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. 

Rendezvous in space

The images of the recent docking of a Dragon spacecraft with the International Space Station were impressive.

The spacecraft gently lined up, moved in slowly and docked. It all looked so graceful and simple. In fact, keeping a rendezvous in space is anything but simple, and not intuitive.

Imagine you are on a spacecraft on its way to the International Space Station (ISS). You are, say 2,000 kilometres behind the space station, lined up in the same orbit. Since you are moving at the same speed as the space station, you will never catch up with it, so you fire the engine for a few seconds.

Your spacecraft accelerates, you see the distance to your destination start to drop, and then you notice something funny. You are moving up into a higher orbit, and slowing down, and the space station is getting further and further away.

This is actually what would happen. Why?

When we try to move a stationary object or change the direction of a moving object, it resists. We call this resistance to change inertia. A curving path, like a spacecraft orbit, is a continuous change in direction. The resistance to this changing direction manifests itself as an outward force, often called centrifugal force.

This force increases with speed and tightness of the curve. A spacecraft moving in orbit is following a balance between gravity pulling down, and centrifugal force pushing out.

For every height above the ground, there is a specific velocity that meets this balance. When we fired our engines, we accelerated the spacecraft, breaking that balance. The centrifugal force then exceeded the pull of gravity, and we moved outward, into a higher orbit.

Climbing against gravity takes energy, which slows the spacecraft down, just like throwing a ball in the air.

The trick is to drop into a lower orbit, by slowing down. As you drop, you speed up. Orbital speeds increase as you get lower, and because lower orbits are also smaller, you overtake objects in higher orbits.

As you overtake the space station, you use the engine to speed up, which makes you rise and slow down, and rendezvous with the space station. Now, it is a matter of docking, using precision sensors and thrusters.

This procedure would work, but it is wasteful. It is better to design the mission from launch to rendezvous, calculating the path of the spacecraft so that the launch process delivers our spacecraft close to the rendezvous point.

Since the actual docking consists of something with a mass of tonnes having a slow-speed collision with something else with an even larger mass, this has to be carefully managed.

It has to be slow and the meeting face on. This is easiest if the docking assembly is at the front of the spacecraft. Having the docking device in front means the astronauts can see directly exactly what is going on, to deal with problems if they arise.

Secondly, that docking assembly, which often includes an airlock, is fairly heavy. So for stability and structural strength reasons it is best located on the centreline of the spacecraft. Since the engine at the rear needs to be on the centreline, the docking assembly has to be on the nose.

The fact that these space-docking procedures are now standard is due to technological improvements over years. In the development leading up to the Apollo space missions, things were more uncertain, and in at least one case, the docked vehicles went into a wild tumble that was only brought under control by prompt action of an astronaut.

Since the Apollo Moon missions depended on multiple docking operations, they had to be as safe and as well understood as possible. That they go so smoothly these days does not mean we should take them for granted.

  • After dark, Saturn and Jupiter lie close together, low in the southwest
  • Mars lies fairly high in the southeast.
  • Venus lies low in the dawn glow.
  • The Moon reaches Last Quarter on Dec. 7. 

Old tech takes us into space

A few days ago, a SpaceX launcher and spacecraft took four astronauts to spend several months on the International Space Station.

The new space systems are safer, more efficient and more sophisticatedly controlled, and boosters can fly home and land for reuse. However, today's launches look much like those of the '50s and 60s. Basically, as yet we have not come up with anything better than the multi-stage rocket, where one rocket piggybacks others partway up.

Surprisingly, the first multi-stage rockets were invented in the 14th Century by the Chinese. They mounted several small rockets on the front of a big one. When the big rocket had burned all its fuel, the smaller ones fired off, having been given a leg up.

Modern multistage space vehicles are still based mainly on the ideas of two people: Konstantin Tsiolkowsky and Wernher von Braun.

If we want to get to the International Space Station, for example, we need a space vehicle system that can lift what we want to deliver to a height of about 410 kilometres and accelerate it to a speed of almost eight kilometres a second.

This takes a lot of fuel, and since a rocket spends most of its operational time in the extreme upper atmosphere and in space, it has also to carry the oxygen needed to burn that fuel.

Then, to contain it we need tanks. To deliver it to the engines we need pumps. The whole lot needs to be supported in a structure that can accommodate accelerations many times that of gravity, handle severe vibrations and stresses, and function in space.

This means lots of weight. Even so, in a modern rocket, the weight of fuel and oxygen can be up to several times the total weight of the hardware.

As the fuel and oxygen are consumed, the tanks become increasingly empty, and more and more of the structure becomes dead weight. There is no point in carrying it all into orbit, and indeed, as yet we have not been able to make a single-stage rocket that is a useful means of getting out into space.

