The Rosetta 67P comet makes its closest approach to the Sun on Thursday, August 13, 2015, promising a flurry of activity as it reaches the special position known as perihelion.
The ball of ice and dust is still home to the plucky lander Philae, although little has been heard from the robot explorer since it woke up in June.
What is perihelion?
Perihelion is used to describe the moment in an object’s orbit when it gets closest to the Sun.
The solar system’s best known comet, 67P/Churyumov–Gerasimenko, travels on a 6.5-year orbit which takes it in to 186 million kilometres from the Sun at its closest approach. That equates to a point halfway between the orbits of Earth and Mars.
At its most distant the comet is at 850 million kilometres from the Sun, out beyond the orbit of Jupiter.
The exact moment of perihelion is at 02.03 GMT on August 13th.
What will happen to the comet?
The comet has been slowly heating up for some time, meaning that the ice inside the body has been sublimating out into space, carrying dust with it. It’s that process that creates the iconic ‘comet tail’ that we can sometimes see from Earth.
ESA’s Rosetta spacecraft has been watching how the comet evolves since it first got close last autumn, and will be watching out for any bursts of activity over the course of August and September. The truth is that nobody really knows what could happen to the comet – it may become even more cracked and could even break apart, its two lobes separating. Or nothing much could happen, just a mild increase in ejection of dust from the surface. That unpredictability is part of the fun of studying comets.
What will Rosetta be doing?
Rosetta will continue to fly alongside the comet during perihelion at a distance of about 150km. The ESA mission team would like it to be as close as possible to the 3km-wide comet while remaining safe from ejected material. It will be monitoring the flow of gas and dust and the plasma environment around the comet, and will pay close attention to the southern hemisphere of the comet, which has only been in sunlight since May 2015. It will send back photos too.
Any news from Philae?
Philae has not found it easy to survive on a comet. The washing-machine sized robot famously bounced down onto 67P last November and carried out some superb science measurements before going silent when its batteries ran out.
There was surprise and jubilation when it woke up again in mid-June, revived by the sun feeding its solar panels. However, since then contact between Rosetta and Philae has been intermittent. The Philae team thinks the lander may have been moved from its position near a cliff face, possibly because of outgassing of dust from the comet surface. They continue to send messages in the hope that if contact is re-established then Philae could send back panoramic images and even re-start science experiments. Philae is also sitting on a treasure trove of data from previous experiments which are yet to be sent back to Rosetta.
What happens to Rosetta after perihelion?
The Rosetta spacecraft has proven itself to be a reliable and trustworthy machine, and it will continue to fly alongside the comet as it travels back into the outer solar system. When the comet’s activity dies down it will be able to fly closer again, and take even more of those stunning images of the surface, allowing the science team to compare and contrast the comet now with the comet in 2014. The mission has been extended by ESA until September 2016, when Rosetta may be too far away from the Sun to charge its batteries from its solar panels. There is an ambition to land Rosetta on the comet at the end of the mission, a stunt which would destroy the spacecraft, but give this hugely successful flight a fittingly dramatic and scientifically interesting end.
Why was Rosetta sent to study a comet?
Rosetta takes its name from the Rosetta Stone, which helped experts decipher the secrets of ancient Egypt. The hope is that Rosetta can do the same for our understanding of the solar system, because comets contain primordial material that hasn’t undergone significant evolution since the formation of the solar system.
For the moment Rosetta has been full of surprises, in particular the finding that the water on comet 67P is not the same as the water on Earth, as had been expected. The comet is more complex and varied than scientists had imagined. The comet can teach us about how planets are formed, and it could help us unravel some of the mysteries of the emergence of life.