Prof. Dr. Ulrich Walter
Institute of Astronautics
TU München, Garching

© 2004

Published under
"Spaceflight – Das absolute Abenteuer" (Spaceflight - The Absolute Adventure)

StarObserver, Issue May 1999, p.14-28

Abbreviations: EST: Eastern Standard Time
SRB: Solid Rocket Booster
MECO: Main Engines Cut-Off
Cue-Card: A checklist where every important activity is described very briefly in just one line.

Separated from the world Kennedy Space Center, Florida/USA, Shuttle Launch Pad 39A, 26. April 1993, 9:50h EST (Eastern Standard Time). Well, that's me now lying on my back, my legs bent upwards, about 60 metres above the Earth in the middeck of the Columbia, one of the American space shuttles, which is supposed to take the seven astronauts into space within a few seconds. This is the place and time I have been working and waiting for ages. I shut the visor and ... I can't hear anything anymore! The only thing I am still able to detect is the radio communication of the air-to-ground, a staccato-like noise reduced to the bare necessities.

You are just like totally separated from the outside world. You don't hear anything anymore, and in the middeck, that is where I am during the start, you can't even see anything, apart from a wall of drawers next to you, or to be more precise, above you; you stare at the wall and hope that during the start it won't release any drawers.

Then the start! Six seconds before takeoff the three liquid engines of the shuttle are ignited. They push the tip of the shuttle forward, because the bolts that retain the system on the ground are fixed to the two white solid fuel boosters a few metres away. During these 6 seconds the astronauts feel like in a swing boat swinging about 1.5 meters forward - and you feel that quite clearly - and then backward again. At the same time the shuttle is vibrating and shaking so much that it sets your teeth right on edge, just like an earthquake. Well, I am inside, I don't hear anything of the overpowering noise that makes the spectators' tummies shiver (the IMAX cinema exaggerates that detail a little bit), and of the light, lashing crashing of the solid fuel boosters (which I miss a little bit in the IMAX cinema). And than the radio tells you: "SRB Ignition - Lift-Off!". The shuttle has lifted off … and what do you feel? Nothing of 3g, the famous strong acceleration three times stronger than the gravitational pull of the Earth! The thrust of the propulsion, that is after all two times 1,200 tons of thrust of the two solid fuel rockets plus three times 185 tons of thrust of the three liquid engines, exceeds the 2,000 tons of the overall system by very generous 50 percent; but the acceleration is not stronger than during the start of an aeroplane.

The solid fuel rockets are now the workhorses pushing the shuttle through the cover of clouds, and their power determines the feeling during the first two minutes of takeoff. As the fuel is inhomogeneously distributed, it burns irregularly, which gives the shuttle fast and strong acceleration knocks, so that it shakes and experiences irregular vibrations. Everything onboard the shuttle is mercilessly and thoroughly shaken. It is like riding at a hundred on cobblestones - and quiet silence reigns. Only very few words are exchanged between mission control and the commander. Everyone involved knows that this is by far the most critical moment of the whole mission. Should something unforeseen happen now, there is absolutely no escape. Even the large amount of improvements after the Challenger disaster could not change that. Solid fuel rockets are like firework rockets - you cannot switch them off. Even if you blew off the boosters, that would not help! Their thrust is so huge that the high aerodynamic drag that would suddenly occur without their thrust, might be able to give the shuttle system such a blow that the whole shuttle would break up! Should the flame jet of a porous booster burn like a flame cutter into the external tank, just as it happened with the Challenger – we would not be able to do anything against it, neither then nor today. During these two minutes the crew is completely and utterly at the shuttle's mercy. That's why we have the quiet silence.

