Asteroids

The asteroid, Eros, photographed by NEAR in February, 2000 appears below in all of its gray glory. The Near Earth Asteroid Rendevous spacecraft went out to this asteroid and enjoyed a remarkably successful mission. This page introduces you to these interesting objects, of which over 7000 have been discovered, and which pose a significant threat to life on planet Earth, as exemplified in such movies as "Deep Impact" and "Armageddon."

On the first day of January 1801, Giuseppe Piazzi discovered an object which he first thought was a new comet. But after its orbit was better determined it was clear that it was not a comet but more like a small planet. Piazzi named it Ceres, after the Sicilian goddess of grain. Three other small bodies were discovered in the next few years (Pallas, Vesta, and Juno). By the end of the 19th century there were several hundred. As of 2002, there are over 7000 named asteroids, and one will be given the name "Albrecht" (after my Uncle Bill) by the end of the year 2002. Asteroids are given names and numbers, as you will see in the reading below. The number corresponds to the order in which it was discovered, and the name appears after. The two asteroids that we know the most about are 243 Ida and 951 Gaspra, which means they were the 243rd and 951st asteroids discovered.

Several hundred thousand asteroids have been discovered and given provisional designations so far. Hundreds more are discovered each year. There are undoubtedly hundreds of thousands more that are too small to be seen from the Earth. There are 26 known asteroids larger than 200 km in diameter. Our census of the largest ones is now fairly complete: we probably know 99% of the asteroids larger than 100 km in diameter. Of those in the 10 to 100 km range we have cataloged about half. But we know very few of the smaller ones; there are probably considerably more than a million asteroids in the 1 km range.

So, it is time to dispell a few of the prevailing myths. Here is an Imperial Star Destroyer moving through an asteroid belt in one of the Star Wars movies. If you have seen "The Empire Strikes Back," you will recall Han Solo deftly weaving the Millenium Falcon through a dense field of asteroids. The result of that movie is that most people believe the Solar System's Asteroid Belt to be this cluttered region. In actuality, there may be a lot of asteroids out there, but they are VERY widely scattered. NASA engineers do not pay much attention to the asteroids when they launch satellites to the outer planets because they are really so far apart from each other. In fact, the total mass of all the asteroids is less than that of the Moon. So, imagine the Moon being shattered and then scattered over an orbit whose circumference is over 2,100,000,000 km! There is no need for such weaving and darting around asteroids as Han Solo did.

243 Ida (above left) and 951 Gaspra (above right) were photographed by the Galileo spacecraft on its way to Jupiter. The NEAR mission flew by 253 Mathilde (below left) on 1997 June 27 returning many images. NEAR (now renamed "NEAR-Shoemaker") entered orbit around 433 Eros (below right) in January 1999 and has so far returned a wealth of images and data. They are the only asteroids which have been studied closely so far.

The largest asteroid by far is 1 Ceres. It is 933 km in diameter and contains about 25% of the mass of all the asteroids combined. The next largest are 2 Pallas, 4 Vesta and 10 Hygiea which are between 400 and 525 km in diameter. All other known asteroids are less than 340 km across.

Astronomers were delighted when the Galileo spacecraft flew by 243 Ida en route to Jupiter and discovered that Ida was not alone out there. In the image to your left, you will see a small 1 km moon, named Dactyl, that is orbiting Ida.

 

 

There is some debate as to the classification of asteroids, comets and moons. There are many planetary satellites that are probably better thought of as captured asteroids. Mars's tiny moons Deimos and Phobos, Jupiter's outer eight moons, Saturn's outermost moon, Phoebe, and perhaps some of the newly discovered moons of Saturn, Uranus and Neptune are all more similar to asteroids than to the larger moons. (The composite image to the left shows Gaspra and Ida bracketing Deimos and Phobos approximately to scale.)

 

 

On March 27, 2014, astronomers in Brazil announced the discovery of a pair of rings around the asteroid Chariklo. Below is an artist's rendition of the asteroid with its rings. Click on the image for a link to more information from Space.com as well as a nice little video about the discovery.

asteroid chariklo with rings

Asteroids are classified into a number of types according to their spectra (and hence their chemical composition) and albedo:

C-type, includes more than 75% of known asteroids: extremely dark (albedo 0.03); similar to carbonaceous chondrite meteorites; approximately the same chemical composition as the Sun minus hydrogen, helium and other volatiles;
S-type, 17%: relatively bright (albedo .10-.22); metallic nickel-iron mixed with iron- and magnesium-silicates;
M-type, most of the rest: bright (albedo .10-.18); pure nickel-iron.
There are also a dozen or so other rare types.
Because of biases involved in the observations (e.g. the dark C-types are harder to see), the percentages above may not be representative of the true distribution of asteroids. (There are actually several classification schemes in use today.)
There is little data about the densities of asteroids. But by sensing the Doppler effect on radio waves returning to Earth from NEAR owing to the (very slight) gravitational tug between asteroid and spacecraft, Mathilde's mass could be estimated. Surprisingly, its density turns out to be not much greater than that of water, suggesting that it is not a solid object but rather a compacted pile of debris.

Asteroids are also categorized by their position in the solar system:

Main Belt: located between Mars and Jupiter roughly 2 - 4 AU from the Sun; further divided into subgroups: Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles and Hildas (which are named after the main asteroid in the group).


