Nature of the Universe

Chapter 10

Small Objects in the Solar System

Apart from the Sun and the first eight major planets, there are also many small objects in our solar system. Among them, the largest is Pluto. Others are comets, asteroids and meteoroids.

Pluto


	Courtesy STScI.

Pluto is not classified into either the terrestrial or Jovian planets due to its special features.

We do not understand much about Pluto partly because it is very far away from us and partly because Pluto is the only major planet that has not been visited by an spacecraft. (In contrast, spacecraft have flew by comets and asteroids.)

Pluto is the smallest major planet in our solar system. Its mass is less than 1% of the Earth, and its diameter is about 2300km. Its density is roughly 2.3 times that of water. Usually, the Jovian planets have density less than 2, while terrestrial planets have density more than 5.

The orbit of Pluto is special. It is inclined 17.2° to the ecliptic. Moreover, part of its orbit lies inside Neptune's. As a result, the most distant planet in our solar system was Neptune from 1979 to 1999.

Pluto's surface and its weak atmosphere are made up of nitrogen. From its density, we know that it is rocky.

Pluto has a satellite called Charon. Its diameter is about 1200km, which is relatively very large, over one half of Pluto's. Moreover, the time for Charon to go around Pluto is the same as the rotational period of Pluto. Therefore, Charon is a synchronous satellite. In other words, you will never see Charon on one half of the surface of Pluto, but you will always see it at a fixed position on the sky on the remaining half of Pluto. Last but not least, it is the only natural synchronous satellite in our solar system.

Comets


	Courtesy NASA.

The photo at the right shows comet West, which visited the inner solar system in 1976.

Comets are usually modeled as ``dirty snowballs''. A comet consists of a tiny nucleus with diameter less than 10km. The nucleus is made up of frozen gases and dust. When it approaches the Sun, some gases will be vaporized and an extended coma will then be produced. When it moves even closer to the Sun, the solar wind (charged particles ejected from the Sun) and the Sun's radiation pressure (See Chapter 11.) push the dust and gases of the comets away, this will result in a beautiful long tail. From this, we know why the comet tail is always pointing away from the Sun.

Orbits of a comet may be open or close. A comet with an open orbit will only visit the Sun once. However, a comet with a closed orbit (actually it is elliptical) will visit the Sun again and again. Perhaps, the most famous one is the Comet Halley, it has a closed orbit with a period of 76 years.

Asteroids

Another name for asteroid is minor planet. Most of them are found in the gap between Mars and Jupiter. There are over 10000 asteroids discovered so far, the largest one is about 1000km in diameter, and most of them are not round in shape. The following is the asteroid Gaspra.

Courtesy JPL/NASA.

This one is the asteroid Ida. Note its little moon (satellite), Dactyl, at the right.

Courtesy JPL/NASA.

When we take long exposure photos, asteroid may show up as a long trail. In fact, most of the asteroids are discovered in this way.

Courtesy STScI.

Meteoroids


	Courtesy NASA.

Meteoroids are small particles in space with size similar to grains of sand. When a meteoroid with a speed of tens of kilometers per second hits the Earth's atmosphere, the friction with air will heat up the meteoroid. The high temperature will cause the surrounding atmosphere to glow, creating a bright streak of light called a meteor. Meteors are usually called ``shooting stars.'' They usually burnt up eventually. But for some massive ones, the remaining parts will hit the Earth surface, they are known as meteorites. If a meteorite is large enough, it could produce a crater on the Earth surface.

Courtesy NASA.

Courtesy NASA and D. Roddy of the US Geological Survey.

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