The hazards of space travel.

Exploration is an important survival strategy in evolution. The migration of expansive species depends on exploring their immediate or distant surroundings for new food sources or safe habitats; it can also come as a result of population pressures or environmental changes. The human species has added another reason for exploration, namely curiosity. This intellectual urge to explore the unknown led the great European explorers to the Americas, Australia and Antarctica between the fifteenth and seventeenth centuries. Inquisitiveness about nature is also the driving force behind humans exploring the polar caps, climbing mountain peaks and diving into the abysses of the oceans. Now, the ultimate frontier to explore in the twenty-first century is space. Astronomical observations and satellites have already yielded immense knowledge about our solar system and the universe beyond. But these technologies can provide only a limited picture of what is out there; eventually humans themselves will have to travel to other planets to investigate them in more intimate detail. Tremendous advances in rocket and spaceship technologies during the past 50 years, driven mainly by national security considerations, the need for better communication or a desire to observe environmental changes and human activity on the ground, have made it possible to send humans into near-Earth orbit and to the Moon. Conceivably, these advances will eventually make it possible to transport astronauts to other planets, and Mars in particular (Fig. 1). But there are significant differences between exploring Earth and exploring space. First and foremost, space is an unforgiving environment that does not tolerate human errors or technical failure. For humans leaving Earth’s orbit for extended periods, there are even more dangers. One is the near absence of gravity in space; the presence of high-energy, ionizing cosmic ray (HZE) nuclei is another. Because both zero gravity and cosmic rays would have severe health implications for astronauts on a Marsbound spaceship, we first need to investigate their effects on cells, tissues and our hormonal and immune systems. However, although we are able to produce HZE nuclei on Earth and study their effects on biological material, we cannot simulate extended periods of low gravity and their additive effects on cells and tissues. Thus, the International Space Station (ISS) will have an enormously important role in assessing the health dangers for humans in space and in the development of potential countermeasures.