There is no turning back now The acceleration, the force that presses you into your seat, has increased steadily as the shuttle system becomes lighter because of the burnt fuel. Just before the final burn-off of the solid fuel booster, exactly two minutes after liftoff, 1.8g is achieved. . The thrust of the burnt-off booster quickly achieves zero and then they are blown off. Once that has been done, the crew breathes a sigh of relief. One or the other utters a "Yeah ..." and everybody thinks the same:

The danger is over! All the problems that may still occur can be handled one way or another, they are not that life-threatening anymore. After this relieving thrust hole when the boosters are being blown off, only the liquid engines produce the thrust. They burn off far more regularly than the boosters. And we have already left the denser, more turbulent area of the atmosphere. You hardly feel any vibrations anymore. The very harmonic power of the propulsion is now only noticed in the permanently increasing force that presses you into the seat.

After 4 minutes 20 seconds we receive the "negative return call" (that means that in an emergency it would not be possible to return to and land at the Kennedy Space Center) from the mission control centre in Houston. After 7½ minutes altogether, when 90% of the large rust-coloured tank has been emptied, and the shuttle system has reached a weight of less than 200 tons, the pressure exerted by the thrust of the 3 liquid engines has reached 3g, and now you have to force yourself to continue breathing, simply because it is easier not to breathe - despite being breathless - than heaving the thorax together with the heavy suit while breathing. The propulsion is now slowed down, and we continue for another 60 seconds with these 3g. Shortly before the whole tank is emptied the commander announces: "In 10 seconds we have MECO" and within just a few seconds the full thrust is reduced to zero. And just as suddenly the pressure of 3g turns into weightlessness – I have reached the outer space!

You are instantly captivated by weightlessness, it is a feeling that does not exist like that on planet Earth. But in the beginning this experience does not cause positive effects in about 70% of the astronauts, they suffer from space sickness. Well, you notice it yourself: Every fast rotation of your body or movement of your head makes you dizzy. The first countermeasure against this feeling is involuntarily ducking your head and hiding it between your shoulders, but this severely limits head movements. It alleviates the situation, but does not necessarily prevent your last meal from showing up again. And it makes the whole problem even more lengthy. Maybe you just have to look for the plastic bag in your breast pocket and let it take its course. Others also suffer from headache, backache and persisting nausea ... If nothing works anymore, the astronauts just ask their colleagues to give them an injection of Phenagran, let their commander declare them "temporarily incapacitated" and look for a quiet little corner for the next few hours to cure the symptoms of space sickness - the best place for that is their bunk bed.

And now the good news: After 36 hours at the latest it is all gone, and then you are able to really enjoy weightlessness. If you close your eyes and slowly drift through the room, arms and legs slightly bend in a very comfortable position, then you don't feel anything that might influence you and you are able to completely concentrate on your own feeling.

The feeling of weightlessness

First I had the feeling that a dream would repeat. When I was young I often dreamt I would be walking down a sloping street just in front of our house. I became lighter and lighter, and suddenly I was lifted off the ground and floated. I did not fly, I floated, and I had never had this feeling in everyday life. And this feeling I had in my dream is almost identical with the feeling you have under weightlessness Psychologists know that this dream of walking, being lifted off the ground and floating is quite common. So is this dream an unconscious experience of weightlessness? How can the human body have such a very realistic dream about something it has never ever experienced? Or is this dream just a joyful variant of the mind with regard to the short, but very dangerous experience of weightlessness when "falling" in everyday life?

What do you feel when you are in the state of weightlessness? First of all you notice that something important is missing. What kind of connection to the environment do I have at the moment? Where is the ceiling with the lamps and where is the floor? I don't know anymore. I don't have a feeling for these things anymore - and indeed top and bottom do not exist anymore!

This lack of connection radically changes my perception. I do not feel embedded anymore into a world that surrounded me just a short while ago, but this being is narrowed down exclusively to myself. How can other things exist, if I do not have any connection to them anymore? And even if there were something somewhere, would it not be the same if it weren't there? I have the elementary feeling that I am alone. I am the world - nothing else!

