This enhanced image, generated from data sent back by Giotto’s camera, showed a peanut-shaped nucleus 15 km long. Image credit: Halley Multicolor Camera Team, Giotto Project, ESA

Aug 26, 2014 The first time we saw a comet close up

Sen—With the European Space Agency’s Rosetta probe now circling a comet, and the prospect of a landing on its solid nucleus to come, I find my mind cast back more than a quarter of a century to the first ever cometary encounter.

Way back in March 1986, another European probe, called Giotto, flew past that most famous of comets, Halley, and gave us our earliest close-up of one of these enigmatic objects.

The 2112 images sent back by Giotto were less detailed than the remarkable views we have been getting from Rosetta of Churyumov-Gerasimenko. But they were astonishing for their time and showed that Halley, a regular bright visitor observed throughout history, was not dissimilar in shape.

Rather than being a spherical snowball, Halley’s 15 km (nine mile) long nucleus was dark and shaped more like a peanut, with bright jets of gas and dust streaming from it. 

The Giotto mission, launched in July 1985 by an Ariane 1 rocket, was ESA's first deep space adventure. It had been put together very quickly for such a project, being given the go-ahead only in 1980. Originally America was also on board for a joint mission, but NASA pulled out due to funding cutbacks. (Sound familiar, anyone?)

I remember watching TV for a special live BBC programme covering the Giotto encounter. I was soon to leave on a six-week trip to New Zealand and Australia to get my own best views of the comet, which visits the inner Solar System every 76 years, and this would be a great appetiser.

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The closest the writer got to Halley's Comet was a trip to New Zealand. This photo shows it above a monument in Wellington, in March 1986. Image credit: Paul Sutherland

In the event, the programme was a rather embarrassing disappointment. There was no instant science, and the studio full of experts and enthusiasts struggled to know what to talk about. It is said that then UK Prime Minister Margaret Thatcher (a qualified chemist) was decidely unimpressed with what she saw and slashed the science budget!

But Giotto was a huge success. When the pictures began streaming in, they were striking for their time. They allowed scientists to measure the size and shape of the nucleus. Seven jets were observed to be spewing three tonnes of material a second. Instruments revealed the composition of outflowing gas and the size, velocity and make-up of dust particles.

Around 12,000 dust impacts were felt by the probe. Then, just before closest approach, at a distance of around 1,372 km (853 miles), a larger piece of comet debris sent Giotto spinning and knocked out its camera.

Despite now flying blind, Giotto’s other instruments continued to send back data. And so, as with Rosetta years later, it was decided to put the spacecraft into hibernation and send it to study a second comet. 

The “Wake up Giotto” moment came in February 1990. ESA scientists found that of the ten onboard instruments, three were fully operational and four partially so. The decision was taken to re-target Giotto to fly past a comet called Grigg-Skjellerup.

In another first, Giotto was sent skimming past Earth on 2 July 1990 to get a speed boost, the first deep space mission to benefit from such a “gravity assist” manoeuvre. Then on 10 July 1992, Giotto flew through the coma of dust and gas surrounding Grigg-Skjellerup, passing less than 200 km (125 miles) from its nucleus.

Since then a number of international probes have visited other comets and some of their asteroid relatives too, boosting our knowledge of these minor members of the Solar System. But Giotto has a place in history as our first ever close-up comet explorer.