Comet 67P has passed the closest point to the Sun

Comet 67P has passed the closest point to the Sun in its 6.5-year orbit, with the European spacecraft Rosetta still in orbit around it. This landmark, called "perihelion", occurred at 03:03 BST on Thursday, when 67P was 186 million km from the Sun - a distance that puts it between the orbits of Earth and Mars. The Rosetta team has been studying the small, icy world as it warms up. It has released dust and gas, including a very bright jet seen on 29 July. Dramatic images of this outburst - the brightest jet seen so far by Rosetta's cameras - were released on Tuesday by the European Space Agency, Esa. "Usually, the jets are quite faint compared to the nucleus and we need to stretch the contrast of the images to make them visible - but this one is brighter than the nucleus," said Carsten Guettler, a member of the Osiris camera team from the Max Planck Institute for Solar System Research in Germany.

The material released by comets, as they become more active on approach to the Sun, is the reason for their characteristic tails when they appear in the night sky. This particular jet, caused by frozen ices turning to gas and pouring out into space, was bright and brief. Three photos, each separated by 18 minutes, capture it appearing and fading.Tumultuous activity like this is not necessarily expected to coincide with perihelion, as any temperature increase is gradual - and also lags behind the comet's actual distance from the Sun. "The solar flux increase between today, tomorrow and the day after is almost immeasurable," said Mark McCaughrean, senior science advisor at Esa. He likened the comet's perihelion to the summer solstice, which is not the warmest day of summer on Earth.

"On the Earth there's a thermal lag, and that's true on the comet too.

 It reaches peak sunlight tomorrow, but it probably doesn't reach peak activity. "We don't know exactly when that peak will be. Nobody's ever done this before." It was just over a year ago, on 6 August 2014, that Rosetta first arrived at Comet 67P/Churyumov-Gerasimenko. It has spent the intervening months in orbit, manoeuvring itself as close as 30km to the surface of the duck-shaped "icy dirtball".

In the last few months, increasing flows of dust around the comet have forced the probe to retreat. "We've now moved back to 300-plus km," Prof McCaughrean said. "Because activity is building up, we are constantly monitoring the navigation cameras, in case they lose lock. And the engineers are pre-emptively moving the spacecraft further away. "Activity will build up further after perihelion, so we'll probably move even further away - then wait to go back in again. Probably by the end of the year we'll be back down at 10-30km. At that point, we get to see what's changed."The Philae lander, meanwhile, is still on the comet's surface, but not communicating. After Rosetta dropped it onto the comet in November 2014, its historic but wayward landing finished in the shade, making it difficult for Philae's solar panels to charge its batteries.

It has only briefly "phoned home" again since, and controllers are unsure if they will ever regain a stable line of communication between Rosetta and its slumbering lander.

source - bbcnews