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however the zero gravity of orbit causes

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a number of undesirable physiological

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changes and weakening of the bones as

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well as creating practical problems with

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liquids etc

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one would therefore want any long-term

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base for human beings to be on a planet

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or moon

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by digging into the surface one would

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get thermal insulation and protection

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from meteors and cosmic rays

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the planet or moon could also serve as

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the source of the raw materials that

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would be needed if the extraterrestrial

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community was to be self-sustaining

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independently of earth

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what are the possible sites of the human

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the most obvious is the moon

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it is closed by and relatively easy to

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reach

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we have already landed on it and driven

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across it in a plaguey

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on the other hand the moon is small and

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without atmosphere or a magnetic field

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to deflect the solar radiation particles

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like on earth

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there is no liquid water but there may

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be ice in the craters at the north and

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south poles

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a colony on the moon could use this as a

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source of oxygen with power provided by

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the moon could be a base for travel to

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the rest of the solar system

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mars is the obvious next target

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it is half as far again as the earth

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from the sun and so receives half the

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warmth

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it once had a magnetic field but it

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decayed four billion years ago leaving

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mars without protection from solar

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radiation

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the strip mars of most of its atmosphere

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leaving it with only one percent of the

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however the pressure must have been

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higher in the past because we see what

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appear to be runoff channels and dried

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up lakes

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liquid water cannot exist on mars now

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it would vaporize in the near vacuum

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this suggests that mars had a warm wet

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period during which life might have

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appeared either spontaneously or through

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panspermia

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there is no sign of life on mars now but

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if we found evidence that life had once

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existed it would indicate that the

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probability of life developing on a

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suitable planet was fairly high

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nasa has sent a large number of

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spacecraft to mars starting with

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mourinho iv in 1964.

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it has surveyed a planet with a number

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of orbiters the latest being the mars

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reconnaissance orbiter

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these orbiters have revealed deep

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valleys and the highest mountains in the

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solar system

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nasa has also landed a number of probes

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on the surface of mars most recently the

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two mars rovers

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these have sent back pictures of a dry

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desert landscape

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however there is a large quantity of

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water in the form of ice in the polar

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regions

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a colony on mars could use this as a

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source of oxygen

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there has been volcanic activity on mars

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this would have brought minerals and

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metals to the surface which a colony

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could use

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the moon and mars are the most suitable

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sites for space colonies in the solar

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system

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mercury and venus are too hot while

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jupiter and saturn are gas giants with

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the moons of mars are very small and

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have no advantages over mars itself

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some of the moons of jupiter and saturn

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might be possible

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in particular titan a moon of saturn is

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larger and more massive than our moon

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the cassini-huygens mission of nasa and

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esa has landed a probe on titan which

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has sent back pictures of the surface

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however it is very cold being so far

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from the sun and i wouldn't fancy living

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next to a lake of liquid method

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what about beyond the solar system

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our observations indicate that a

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significant fraction of stars have

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planets around them

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so far we can detect only giant planets

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like jupiter and saturn but it is

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reasonable to assume that they will be

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accompanied by smaller earth-like

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planets

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some of these will lie in the gold

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deluxe zone where the distance from the

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star is in the right range for liquid

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there are around a thousand stars within

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30 light years of earth

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if one percent of these have earth-sized

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planets in the gold deluxe zone we have

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we can't envisage visiting them with

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current technology but we should make

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interstellar travel a long term aim

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by long term i mean over the next 200 to

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500 years

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the human race has existed as a separate

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species for about 2 million years

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civilization began about 10 000 years

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ago and the rate of development has been

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steadily increasing

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if the human race is to continue for

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another million years we will have to

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thank you for listening

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thank you professor hawking

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for uh

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that series of insights and and a

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challenge to us all

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uh

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i believe now for those of you that

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wanted to do flash photography it would

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be okay for a few moments

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and i invite you all to head upstairs

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for a very nice reception courtesy of