Exploration of the Solar System

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The discovery of the Earth's radiation belts is a classic example of an in situ measurement, where detectors on the Explorer 1 satellite were measuring the immediate environment through which the spacecraft was flying. This is the main way in which we have gathered information on the Earth's magnetosphere, the cocoon carved out of the solar wind by the Earth's magnetic field. An up-to-date example is the European Space Agency's Cluster mission (launched in 2000), in which four identical spacecraft equipped with an array of sensitive instruments fly in formation through the magnetosphere. Having measurements from more than one adjacent spacecraft means that we can determine how plasma and magnetic fields are moving in space, as well as their properties at any given point.

As the techniques of space flight were mastered, the range of in situ measurements was rapidly extended, first to the Moon, and then to other bodies in the Solar System. In September 1959, the Soviet Lunik '2 spacecraft measured solar wind particles for the first time, confirming the existence of the solar wind, which until that time had been inferred only indirectly. A further advance in our knowledge of the solar wind came from the flight of the US Mariner 2 spacecraft to Venus. Mariner 2, launched in August 1962, measured the velocity of the solar wind and identified both slow and fast components. In common with many planetary probes, it contained both in situ and remote sensing devices. The latter were in the form of microwave and infrared radiometers, which allowed us to measure the temperature and composition of Venus and its atmosphere as the spacecraft flew by the planet.

A common remote sensing device is some sort of camera, an early example of which was an instrument on Lunik 3 that photographed the far side of the Moon for the first time in 1959. Close-up photographs of the Moon were obtained with the Ranger spacecraft in the last minutes before they crashed onto the surface, and the whole Moon was surveyed from the Lunar Orbiter spacecraft in 1966 and 1967. Several Surveyor spacecraft landed on the Moon between 1966 and 1968 and obtained photographs from its surface, as well as probing the terrain with mechanical scoops (Figure 1.1). The exploration of the Moon in the 1960's culminated in the landing of astronauts, who conducted a range of investigations including the deployment of seismic detectors and laser ranging targets, and collecting rock and dust samples that were returned to the Earth for analysis. The results of this exploration, the Apollo programme, have, over the years, contributed greatly to our understanding of the Moon's origin, and of the formation of the Solar System.

Turning further afield, spacecraft have now been sent to every planet in the Solar System bar Pluto/Charon. The Mariner 4 spacecraft discovered craters on Mars during a flyby in July 1965, while a large part of Mars was mapped using

Figure 1.1 The Surveyor 3 spacecraft landed on the Moon in April 1967. Astronauts Conrad and Bean later visited Surveyor 3 in November 1969 during the Apollo 12 mission. Here Conrad is pictured examining the camera on Surveyor 3. The Apollo 12 lunar excursion module 'Intrepid' is visible in the background. (NASA)

Figure 1.1 The Surveyor 3 spacecraft landed on the Moon in April 1967. Astronauts Conrad and Bean later visited Surveyor 3 in November 1969 during the Apollo 12 mission. Here Conrad is pictured examining the camera on Surveyor 3. The Apollo 12 lunar excursion module 'Intrepid' is visible in the background. (NASA)

Mariner 9, which was placed in orbit about the planet in 1971, and two Viking spacecraft, which arrived at Mars in 1976. The Viking's also carried landers, which obtained the first pictures from the Martian surface. The in situ exploration of the planet was continued in 1997 from the Mars Pathfinder lander, which included a rover vehicle. Meanwhile surveys of the planet from orbit have continued using the Mars Global Surveyor (arr. 1997) and Mars Odyssey (arr. 2001) spacecraft.

Mariner 10 visited both Venus and Mercury, obtaining the first detailed view of the latter and revealing Moon-like craters in three encounters with the planet between 1974 and 1975. As noted above, the first spacecraft to visit Venus was Mariner 2 in 1962. It was followed by many others (more than 20 in all so far), including Pioneer Venus, which made the first detailed map of its surface, and the Soviet Venera 7, the first spacecraft to land on another planet. Another Soviet lander, Venera 9, returned the first photographs of the surface of Venus, while the lander Venera 13 survived for over two hours in the hostile Venusian environment, returning colour pictures. In the early 1990's, the orbiting US spacecraft Magellan produced detailed maps of Venus' surface using radar.

Exploration of the outer Solar System began with NASA's Pioneer 10 and 11 spacecraft, launched in 1972, followed by the pair of Voyager spacecraft, which took the first close-up photographs of the outer planets. Galileo, launched by NASA to Jupiter in 1989, included an atmospheric probe as well as an orbiter, while the NASA Cassini spacecraft, currently on its way to Saturn, also contains an atmospheric probe, known as Huygens, which was built by ESA. Spacecraft have explored minor Solar System bodies as well as the main planets. Galileo was the first mission to make a close flyby of an asteroid (Gaspra), and also the first mission to discover a satellite of an asteroid (Ida's satellite Dactyl; Figure 3.20). Another example is the ESA Giotto mission, which made a close approach to Comet Halley in 1986, and subsequently also encountered Comet Grigg-Skjellerup, in 1992.

There are future aspirations to return samples from a comet, which are expected to be representative of the primitive state of the Solar System, and to determine the composition of the Martian surface, first by in situ robotic methods (as planned for example with the UK Beagle 2 lander which will shortly be launched on ESA's Mars Express) and later by returning samples to the Earth. A major goal is to identify how and where life originated in the Solar System by searching for evidence that primitive life once evolved on Mars. The ultimate aim is manned exploration of the planet.

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