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Ken Tapping, August 10, 2011
In the sky this week...
> Saturn is low in the southwest after sunset.
> Jupiter rises around midnight and Mars about 2 a.m.
> The Moon will be full on the 13th.
Look for the Perseid Meteor shower on the night of 12-13th August.
The impact of being able to put devices into Earth orbit and send them to other planets has been huge. We have explored much of the Solar System and have landed on other worlds. We have a better idea of what our planet is like and how it works than ever before. Putting telescopes into orbit has revolutionized astronomy. Finally there are the countless services and commercial applications, such as communications, GPS navigation and broadcasting satellites. However, no matter what the application is, deploying it involves lifting things into orbit. We call what we want to put into orbit or to send off into space the "payload." Inevitably there is always some other stuff that ends up in orbit too.
Putting things into orbit comprises two main jobs: lifting the payload to a height where the air resistance is low, and then accelerating it about 30 000 km/h, which is fast enough for it to fall in a long arc around the Earth without hitting the ground. This requires a lot of energy, which at the moment we get from burning large amounts of highly explosive chemical fuels.
Since each kilogram of mass requires many kilograms of fuel to lift it into orbit, we try to eliminate all unnecessary mass. This involves getting rid of all the parts of the launch vehicle when they have done their job and are no longer needed, such as large, empty fuel tanks and their support structure. In addition, the large engines needed to handle the first part of the launch are not as useful later. Rocket motors work best at full throttle, so when we need a smaller thrust to accelerate the payload into orbit, the best thing is use a smaller engine, and dump that big one.
The usual solution is to use multi-stage rockets. These consist of a stack of two, three or four rockets on top of one another. The first stage lifts the whole vehicle and accelerates it. When it runs out of fuel it is detached, and the second stage fires and continues the launch. There may be a third and even a fourth stage needed to put the payload at the right height, with the right speed.
The last stage has shut down, and our spacecraft is in orbit. However, the final stage is leaking gases and the unburned fuel could explode. Therefore, springs, small rockets or other devices detach the payload, so that it slowly moves away. However, the last rocket stage and all the discarded bits remain in orbit too, as space junk.
The Space Shuttle was rather different. In this case the Shuttle's engines and two large solid-fuel boosters all fired at launch, with the Shuttle's main engines getting their fuel from a large external tank. Initially most of the work was done by the solid-fuel boosters. Later, the burned-out boosters were dropped and the main engines used the remaining fuel in the tank to continue the launch. A nice thing about the Shuttle was that all the dropped-off bits returned to the Earth, and the astronauts could test satellites before deploying them. Now, after more than 30 years of service the Shuttle has been pensioned off. On one side, this sadly marks the end of an exciting era. However, it also opens the door on a new one, offering opportunities to develop new ways to access the Final Frontier.