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the Kuiper Belt, the birthplace of most short-period and water-rich comets. This fact, and the peculiar retrograde orbit, tell that Phoebe is likely a KBO captured by Saturn. Similarly rich in water is the KBO Quaoar [Jewitt and Luu, 2004].

Some of the planetoids already observed have fascinating features. Table 8.1 compares two, Sedna and the recently discovered DW2004, to Pluto: Sedna shuttles back and forth from way beyond the Kuiper Belt (in fact, near the edge of the Oort Cloud) to the Sun. Its extremely eccentric orbit may be explained by an encounter with a star [Kenyon and Bromley, 2004]. A very reasonable conjecture is that Sedna must carry traces of its immense journey on its surface. Sedna would be a very desirable mission target indeed: some comets may travel even farther, but are not as large, which poses the question of how Sedna and other planetoids came to be. An even more interesting body discovered in January 2005 is 2003 UB313, a KBO bigger than Pluto [The Planetary Report, 2005]. Its orbit passes inside that of Pluto and is tilted 45° with respect to the ecliptic plane.

In the tentative budget of the ''New Frontiers'' NASA program [NASA, 2008] there is in fact included a ''New Horizons'' 2 mission (NH 2) [Spencer et al., 2003] to explore some near KBO. One of the candidate objects is called 1999 TC36: it consists of twin bodies, some 400 to 500 km across. TC 36 is similar, albeit smaller, to the Pluto-Charon system. As planned, right now this NH 2 mission will utilize gravity assists from Jupiter and Uranus, reaching TC 36 in 2014, and is considered a ''very fast'' mission. Meanwhile, the first New Frontiers mission to Pluto, launched in late 2005, has just crossed the Saturn orbit, and at a leisurely 18.2 km/s will cross that of Uranus in March 2011, reaching Pluto in 2015, ten years after launch [Space News, 2008]. Such is the pace of missions powered by chemical propulsion ... but such a mission may not satisfy the appetite for discovery recently sharpened by analysis of Sedna and other similarly ''strange'' KBOs. Their odd orbits might be explained by the existence of a planet bigger than Pluto and much farther away. This ''plutoid'' has been postulated by astronomers P. Likawka and T. Mukai, at the University of Kobe, Japan [Than, 2008]. KBOs and their features are becoming a source of novel ideas, as they are beginning to disrupt the conventional understanding of how our Solar System came to be, beside being a new and exciting research area.

The heliopause is a region vastly more distant from the Sun than the Kuiper Belt. The solar wind is an isotropic flow of plasma (mostly protons) moving at 300 km/s to 700 km/s (i.e., at supersonic speed with respect to plasma acoustic speed). In interstellar plasma, crossed by the Sun and all its planets, this supersonic flow creates a shock that has been detected by its radio emission [Gurnett et al., 1993]. This immense shock separates the Solar System from interstellar space and bounds a bubble-like region called the heliosphere, its characteristic size of the order of 100 AU to 150 AU. In fact, the size of the heliosphere depends on the Sun cycle, the space magnetic field, and the presence of neutral particles [Encrenaz et al., 2004, Section 5.1.5]. The entire Solar System is inside the heliosphere, and has no contact with true interstellar space: from Earth, as well as from all other planets, we are looking at "space" like fish from inside a glass bowl. There is keen interest among scientists in investigating the properties of true space (i.e., far from the influence of our Sun).

As the density of solar wind plasma decreases with the cube of distance from the Sun, so does the strength of the shock separating the heliosphere from the true space environment. Thus, the solar wind eventually becomes subsonic, slowing down abruptly. The region where this occurs is called the heliosheath, of great scientific interest as well, because this is where the solar wind starts interacting with interstellar plasma and gets hotter. Another sign of this interaction is the increasing magnetic field recorded by the Voyager 1 probe, which reached the heliosheath a few years ago [Britt, 2005].

Still farther away from the Sun, the heliosheath ends at the so-called heliopause, beyond which is "uncontaminated" interstellar space. The heliopause is a peculiar environment, characterized by a hydrogen plasma (protons and electrons) with a density of the order of 1 per cubic centimeter immersed in a weak magnetic field. At the time of writing, the Voyager 2 probe, launched by the US on August 20, 1977, has just crossed into the heliosheath [Jokipii, 2008]; it will travel for another ten years before reaching the heliopause. Thus, it has taken more than 30 years for a man-made object to experience interstellar space. There is indeed no way to simulate in a laboratory the conditions near the heliopause or in true space, hence the interest of astrophysicists in reaching it.

A third deep-space mission of interest is associated with relativistic effects of massive bodies on starlight propagation, and goes under the name of gravitational "lensing" [Wambsganss, 2001]. It is known from the General Theory of Relativity that a gravitational field bends light, ever so slightly. Our Sun does that with the light of every star grazing its apparent disc. In fact, rays of parallel light from such stars are bent by an angle e given by

rc 2

where e is the deflection of the electromagnetic wave, G is the universal gravitational constant, M is the mass of the Sun, c is the speed of light, and r is the distance of the [parallel] rays from the Sun center.

The nearer the light rays to the solar disc, the sharper the bending angle e (see Figure 8.2). The Sun acts as a lens not just for visible starlight, but for all electromagnetic waves.

Viewed from Earth, the rays focus at a point that depends on the distance r and is "to our back'' when looking straight at the Sun. The minimum r is of course the h

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