Voyage To The Planets: Remote Sensing

NASA/JSC

Just as in the 15th and 16th century, when the great geographic discoveries were made by only a few countries, the exploration of space was initially dominated by the United Sates of America and the Soviet Union. More recently Europe and Japan have played a significant part and as time passes more and more countries (China, India and Brazil, for example) are becoming interested. The political influence on national budgets has always been important. In the late 1950s and the early 1960s the Moon was the main target. The USSR’s spacecraft Luna 3 imaged the far side of the Moon for the first time in October 1959. Astronomers were surprised to realise that the face that pointed away from Earth was not the same as the face that pointed towards us. On the other side of the Moon there were very few lava-filled basins and volcanic activity had been negligible. Both of these observations indicated that the surface solid rock layers must be thicker than they are on the Earth facing side. Luna 9 soft landed on the Moon in January 1966, followed by the USA’s Surveyor 3 in April 1967. Everyone was relieved to learn that the soil-strewn lunar surface was not a ‘quick sand’ but could support weight and was safe to walk on. Lunar 10 orbited the Moon in March 1966 followed by the USA’s Lunar Orbiter in August of the same year. There were five Lunar Orbiters in total, three passing around the equatorial regions and two over the poles. With the Moon spinning slowly below, the narrow-angle and wide-angle cameras produced detailed maps of the whole surface, and the techniques learnt at the Moon paved the way for the detailed mapping of other planets.

The 1970s saw a considerable expansion of effort. Amongst the major milestones was the USSR Venera spacecraft that parachuted gently down through the dense atmosphere of Venus obtaining about one hour’s data from the surface. After an hour they were eaten away by the sulphuric acid rain drops picked up from the all-encompassing high cloud layers. Then there was the Mariner 10 flyby of Mercury in 1974 which imaged half the surface of this bakingly hot planet. The Viking orbiters and landers went to Mars in 1975 and showed the surface to be a dead, dry, rusty dessert. Pioneer 10 and 11 were launched in 1972 and 1973 and passed Jupiter and Saturn 21 and 75 months later.

In the 1980s the sophistication of the spacecraft increased, the instruments becoming more plentiful and more sensitive. The two Voyager spacecraft took full advantage of the changes that could be induced in their orbits by the gravitational fields of the planets they passed. Voyager 2 left Earth in August 1977 and its gravity-assisted flight path sent in on a Grand Tour, passing Jupiter in July 1979, Saturn in August 1981, Uranus in January 1986 and Neptune in August 1989. The data sent back was impressive. For example, 17500 images of Saturn and its moons Phoebe, Iapetus, Hyperion, Tethys and Enceladus were returned as well as 6000 images of Uranus and its moons Miranda, Ariel, Umbriel, Titania and Oberon.

Voyager is a telling example of the duration and difficulty of a modern space mission. It took about six to eight years to plan, build and test the Voyager spacecrafts. After launch it took a further twelve years to reach Neptune, during which time the instrumentation had to work efficiently and reliably and the science team had to remain motivated. Ground-based radio receivers had to be sensitive enough to pick up signals from a spacecraft that was about 4500 million kilometres away from Earth. The radio dish on the spacecraft was 3.7 m across. The spacecraft had to be powered continually, and the Voyager crafts did not used solar cells but carried with them small plutonium systems, the heat from the radioactive decay being converted into electricity. The surfaces of the planets and satellites that were passed were investigated using narrow and wide angle high resolution solid state cameras, mounted on a tracking platform. The polarisation and intensity of the light was carefully monitored. Spectra were taken in the ultraviolet and the infrared. The suite of instruments provided information about the physical and chemical form of the planetary and satellite surfaces and atmospheres. The surroundings of the planets were investigated using instruments that measured the strength and direction of the magnetic fields and the energy, velocity and composition of the charged particles that occupied the planetary magnetospheres. But we still only got snapshots. The spacecraft were only close to the planets for a day or so. The way in which characteristics varied with time could not be discerned

NASA/JSC

The next stage of planetary exploration clearly required orbiters. Pioneer Venus went into orbit around the planet in 1978 and mapped the surface using radar to a resolution of about 100km. Eleven years later we had the orbiter. The Magellan planetary probe was released from the cargo bay of the space shuttle Atlantis in May 1989 and rocketed off to Venus. After breaking hard it started to orbit Venus on 10 August 1990. Two years later the Magellan craft had been round the planet 5500 times and its radar system had mapped over 98% of the surface. Features down to 120m across could be resolved and heights could be measured to an accuracy of 10m. Continents and plains were revealed in detail, as were the multiple lava-flows, and the apparently random scattering of volcanic and impact craters. Interestingly the present surface seemed to have an age that was only about 15% that of the planet.