This concentration on myself leads me to doing some soul-searching. What has changed with me? I noticed that I do not feel loaded anymore. Your clothes, that still warm you, are floating around your own body like a shell, they hardly have any contact with your body. The clothes are weightless as well and do not rest on your shoulders or elsewhere. It is quite a strange and unusual thing that you have to shake your shoulders a little bit, just to feel that your clothes are still there. But it is not just the load of your clothes that is missing, it is also the load of your own body that has disappeared. No more body pressure on the soles of your feet when you are standing, or on your behind when you are sitting. Your arms do not rest somewhere just as usually. It is quite strange: Only when you are in such a situation, where you do not feel anything at all of your body, you actually recognise the loads your body has to carry on the Earth, although it should actually be the other way round! This experience makes me now conscious of the hardly perceptible hanging down of my cheeks. And this feeling like having butterflies in your tummy is, as I now know, the pulling of my intestines under the influence of gravity. In the state of weightlessness you do not feel any of these things. You are "absolutely unloaded" in the real sense of the word.

Absolutely unloaded. Well, how do I know that I actually still have a body, if it is not because of these external impressions? And the answer is quite baffling: It seems as if indeed I didn't have one anymore! Nothing at all gives me a hint that I still have a body. A very strange feeling, a being without a body! But what is that feeling that I think is my existence? On Earth I had my own body, and just now with hindsight I actually notice that I used to define my own existence just by the perception of my own body. I waggle my shoulders and touch my index fingers with my thumbs. Yep, it is still there - I am still there! But now, without it, I am still there? Of course I am, I feel it, otherwise I would not be able to ask that question! But that is exactly what it's all about! The only thing that remains, that is my existence, is thinking. I think, therefore I am!

That was exactly what I thought at that moment, and it was a remarkable feeling, something I have never forgotten. That is what is special with weightlessness: It narrows you down to your own self, to your own mind.

And then of course you have that awesome view of the Earth.

I look outside full of expectations and I see ... water! Nothing but deep blue water! My daily experience that tells me that the Earth is made up almost exclusively of land is deeply shaken. Two thirds of the Earth's surface is water and not land! I only fully understand that here in this place. It is probably the Pacific, and it will remain the Pacific for the next half hour, i.e. the next 15,000 km. You see little, but that's enough to look around in astonishment. Luminous white formations of clouds artistically conceal the blue of the sea. You may think the Earth from outer space is a caprice of Bavarian invention: The clouds together with the sea form a composition of the Bavarian national colours in front of the background of the pitch black universe.

From that distance of 300 km you are not able to see the Earth as a globe, but the Earth's curvature runs right along the upper field of vision if you have the right arrangement of windows. Now you can also grasp what it means that the Earth has a diameter of 12,750 km, the atmosphere however only a width of about 20 km.

If you compare these two dimensions, our earthly protective cover seems just like a paper-thin ring layer, so fragile that you may think just a breath of wind would be enough to simply sweep it a way, and that every touch, just the smallest impact may lead to bad scratches. It is exactly that fragile, tender layer where our so-called life takes place. A life which is a balancing act between the powerful, impenetrable mass of the Earth and - have a look to the side - the life-threatening nothing of the universe! And human beings don't even populate the whole of the Earth. Mankind seems to be only an inconspicuous bacterium in a soap bubble around the Earth within the infinite ocean of the universe.

After a few days you get to know "your" Earth and you start to see relations, general features that you would never have expected before. I.e. you have learned to distinguish the continents by means of the colour. Whenever you look down and you see land, you now at which continent you are looking at the moment

, because every continent has its characteristic colour!

South America for example is dark green. The colour of the rain forest is the dominating colour of this continent. Africa with its extensive desert Sahara and the surrounding steppes and savannahs presents itself in an ochre brown colour. Australia: the whole continent a deep purple red! Indonesia with its large number of islands with their hazy rain forests is also dark green sea of colours. Europe? In the south a friendly light brown, the rest is rather grey green - if you are able to catch a glimpse of the land through the equally dreary clouds. Even the clouds are in a dreary grey. And for the first time you start to think up the simple, but accurate rule of thumb of astronauts:

Where humans cannot live, in the ice and sand deserts, the world is just beautiful, and where humans do live, are able to live, the world is not beautiful (anymore)!

