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  1. Science & Tech
12 November 2014updated 01 Jul 2021 11:42am

Rosetta, Philae and the comet: what you need to know about today’s historic space mission

Some background while we wait for the ESA's probe to touch down on the comet's surface.

By Ian Steadman

UPDATE (16:04 GMT): Philae has landed!

This morning at 09:03 (GMT) the Rosetta probe successfully detached its main payload – the Philae lander. About the size of a washing machine, Philae is now drifting down towards the surface of the comet 67P/Churyumov-Gerasimenko, which is currently out beyond the orbit of Mars, some 670 million km from the Sun.

It will take seven hours or so for Philae to become the first spacecraft to land on a comet. The European Space Agency is running informational programs all day on a livestream on its website – which you can watch here – and touchdown is expected to come at around 16:00 (GMT). It’s going to be worth tuning in to the livestream beforehand to watch it happening in real-time.

Here’s some background to today’s big event:

The comet: The target of the mission today is a comet called 67P/Churyumov-Gerasimenko, named after the two Russian astronomers who discovered it in 1969. It’s just over 4km across, takes 6.45 years to orbit the Sun, and one of the most wonderful discoveries of the mission so far is that it looks a little bit like a rubber duck. Here’s what it would look like if it was hovering over central London:

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Image: ESA

Rosetta: The European Space Agency’s probe was first planned out in the mid-1990s, and launched in 2004. Today’s event has been at least 20 years in the making, if not longer, and it is an extraordinary achievement of science and engineering. 67P/C-G is currently on the inward part of its orbit around the Sun, but like many comets it orbits in a highly elliptical fashion relative to Earth. The reason it’s taken ten years to get Rosetta in position is that the probe had to make multiple passes around the Sun, Earth and even Mars to give it gravity boosts in the correct direction. Here’s a video show what happened:

Now, Rosetta is orbiting the Sun along the same orbital path as the comet – but the final steps to get into that position were extremely tricky, with the engines required to make adjustments with a series of triangular orbits to ensure it got as close as possible. The comet’s gravity is quite weak, so it was never going to be as easy as it is to, for example, put a satellite into orbit around the Moon:

Image: ESA

Of the data expected to be collected over the duration of the entire mission, most will come from Rosetta. It has extensively mapped the comet’s surface and density from different heights, measured how it reacts to the solar wind, and analysed the dust and gasses it has flown through in its wake. The expectation is that Rosetta will remain in a tight orbit of 10km in height around the comet as it travels in towards and around the Sun, giving crucial insight into the development and structure of the tail that will form as it heats up and sheds mass. ESA scientists hope for at least another year and a half of study from Rosetta until either the damage from the flying debris gets too great or the solar panels are coated with too much dust to provide energy – but there is every possibility it could survive longer than that.

Comets are fragments of the early Solar System – examples of the kinds of lumps of rock and ice which collided together billions of years ago to form the planets as we know them, and as such analysing them can reveal hugely significant facts about what our place in the universe was like when the Sun was first born. There is even the possibility of settling the question of whether life-giving minerals and chemicals – including water – were “seeded” on Earth by comets in its early past, making them essential building blocks for life. It is very literally a mission of immense historic importance.

It’s also worth noting the symbolism of this mission, being as it is arguably the European Space Agency’s finest work so far (or, at least, fingers are crossed for the landing). Philae’s landing will come a day after the hundredth anniversary of the European continent tearing itself apart in one of the bloodiest conflicts ever known; a hundred years later, the nations of Europe are working together for the betterment of scientific research, and to further understand our shared origins. We should celebrate this.

Philae: The lander doesn’t have any engines or thrusters of its own, so mission controllers this morning had to time its release from Rosetta extremely carefully. We know that it worked successfully, and Philae is currently drifting like an extremely slow baseball pitch towards the comet’s surface.

Since it doesn’t have any way of propelling itself, the plan for when it reaches the surface is like something out of a movie. It’s going to fire a harpoon gun at the surface to anchor itself, and then, when it manages to winch itself down, drills in each of its three legs will dig downwards as well, giving Philae a solid chance of not bouncing off back into space, helpless. (This is a real worry – the Japanese probe Hayabusa-1 had a lander called Minerva which was meant to land on an asteroid in 2005, but it missed and floated away.)

Signals from Earth also take roughly half an hour to reach both Rosetta and Philae, so it has to do this completely on auto-pilot. Part of the problem with Rosetta being such a success so far is that it has revealed the existing dichotomy between “asteroid” (a big rock) and “comet” (a big snowball) isn’t quite right – the stunning pictures that Rosetta has taken so far show it to be quite a rocky and dusty place. Philae’s harpoon was designed for ice, not rock, or dust. ISS astronaut Alexander Gerst filmed a video explaining the harpoon, which is nice:

What next?: You can watch the ESA livestream, which should be returning video and images from Philae as it happens. Randall Munroe of xkcd is also doing a “live cartoon” of sorts, if you want something a bit different. And, while waiting, some of the ESA mission scientists are shaving their heads in support for pancreatic cancer research – feel free to support them.

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