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Figure 8.1. The nearest stars. (Note: for historical reasons, between one magnitude and the next the light ratio is 2.512. The more negative the magnitude, the larger the apparent star diameter).

Figure 8.1. The nearest stars. (Note: for historical reasons, between one magnitude and the next the light ratio is 2.512. The more negative the magnitude, the larger the apparent star diameter).

In this picture, the basic unit of distance is no longer the size of our Solar System, or the AU, but rather 1 light-year. For comparison, our Sun's closest star (Proxima Centauri) is 4.2 light-years away, or 4,000 times the diameter of our solar system measured at Pluto's orbit. If we had means to reach Pluto in a few months, reaching Proxima Centauri at the same speed would take of the order of a millennium.

Lying behind these considerations is the question of why cross these immense distances, and which star to visit. Proxima Centauri is a star of spectral type M5e, very different from our Sun (its type is G2 V). The symbols identifying star type were invented to classify the star's electromagnetic spectrum, which may give an idea of what sort of light one would see on a hypothetical planet orbiting a star. For instance, the Sun "surface", or disc we see, emits light as if it was a black body radiating at the temperature 5,800 K, the yellow-green peak of its spectrum imparting that warm quality humans associate to its light. An M-type star such as Proxima Centauri would have a cooler surface temperature, about 3,600 K, its hue shifted towards the red-yellow, and having probably a large, and fascinatingly unknown, effect on life forms [Kiang, 2008]. In this context, another fundamental question is whether life as we see it on Earth is the only possible type of life. So, what are life's ultimate boundaries? [Baross et al., 2007]. This question may be extended to the search of life in the most general sense (e.g., "growing and adapting"), according to Loeb's classical definition, and much hay has been made in this area by science fiction writers.

However, without planets to orbit around or to land on, it is hard to conceive the motivation of such immensely long and expensive trips. Human beings have always been driven to explore faraway places by the hope of finding new life-forms and scenery, not just light. The star to reach and the distance to cross will in the end be chosen on the basis of hints or information about the existence of planets, rather than solely by scientific curiosity about stars [Lissauer, 1999]. In fact, the number of planets found orbiting stars is steadily increasing, although the vast majority belong to the hot gaseous giants similar to Jupiter or Saturn [Schneider, 2005; Encrenaz et al., 2004]. This means that distances at which planets have actually been observed, or are suspected to be, may be even greater than those in Figure 8.1, perhaps tens or even hundreds of light-years. The thought of finding not just life, but also intelligent life might be a powerful motivation if people were actually convinced of the likelihood of its existence. However, this seems not to be the case, or at least is considered a remote possibility; see [Crawford, 2000]. These thoughts should give pause to the discussion of propulsion systems for stellar missions.

Scientifically speaking, however, there are objects and regions of space that are much farther than our known planetary system, but closer than stars, and at the same time of great interest to science. Perhaps with some exaggeration, these destinations could be dubbed quasi-interstellar (QI) destinations. Among them some of the most interesting are (in order of their known distance from Earth) the Kuiper Belt, the heliopause, the gravitational Sun lens region, and the Oort Cloud. Interstellar precursor missions to these regions are very attractive; the reasons are given briefly below.

8.1.1 Quasi-interstellar destinations

Loosely speaking, the Kuiper Belt is the region of space beyond the orbit of Neptune or Pluto conventionally extending up to 100 AU from our Sun. Until the 1950s astronomers thought Pluto was more or less the last "planet": with the exception of comets, perhaps only one or two other objects might be lying beyond its orbit. In 1951 the Dutch astronomer Gerard Kuiper started wondering about the place of birth of short-period comets, since each of their passes near the Sun subtracts 0.01% of their mass; their lifetime should be also very short, some 10,000 passes, or only half a million years [Luu and Jewitt, 1996]. Since the Solar System is more than 4.5 billion years old, no comet should have survived ever since.

After discovering "planetoids", bodies orbiting the Sun, even larger than Pluto's moon Charon and with extremely long orbital period, we know now that the space beyond Neptune and Pluto is populated. The density of objects there is much too low to form larger bodies by mutual gravitational attraction; however, the large planets (Jupiter, Saturn, and especially Neptune) can draw and pull these objects toward the Sun along highly elliptic orbits. If, as it seems, this is a realistic picture, the Kuiper Belt is a reasonably close region of space where we could find objects (KBO, or TNO, Trans Neptunian Objects, for short) dating back to the formation of the Solar System, including most short-period comets [Hahn, 2005].

In fact, during its Saturn flyby, the Cassini spacecraft took pictures of one of the Saturn satellites, Phoebe, from 13,800 km. Phoebe has a retrograde orbit and an average diameter of 220 km. The pictures Cassini took were fairly good, and indicated the presence of water [Porco, 2004]. The inference is that Phoebe did not come from the rocky, "dry" asteroid belt between Mars and Jupiter, but rather from

Table 8.1. Comparing orbits of Pluto and of KBO.

Diameter

Distance from the Sun

Orbital period (years)

Pluto

Sedna

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