March 1982:the future of planetary exploration

The future may see the 1970's as the first golden age of planetary exploration. Mariner 9 and later Viking revealed the complex world of Mars to our gaze. Venus was unveiled by Pioneers 12 & 13 and the Russian Venera probes. Mariner 10 made three close approaches to Mercury in a game of interplanetary billiards. And Pioneers 10 & 11. followed by the two Voyager spacecraft, revealed the diverse worlds of Jupiter and Saturn and their attendant satellite systems.

The Solar System has proved to be a far more interesting place than we had imagined. True, there were no green men on Mars, nor hot swamps beneath the clouds of Venus, but neither were these other worlds mere dull and barren repetitions of what we had discovered on the Moon.

By contrast, the 1980's have opened with a paucity of planned missions. The Europeans have actually become more active than the Americans, with Giotto planned for Halley's comet, active funding of their International Solar Polar Mission spacecraft, and German participation in the US Galileo probe to Jupiter. Only the Venus Orbiting Imaging Radar is a wholly US mission. And beyond those missions there are only proposals and studies. However, absence of missions is not due to absence of targets. Indeed, the very variety of targets may be making planning more difficult.

Mars is a good example. Follow-on missions to Viking have proposed a polar geophysical satellite, surface rovers, subsurface penetrators, robot aircraft with a planet-wide range, and surface sample return missions similar to Luna 16. The Mars Science Study Group highlighted the most expensive of these - surface sample return - as being of the greatest value, but there is no doubt that even this would only provide a small fraction of the information we would like to know about our neighbour planet. And in the absence of funding for a surface sample return mission, the next Mars mission (possibly an ESA funded mission) is likely to choose one of the lesser options. These sort of considerations led the author to re-examine the prospects for a manned mission to Mars in the mid-1990's which could carry out a comprehensive survey of the planet. The conclusion of that study was that, using the hardware that might be available by the end of the century, such a comprehensive mission actually be more cost-effective than a series of specialised robot probes.

This conclusion may have wider implications for the long-term exploration of the Solar System. With developments for operations in near-Earth space - launching heavy platforms into geostationary orbit and carrying out orbital construction - the space transportation technology of the early twenty-first century may well favour launches of single, comprehensively equipped missions to individual planetary systems rather than a succession of specialised smaller craft. This article is a personal speculation on some of the proposals and possibilities.

Apollo demonstrated the importance of surface sample return missions. While it is easy to make overall chemical and geological analyses of the local rocks on-site, a real understanding of the make-up of another world demands that samples be returned for study by the most advanced laboratory facilities available on Earth. Furthermore, on-site measurements do not lend themselves to follow-up measurements suggested by the original investigation. And to get maximum advantage from such samples, they should be selected from diverse sites over the target planet - often from sites which are not particularly optimum from the point of view of landing a probe. It may even be desirable to drill beneath the surface to reveal the inner structure of the planet. This all means that while robot missions may well remain smaller in mass than a manned mission, the robot probes will have to grow considerably in size and complexity.

Now, even with a Centaur upper stage, the throw mass of the Shuttle for interplanetary missions is essentially unchanged from that of the Titan/Centaur combination which launched Viking and Voyager. Much larger missions will demand improved operational concepts, improved propulsion systems, or on-orbit assembly of large vehicles.

Asteroids and Comets

While we tend to think of the asteroids as lying "beyond Mars," some of them are actually the closest objects to approach Earth besides the Moon. Interest in such "Earth approaching" asteroids has been rekindled by the Space Colonizers, who see them as potential sources of raw material - possibly containing volatile and carboniferous materials. They are also possible targets for the amateur astronomy community, who can scan the heavens extensively in search of such "targets of opportunity."

However, the fact that they are relatively close to the Earth does not make these asteroids easy targets. Most have elliptical orbits which have significant inclinations to the plane of the ecliptic, and getting above the plane of the planets can be expensive in "delta-V." Earth and Venus swing-by trajectories may provide useful assistance in this area.

One target which has been studied already is the asteroid 1943 Anteros, which has an aphelion only 1.4 times the Earth's distance from the Sun, and an orbital inclination of 8.7°. Hulkower and Ross have studied a possible rendezvous mission with a launch window in May 1985, which would allow a Titan 34D/IUS combination to deliver a 615 kg spacecraft to Anteros some 402-455 days later. Another launch window is available in October 1987. The authors suggest a modified Tiros N as the probe vehicle carrying 50-100 kg of scientific instruments for our first close-up view of an asteroid.

Return missions to asteroids are likely to be more complex because of the lack of synchronisation between the elliptical, inclined asteroid orbit and the Earth, but once again planetary swing-by manoeuvres may be the key. Some asteroids may be easier in this respect than others, because they are locked "in resonance" with the Earth, making regular close-approaches to our planet.

