Planetary astronomy was transformed by the Italian scientific genius Galileo Galilei in 1609. Before Galileo the planets were merely points of light that periodically moved around against the background of stars.
Plotting positions and endeavouring to calculate orbital parameters was the main occupation. Galileo’s new-fangled astronomical telescope changed everything.
The Moon, instead of being a perfectly smooth sphere, as Aristotle had suggested nearly 2000 years before, actually had mountains, craters and ‘seas’; Saturn was girdled with an equatorial ring and Jupiter had four small satellites.
The years 1781, 1846 and 1930 saw the solar system dramatically expand, these being the years in which Uranus, Neptune and Pluto were discovered. As telescopes increases in size and sophistication more and more planetary surface details were revealed.
The canals of Mars
A major breakthrough occurred in 1878. Giovanni Schiaparelli, the director of the Milan Observatory and a man with super eyesight, used his meticulous observations of Mars to produce a map of the surface. This seemed to be covered in groove-like channels. In Italian the word is ‘canali’.
Impetuous English speakers immediately translated this as ‘canals’ and soon other planetary astronomers, such as the rich American Percival Lowell, were producing books and articles that overflowed with descriptions of artificial waterway constructed by intelligent Martians irrigating the equatorial regions of their planet with water from the melting polar ice-caps.
Planets were big news, they were alive, they were fascinating, they must be studied in detail and we must try and devise ways of visiting them. The likes of Jules Verne and H. G. Wells soon had us worrying about interplanetary conflict and interplanetary cooperation.
Sputnik changes the rules
A century ago science fiction was obsessed with travel and contact, and we read of sending people to the centre of the Earth, the Moon or beyond, as well as receiving visitors from nearby planets. But until recently astronomy has been very much a ‘hands-off’ science. Astronomers have been able to look but not touch.
They stand at the bottom of the telescope and have to accept what arrives down the end of the tube. But Sputnik 1, launched by the USSR in October 1957, changed that - especially for solar system astronomers.
The space age and space race started. Large rockets were developed that could hurl space probes way beyond the Earth’s atmosphere, past the Moon and the planets and out towards the stars. Soon those rather fuzzy, turbulent, indistinct images of planetary surfaces, taken from huge distances, were being replaced by crystal-clear close-ups.
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.
Phoenix Mars Lander [Image: NASA/JPL/UA/Lockheed Martin]
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.
Next we have sample return. An actual piece of the soil and rock is picked up, stored carefully and then brought back to the Earth-based laboratory.
Back here on Earth, huge sensitive instruments can be used to probe the material in intricate detail, and we do not have to rely on the remote manipulation of small hardware designed to cope with the rigours of space.
Over 350 kg of lunar material have been returned to Earth by the USA and the USSR from nine specific locations. How typical this is of the Moon in general is an open question.
The ‘planetary surface’ dilemma still remains. It is one thing to scrape a sample from a planetary surface. We still have little idea as to what the planet is like inside.
The final stage, stage six, is sending someone. Here you have all the advantages of the clever, inquisitive, enquiring human noticing the unusual and taking advantage of the unexpected. But as yet only 12 people have walked on the Moon.
This celestial body is only 380,000 km away, and we can get there in three days. And it all happened a long time ago, between July 1969 and December 1972.
Why have we gone into space to observe the planets? It’s more than just our adventurous spirit and unbounded curiosity. Military progress and rivalry between governments played a vital role. Rocketry was designed so we could hurl bombs from one continent to another.
The first USSR spacecraft underlined the progress of the communist system and the USA raced to catch up. “Achieving the goal, before this decade (the 1960s) is out, of landing a man on the moon and returning him safely to earth” became the Kennedy rallying call.
But there were many before Kennedy pointing to the distant frontier. Percival Lowell’s observation suggested that the surface of Mars was criss-crossed with canals irrigating the equatorial regions with melted polar snow. Life was out there. We had to go and look, and say hello. Space is the final frontier and it would be inhuman not to explore.
Find out more
One further way to explore the Solar System: Use our desktop guide to the planets