Getting back home

We have undocked from the space station, en route home.

Since we don't want to hit any part of the space station physically or with rocket exhaust, we gently move away to a good, safe distance.

If we do nothing more, we will continue to orbit the Earth, at the same height and speed as the space station. To get home, we have to change our orbit to an elliptical one taking us down to or close to ground level.

We do this by slowing ourselves down by a precisely calculated amount. This is done using the manoeuvring thrusters or a special retro-pack provided for that purpose.

They change our orbit just a little, putting us on a downward curving path. For much of our space mission, the Earth's atmosphere has been a problem, making launching spacecraft more difficult because of air drag and aerodynamic stresses.

Now, on the return trip, the atmosphere and air drag save us a lot of difficulty and expense.

If the Earth had no atmosphere, and we did nothing to slow ourselves down, our spacecraft would hit the ground at over eight kilometres a second. In the absence of an atmosphere we would have to use a rocket.

Because it will have to more or less cancel all the energy needed to put us into orbit, the slowing down rocket would need to be almost the same size as the one that took us to orbit.

It gets worse, because putting something that heavy into orbit would require a much bigger launcher. This is where our nice deep atmosphere comes in.

Our descent is taking us into denser and denser atmosphere. At speeds 25-30 times the speed of sound, the spacecraft gives no time for the very thin atmosphere to get out of the way.

It gets compressed in front of the spacecraft and gets very hot, many thousands of degrees, before it spills past the front of the spacecraft as it gets shoved aside. It is this hot, ionized air that gives the light show returning astronauts experience on the way down.

The work of compressing this air and pushing it out of the way rapidly slows the spacecraft down.

What might be surprising is that although we put sharp, pointed noses on supersonic aircraft, we make spacecraft with blunt noses, as with the space shuttle, or we give spacecraft really blunt back ends, and return to Earth backward.

The reason is that a sharp, pointy nose would melt, and also that we want the air drag to slow us down. We pick our path back through the atmosphere to keep the heating rate tolerable and to limit the deceleration stresses on the astronauts, which might peak at many times the Earth's gravity.

Handling the heat requires special engineering. Like almost all other spacecraft, our vehicle uses a heat shield. This is a saucer-shaped disc of material covering the rear of the spacecraft.

It is made of a highly heat-resistant substance that can absorb a very large amount of heat and then burn off, taking the heat with it, revealing a new, cold surface beneath. It is thick enough to deal with the expected heat load with a large safety margin.

Air at a temperature of thousands of degrees ionizes; the atoms partially break up, leaving clouds of free electrons. These completely block radio signals, causing the familiar radio blackout, which is a feature of all our returns from space

As we get lower and slow down more, the heat declines, the ionization dissipates, and we regain radio communication.

Little puffs by the attitude control thrusters keep us the right way up. We eventually go subsonic and soon after, the drogue chutes open. These slow us more, stabilize us and help deploy the main chutes. After these have opened, we can take things easy until we splashdown in the ocean.

• After dark, Saturn and Jupiter lie very close together, low in the southwest.
• Mars is fairly high in the southeast
• Venus is low in the dawn glow.
• The Moon will be New on the 14th. 

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.