The Rosetta space mission of the European Space Agency is heading for a terrific show in November.

The Rosetta mission is an attempt to land a space probe on a comet, in outer space.

Imagine taking a radio-controlled model helicopter and flying it 10 km, out of sight, and then, by using an on-board camera, looking down, you try to land the helicopter on the back of a cow.

Rather difficult. Well, the Rosetta mission is much more difficult than that.

The Rosetta craft has a detachable lander called Philae. The target is the comet 67P Churyumov-Gerasimetnco, and the plan is to land on this comet, as it hurdles toward the sun.

Rosetta was launched on March 2, 2004, from the Guiana Space Centre, in French Guiana, and has been hurtling towards the comet for ten years.

On August 6, 2014, Rosetta arrived at the comet. What this means is that, at about 100 km out, Rosetta slowed down to a relative velocity of 1 m/s. By doing this, it became the first-ever spacecraft to rendezvous with a comet in space. Other spacecraft have passed comets and photographed them but Rosetta is now creeping towards the comet as they both race through space.

This is all amazing science and engineering.

Okay, why the interest in the comet? For centuries, comets have been mysteries. They would appear in the night; after night, they would grow long tails as they approached the sun. People were scared of comets, and popular wisdom of the past was that comets brought bad news. Kings would die or nations would go to war.

Famously, Sir Isaac Newton, when working on his theory of gravity, stated that comets orbited around the sun in elliptical orbits. He said that some comets were actually the same comet coming back time after time on a predictable orbit. He then predicted that a particular comet which had been seen nearly a century before would come back again, and he named the date. He turned out to be right – it was the comet today known as Halley’s Comet. It was named after Sir Edmund Halley, because Halley helped Newton to publish his famous book. Halley’s Comet returns to earth every 76 years.

Many comets are on very elliptical orbits, which means that they make very far out in space at their furthest point. This can be beyond the outer planets, where the comet slows down to about walking speed. It then turns and starts to fall towards the sun again, picking up speed as it goes. At that stage, it is a cold dark blob. As the comet nears the sun, the sun starts to heart it up, and it starts to evaporate. This evaporation leads to the spectacular tail on some comets as they near the sun.

Interestingly, the tail is debris being pushed away from the comet by the ‘pressure’ of sunlight and nuclear particles streaming from the sun. So, the tail always points away from the sun. At times, in the sky, when viewing a comet, it can be travelling tail first. The tail does not stream out behind as it travels.

What is a comet? That is the burning question. Science knows a lot more now than just a few decades ago, but there is a lot to learn. That is why Rosetta is so important.

Current theory is that comets are giant dirty snowballs. It is thought that comets may originate form the dirt of the solar system. They could be balls of ice and snow rolled up with sand, rocks and whatever else.

As they approach the warming sun, the ice surface melts and releases all the sand and other crud. We want to know what the crud is. By the way, the target comet for this November is far from being a ball. It is the shape of a toy rubber duck which floats in your bath. So, the Rosetta team have had a huge headache in trying to find a site for Philae to land. Photos now show that the comet has cliffs, valleys, boulders and undulating ground.

A landing site on the ‘duck’s head’ has been chosen as the main site. It is rough terrain near some cliffs, but it offers a reasonably smooth plain where slopes are less than 30º.

There are backup sites, so the guys in charge may change their minds as their information improves.

It will take seven nail-biting hours for the final landing. It now appears there may be two water vapour volcanoes very close to the landing site.
We wait with excitement to see what we may soon be seeing on TV. If only Newton were here to see it.