Manned Exploration of the Solar System

Looking to the future of manned exploration of the solar system beyond Mars becomes a bit of a crystal ball-gazing exercise. For national space agencies, the far future in terms of space program planning means the years 2030 to 2040. Consequently, these plans include a manned mission to Mars, but frankly nothing much beyond that. So to talk about future missions involves guesswork, most of which will miss the mark. However, there are some issues concerning manned spaceflight that are common to the missions we have discussed so far. For such missions to be adequately justified, and ultimately to succeed, there are four key components:

• A good supporting case. These questions need to be addressed: What's it for? Why go? What are the benefits of doing it? Most space exploration is justified on the basis of scientific goals, but other political factors, such as national prestige, work force utilization, and economic benefit through spin-off need to be recognized as valid driving influences.

• An effective team with a common vision of the objectives and how the program can achieve them. With the kind of huge-scale space engineering projects that we have been discussing, such a team will most likely consist of a large number of different nations to share technical responsibilities and program costs.

• The means to go—in other words the technology required to achieve the objective, such as an appropriate launcher capability and manned spaceflight infrastructure.

• An appropriate source of funding. Such missions generally require a huge source of funds sufficient to finance the program, and consequently complete financial planning for the program is required to ensure success. All too often in the past, space projects have suffered from a "stop-go'' mentality in terms of funding, governed by political short-termism.

Given the projected cost of a manned landing on Mars, for example, it seems inevitable that deep space missions in the longer-term will continue to be government-sponsored (as opposed to privately funded), and mostly motivated by scientific objectives. Also, we have seen that a good proportion of this cost is driven by the problem of access to Earth orbit—the cost of launch. Currently, this is estimated to be somewhere between about $2000 and $5000 per kilogram launched into low Earth orbit. The other major technical challenge is the development of new space propulsion systems for

Table 10.1: A wish-list of proposed manned space exploration in the 21st century




Return to the Moon


Manned landing on a near-Earth object*


Permanent lunar base


Manned landing on Mars


Introduction of a single-stage-to-orbit man-rated launch vehicle


Manned landings on the moons of Jupiter (Europa)


Permanent Martian base


Manned landings on the moons of Saturn (Enceladus)

* A near-Earth object is a small body (such as an asteroid or a comet) which has an orbit that comes close to, or crosses the orbit of Earth. Such objects pose an impact threat to Earth (see Chapter 11).

* A near-Earth object is a small body (such as an asteroid or a comet) which has an orbit that comes close to, or crosses the orbit of Earth. Such objects pose an impact threat to Earth (see Chapter 11).

use when the manned spacecraft are beyond Earth orbit. We will discuss these aspects in the next section.

However, let's return briefly to our crystal ball-gazing activity, and ask what kind of missions might be achieved in the 21st century. Table 10.1 lists the anticipated missions over this time period, although it is probably better to call it a wish list, given the shortcomings of the process of so-called prediction in the space business.

Looking at manned missions beyond Mars, exploration of the icy moons of Jupiter would seem to be the next most obvious step. The four major moons of Jupiter—Io, Europa, Ganymede, and Callisto—were discovered when Galileo turned the first telescope in Jupiter's direction about 400 years ago. With Jupiter being five times more distant from the Sun than Earth, the level of solar illumination and heating is around 25 times less (the inverse square law again!) than at the Earth. Generally Jupiter's moons are rather cold, inhospitable places, with the surface temperature of Europa, for example, being around -160°C. However, as unlikely as it might seem, Europa has been identified as a place in the solar system where life may have evolved, and scientists are enthusiastic about the idea of sending robotic and ultimately manned missions there.

The story of potential life on Europa is an intriguing one, which began with the entry into Jupiter orbit of the unmanned Galileo spacecraft in September 1995. Soon afterward, the spacecraft returned images of the icy surface of Europa, such as that shown in Figure 10.8. At first glance, the image looks rather uninteresting. But a more careful examination shows that the icy surface of Europa has fragmented at some time in the past into

Figure 10.8: An image of the icy surface of Europa, taken by the Galileo spacecraft. (Image courtesy of NASA/Jet Propulsion Laboratory [JPL]—Caltech.)

icebergs floating on a liquid ocean, which appear to have drifted off-shore before the ocean refroze. The implication of this is the belief that beneath the icy crust of the moon there is an ocean of liquid water! It is thought that the water remains liquid because it is heated from below by hot volcanic vents on the seabed. As Europa orbits Jupiter, it is subject to tidal forces that squash and stretch the moon, and it is thought that this drives the volcanic activity. How does this lead us to believe there is life there? Well, similar volcanic vents have been found deep in Earth's ocean trenches. These vents are so deep, in fact, that there is no solar energy to sustain life, but marine biologists have found life there, nurtured and maintained by the heat and energy from the volcanism. Scientists believe that a similar process may be happening in Europa's ocean, and the exciting thing is that the life there may have evolved beyond the purely microbial. Arthur C. Clarke picked up on this fascinating idea some years ago in his Odyssey series of novels in which he crafts a fine drama around this speculation about extraterrestrial life.

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