172
3 A Brief History of Space Flight
Neptune in 1992, when the existence of the Kuiper Belt was first confirmed by the
observation. Up to now, about 2000 small celestial bodies have been discovered in
the Kuiper Belt, and their surfaces are covered with icy-ice made up of nitrogen,
methane, ammonia and water.
In 1951, the Dutch astronomer, Jan H. Oort (1900−1992), put forward a conjecture
that in the region about 5.0 × 10
4
−1.5 × 10
5 AU away from the Sun, there might be
a huge cloud composed of long-period comets, including 100 billion comets. Later,
the region was officially called Oort cloud by astronomical community. The comets
with the orbital periods of millions of years move around the Sun in very flat elliptical
orbits. As a result, they move very fast near the perihelion and stay for a short time;
they move very slowly near the aphelion (the Oort cloud) and stay for a long time.
The long-period comets spend most of their time near the aphelion, so although there
are many comets in the Oort cloud, few are observed near the Sun. Certainly, for
such distant objects, since they are greatly affected by the gravitation from other
stars, it can not be determine whether they will return to the solar system again. The
Oort cloud is the critical region of the gravitational interaction between the Sun and
other stars, and also the gravitational boundary of the solar system. Although the
Oort cloud has not been observed directly, the conjecture is still accepted by most of
astronomical specialists.
3.6.1.2 Physical Boundary
The physical boundary of the solar system is the interface between the solar wind and
the interstellar medium. In general, the Sun is divided into six layers from the center
out, subsequently solar interior core, radiative zone, convective zone, photosphere,
chromo-sphere and corona. Most of the Sun’s mass is concentrated on the solar
interior core, which occupies the innermost quarter or so of the Sun’s radius, and
more than 99% of the Sun’s energy comes from this zone. The corona is located in the
outermost layer of the Sun, belonging to the outer atmosphere of the Sun. Under the
action of millions of degrees high temperature in the corona, hydrogen and helium
atoms are ionized into high-speed charged particle streams such as positively charged
protons, helium nuclei and negatively charged free electrons, which constantly break
free of the Sun’s gravity and shoot at the speed of 200−800 km/s to the outer space
of the Sun, thus forming so-called solar wind. The interstellar medium refers to
the general term of matter and radiation field between galaxies and stars, which
is composed of the interstellar gas and dust. The interstellar gas mainly includes
atomic, molecular, electronic and ionic states of gaseous hydrogen and helium. The
interstellar dust mainly includes solid particles with a diameter of about 0.1 µm,
such as ice, graphite and silicate, which are dispersed in the interstellar gas, and its
mass accounts for about 10% of the interstellar gas.
In general, the vast, bubble-like region of space “blown” by the solar wind is called
heliosphere, as shown in Fig. 3.5. Outside the heliosphere, there is the tremendous
interstellar medium that the solar wind can no longer shake, where the solar wind
(also known as solar plasma) gives way to the interstellar plasma permeating our
3 A Brief History of Space Flight
Neptune in 1992, when the existence of the Kuiper Belt was first confirmed by the
observation. Up to now, about 2000 small celestial bodies have been discovered in
the Kuiper Belt, and their surfaces are covered with icy-ice made up of nitrogen,
methane, ammonia and water.
In 1951, the Dutch astronomer, Jan H. Oort (1900−1992), put forward a conjecture
that in the region about 5.0 × 10
4
−1.5 × 10
5 AU away from the Sun, there might be
a huge cloud composed of long-period comets, including 100 billion comets. Later,
the region was officially called Oort cloud by astronomical community. The comets
with the orbital periods of millions of years move around the Sun in very flat elliptical
orbits. As a result, they move very fast near the perihelion and stay for a short time;
they move very slowly near the aphelion (the Oort cloud) and stay for a long time.
The long-period comets spend most of their time near the aphelion, so although there
are many comets in the Oort cloud, few are observed near the Sun. Certainly, for
such distant objects, since they are greatly affected by the gravitation from other
stars, it can not be determine whether they will return to the solar system again. The
Oort cloud is the critical region of the gravitational interaction between the Sun and
other stars, and also the gravitational boundary of the solar system. Although the
Oort cloud has not been observed directly, the conjecture is still accepted by most of
astronomical specialists.
3.6.1.2 Physical Boundary
The physical boundary of the solar system is the interface between the solar wind and
the interstellar medium. In general, the Sun is divided into six layers from the center
out, subsequently solar interior core, radiative zone, convective zone, photosphere,
chromo-sphere and corona. Most of the Sun’s mass is concentrated on the solar
interior core, which occupies the innermost quarter or so of the Sun’s radius, and
more than 99% of the Sun’s energy comes from this zone. The corona is located in the
outermost layer of the Sun, belonging to the outer atmosphere of the Sun. Under the
action of millions of degrees high temperature in the corona, hydrogen and helium
atoms are ionized into high-speed charged particle streams such as positively charged
protons, helium nuclei and negatively charged free electrons, which constantly break
free of the Sun’s gravity and shoot at the speed of 200−800 km/s to the outer space
of the Sun, thus forming so-called solar wind. The interstellar medium refers to
the general term of matter and radiation field between galaxies and stars, which
is composed of the interstellar gas and dust. The interstellar gas mainly includes
atomic, molecular, electronic and ionic states of gaseous hydrogen and helium. The
interstellar dust mainly includes solid particles with a diameter of about 0.1 µm,
such as ice, graphite and silicate, which are dispersed in the interstellar gas, and its
mass accounts for about 10% of the interstellar gas.
In general, the vast, bubble-like region of space “blown” by the solar wind is called
heliosphere, as shown in Fig. 3.5. Outside the heliosphere, there is the tremendous
interstellar medium that the solar wind can no longer shake, where the solar wind
(also known as solar plasma) gives way to the interstellar plasma permeating our