MISSIONS TO COMETS

Until the beginning of the Space Age, all of our knowledge and understanding of comets came through the eyepieces of ground-based telescopes.

But once robotic probes started being sent out into deep space to study Earth’s sister planets, it was only a matter of time until some were aimed at comets, too. Launched in 1978, NASA’s International Comet Explorer (ICE) mission sent a probe towards Comet 21P/ Giacobini-Zinner. But studying the comet wasn’t the probe’s primary mission – that was to study Earth’s magnetic field and its interactions with the solar wind. When it reached its target in 1985, it raced by without stopping. The probe carried no cameras, so no images of the comet were taken, but it did take many useful measurements as it flew through the comet’s plasma tail. Six months later, ICE was part of an international armada of probes which studied Halley’s Comet, flying through this famous comet’s tail as well.

In March 1986, Halley’s Comet was almost invaded by probes from Earth. First, a pair of Russian Vega probes flew past it after swinging by Venus. They took many measurements of the size and composition of the comet’s icy nucleus and sent back around 1,500 images. Two days after the Vegas dashed past Halley, the Japanese Sakigake probe did the same, passing the nucleus at a distance of seven kilometres (4.3 miles).

Two days after that, the European Space Agency (ESA) probe Giotto flew past Halley’s Comet. By far the most advanced of all the probes sent out to meet the comet, Giotto passed the icy nucleus and sent back many images of its surface. Famously, when the first raw images were shown live on a BBC TV program there was a lot of bafflement and even disappointment that they just looked like coloured splashes and blobs. When the pictures had been processed and calibrated properly they showed fascinating detail on the surface – craters and ridges and jets of dust shooting out of it.

But the most exciting and scientifically important comet mission so far has been the European Space Agency’s Rosetta mission. Rosetta was a daring and hugely ambitious mission which not only aimed to rendezvous with and orbit a comet, but drop a lander down onto its surface. Many considered it to be Europe’s equivalent to Apollo, since it was so challenging. The probe was launched in 2004 and had a long, meandering trip across the Solar System, much of it spent fast asleep,