Asteroids are not the only smaller bodies found in the Solar System. Another major class of such objects is comets. These regularly travel through our cosmic neighbourhood, and very occasionally become bright enough to be spectacular.
The appearance of the brighter comets throughout history earned them a fearsome reputation. Today we know they are minor, insubstantial bodies compared to planets. But they are still regarded with awe by scientists because they are pristine chunks left over from the formation of the Solar System, 4.5 billion years ago.
Analysis has shown that their chemistry is organic including water ice, methane and carbon compounds beneath a dark crust. Our understanding of comets has been boosted by Europe’s recent Rosetta mission to explore and land on one.
Hairy, scary stars
The word comet means “hairy star” because the ones bright enough to see before the age of the telescope grew tails resembling flowing hair. They were unpredictable, unlike the planets in their steady courses, so the sudden addition of a bright interloper, with a tail stretching across the sky, unnerved court astrologers in early civilisations who saw them as portents of cataclysmic events.
The brightest comets are still unpredictable, because many are on their first journey into the Solar System. A spectacular example in the 21st Century was Comet McNaught which delighted southern hemisphere observers in 2007. Ten years earlier, another visitor, Comet Hale-Bopp, became a brilliant object with a fine tail for those at northern latitudes.
Astronomers now believe that most originate either from a vast zone of icy debris called the Oort Cloud, stretching perhaps a quarter the way to the nearest star. Something, such as a passing star long ago, disrupted the cloud nudging some of this debris so that it fell in towards the Sun. Other comets, with shorter periods, are thought to be from the Kuiper Belt.
Comets typically have extremely stretched orbits that bring them in close to the Sun before sending them back out into space, sometimes never to return. But throughout history, Jupiter has wielded its gravitational influence to steer a number into much smaller orbits. They now orbit the Sun in periods of as little as a few years, and their appearance can be predicted.
Bright comets have always made the cosmic headlines. However the vast majority are much fainter. Many are tracked across the sky every year by astronomers, but only a few become bright enough to be seen with binoculars, let alone the naked eye.
What is a comet?
The bundle of ice and dusty material that produces a comet is known as the nucleus and is usually no more than a few tens of kilometres wide. As it heads into the Solar System, it becomes warmed by the sunlight, thawing the ice and causing gases to escape and expand into a ghostly “atmosphere” called a coma. Only the more active comets form tails, giving off a straight tail of gas that points away from the Sun due to pressure from the solar wind, and a second curved flow of dust released by the warmed nucleus. When the Earth passes through streams of dust left by comets, they produce showers of meteors, or “shooting stars”.
The classic example of a comet disturbed by Jupiter is the well-known Halley’s Comet, which now travels into the inner Solar System from beyond Neptune every 76 years and was last close by in 1985-1986. Comets usually take the names of their discoverers, but Halley’s Comet was named after English astronomer Edmond Halley who realised that certain comets recorded throughout history were one and the same, and so successfully predicted its return of 1758. We will next see Halley’s Comet in 2061.
Though comets have visited us throughout history, the last return of Halley’s Comet marked the first time space scientists went to visit them! Five spacecraft – two Soviet, two Japanese and one European – flew out to study it as it passed through the inner Solar System. ESA’s Giotto flew closest, at a distance of just 596 km, and sent back the first images of the comet’s nucleus, showing that it was shaped like a peanut.
In 2001, NASA’s Deep Space 1 flew past Comet Borrelly and in 2004, the Stardust probe sped through the coma of Comet Wild 2, collecting particles that it sent back to Earth in a capsule. NASA’s most dramatic comet encounter came when a probe called Deep Impact fired a missile into Comet Tempel 1 on July 4, 2005. Scientists had wanted to know what lay within, but the explosion was so great that it hid the view. Deep Impact, now renamed EPOXI, flew on to view Comet Hartley 2 in October 2010 – but without the fireworks this time.
The Rosetta mission
The biggest boost to our knowledge about comets came from a European-led mission called Rosetta, which flew alongside one called 67P/Churyumov-Gerasimenko for more than two years, studying it in incredible detail.
Rosetta launched from the European Space Agency’s site at Kourou, French Guiana, in March 2004. The spacecraft carried a small companion probe, Philae, to land on the comet’s nucleus. With its huge solar panels, Rosetta stretched 32 metres in length. Philae was the size of a tea caddy.
The comet, named after its two discoverers, is another example of one with an orbit shortened by Jupiter. It now travels in from beyond Jupiter to a closest point to the Sun, called perihelion, that lies between Mars and Earth. A single circuit takes six and a half years.
Rosetta took a roundabout route to reach the comet, swinging past Earth and Mars to build up speed. On one such approach to Earth, the spacecraft was briefly mistaken for an incoming threatening asteroid! After visiting two asteroids en route, Steins in 2008 and then Lutetia in 2010, Rosetta was put into hibernation to conserve power in July 2011. Scientists were elated and relieved when it responded to a wake-up signal two and a half years later in January 2014.
Then, as the comet grew from being a tiny speck to show some form, the team realised they faced a new challenge. Rather than have being round, it had a twin-lobed shape that resembled a duck. Landing Philae was going to be tricky already, since a comet has such a small gravitational pull, but it just got a lot more difficult.
Rosetta rendezvoused with 67P in August 2014 and began to fly around it at varying distances, both to study it, but also to map its irregular form and decide where to send Philae.
In November the probe was sent on its descent to touch down on the head of the “duck”. But after harpoons failed to fire and anchor it securely, it bounced twice before ending up in the shadow of a cliff. Philae sent back much useful data for several hours before its batteries died and, without sunlight to recharge them, contact was effectively lost.
Meanwhile, Rosetta continued its work as the comet got closer to the Sun and began fizzing with activity, sending out jets of gas and dust. It reached perihelion in August 2015 before beginning the slow retreat into the outer Solar System again. Rosetta could have continued orbiting 67P but its controllers decided to end the mission in style by landing Rosetta close to Philae on the comet in September 2016.
What we learned from Rosetta
Traditionally scientists have seen comets as “dirty snowballs”, but 67P showed their make-up is more complex. Comet 67P is certainly not shaped like a snowball, but appears to be formed from two objects which merged, billions of years ago.
The nucleus was found to be very low density, porous, and covered in sinkholes where the surface has collapsed as gas escaped to form the comet’s coma and tail. Much of its surface is covered in powdery dust, within which were detected some of the organic compounds that are important for life as we know it. But the water in the comet was of a different “flavour” to that found in Earth’s oceans, suggesting that our seawater may not have been delivered by the bombardment of comets like 67P.
A big surprise was the discovery of molecules of oxygen, which had not been expected to be able to survive so long in a comet — another way Rosetta is changing the way we understand how the Solar System formed.