Robot probe Rosetta could provide dramatic images as 67P reaches perihelion, its orbital highpoint – and threatens to break up.
European scientists are preparing to celebrate an unusual midsummer festival – on a comet that is currently 165 million kilometres from Earth. The comet, 67P/Churyumov–Gerasimenko, is set to reach the closest point to the Sun in its 6.5-year orbit on Thursday and Europe’s robot probe Rosetta is poised to provide scientists with a ringside view.
The spaceship has already been in orbit around Comet 67P for a year and has studied dust and gas as they have started to erupt from its surface as it has approached the Sun. On Thursday, when the comet reaches perihelion, its closest point to the Sun, that activity will reach its peak.
“Perihelion is an important milestone in any comet’s calendar, and even more so for the Rosetta mission because this will be the first time a spacecraft has been following a comet from close quarters as it moves through this phase of its journey around the solar system,” said Matt Taylor, the European Space Agency’s Rosetta project scientist.
Comets are made of the detritus of the primordial solar system that formed around the Sun more than 4.5bn years ago. When a comet gets close to the Sun, it produces streams of sublimated ice, dust and gas that form a fuzzy atmosphere known as a coma and a spectacular tail which can stretch for millions of miles.
If this takes place near Earth, it can make a spectacular sight – as comet Hale-Bopp did in 1997. However, Comet 67P is currently so far away from our world that it can only be observed through a telescope.
By contrast, Rosetta and its little lander Philae – which were launched together in 2004 – have managed to get up close and personal with 67P and have made some spectacular discoveries, including the presence of a rich brew of 14 organic chemicals including methyl isocyanate, acetone, propionaldehyde and acetamide in the comet’s soil.
This discovery gives support to the idea that comets, crashing on Earth, might have played a key role in providing materials that led to the appearance of rudimentary life on our world. Because of awkward landing conditions, however, space engineers are unsure if Philae will be able to provide any more information for them.
Rosetta has also discovered that the interior of 67P is extraordinarily light. At least 75% of it is empty space. Scientists speculate that much of its interior material may have been vented into space as the comet has warmed up during previous close encounters with the Sun.
“When a comet is closest to the Sun, the maximum amount of heat percolates through its dusty surface and stimulates activity,” said Mark McCaughrean, a senior scientist at the European Space Agency.
As a result, ice, carbon dioxide and carbon monoxide below the surface evaporate and vent into space, carrying with them dust that covers the comet. “Often this produces deep pits into which surface ice and dust collapse, creating sinkholes all over the comet,” added McCaughrean.
After perihelion, 67P will swing away from the Sun and start to cool. Rosetta will follow it for another year to study changes that took place on its surface. These could include the appearance of many new sinkholes.
“Our problem at this stage is getting close to 67P,” said McCaughrean. “When it is at its most active, the comet produces a great deal of dust, and those particles confuse Rosetta’s onboard star tracker. It uses the positions of stars to align itself, but gets confused when lots of new pinpricks of light appear.
“As a result, we have to keep Rosetta relatively high above the comet, which means we cannot study it in the detail that we might have wished for. On the other hand, we are – for the first time – getting an unprecedented view of a comet as it sweeps round the Sun.”
In addition, there is a prospect that a far more spectacular event could occur on 67P. The comet is shaped like a huge rubber duck and Rosetta has found that the neck has developed a 500-metre fracture. This could widen and deepen as more and more material is vented from its interior and could cause the comet to split in two.
Photographs of the huge object splintering apart would be some of the most dramatic images ever recorded by space scientists.
In the end, after another year of tracking 67P, ESA space engineers will allow the craft to spiral in on 67P and gently crashland on it. “After the summer of 2016, communications with the probe will start to break up, so before that happens we will spiral it very slowly down on to the surface of 67P, going down from 20 to 10 to 5 to 2 kilometres before landing it gently. We may not be able to keep our communications link at the very end, but it will be worth trying to get one last, really close-up view of the comet.”
■ Comet 67P is almost pitch black, reflecting only 6% of incoming light.
■ Its soil is rich in complex organic chemicals such as acetone, propionaldehyde and acetamide.
■ It is incredibly light. At least 75% of comet 67P is empty space.
■ It has a highly textured landscape covered with steep cliffs, boulders – and sinkholes.
■ It has an odd, two-lobed shape that resembles a duck and may have formed out of two separate pieces called cometesimals.