On 20 July 1969 NASA’s Apollo 11 mission landed on the surface of the Moon. Apollo was done, to paraphrase US President John F Kennedy, because it was hard, and human spaceflight still remains very hard. Indeed, since the sixth and final Apollo lunar landing in December 1972, all of human spaceflight has been constrained to low Earth orbit – just a few hundred miles above the ground.

Yet over that same period, robotic science missions have studied the Sun, comets, asteroids and moons, and have reached every planet in the solar system. (We are still awaiting the arrival of the New Horizons mission to recently demoted Pluto.) They have also probed the solar wind and explored the rest of the universe in the electromagnetic spectrum from radio to gamma rays – right back to the Big Bang and the cosmic microwave background.

While Apollo provided insights into the geology of six small areas on the Moon, hardly any of the science achieved since then could have been done by a manned mission. There are many reasons for this, but the principal issue is the difficulty of keeping people alive in space and returning them safely to Earth. Crew safety has to be paramount, so science can never be the priority of a manned mission. Science is always scaled back when cutbacks are needed, well before anything that might affect the safety of the crew.

Scientific space missions and human spaceflight take place on very different scales. Apollo is estimated to have cost about $25bn at 1969 prices, equivalent to $145bn at today’s prices. The plan to return astronauts to the Moon is currently estimated to cost $97bn, though there are likely to be over-runs beyond this, and a crewed mission to Mars is expected to cost several times more. The cost of Apollo alone is comparable to the combined cost of all robotic space missions since we left the Moon. If science were the primary goal of these successors to Apollo, then it could be done cheaper, better and faster without a human crew.

To take one example, the Spirit and Opportunity rovers have been driving around Mars for the last five years. To date, the mission has cost just less than $1bn. Although each rover has only travelled about 10 miles on the red planet, for the cost of a human mission to Mars we could send about 600 such vehicles and conduct something approaching a geological survey of the whole planet (this of course ignores economies of scale and technological advances since the rovers were designed). This is something a human mission to Mars, even with many months on the surface, could not achieve.

Even the success stories that combine human spaceflight with science do not survive close inspection. The most famous of these is the Hubble Space Telescope, which has spent many years beaming back spectacular images of the universe. It is, of course, a robotic mission, but it has been repaired and upgraded five times by astronauts using the Space Shuttle. The original cost of Hubble was about $1.5bn, including contributions from both NASA and the European Space Agency. The subsequent servicing missions have cost an additional $3–4.5bn, largely as a result of the expense of shuttle launches. For the cost of the upgrades and repairs we could have launched two or three extra Hubble Space Telescopes. Hubble’s successor, the James Webb Space Telescope, will not even need to be serviced, principally because it will be positioned well beyond the Moon, some 1.5 million kilometres away.

Given all of the above, it is fair to ask why we bother with human spaceflight at all. It simply does not deliver as much science per pound, dollar or euro as robotic missions. The issue is that human spaceflight has never been about science. From the beginning, with the Soviet Union’s Vostok mission and NASA’s Mercury project each seeking to send a human into space, it was, and still remains, about national prestige and spectacle rather than science. Science can be a useful label to help market a space mission, but prestige has always been the main driver. A human mission to Mars or the Moon would be the ultimate reality TV spectacle, but would it be worth the hundreds of billions it would cost? This is the question that the Augustine Commission is asking in the US, and its answers might make unpleasant reading for the human-spaceflight community.

Admittedly, human spaceflight is one of the most spectacular achievements of the 20th century, but science has never been, and can never become, the driving force for a manned programme. The most effective way of achieving scientific goals in space is through the use of unmanned robotic probes.

Human spaceflight tends to be a controversial issue because many scientists believe that the limited resources available for space exploration would be better invested in robotic missions. On the other hand, human beings are uniquely qualified to undertake several key scientific investigations in space ranging from life- and physical-sciences research in microgravity to geological and biological fieldwork on planetary surfaces.

It is true that most astronomical observations have, since the start of the space race, benefited from robotic spacecraft placed above the obscuring effects of the Earth’s atmosphere. However, one of the principal lessons from the most successful of these instruments, the Hubble Space Telescope, is that access to a human-spaceflight infrastructure can greatly increase the lifetime and efficiency of space-based astronomical instruments. Since its launch in 1990, Hubble has been serviced by five Space Shuttle missions, without which it would have been a much shorter lived, and far less versatile, instrument.

Perhaps the Apollo missions to the Moon were the best demonstration of astronauts as explorers of planetary surfaces. They underlined how humans bring an agility, versatility and intelligence to exploration in a way that robots cannot. Although it is true that humans will face many dangers and obstacles operating on other planets, mostly due to their physiological limitations when compared with robots, the potential scientific returns (resulting from rapid sample acquisition, the ability to put data and past experience into a coherent picture, and the on-the-spot ability to recognize observations that may be of importance) is more than sufficient to justify employing astronauts as field scientists on other planets.

Indeed, these advantages were recognized in a report by the UK’s Royal Astronomical Society in 2005, which concluded that “profound scientific questions relating to the history of the solar system and the existence of life beyond Earth can best, perhaps only, be achieved by human exploration on the Moon or Mars, supported by appropriate automated systems”.

Closer to home, the microgravity environment of low Earth orbit such as that on the International Space Station (ISS) gives a unique opportunity for research in the life sciences (including human physiology and medicine), materials science and fundamental physics. This type of environment can provide unique insights into areas such as gene expression, immunological function, bone physiology and cardiovascular function, which are important for understanding a range of terrestrial diseases such as osteoporosis, muscle atrophy and cardiac impairment. As humans are the subjects of these physiological experiments, robots can never be a substitute. Further progress in these areas will rely on funding for the ISS being maintained well into the coming decades. Although science will be a major beneficiary of having people in space, it is not, and is never likely to be, the sole motivation for human space exploration. Other benefits of investing in human space exploration include the stimulus it gives to scientific and technical education: space exploration is inherently exciting, and is an obvious way to inspire young people to take an increased interest in science and engineering. Human spaceflight is also good for hi-tech jobs and can foster innovation. Finally, it provides a focus for international cooperation that could help to build a more stable geopolitical environment.

The multiple benefits of human space exploration were recognized by the publication in May 2007 of the Global Exploration Strategy report by 14 of the world’s space agencies. Developing a global exploration programme, with the ultimate aim of sending astronauts back to the Moon and on to Mars, is a noble vision for the 21st century, the realization of which would confer significant scientific, economic and cultural benefits on our world. As such, human spaceflight should be seen as an investment in the future of humanity and it deserves the full support of the scientific community, and indeed of all citizens of our planet.