There are six approaches to planetary exploration in the space age. The first, and most unfortunate, is to ignore the object all together. This has been applied to Pluto, the tiny outrider of our system that is about 40 times further away from the Sun than the Earth and a quarter the mass of our Moon. This is no place to debate whether Pluto is a planet or not, but to ignore it completely is a disgrace. To date no spacecraft has been anywhere near Pluto, and our knowledge of its surface features is negligible.

Stage two is the flyby. Here a spacecraft takes a snapshot of a planet, moon, comet or asteroid as it hurtles past, typically at a speed of about 50,000 mph, missing the planet by a few hundred kilometres. Only half the object can be seen and any changes of its features with time are clearly missed. All the planets, with the exception of Pluto, have been flown by.

Stage three is the orbiter. The spacecraft become a moon and collects planetary data indefinitely (or until its instrumental system breaks down, the money to collect the data and analyse the results runs out, or it is crashed into the surface to get it out of the way of subsequent missions). The evolution of cloud systems and volcanic activity can be monitored, the surface can be accurately mapped, spectroscopic details can be collected these indicating the composition of surface features and atmospheric gasses and the cloud particles. Also the atmosphere, and the ionosphere, and the interaction of the planetary magnetic field with the wind of particles being blown away from the Sun, can be investigated in three dimensions and as a function of time. Orbiters have been employed at Venus, Mars, Jupiter, Saturn and our Moon.

The forth exploration stage is the lander. A space probe gently touches down on the surface, causing as little disturbance as possible, and then, after looking round starts digging into the soil and analysing nearby rocks and gasses. Some of these landers even travel about and can be moved towards interesting features. So far we have only landed on Mars, Venus and the Moon. The question of where to land is always problematic. Should it be the pole, the equator or the tropics? Should it be on top of a mountain, in the middle of a dessert or near a dried-up river bed? Maybe the feature that you search for, evidence of life on Mars for example, is not where you are, but is just over the hill, or at a completely different latitude and longitude.

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