Surface temperatures on Mercury vary from 430°C to -180°C. Venus is even hotter at 465°C. It also has a crushing surface pressure 90 times that of Earth and it rains sulphuric acid. None of the early rocket probes landing on Venus survived more than a few minutes. And man will never set foot on the gas giants or frozen giants. Exploration of the worlds beyond Mars will involve journeys of enormous length and duration to frozen and dimly lit destinations. Almost certainly they will be carried out by robots and will depend on advances in artificial intelligence (AI).

It is humbling to note that only in March 2025 did astronomers at the France-Canada-Hawaii telescope discover that Saturn had 128 more moons than originally thought – bringing its total moons to 274, though there could still be a few more. Jupiter has 95 known moons, Uranus 28 and Neptune 14. These are the worlds on which attention will fasten once Mars has been reached. It is even more humbling to recall that Jupiter’s four largest moons (Io, Europa, Ganymede, and Callisto) were discovered by Galileo in 1610 – when one considers his poor equipment and the rudimentary state of knowledge at the time, a truly astonishing achievement.

Io is the most volcanically active world in the solar system – driven by Jupiter’s immense tidal forces – and it lies within Jupiter’s most intense radiation belts. This hostile combination means it will almost certainly be avoided.

Europa is a far more compelling target, with a vast ocean of salt water – estimates are that it could be 100 kilometres deep – which could harbour life forms, but it too is exposed to Jupiter’s fierce radiation. Any human venturing to Europa would need to burrow deep under the ice sheet to protect himself from radiation. And drilling through an ice sheet many miles thick to get to the water beneath - would be extremely difficult.

Ganymede is the largest moon in the solar system, indeed, it is larger than Mercury. It also probably has a subsurface ocean and, because it has its own magnetosphere, is somewhat less exposed to radiation. But Callisto is still further out and lies beyond Jupiter’s main radiation belts. It too has sub-surface water and undoubtedly offers the most benign environment for exploration. If there is to be an outpost in the Jovian world it will be on Callisto.

Saturn’s Moons

Among Saturn’s moons two stand out, Titan and Enceladus. Titan has a dense atmosphere of nitrogen, atmospheric pressure only 1.5 times that of Earth, subsurface water, and a rich organic chemistry. Its dense atmosphere acts as a shield against radiation and provided an astronaut was protected against its extreme cold, he or she would be able to move about on the surface.

Enceladus is a small icy moon whose secrets were revealed by the Cassini probe: it shot out plumes of warm water into space, clearly from a heated subsurface ocean. This heating doubtless derives from the strong push-and-pull effects of Saturn’s gravity and the counter-effects of the gravitational pull of other moons. Indeed, Cassini detected tiny particles of silicates within the plumes of water. On Earth that would indicate water heated to 90°C or above, with a strong suggestion that there are hydrothermal vents on the ocean floor of Enceladus. This made it a prime candidate for possible life forms. On Earth such hydrothermal vents on the ocean floor shooting out warm water are home to all manner of life forms (despite the lack of sunlight) and there is speculation that the same may be true on Enceladus.

Uranus, like Saturn and Jupiter, seems to have significant amounts of helium-3 and deuterium in its atmosphere – both major fuels for nuclear fusion. Uranus’s moons – all named after characters from Shakespeare or Alexander Pope – are less well known though all of them seem to have large amounts of water-ice.

Finally, there is Neptune, a dark and freezing world. Among its moons, one stands out – Triton, both because it is large and because, uniquely, it revolves in the opposite direction from Neptune itself.

However, beyond Neptune lies the Kuiper Belt, a vast area containing primordial material left over from the age of planetary formation. The Belt includes over a trillion very diverse objects ranging from the nuclei of comets to lumps of rock or ice to dwarf planets like Pluto. Geologically, the Belt doubtless contains all the secrets of how the solar system was created and how it evolved, but the sheer diversity of the Belt’s constituents more or less guarantees that it is also rich in all manner of minerals.

Almost certainly this helps to explain the peculiarities of Triton, very probably not an organic part of Neptune’s universe but instead a captured Kuiper Belt object.