If asteroids represent difficult targets, then comets are even worse. The ESA Giotto probe will pass Halley's comet at a relative speed of 60 km/sec, and will use real-time transmission to return its data before it enters the comet's tail. Missions to comets with shorter periods are easier, but less interesting, as these represent "old" comets that have lost much of their volatile material. Solar-electric propulsion would be useful to allow an approach to a true "rendezvous," but even here on-board intelligence and short communications times may be needed if the spacecraft is to manoeuvre accurately and "explore" the nucleus in detail.

Venus

Despite numerous missions to Venus, the true features of that planet have only just begun to appear from the scan provided by the radio-altimeter aboard Pioneer 12. The Venus Orbiting Imaging Radar spacecraft should provide us with a detailed map of the surface of Venus for the first time, but evidence already in shows an active world with rift valleys,meteor scars and possibly volcanoes.

Although it is the nearest planet to the Earth, Venus is going to be immensely difficult to explore. It is simple enough to observe it from orbit, or even to drop probes or balloons into the upper atmosphere, but any probe intended for the surface must be heavily insulated and protected against the high pressure atmosphere if it is to operate at all - even for a few minutes.

As far as I know, nobody has yet attempted to design a surface sample return mission for Venus, although such a mission would be of immense scientific interest. The orbital velocity which would have to be achieved by the ascent vehicle is less than that for Earth, and in terms of sample sizes considered for a Mars sample return mission a three-stage solid-propellant vehicle with a mass of about three tonnes would appear to be sufficient. The problem is, however, that this vehicle would first have to come up through 60 or 70 km of dense, hot atmosphere slowly. Balloons or ducted rockets might help, but there is a trade-off between ascending slowly enough to avoid wasting propulsive energy on drag, and fast enough to avoid the external heat soaking into the vehicle components.

Mercury

The planet Mercury has remained unvisited by a spacecraft since the fly-by mission of Mariner 10 in 1974/5. The closest planet to the Sun, Mercury has a special place in the physics of the Solar System on account of its high density and its magnetic field. In addition, the massive event that formed the Caloris basin (perhaps a 70 km asteroid with a collision velocity of 50 km/sec) generated temperatures and shock levels unmatched in the Solar System. The Caloris fireball melted the Mercurian landscape in depth from horizon to horizon, and the effects of the shock .are visible on the far side of the planet.

NASA has looked at the possibility of a Mercury orbiter mission using a Venus swing-by similar to that of the Mariner 10 probe. The spacecraft would actually be a dual probe - separating as it approached Mercury to give a low level (105 km altitude) orbiter for geophysical mapping, and a high level orbiter in an elliptical orbit to make particle and field measurements on the planet's magnetosphere. While the scientific payloads of the two spacecraft are distinct, the supporting telecommunications, data handling and control, attitude control, thermal control and structure would be common.

Thermal control is likely to be the major problem faced in a Mercury orbiter design. Not only is direct radiation from the Sun more than ten times that at Earth, but reflected thermal radiation from the planet is over twice that from the Sun at the Earth. The spacecraft design favoured by JPL has a long,thin structure with its axis aligned towards the Sun, with "planet shading" wings extending from the sides to protect the thermal radiation surfaces from the reflected glare of the planet.

Particular mission opportunities exist for 1986 and 1994 which would be sufficiently favourable to allow a small "rough lander" probe to be included in the mission.

Jupiter System

Jupiter and its system of satellites represents a particularly attractive target for future planetary exploration. The Galileo mission is intended to drop a "sounder" probe deep into the Jovian atmosphere, and then use the orbiter to patrol the satellite system over an extended period using "gravity assist" to move from one satellite to another.

The Voyager missions revealed the diversity of the inner satellites, and an understanding of their varied structures is now felt to be vital in forming a picture of the early events in the formation of the Solar System. The outer satellites, which lie in orbits highly inclined to the equatorial plane, are thought to be captured asteroids and are as yet unexamined. Not surprisingly, lander missions to the four Galilean satellites are already being studied. One idea which has merit for Europa and Ganymede - the surfaces of which are largely ice - is to carry a nuclear-heated probe which could melt a deep borehole into the surface of these worlds to investigate the sub structure. Surface sample return missions from the Jovian satellites have also received some attention.

Europa is the preferred target,with a guarantee of surface ice from which oxygen and hydrogen can be electrolyzed. If only oxygen were recovered - a relatively easy process since the surface temperature of Europa is within the boiling point range of oxygen - and were used with methane fuel transported from Earth for the return propulsion, the launch mass of a Europa surface sample return mission would be reduced by about 4 tonnes. If both hydrogen and oxygen were utilised', the processing plant would increase in size but the Earth launch mass might be reduced by a further 550 kg.