Why go to space?
Stephen Hawking makes the case for exploring beyond the bounds of our planet.
Why should we go into space? What is the justification for spending all that effort and money on getting a few lumps of Moon rock? Aren't there better causes here on Earth?
In a way, the situation was like that in Europe before 1492. People might well have argued that it was a waste of money to send Columbus on a wild-goose chase over an almost unimaginable distance. Yet, the discovery of the New World made a profound difference to the old one.
Spreading out into space will have an even greater effect; it will completely change the future of the human race and maybe determine whether we have any future at all.
It won't solve many of our immediate problems on Earth, but it will give us a new perspective on them and cause us to look both outwards and inwards. With luck it could unite us to face a common challenge.
This would be a long-term strategy – and by long term, I mean hundreds or even thousands of years. We could have a base on the Moon within 30 years; reach Mars within 50 years and even the moons of the outer planets within 200 years.
By 'reach', I mean with manned space flight. We've already driven rovers on Mars and landed a probe on Titan, the largest moon of Saturn, but if one is considering the future of the human race, we have to go there ourselves.
Going into space won't be cheap, certainly, but it will take only a small proportion of world resources. NASA's budget has remained roughly constant in real terms since the time of the Apollo landings, but it has decreased from 0.3 per cent of U.S. GDP in 1970 to 0.12 per cent today.
Even if we were to increase the amount spent on space endeavours internationally by 20 times, to make a serious effort to send people into space, it would only be a small fraction of world GDP.
There will be those who argue that it would be better to spend our money solving the problems of this planet, like climate change and pollution, rather than wasting it on a possibly fruitless search for a new planet. I am not denying the importance of fighting climate change and global warming, but we can do that and still spare a quarter of a per cent of world GDP for space. Isn't our future worth a quarter of a per cent?
Everyone thought space was worth a big effort in the '60s. In 1962, President Kennedy committed the U.S. to landing a man on the Moon by the end of the decade. This was achieved just in time, by the Apollo 11 mission in 1969.
The space race helped to create a fascination with science and led to great advances in technology, including the first large-scale integrated circuits, which are the basis of all modern computers.
However, after the last Moon landing in 1972, with no future plans for further manned space flight, public interest in space waned. This coincided with a fall in enthusiasm for science in the West because, although it had brought great benefits, science had not solved the social problems that increasingly occupied public attention.
A new manned spaceflight program would do a lot to restore public enthusiasm for space and for science generally.
Robotic missions are much cheaper and may provide more scientific information, but they don't catch the public imagination in the same way, and they don't spread the human race into space, which I argue should be our long-term strategy.
A goal of a base on the Moon by 2020 and of a man landing on Mars by 2025 would reignite a space program and give it a sense of purpose in the same way that President Kennedy's Moon target did in the 1960s.
A new interest in space would also increase the public standing of science generally. The low esteem in which science and scientists are held is having serious consequences. We live in a society that is increasingly governed by science and technology, yet fewer and fewer young people long to go into science.
One important question is: what will we find if we do make the effort to go into space? Is there alien life out there, or are we alone in the universe?
We believe that life arose spontaneously on the Earth. So it must be possible for life to appear on other suitable planets, of which there seem to be a large number in the galaxy.
But we don't know how life first appeared. The probability of something as complicated as a DNA molecule being formed by random collisions of atoms in the ocean seems incredibly small. However, there might have been some simpler macromolecule that was a building block for DNA or another molecule capable of reproducing itself.
Even if the probability of life spontaneously appearing on a suitable planet is very small, since the universe is infinite, life most likely would have appeared somewhere else too. If the probability is very low, the distance between two independent occurrences of life could be very large.
However, there is a theory known as ‘panspermia’, which suggests that life could spread from planet to planet or from stellar system to stellar system carried on meteors. We know that Earth has been hit by meteors that came from Mars, and others may have come from further afield. We have no evidence that any meteors carried life, but it remains a possibility.
An important feature of life spread by panspermia is that, at least in the neighbourhood of Earth, it would also have DNA as its basis. On the other hand, an independent occurrence of life would be extremely unlikely to be DNA-based.
One piece of observational evidence on the probability of life appearing is that we have fossils from 3.5 billion years ago. The Earth was formed 4.6 billion years ago and was probably too hot for about the first half-billion years or so. So life appeared on Earth within half-a-billion years of it being possible, which is short compared to the 10-billion-year lifetime of an Earth-like planet.
This fact suggests either panspermia played a role in the formation of life on Earth, or that the probability of life appearing independently is reasonably high. If it the probability was very low, one would have expected it to take most of the 10 billion years available.
While there may be primitive life in another region of the galaxy, there don't seem to be any advanced intelligent beings. We don't appear to have been visited by aliens. I am discounting reports of UFOs, of course – my main reason for this being, why would they appear only to cranks and weirdos?
If there is a government conspiracy to suppress the reports and keep for itself the scientific knowledge the aliens bring, it seems to have been a singularly ineffective policy so far.
Furthermore, despite an extensive search by the SETI project, we haven't heard any alien television quiz shows. This probably indicates that there are no alien civilisations at our stage of development within the radius of a few hundred light-years. Issuing an insurance policy against abduction by aliens seems a pretty safe bet.