Propulsion

Any consideration of exploring these worlds beyond Mars has to take into account the need for other types of rocket propulsion. Our current chemical rockets have a fatal limitation, deriving from the Tsiolkovsky rocket equation. Konstantin Tsiolkovsky (born in1857), the great Russian pioneer of astronautics, showed how the speed of a rocket depended on the exhaust velocity of the fuel it burned – and all chemical reactions have inherent limits to their exhaust velocities.

Perhaps the most promising alternative is a nuclear thermal rocket, which could achieve approximately double the speed of our current rockets, though even that may be regarded as too slow given the enormous distances involved. Research will doubtless continue into other forms of propulsion. In science fiction this problem is solved by “moving to warp speed” or “the leap into hyperspace” but, unhappily, these are mere comic book inventions.

Probably the best hope is a rocket using nuclear fusion. This could reduce the nine months that current rockets would take to get from Earth to Mars to merely a few weeks.

Given how hostile many of the environments that a space traveller would face are, this has given rise to ideas of terraforming worlds to recreate Earth-like conditions. However, as could be seen from our earlier description of what terraforming Mars would involve – including changes to the atmosphere taking many thousands of years to complete – there has instead arisen the idea of para-terraforming. Instead of trying to transform a whole planet, this would involve the creation of large, airtight domed settlements. Inside the dome the air would be warm and breathable and there would be a sealed biosphere, allowing for Earth-like agriculture and comfortable human habitation. Just such a domed settlement was seen in the Arnold Schwarzenegger film Total Recall – though at the film’s end Mars becomes perfectly terraformed and humans can breathe the air outside the domes.

In all conscience the construction of such domed cities would be complex enough, though far less ambitious than full terraforming. One can imagine that such domed structures might be built with the idea that full terraforming would follow, but that nothing lasts so well as the provisional – and the temporary would become permanent.

Knowledge

What is moving fast is our knowledge of outer space. The first exoplanet was not discovered until 1992 and the first such planet in a habitable (“Goldilocks”) zone not until 2001. A habitable zone: that is, in a zone not too close or too far from the star that warms it and with the possibility of having a temperate atmosphere and water.

But all such planets were some 3 000 light years away, until 2016, when Proxima Centauri b was discovered revolving round the nearest star to our Sun, Alpha Centauri B. This is only 4.24 light years away – still an enormous distance. Moreover, this Earth-sized planet is very close to its star (a red dwarf) and revolves round it so fast that its year lasts only 11.2 days. Moreover, one side always faces its star, making it horribly hot, while the other side freezes. The only potentially habitable area where liquid water might exist is the thin temperate strip separating these two zones, but we don’t yet know if this planet has an atmosphere. Discoveries of this sort have been enormously assisted first by the Hubble telescope and even more by the newer James Webb telescope, both of which stationed in space and therefore not obstructed by cloud or the alternation of Earth’s day and night.

Proxima Centauri b may well turn out to be a disappointment, but already preparations have begun by Breakthrough Initiatives (founded by Stephen Hawking, Mark Zuckerberg, and Yuri Milner) to send a probe there. The idea is to send a fleet of light-sail interstellar probes known as Starchip to fly by the exoplanet and doubtless scout for other such planets in the vicinity. Starchip is to be driven by 100GW worth of phased array and steerable lasers on Earth, which will gradually accelerate Starchip up to 20% of the speed of light, with the laser beams insistently pushing the sail. Even so, its journey will last at least 20 years and each radio message it sends back will take over four years to arrive.

But one can’t help wondering if this will really happen, because other exoplanets are being discovered at a great rate – we now know of 6 128 of them, with another 8 000 candidates awaiting confirmation – and other, more suitable, candidates for a probe visit could well emerge. Moreover, laser-powered flight is still in its proof-of-concept phase. And 100 gigawatts is the total output of a large nuclear reactor.

As may be seen, by this stage we are having to envisage the development of new technologies – in robotics, AI, nuclear fusion and laser-powered flight. Given the speed and creativity of modern technological development, that is not unreasonable, but it does move us into the realm of science fiction. On the other hand, the history of astronautical development in the last century has already turned science fiction into reality and in many cases exceeded what was only imagined.