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Rosetta’s rendezvous with a comet

Rosetta and the lander Philae ready to encounter their strange companion. ESA

After a decade of travelling through space, and years before that of mission planning, the European Space Agency’s (ESA) Rosetta spacecraft is right on track to deliver a superb mission.

Already it has revealed exciting views of its target – Comet 67P/Churyumov-Gerasminko – and on Wednesday August 6, Rosetta will complete the final of a series of ten manoeuvres that will bring it to within 100km of the comet.

All going well, confirmation of the rendezvous is expected to come through at 7:45pm (AEST) August 6, from ESA’s space operations centre in Darmstadt, Germany.

Reaching new heights

Rosetta is set to be the first spacecraft to orbit a comet and later this year will be the first to deploy a small lander, called Philae, to touch down on the comet’s surface.

Comets represent some of the most primitive material in the solar system, unchanged by the dramatic processes that built up the moons and planets. They can tell us what ingredients were around when the solar system formed 4.6 billion years ago and they may have provided Earth with the water and organic material necessary for life to develop.

Rosetta is catching up with Comet 67P/Churyumov-Gerasminko, a short period comet that takes 6.5 years to orbit the sun. It travels from just beyond the orbit of Jupiter to within the orbits of Mars and Earth.

It likely originated from within the Kuiper Belt, a region out beyond Neptune, but was ejected at some point. Now it belongs to the realm of Jupiter-family comets, known as such because the orbits of these comets are highly influenced by Jupiter’s gravity.

Are we there yet?

The comet is currently located between the orbit of Mars and Jupiter at around 500 million km from the sun and it’s taken Rosetta a long time to catch up to it.

Since being launched from Europe’s Spaceport in Kourou, French Guiana on 2 March 2004, Rosetta has travelled more than six billion kilometres on a lengthy journey that included three fly-bys of Earth, one of Mars and two bonus glimpse of asteroids in the asteroid belt – the small asteroid 2867 Steins from a distance of 800km and the much larger 20 Lutetia viewed from around 3,000km.

It spent almost the past three years of its journey in hibernation before waking up in January this year.

Over the past few weeks as the kilometres between Rosetta and the comet began to rapidly slip away, dramatic images have shown that Comet 67P/Churyumov-Gerasminko is unlike any of the other handful of comets that have been seen from up-close in space.

From August 3, with Rosetta at 300km from the comet. ESA/Rosetta/NAVCAM

This comet, which is estimated to be 4km across, consists of two distinctly-shaped parts. It could be that a very slow collision has bound two separate objects together.

But it’s also possible that this is a single object that’s either been warped out of shape by the gravitational pull of something big or perhaps its outer layers have been removed over time, leaving only the most dense material behind.

What’s exciting is that we will soon know so much more about this comet. However, it’s certainly not plain sailing from here. In fact, the challenges are, in some ways, just beginning.

Three-sided orbit?

The rendezvous puts Rosetta into a strange triangular orbit best seen in the video below. Every Wednesday and Sunday, small thruster burns will bring the spacecraft back around to travel another side of the triangle.

Rosetta’s triangular orbit.

Eventually, Rosetta will reach within 30km from the surface of the comet and by that point the comet’s weak gravity should be able to take over and keep Rosetta in orbit.

At that time, the surface will be mapped in amazing detail to search out the most ideal location to send Rosetta’s lander, Philae. To make the deployment itself, Rosetta will need to come within just 2.5km of the comet’s nucleus.

But there’s more. Now is an exciting time to hook up with the comet, because Rosetta will be travelling with Comet 67P/Churyumov-Gerasminko as it makes its journey around the sun. The comet will come closest to the sun in August next year.

Sunward bound

It’s on this journey towards the sun that things really start to happen for a comet. As a comet warms up, the ice on its surface sublimates, turning directly into a gas. This forms a fuzzy coma around the comet and as the gas escapes, it also takes along with it tiny dust particles.

The warmer and more active the comet gets, the more gas and dust is released into space. This material forms the comet’s tail and is pushed back away from the comet by the pressure of the solar radiation.

Each comet develops in its own peculiar way, based on how compacted the comet might be, how much volatile material it contains and which areas of the comet are being heated by the sun.

Already, even though Comet 67P/Churyumov-Gerasminko is well beyond Mars, it is showing signs of out-gassing water.

ESA is not taking any chances with Rosetta and the spacecraft is currently travelling slightly ahead of the comet, staying out of the way of any such out-gassing material.

It is sure to be an exciting and challenging mission to see just how close the spacecraft can get to the comet, and the amazing science it will do, while keeping Rosetta out of harm’s way.