And why haven't we heard from anyone out there? One view is expressed in a Calvin and Hobbes cartoon. The caption reads: "Sometimes I think that the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us."
More seriously, though, there could be three possible explanations of why we haven't heard from aliens. First, it may be that the probability of primitive life appearing on a suitable planet is very low.
Second, the probability of primitive life appearing may be reasonably high, but the probability of that life developing intelligence like ours may be very low.
Just because evolution led to intelligence in our case, we shouldn't assume that intelligence is an inevitable consequence of Darwinian natural selection. It is not clear that intelligence confers a long-term survival advantage. Bacteria and insects will survive quite happily even if our so-called intelligence leads us to destroy ourselves.
There is a third possibility. Life appears, and in some cases develops into intelligent beings, but when it reaches a stage of sending radio signals, it will also have the technology to make nuclear bombs and other weapons of mass destruction. It will, therefore, be in danger of destroying itself before long.
Let's hope this is not the reason we have not heard from anyone. Personally, I favour the second possibility; that primitive life is relatively common, but that intelligent life is very rare. Some would even say it has yet to occur on Earth.
Another question is: Can we exist for a long time away from the Earth?
Our experience with the International Space Station (ISS) shows that it is possible for human beings to survive for many months in space, but that zero gravity causes a number of undesirable physiological changes such as weakening bones.
One would therefore want any long-term base for human beings to have gravity, such as a planet or moon, with gravity.
By digging into the surface, one would get thermal insulation and protection from meteors and cosmic rays. The planet or moon could also serve as a source of the raw materials that would be needed if the extraterrestrial community were to be self-sustaining and independent of Earth.
What are the possible sites of a human colony in the Solar System? The most obvious is the Moon. It is close by and relatively easy to reach. We have already landed on it and driven across it in a buggy.
On the other hand, the Moon is small and without atmosphere or a magnetic field to deflect the solar radiation particles, like on Earth. There is no liquid water, but there may be ice in the craters at the north and south poles. A colony on the Moon could use this as a source of oxygen with power provided by nuclear energy or solar panels. The Moon could also be a base for travel to the rest of the Solar System.
Mars is the obvious next target. It is half as far, again, as the Earth from the Sun and so receives half the warmth. It once had a magnetic field, but it decayed four billion years ago, leaving Mars with no protection from solar radiation. This stripped Mars of most of its atmosphere, leaving it with only one per cent of the pressure of Earth's atmosphere.
However, the pressure must have been higher in the past because we see what appear to be runoff channels and dried-up lakes. Liquid water cannot exist on Mars now, as it would vaporise in the near-vacuum. This suggests that Mars had a warm and wet period during which life might have appeared either spontaneously or through panspermia.
There is no sign of life on Mars now, but if we found evidence that life had once existed, it would indicate that the probability of life developing on a suitable planet was fairly high.
NASA has sent a large number of spacecraft to Mars, starting with Mariner 4 in 1964. It has surveyed the planet with a number of orbiters, the latest being the Mars Reconnaissance Orbiter. These orbiters have revealed deep gullies and the highest mountains in the Solar System.
NASA has also landed a number of probes on the surface of Mars, most recently the Phoenix Mars Lander. The pictures they have sent back are of a dry desert landscape.
However, there is a large quantity of water in the form of ice in the polar regions. A colony on Mars could use this as a source of oxygen, at least. There has been volcanic activity on Mars, too. This would have brought minerals and metals to the surface, which a colony could utilise.
The Moon and Mars are the most suitable sites for space colonies in the Solar System. Mercury and Venus are too hot, while Jupiter and Saturn are gas giants with no solid surface. The moons of Mars are very small and have no advantages over Mars itself.
Some of the moons of Jupiter and Saturn might be possible. In particular, Titan, a moon of Saturn, is larger and more massive than other moons and has a dense atmosphere.
The Cassini-Huygens Mission of NASA and ESA landed a probe on Titan in 2005, which sent back pictures of the surface. However, it is very cold, being so far from the Sun, and I wouldn't fancy living next to a lake of liquid methane.
What about beyond the Solar System? Our observations indicate that a significant fraction of stars have planets around them. So far, most planets detected are giant planets like Jupiter and Saturn, but some planets are within the range of Earth-like size and it seems reasonable to assume Earth-like planets will be found. Some of these will lie in the habitable zone where the distance from the stars is the right range for liquid water to exist on their surface.
There are around a thousand stars within 30 light-years of Earth. If just one per cent had Earth-sized planets in the habitable zone, we would have 10 candidate new worlds.
We can revisit it with current technology, but we should make interstellar travel a long-term aim. By long term, I mean over the next 200 to 500 years.
The human race has existed as a separate species for about two million years. Civilisation began about 10,000 years ago, and the rate of development has been steadily increasing. But, if the human race is to continue for another million years, we will have to boldly go where no one has gone before.
This article is an edited extract of a lecture given in 2008 at George Washington University in Washington, DC, to mark the 50th anniversary of the US space agency NASA.