3.6 Flying Out of the Solar System
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3.6.1 Boundaries of the Solar System
To judge whether a man-made vehicle flies out the solar system or enters the interstellar space, it involves the determination of the boundary of the solar system. Where
is the boundary of the solar system, and what criteria should be adopted to determine the boundary of the solar system? In the past twenty years, there has been a
popular debate about whether Voyager-1 enters the interstellar space or flies out of
the solar system. Currently, in the field of astronomy and astrophysics, there are two
main criteria to determine the boundary of the solar system: one is the gravitational
boundary, and another is the physical boundary.
3.6.1.1 Gravitational Boundary
The gravitational boundary refers to a spherical boundary based on the gravitational
action range of the Sun; within the spherical boundary, the gravitational attraction of
the Sun rather than that of the rest of the Galactic System controls the orbit motions
of celestial bodies, such as planets and comets. In general, the gravitational action
range of a celestial body is relative to that of another celestial source with stronger
gravitation field. The Sun is rotating around the center of the Milky Way system, and
the center is the stronger gravitational source compared to the Sun. From numerical
analysis results, it is shown that the gravitational range of the Sun relative to the Milky
Way is approximately 6.0 × 10
4 AU, which is equal to one fourth of the distance
(about 4.22 light years) between the Sun and the nearest star, Alpha Centauri.
According to the view of dynamics, under the action of solar gravitation, the
celestial bodies in the solar system move around the Sun. As a result, the orbits
of the outermost planets in the solar system had been ever used as the boundaries.
With the discovery of Uranus, Neptune and Pluto, the solar system boundary was
expanded respectively from 19.2 AU to 30.1 AU and 39.5 AU. Of course, the Pluto
has been downgraded to a mere dwarf planet at the IAU General Assembly in 2006.
In 1943, an Irish astronomer, Kenneth E. Edgeworth (1880−1972), put forward the
hypothesis that the material distribution of the nebula beyond the Neptune was so
sparse that a large amount of the celestial bodies with smaller mass could be brought
up. Later, a Dutch-American astronomer, Gerard P. Kuiper (1905−1973), further
developed Edgeworth’s hypothesis, and believed that there was a zone of ice-filled
objects running on the edge of the solar system beyond the Neptune’s orbit, where the
remnants of the original solar nebula gathered, and also the birthplace of asteroids and
short-period comets. This is the famous Edgeworth−Kuiper Belt, simply referred
to as Kuiper Belt, which is a flat ring of icy small bodies that revolve around the
Sun beyond the orbit of the Neptune, with 30−50 AU away from the Sun. Kuiper
considered that comets should also be included within the solar system’s boundary
to be determined. In this way, the boundary of the solar system was extended by 10
AU. In the search for the solar system boundaries, the Pluto had been the only known
object in the Kuiper Belt until another object, 1992QB1, was discovered outside the
171
3.6.1 Boundaries of the Solar System
To judge whether a man-made vehicle flies out the solar system or enters the interstellar space, it involves the determination of the boundary of the solar system. Where
is the boundary of the solar system, and what criteria should be adopted to determine the boundary of the solar system? In the past twenty years, there has been a
popular debate about whether Voyager-1 enters the interstellar space or flies out of
the solar system. Currently, in the field of astronomy and astrophysics, there are two
main criteria to determine the boundary of the solar system: one is the gravitational
boundary, and another is the physical boundary.
3.6.1.1 Gravitational Boundary
The gravitational boundary refers to a spherical boundary based on the gravitational
action range of the Sun; within the spherical boundary, the gravitational attraction of
the Sun rather than that of the rest of the Galactic System controls the orbit motions
of celestial bodies, such as planets and comets. In general, the gravitational action
range of a celestial body is relative to that of another celestial source with stronger
gravitation field. The Sun is rotating around the center of the Milky Way system, and
the center is the stronger gravitational source compared to the Sun. From numerical
analysis results, it is shown that the gravitational range of the Sun relative to the Milky
Way is approximately 6.0 × 10
4 AU, which is equal to one fourth of the distance
(about 4.22 light years) between the Sun and the nearest star, Alpha Centauri.
According to the view of dynamics, under the action of solar gravitation, the
celestial bodies in the solar system move around the Sun. As a result, the orbits
of the outermost planets in the solar system had been ever used as the boundaries.
With the discovery of Uranus, Neptune and Pluto, the solar system boundary was
expanded respectively from 19.2 AU to 30.1 AU and 39.5 AU. Of course, the Pluto
has been downgraded to a mere dwarf planet at the IAU General Assembly in 2006.
In 1943, an Irish astronomer, Kenneth E. Edgeworth (1880−1972), put forward the
hypothesis that the material distribution of the nebula beyond the Neptune was so
sparse that a large amount of the celestial bodies with smaller mass could be brought
up. Later, a Dutch-American astronomer, Gerard P. Kuiper (1905−1973), further
developed Edgeworth’s hypothesis, and believed that there was a zone of ice-filled
objects running on the edge of the solar system beyond the Neptune’s orbit, where the
remnants of the original solar nebula gathered, and also the birthplace of asteroids and
short-period comets. This is the famous Edgeworth−Kuiper Belt, simply referred
to as Kuiper Belt, which is a flat ring of icy small bodies that revolve around the
Sun beyond the orbit of the Neptune, with 30−50 AU away from the Sun. Kuiper
considered that comets should also be included within the solar system’s boundary
to be determined. In this way, the boundary of the solar system was extended by 10
AU. In the search for the solar system boundaries, the Pluto had been the only known
object in the Kuiper Belt until another object, 1992QB1, was discovered outside the