We have standardized on the use of multi-stage rockets. We make a stack of rockets and put what we want to deliver into space, the payload, usually on top.

At launch, we fire the bottom, or first stage. Because it has the most weight to lift, it is usually the biggest. Sometimes we need to strap on additional rockets — boosters — in order to get more thrust. The first stage gets us to 50-80 km, above most of the atmosphere and up to a speed of a several thousand kilometres an hour.

When its fuel and oxygen is all used up, the first stage is dropped. In the past it used to be discarded. In the modern, SpaceX system, some fuel and oxygen are retained in the first stage and used to land it safely back on Earth.

Since this is a large and expensive piece of hardware, being able to reuse it leads to large cost savings. The second stage now fires. At the same time the path of the spacecraft is tilting down, until when it reaches orbital speed, it is moving parallel to the ground.

When this has exhausted its fuel, or the required velocity has been reached, it too is discarded. Leaving it attached to the spacecraft just means more dead weight to manoeuvre and also the risks associated with any unburned fuel.

The spacecraft is now freely in orbit. It is equipped with low-power thrusters that can be used to make the small changes in speed, direction or attitude needed to rendezvous with the space station. If we want to go further, for example, to the Moon, we can add an extra stage.

Reducing the amount of excess weight we have to lift or accelerate reduces costs and in many cases make the missions possible. Even after decades, the ideas of Tsiolkowsky and von Braun still define how we access space.

  • After dark, Saturn and Jupiter lie close together, low in the southwest
  • Mars is rising in the east.
  • Venus lies in the dawn glow with Mercury below and hard to spot. 
  • The Moon is Full on the 30th. 

Moon's a watery world

For many years astronomers have been puzzling over the question "Why is the Earth so wet?

Two thirds of our planet is covered with water, and the parts of the Earth above the water level are covered with features shaped by water. Astronauts on the moon looked back on a blue world, patched with pure white clouds, evidence of a uniquely wet world. 

However, is it uniquely wet?

The Apollo astronauts brought back a large collection of samples of lunar soil and rocks. One thing they had in common was that they were all very dry. This makes sense.

The moon has almost no atmosphere, so there is no greenhouse effect and all the incoming solar energy hits the ground. The result is daytime temperatures reaching about 130 Celsius, and then falling at night to -170 Celsius.

These temperatures vary with latitude, as they do on the Earth, There are some places, such the depths of craters that never see the Sun where the temperature is as low as -250 Celsius.

This combination of baking and freezing in a vacuum is a good way to dry something, especially if this process has been repeating since the moon and Earth formed, about 4.5 billion years ago.

However, given that both formed from the same ingredients, the moon must also have been a watery world once. Is it still?

If you were to visit the Arctic during the summer, you would first notice the mosquitoes. The second would be that the weather is nice and warm. However, if you were to push a thermometer into the ground, you would find that some distance below the surface, the temperature is below freezing, even in high summer.

This is the permafrost, because it stays frozen throughout the year. Under the insulating layer of soil, the temperature hovers around an average value, staying constant over the year. If this average is below freezing, water down there is always frozen.

On the same basis, if we were to stick a thermometer into the surface of the moon, at some depth we will find the temperature unchanging, around a chilly -40 C.

We have measured the moon's average temperature in other ways, such as using radio telescopes. Short radio wavelengths tell us the temperature of the surface layers. Longer ones tell us the temperatures deeper down, where they are unchanging.

During the early history of the Earth, a lot of water mixed in with the construction material was ejected into the atmosphere, initially as superheated steam. Eventually temperatures fell enough for the first rain to fall, and it rained for a very long time, forming the oceans.

Even today, most of the Earth's water is buried deep in the planet. The moon is smaller than the Earth, leading to the loss of its atmosphere to space, and it cooled off faster.

However, it did form from the same mixture of construction materials, so we should expect there to be quite a lot of water on the moon, somewhere.

Sensitive instruments have detected water molecules at the moon's surface, probably from somewhere inside. Solar radiation would break these up into hydrogen and oxygen, which are lost to space.

However, there are accumulations of ice in deep craters, particularly in the polar regions, in places where the sunlight never reaches. This is certainly encouraging from the point of view of space exploration, because it means people on the Moon can use the local product.

This water would be good for more than just drinking. It can be chemically split into hydrogen and oxygen, yielding components for rocket fuel-and breathable air. If there are large quantities of water ice buried deep in the moon, life for the visitors will be far easier. However, building in permafrost is a challenge.

  • After dark, Saturn and Jupiter lie low in the southwest
  • Mars is rising in the east.
  • Venus lies high in the dawn glow with Mercury below. 
  • The Moon will reach First Quarter on the 21st. 

More Skywatching articles

About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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