largest discrete structure in the Solar System. If it could be seen, it would be the largest object in the Earth's sky. No longer exposed to the solar wind, the spacecraft proceeded to map the intensity, direction and structure of the planet's own magnetic field. Over the next few days, the fluxes and energies of the trapped charged particles increased. After a week, by which time the spacecraft was very close to the planet, the radiation was so intense that several of its instruments were reporting 'off-scale-high', having been saturated. On 3 December, Pioneer 10 flew 130,000 kilometres above the Jovian cloud tops, which, in terms of planetocentric distance, was slightly less than 3 radii.
The Photopolarimeter scanned narrow strips of Jupiter's disk as the spacecraft rotated. The resulting imagery not only far exceeded the best telescopic pictures, the 500-kilometre surface resolution revealed fine structure in the atmosphere which not even dedicated observers had glimpsed during rare moments of perfect seeing. The Infrared Radiometer, whose field of view became tighter than the planet's disk only for an hour or so in the run up to closest approach, measured the temperature field at the cloud tops, thereby confirming that the planet emits more energy than it receives from the Sun.4 The Ultraviolet Photometer recorded 'glows' around Jupiter
from neutral hydrogen, and from auroral activity. As the spacecraft slipped behind Jupiter's limb, the way in which its radio signal was refracted and attenuated profiled the physical and chemical properties of the upper atmosphere. Tracking of the fly-by charted the gravitational field, which measured the planet's moment of inertia, in turn providing insight into its interior structure.
In addition, Pioneer 10 secured the first close imagery of the four large satellites. The trajectory had been carefully set up so that the spacecraft would pass behind Io, the innermost of the four, and this revealed that the moon has a tenuous ionosphere. Furthermore, the Ultraviolet Photometer revealed that a large region of space near Io glows. In early telescopic reports, Io had been thought to be too small to retain an atmosphere, so the moon became one of the mysteries of the Jovian system that the Voyagers would investigate.
As a result of its encounter with Jupiter, Pioneer 10 was accelerated away from the Sun, to report on the outer heliosphere.
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