It is also very satisfactory to notice how trivial the apparently very important human problems are. Just think of the TV news, full of government, belligerent and diplomatic disputes. But from the perspective of the universe, the Earth has a totally different face. Humans don't count at all. The Earth would do good, maybe even do better without that species. The Earth has a stoic calmness, and humans have about the same importance as bacteria have for humans. National borders? Nothing of the kind is visible. What counts are countries and continents. Maybe with two exceptions: The dead straight borderline between Israel and Egypt - it is visible on the eastern fringe of the Sinai, and the just as straight borderline between Angola and Namibia, 200 km north of the Etosha Pan in southwest Africa. But in both cases it is not the borderline as such which is recognisable, but the stark contrast between the extensive use of the land between the bordering states.

Fascination of the night

The onset of a night of three quarters of an hour may at first seem to be a waste of time for the astronaut who simply wants to look at the Earth. But a little later, once your eyes have adjusted to the dark, the Earth by night is a very special spectacle. First you can observe the heat thunderstorms going on until the earthly morning. The play of light of the flashes of lightning dimmed by the clouds reminds me of the flashing of detonating bombs during night raids seen from a plane in old films of the Second World War. Despite its destructive impact they still have a magic fascinating spell. You can see quickly alternating lightning here and there without any connection. But sometimes you also see lightning flashing through the clouds up to one hundred kilometres long, followed by its meandering trace. In contrast to frightening thunderstorms on the Earth, a thunderstorm observed from space is something rather eerie, because a quite earthly ingredient is missing - just like observing bombings from a bomber - the thunder!

If aliens ever consider the Earth to be populated by intelligent beings - it is of course disputable whether intelligence really exists on the Earth -, they will surely come to that conclusion when it is night on Earth. Because it is at night, when clouds don't obscure the view, that humans determine the picture of the Earth. The bright, sharply limited lights of the cities, together with the cobwebs of streetlights in the suburbs are a striking sign that higher beings actually exist. Humans have conquered the night. There is no better place to acknowledge that fact than outer space. Civilisation presents itself as a branched lymphatic system going through the land and around the edges of the oceans, because humans usually prefer to settle along the coastlines.

Milky Way. This term recovers its original meaning in space. To be able to fully enjoy the splendour and beauty of the starry sky, the lights on the flight deck need to be completely dimmed. Not only the enormous multitude of stars is fascinating, but also their merciless clarity. They are unalterably nailed on the firmament just like the finest stitches in a velvet carpet illuminated from behind. The twinkling does not bring any apparent life into them. Their silent existence is a simple expression of the infinite silence of the universe.

Looking at the Earth might be a very beautiful sight, but the largest part of the mission as an astronaut and especially as a scientific astronaut is spent working. Well, it is always like that with remembering: Only the most beautiful and vivid experiences are remembered, you very quickly forget the monotonous work, and time just flies by in the true sense of the word.

Saying good-bye to the universe

After ten very busy, but also wonderful mission days, I go to my seat and prepare for re-entry into gravity, I read through the checklist, especially the cue card for emergencies, just as I did before the start. This reassures me that everything is in order. Now I just need to do one thing as a precautionary measure: Astronauts are supposed to strongly increase the quantity of liquid in their body in order to prevent a circulatory collapse when getting up for the first time after landing. They are supposed to swallow several salt tablets and wash them down with a lot of water. Salt adsorbs water in the body and prevents the kidney from discharging it. And this procedure is anyway much more pleasant than drinking several litres of salt water. We are now ready for re-entry into the Earth's atmosphere.

