Thirty years ago, a European spacecraft completed a pioneering mission for cometary science: Giotto flew by Halley's Comet, returning the first close-up observations of a comet's nucleus and paving the way for future cometary exploration such as the ongoing Rosetta mission.
"All our understanding of comets when Rosetta was realised in the 90s went back to what we learned from Giotto and to some extent the Vega missions," said Stephan Ulamec, project manager for Rosetta's Philae lander, in a phone call. "All of what we produced for engineering models to design the orbiter, to design the lander in particular—all of this goes back to the Giotto mission."
On 13-14 March 1986, the Giotto probe passed by Comet 1P/Halley at a distance of 596 km—thousands of kilometres closer than the two Soviet Vega probes that intercepted Halley earlier in the month—and gave us the first real look at a comet. Giotto gave us the first evidence that comets contained organic material, which is in large part why we're so interested in them: It's thought comets could reveal more about the formation of the planets and potentially answer mysteries like how water got to Earth.
Giotto's specific target was suitably worthy. Halley was the first comet to be recognised as "periodic" when Edmond Halley showed in the 18th century that it was reappearing at regular 76-year intervals. But because of this, Giotto was under a big time constraint.
"The Giotto mission was started at the end of the 70s with the arrival of Halley's comet in the vicinity of the Sun a few years later, which left very little time to prepare for an encounter," recalled astrophysicist Roger-Maurice Bonnet in a phone call. Bonnet was ESA's director of scientific programmes at the time and now works at the International Space Science Institute at Bern.
Originally planned as a joint mission with the US, Giotto became the European Space Agency's first ever planetary mission when NASA pulled out due to budget cuts. ESA gave the go-ahead in 1980. "It was cheap, it was done fast, it was very risky—we launched a mission with some detectors which had never been flown before in space," summarised Bonnet. "We had guts to do Giotto."
But Giotto was a success, and its legacy lies in Europe's continued thirst for cometary exploration and most notably the Rosetta mission, which got even closer to a comet in 2014 when it put an actual lander on the surface of Comet 67P/Churyumov-Gerasimenko.
"It triggered the interest in Europe in cometary missions and it triggered the interest to not just have a flyby within hours, but to really visit a comet and observe it over months or even years and land on it," said Ulamec.
The seeds of the Rosetta mission were already being planted before Giotto had returned any data; ESA committed to a "comet nucleus sample return mission" as early as 1984. As the Rosetta mission developed, the team was able to use Giotto's findings to refine its design. Many instruments on Rosetta are specifically looking for the kind of organic chemistry Giotto first found on Halley, and Giotto's characterisation of the comet's composition helped inform the design of the Philae lander.
Rosetta was launched in 2004 and reached it target 10 years later; the orbiter is still following Comet 67P.
The contexts of the Giotto and Rosetta missions differ largely in several ways. For a start, Rosetta's flyby is more like a slow ride-along; the orbiter is following 67P as the comet develops and even had the chance to drop a lander. Giotto, meanwhile, only got a fleeting glimpse of Halley. Given Halley travels in retrograde (in an opposite direction to the Sun), Giotto and the comet were travelling towards each other at 245,000 km per hour.
"In the case of Rosetta, because we were going in the same direction as the comet, we could observe the evolution of the activity of the comet—how the dust was ejected into interplanetary space, how the comet was heated, and whether we could measure the dust particles coming out of the comet," said Bonnet. "While with Giotto we took the particles in full face, like in a car accident or a plane accident."
A special dust shield kept Giotto safe so it could take pictures of the comet nucleus as it flew by, and indeed the spacecraft not only survived the journey but lived to go into hibernation for a couple extra missions, such as a flyby of Comet Grigg-Skjellerup in 1992.
Rosetta still has months left before its mission ends with a dive into its target comet, but its legacy clearly carries on the work of its comet-chasing forefather.
"Both missions are landmarks in the history of the evolution of the Solar System," said Bonnet. "There will be others in the future, but certainly both of them have made history when they obtained their first results."