Near-Earth Asteroids (NEAs): ones that closely approach the Earth
Atens: semimajor axes less than 1.0 AU and aphelion distances greater than 0.983 AU;
Apollos: semimajor axes greater than 1.0 AU and perihelion distances less than 1.017 AU
Amors: perihelion distances between 1.017 and 1.3 AU;


Trojans: located near Jupiter's Lagrange points (60 degrees ahead and behind Jupiter in its orbit). Several hundred such asteroids are now known; it is estimated that there may be a thousand or more altogether. Curiously, there are many more in the leading Lagrange point (L4) than in the trailing one (L5). (There may also be a few small asteroids in the Lagrange points of Venus and Earth (see Earth's Second Moon) that are also sometimes known as Trojans; 5261 Eureka is a "Mars Trojan".)


Between the main concentrations of asteroids in the Main Belt are relatively empty regions known as the Kirkwood gaps. These are regions where an object's orbital period would be a simple fraction of that of Jupiter. An object in such an orbit is very likely to be accelerated by Jupiter into a different orbit.


There also a few "asteroids" (designated as "Centaurs") in the outer solar system: 2060 Chiron (aka 95 P/Chiron) orbits between Saturn and Uranus; the orbit of 5335 Damocles ranges from near Mars to beyond Uranus; 5145 Pholus orbits from Saturn to past Neptune. There are probably many more, but such planet-crossing orbits are unstable and they are likely to be perturbed in the future. The composition of these objects is probably more like that of comets or the Kuiper Belt objects than that of ordinary asteroids. In particular, Chiron is now classified as a comet.

4 Vesta has been studied recently with HST (left). It is a particularly interesting asteroid in that it seems to have been differentiated into layers like the terrestrial planets. This implies some internal heat source in addition to the heat released by long-lived radio-isotopes which alone would be insufficient to melt such a small object. There is also a gigantic impact basin so deep that it exposes the mantle beneath Vesta's outer crust.

Though they are never visible with the unaided eye, many asteroids are visible with binoculars or small telescopes.

Asteroid table


A few asteroids and comets are listed below for comparison. (distance is the mean distance to the Sun in thousands of kilometers; masses in kilograms).

No.

Name

Distance (km )

Radius (km)

Mass (kg)

Discoverer

Date

2062

Aten

144,514,000

0.5

?

Helin

1976

3554

Amun

145,710,000

?

?

Shoemaker

1986

1566

Icarus

161,269,000

0.7

?

Baade

1949

1862

Apollo

220,061,000

0.7

?

Reinmuth

1932

2212

Hephaistos

323,884,000

4.4

?

Chernykh

1978

951

Gaspra

330,000,000

8

?

Neujmin

1916

4

Vesta

353,400,000

265

3.0x10^20

Olbers

1807

3

Juno

399,400,000

123

?

Harding

1804

15

Eunomia

395,500,000

136

8.3x10^18

De Gasparis

1851

1

Ceres

413,900,000

466

8.7x10^20

Piazzi

1801

2

Pallas

414,500,000

261

3.18x10^20

Olbers

1802

243

Ida

428,000,000

35

?

?

1880

52

Europa

463,300,000

156

?

Goldschmidt

1858

10

Hygiea

470,300,000

215

9.3X10^19

De Gasparis

1849

511

Davida

475,400,000

168

?

Dugan

1903

911

Agamemnon

778,100,000

88

?

Reinmuth

1919

2060

Chiron

2,051,900,000

85

?

Kowal

1977

The spacecraft DAWN, is a specially-designed propulsion system craft that will explore the asteroids Ceres and Vesta.

NASA also has plans to send a spacecraft that will capture an asteroid and brind it back toward Earth. Mission paramaters can be found here. Plans to place it near the moon for further study were being narrowed down as recently as March, 2014.

Asteroid Capture Mission

 

Further Asteroid Study:

Meteors ... the first site to move to from this Asteroid Page.

Impacts ... the second site to move to from this Asteroid Page. There is some really interesting information embedded within this section.

Mining ... the final site to move to from this Asteroid Page.

Once you have looked over these asteroid pages, do you think you now know the answers to these questions?

1) Give a brief description of the physical appearance of Pluto.

2) Who discovered Pluto and when was it discovered?

3) What do Pluto and its Moon Charon share some of in common?

4) What two places in the solar system do scientists believe comets may come from?

5) What causes the comet to have a tail when it comes near the Sun?

6) How is it possible that the tail of the comet goes first when it is leaving the Sun?

7) While many astronomers believe the oceans of Earth may have been made by comet impacts, what else may comets have brought to Earth?

8) What are the three kinds of asteroids in terms of physical composition?

9) Name the 3 places where asteroids can be found, and describe why NEAs are of such great interest?

10) What are meteors and how can astronomers predict something called a "meteor shower?"

If you think you can correctly answer these questions, go to the Pluto, Comet, and Asteroid Quiz and send your responses to me.

You can go back to Comets, or the Planet Introduction, BUT before you go on to the Sun, there is a short assessment for you to complete that will demonstrate that you have learned something or two during this fabulous unit on the planets. It is called Planet Assessment, a sort of catchy name if I might say so myself. Then, you can move on to the Sun, or regettably back to the Syllabus.

 


| Home | Course Information | Assignments | Teacher Bio | Course Units | Syllabus | Links