Space shuttle Atlantis was also used to launch the Galileo probe to Jupiter in October 1989. Gravity assists from Venus (in February 1990) and Earth (in December 1990 and December 1992) increased the size of the orbit, and the spacecraft eventually rendezvoused with Jupiter in December 1995. On its way through the Asteroid Belt, Galileo flew past Gaspra and Ida, taking superb colour pictures. As it was getting close to Jupiter Galileo released a small instrumented probe. With both parachute and heat-shield braking, this probe descended through 200km of the jovian clouds, transmitting about 70 minutes of data as the ambient atmospheric pressure changed from 0.08 to 30 times that of the Earth’s ground level pressure. As often happens with space missions, there were many surprises. The atmosphere was denser than expected and it was also less rich in helium and neon. The main Galileo spacecraft started its Jupiter investigation on a highly eccentric 230 day orbit. Successive passes of Jupiter slightly altered this orbit and led it on a series of swing-by manoeuvres, shaped like flower-petals, that took it on low altitude flybys of the major satellites Io, Europa, Ganymede and Callisto. Much time was spent investigating the high-pressure anti-cyclonic Great Red Spot in Jupiter’s southern hemisphere cloud band, the tenuous Jovian ring system and the Io plasma torus. It was clear that the sulphurous volcanic landscape of Io had changed considerably in the 17 years that separated the Voyager and Galileo missions. Visiting planets once is clearly not good enough. Many features need to be continuously monitored.

The success of the Galileo mission paved the way for the funding of an orbiter of the next planet from the Sun. The Cassini spacecraft was launched on top of a Titan 4 Centaur rocket on 15 October 1997, finally reaching Saturn in July 2004. During the nominal mission of 4 years it will orbit Saturn 63 times and will fly above Titan, the big (Mercury-sized) moon, on 33 occasions. There will also be close flybys of Iapetus, Enceladus, Dione and Rhea and more distant observations of Tethys, Mimas and Hyperion. One of the highlights of the mission will be the Huygens probe descent to the surface of Titan. Titan appears from afar like a rather uniform ball of orange fog. Voyager 1 data indicated that it has a very dense atmosphere of about 80% nitrogen, 6% methane plus hydrogen and other. It is thought that this atmosphere might be similar to the original atmosphere of Earth, the atmosphere that was subsequently oxygenated by life-forms. Temperature, pressure, density, composition, winds and aerosol content will be measured as the probe parachutes down to the surface. Images and spectra will be obtained. It is hoped that the Huygens probe will continue to function for several minutes after landing and the onboard instrumentation is designed to measure the characteristics of the surface in detail. It is not known, however, whether the probe will hit methane icy glaciers, solid rock mountains or a liquid hydrocarbon-methane oceans when it touches down. Images taken by the Hubble space telescope of Titan through one of the atmospheric ‘windows’ in the near infra-red part of the spectrum indicate that the surface reflectivity varies by about 10% as one moves from bright parts, that might by clean ice surfaces, to dark parts, that might by hydrocarbon seas. Hopefully data from the Huygens probe will solve this mystery.

NASA JPL

The 1990s were uncertain times in space exploration. The USSR / Russia essentially pulled out of the game. In the USA NASA’s mantra became ‘faster, better, cheaper’, and the two awful space shuttle disasters cast doubt on the reliability of that craft. Behind ‘faster, better, cheaper’ was a desire to reduce the lead time between mission conception and launch from eight years to five. Instead of putting tens of experiments on one huge expensive spacecraft the missions were broken up into smaller less expensive dedicated probes. More bang for the buck was the aim. There were some very notable successes. One was the low-cost Discovery mission NEAR-Shoemaker to the asteroid Eros. This orbited Eros for a year and finally landed on the surface on February 12 2001. And there was the Stardust mission to Comet Wild 2. This flew to within 236 km of the cometary nucleus on January 2 2004 and is returning to Earth with samples of cometary dust onboard, and will soft-land in the Utah dessert.

There are though, places that spacecraft cannot safely go. The rings of Saturn are full of ice-covered rocks and a spacecraft travelling through these rings would have too high a chance of hitting one of them. When the Giotto spacecraft visited Comet Halley in March 1986 it passed within about 600km of the nucleus, on the side facing away from the Sun. This miss-distance was picked because it gave the craft a 50/50 chance of survival. The minute cometary dust particles were approaching at a speed of 65 km s-1 and their kinetic energy was such that each impact could easily destroy the spacecraft. A special ‘bumper shield’ (devised by the famous American astronomer Fred Whipple) was used to break up the particles and spread their energy over a large area of thick ‘bullet-proof’ material. Small rocket jets were used to damp out the rolling effects caused by these impacts. The craft just survived, and subsequently went on to visit another comet Grigg-Skjellerup in 1992.

There are many things that spacecraft can do that are impossible from the surface of Earth. Travelling down through the clouds of Venus and Titan, and imaging the back of the Moon are perfect examples, as is the detection of the radiation belts and magnetic fields of distant planets.

And space exploration has been full of surprises. My favourites were the completely unexpected discoveries of volcanoes on the jovian satellite Io, the magnetic field of Uranus that is inclined at 60º to its spin axis and the huge Caloris crater on Mercury, the production of which nearly broke the planet apart. Even our nearby Moon was found to have strange enhancements of gravitational and magnetic fields that we called mascons and magcons.

Flybys and orbiters are fine for mapping surfaces and probing atmospheres and magnetospheres, but there is nothing quite like getting up-close and personal. For this you have to take the next step - land, rove about and start digging.