75 minutes or half a rotation of the Earth before landing the commander turns the shuttle's tail to the front. That doesn't matter for us, as in weightlessness we don't even now where bottom or top is. Exactly one hour before landing the orbit propulsion engines are switched on against flight direction for three minutes, and the orbit velocity is decreased by only 300 km/h: Instead of 28,000 km/h we now fly at a speed of only 27,700 km/h. This apparently insignificant change is indeed enough to take the shuttle on a slightly elliptical orbit into deeper layers of the Earth's atmosphere. Meanwhile the commander has returned the shuttle to its normal flight position and put it 35 degrees against flight direction. The shuttle looses more and more height, and the onboard computer controls the shuttle during this flight phase, so that the speed will remain 27,700 km/h during the next half hour. The aerodynamic drag in these heights is used exclusively to decrease the flight altitude at a constant speed. You don't notice this stage of approach very much. The aerodynamic drag forces are so low that the corresponding gravitational forces still remain below 0.2g, and as you are securely fixed into your seat with the belts, you cannot yet feel these feeble forces. Only if you lightly throw an object in the direction of the ceiling, e.g. a pen, you notice how deeply you have already entered the atmosphere. If it doesn't hit the ceiling anymore, but slowly reverses from its trajectory, you know that things are going downhill.

25 minutes remaining until touchdown. The air friction forces have increased considerably, the tiles on the lower side of the shuttle start glowing at 1500°C. The air is strongly ionised, even radio communication is interrupted for the time being. You hardly notice the temperature increase in your spacesuit. You may sweat because you are nervous, as the shuttle started shaking considerably. The air density at the altitude of 120 km has increased so much that the shuttle acquires aerodynamic features and the gravitational forces have increased so much that the anti g suit (the anti g suit uses the pressure of air cushions on the legs and intestines in order to avoid that the blood sinks into the lower body and thus leads to an inadequate blood supply, i.e. a blackout of the brain) needs to be inflated. This is the moment the commander takes over the control of the shuttle. He also reduces the speed of the shuttle by means of different roll and turn manoeuvres.

12 minutes before touchdown the heat of the tiles has reduced so much that radio communication is possible again. The shuttle has an altitude of 55 km at a speed of 12,000 km/h and is still 900 km away from the runway of the Edwards Air Force Base. I have inflated my anti g suit again because the g strain increased to 1.3g. It is quite unusually demanding after weightlessness in space. You hear the commander's announcements in a choked voice, he is also fighting against physical weakness, and I am glad that I am in my seat and don't need to carry my own weight standing.

5 minutes left. Now we start the approach of the runway. The shuttle races towards the runway at a distance of 25 km and with a speed of Mach 2.5 in inclined downward approach, carries out an exactly determined turn manoeuvre directing the shuttle exactly in the direction of the runway. Now the commander merely needs to adjust the incidence angle of the shuttle so that it flies in an angle of approach of 22°, for a pilot almost like a stone, in the direction of the touch-down.

The velocity has reduced to 700 km/h. 30 seconds before touch-down the commander pulls up the nose of the shuttle reducing the angle of approach to 1.5° and decreases the velocity to the landing velocity. Only 15 seconds before landing the landing gear is extended, because it could have been ripped off before because of the high velocity. Finally the shuttle lands with almost exactly 400 km/h: touchdown.

But we hardly noticed the landing because of the commander's very smooth landing operation. The pilot counted down the altitude, that's how you know how far away we still are from the touchdown. The commander continues to hold the shuttle's nose into the airstream also after landing to reduce speed. Once the front landing gear touches the ground, the brake parachute is released, and you clearly notice its effective braking power in the shuttle. Exactly one minute after touch-down the shuttle comes to a standstill I lean back, feel relaxed, and I know:

We are back on Earth!

If you want to know more about the adventure of a spaceflight with all its preparations and if you want to experience the Earth with many impressive pictures, we recommend the book "In 90 Minuten um die Erde" (Around the World in 90 Minutes) written by Ulrich Walter, published by Stürtz Verlag (ISBN 3-8003-0876-2). If you want to admire the whole beauty of planet Earth, go on an interactive journey with the CD ROM "Mission Erde", United Soft Media Verlag, 2000, (ISBN 3803217016) by Ulrich Walter, it is just like the journey experienced by astronauts during their shuttle flights and on the International Space Station.