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Chapter 2. The Solar System Part 3. Asteroids (Minor Planets). Objects in the solar system smaller than a planet that are made of rock.

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Chapter 2

Chapter 2

The Solar System

Part 3

Asteroids minor planets
Asteroids (Minor Planets)

  • Objects in the solar system smaller than a planet that are made of rock.

  • The first asteroid in space, named Ceres, was discovered in 1801. Since then over 250,000 asteroids have been discovered and that number continues to rapidly increase every year. This despite that probably many asteroids are too small to be seen from Earth.


  • Ceres is still by far the largest known asteroid with a diameter of 623 miles. Only 26 have been discovered larger than 120 miles. Most are smaller than a mile across.

  • Asteroids are referred to by a number and its name. The number represents the order in which it was discovered. For example, Ceres is referred by 1 Ceres since it was the first one discovered.


  • The space probe Galileo provided the first close images of an asteroid when it passed near and photographed 951 Gaspra and 243 Ida in the Main Asteroid Belt.

  • Galileo’s pictures revealed that Ida has a natural satellite, Dactyl. Ida is about 35 miles long and 15 miles in diameter. Its tiny moon is about a mile in diameter and orbits about 60 miles above Ida. Since then several other asteroids have been found to have companions, leading astronomers to believe that it may not be uncommon.


  • Most asteroids reside in the Main Asteroid Belt between Mars and Jupiter. It is hypothesized that they are bits of primordial solar system material that was unable to form a planet due to Jupiter’s large gravitational influence.

Near earth asteroids
Near Earth Asteroids

  • Gravitational perturbations mostly from Jupiter cause asteroids to occasionally leave the Main Asteroid Belt and wander the solar system. Those that come near Earth are called Near Earth Asteroids.

  • Asteroid collisions with Earth are known to be common in its history. Until the 1990s though, it was not recognized that they could significantly change conditions on Earth. This was brought about by the theory that an asteroid impact 65 million years ago was responsible for the extinction of some 85% of all species, including all of the dinosaurs.

Near earth asteroids1
Near Earth Asteroids

  • There are currently 3168 discovered near earth asteroids. They are classified into 3 categories:

    • Apollo – Asteroids that cross the orbits of both Earth and Mars.

    • Amors – Asteroids that cross the orbit of Mars but not Earth.

    • Atens – Asteroids that cross the orbit of Earth but not Mars.

    • The last close encounter happened last year when the peanut-shaped 3-mile long asteroid named 4179 Toutatis passed within a million miles of Earth.

Near near earth asteroid rendezvous shoemaker
NEAR (Near Earth Asteroid Rendezvous)-Shoemaker

  • Mission launched in February 1996 to study near earth asteroids 253 Mathilde and 433 Eros.

  • First attempt to insert NEAR-Shoemaker into Eros’ orbit failed in 1998. The second attempt in 2000 was successful though.

  • NEAR-Shoemaker conducted the first long-term close-up study of an asteroid: mass, structure, geology, composition, and gravity.

Near shoemaker

  • Near-Shoemaker discovered that Eros has an ancient surface covered with craters, grooves, layers, house-sized boulders, and other complex features. It also seems to be a fragment of a larger body.

  • In February 2001, the NEAR spacecraft landed on asteroid Eros, after transmitting 69 close-up images of the surface during its final descent.

Asteroid collision prevention
Asteroid Collision Prevention

  • Efforts to find near earth asteroids stepped up in 1998 with highly efficient automated systems that consist of cameras and computers directly connected to telescopes. Since then a large majority of the asteroids have been discovered by such automated systems .

  • Several theories exist as to how to prevent a collision with an asteroid. The most popular is the one to deflect the asteroid such that it misses Earth. How to exactly accomplish this is not exactly known.


  • Unlike the other small bodies in the solar system, comets have been known since antiquity. There are Chinese records of Comet Halley going back to at least 240 BC.

  • Comets are sometimes called dirty snowballs. They are a mixture of ices (both water and frozen gases) and dust that for some reason didn't get incorporated into planets when the solar system was formed. This makes them very interesting as samples of the early history of the solar system.


  • Comet Machholz

  • Comet Hale-Bopp

  • Comet C 2003 K Linear

  • Comet Neat

  • Comet Bradfield

  • Comet Halley

  • Comet Wild 2

  • Comet Shoemaker-Levi 9


  • Comets have several distinct parts:

    • nucleus: relatively solid and stable, mostly ice and gas with a small amount of dust and other solids.

    • coma: dense cloud of water, carbon dioxide and other neutral gases sublimed from the nucleus;

    • hydrogen cloud: large but very sparse envelope of hydrogen surrounding the comet and sometimes trailing it;


  • dust tail: very long white tail composed of smoke-sized dust particles driven off the nucleus by escaping gases; this is the most prominent part of a comet to the unaided eye;

  • ion tail: very long bluish tail composed of ionized gas caused by interactions with the solar wind. It always points away from the Sun.


  • Since both the coma and the tails are composed of material driven off the nucleus by solar heating, comets are invisible except when they are near the Sun.

  • Most comets have highly eccentric orbits which take them far beyond the orbit of Pluto; these are seen once and then disappear for millennia. Only the short- and intermediate-period comets (like Comet Halley), stay within the orbit of Pluto for a significant fraction of their orbits.


  • Short-period comets are believed to originate in the Kuiper Belt. Other comets are theorized to come from the Oort cloud or perhaps from outside our solar system.

  • After 500 or so passes near the Sun off most of a comet's ice and gas is lost leaving a rocky object very much like an asteroid in appearance. (Many of the near-Earth asteroids may be "dead" comets.) A comet whose orbit takes it near the Sun is also likely to either impact one of the planets or the Sun or to be ejected out of the solar system by a close encounter (esp. with Jupiter).

Meteor showers
Meteor Showers

  • A meteor shower sometimes occurs when the Earth passes through the orbit of a comet. Some occur with great regularity: the Perseid meteor shower occurs every year between August 9 and 13 when the Earth passes thru the orbit of Comet Swift-Tuttle. Comet Halley is the source of the Orionid shower in October.

Comet machholz
Comet Machholz

  • Good views of Comet Machholz are in store for northern hemisphere comet watchers in January. Now making its closest approach to planet Earth, the comet passed near the Pleiades star cluster on January 7th and the double star cluster in Perseus on January 27th as Machholz moves relatively quickly through the evening sky. Currently just visible to the unaided eye from dark locations, the comet should be an easy target in binoculars or a small telescope.