Module 14: Beyond the Terrestrial Planets Activity 1: The Asteroid Belt

Summary : In this Activity, we will investigate (a) the asteroid belt - vital statistics, (b) asteroids as failed planets, (c) asteroid orbits, (d) asteroids outside the Asteroid Belt, and (e) properties of asteroids.

After the discovery of Uranus in 1781, the search began for a planet between the orbits of Jupiter and Mars as predicted by Bode’s law*. (a) The Asteroid Belt - vital statistics * For more information on Bode’s law, click hereclick here On new year’s day in 1801, Italian astronomer Guiseppe Piazzi discovered this missing planet at about 2.8 AU and named it Ceres. However it soon became apparent that this object was too small be be a fully fledged “planet” – its diameter is only about 1000 km.

Ceres has diameter of about 30% of that of the Moon. Earth Ceres These figures show Ceres to the same scale as the Moon and the Earth. Moon

Asteroid Gaspra from the Galileo spacecraft which passed within 1600 km By 1849, 10 such objects were known, and in 1868 the 100 th asteroids or minor planets was announced. By 1923, more than 1,000 asteroids were catalogued So the search for the missing planet continued. In March 1802, German astronomer Heinrich Olbers found a second small body at a similar distance to Ceres, called Pallas. Pallas is even smaller and fainter than Ceres. Well over 20,000 asteroids orbiting the Sun have since been discovered.

The asteroid belt is a region between the orbits of Mars and Jupiter lying between about 2.1 to 4.1 AU from the Sun. The overwhelming majority of asteroids are located in the asteroid belt. The Asteroid Belt Not to Scale!

However, the theory lacks a plausible cause of this disintegration… asteroid belt Asteroid distribution Orbital eccentricity Why are asteroids found so predominantly in this region? One theory is that the asteroids are the debris of the disintegration of a planet that once existed between Mars and Jupiter.

Asteroid-like objects are believed to have filled the early Solar System. (b) Failed Planets? Computer simulations provide evidence that Jupiter’s strong gravity and tidal effects disrupted the orbits of these planetesimals within the asteroid belt. As a result much of this material was ejected from the Solar System.The total mass of all asteroids in the asteroid belt is less than that of our Moon.

It is now believed that Jupiter’s gravitational field “cleared” the asteroid belt before a planet was able to form. Simulations suggest that without this clearing effect an additional planet would have formed between Mars and Jupiter. In this context, the asteroids could be considered to be a failed planet.

Kirkwood Gaps Jupiter has an orbital period of 11.9 years. In 1867 Daniel Kirkwood observed that very few asteroids have orbital periods which correspond to simple fractions of 11.9 years. (c) Orbits Kirkwood gaps, as these features are known, exist where asteroids’ orbital periods would be 1/3, 2/5, 3/7 and 1/2 that of Jupiter. asteroid belt Orbital eccentricity :12:1 5:2

Why do Kirkwood gaps exist? Consider an asteroid with an orbital period exactly 1/2 that of Jupiter. The asteroid circles the Sun twice in the time Jupiter circles the Sun once. (This is also called a 2:1 resonance.) Consequently the asteroid lines up between Jupiter and the Sun at the same location every second time it orbits the Sun. These repeated alignments result in the asteroid being deflected from its orbit and ejected from the Solar System by Jupiter’s gravitational field. Click hereClick here to see an animation showing an asteroid with a period half that of Jupiter.

So if the Kirkwood gaps are the places where we don’t find asteroids, where do we find them?? Most of the Solar System’s asteroids are found in the Main Belt between about 2.1 and 4.1 AU. The majority of main belt asteroids follow slightly elliptical stable orbits, orbiting the Sun in the same direction as the Earth. Typically the orbital periods of these asteroids range from 3 to 8 years. General Orbits There are also a few special resonances where asteroids like to group together, such as the 3:2 resonance at 3.97 AU (with periods 2/3 that of Jupiter) where we find the Hilda group. And the Trojan asteroids are found at the 1:1 resonance, which means they have the same orbital period as Jupiter.

Shown are the orbits of the first three asteroids to be discovered: Ceres, Pallas, and Juno. Pallas Ceres Juno

Here we show the asteroid distribution again, but this time with the various asteroid groups noted.

(d) Asteroids outside the asteroid belt Two groups of asteroids orbit the Sun at distances similar to Jupiter, outside the asteroid belt. These are the Trojan asteroids. Over 1,600 Trojan asteroids are catalogued. Estimates of the their total number go up to tens of thousands. (i) Trojan asteroids

One group of Trojan asteroids is located at a stable point that trails Jupiter’s orbit by 60°. Another group leads Jupiter by 60°. Trojan asteroids leading group Trojan asteroids trailing group There are slightly more leading that trailing Trojans.

There are also three unstable points in such a system. The two stable Lagrange points are labelled L 4 and L 5. The combined gravitational forces of Jupiter and the Sun in this 3-body rotating system produce these two regions where small bodies can have stable orbits. French mathematician Louis Lagrange predicted the existence of these stable orbits, called Lagrange points, in The first Trojan asteroid was discovered in L4L4 L5L5

Jupiter is not the only planet to host Trojan asteroids. These two stable points predicted by Lagrange actually apply to all planetary bodies. Martian and Neptunian Trojan asteroids To date Trojans have also been found around Mars and Neptune, and there are currently 6 known Martian Trojan* asteroids and 1 Neptunian Trojan asteroid. * It is more difficult to determine the orbits of the Martian Trojans than the Jovian or Neptunian Trojans, so there is some uncertainty in this number.

(ii) Near Earth Asteroids As the name suggests, some asteroids are also found near the orbit of the Earth. There are three subclasses: – Atens, with average orbital distances < 1 AU (i.e. closer to the Sun than the Earth); – Apollos, which are Earth-crossing; and – Amors, with average orbital distances > 1 AU and < 1.3 AU (i.e. between the Earth and Mars). There are more than 2,800 known Near Earth Asteroids.

Since the Apollo asteroids cross the Earth’s orbit…. Within tens of millions of years many of the Apollo asteroids will strike the Earth. Typical orbit of an Apollo asteroid Sun Earth’s orbit …it is just a matter of time before these asteroids collide with the Earth.

But the Solar system is billions of years old, far older than tens of millions of years. So why haven’t the Apollo asteroids all been destroyed by colliding with the Earth already? Apollo asteroids have not been in their current orbits since the early stages of the Solar System. The asteroid belt acts as a continual source of Apollo asteroids. The influence of Jupiter affects the orbits of certain asteroids in the asteroid belt. If the orbit of an asteroid subsequently passes close enough to Mars, it can be pulled deeper into the Solar System.

(iii) Centaurs The Centaurs are asteroids that generally lie between the orbit of Saturn and Neptune, with average orbital distances between 10 AU and 30 AU. The first object to be classified as a Centaur was Chiron (not to be confused with Charon, Pluto’s moon!), in 1977, the most distant asteroid at the time. A surprise came in 1988, where Chiron’s orbit carried it closer to the Sun and its brightness nearly doubled. Chiron was behaving like a comet! Being so far from the Sun, the Centaurs generally contain some ices such as CO 2 which sublimate* as they move closer to the Sun. * Sublimate means to go straight from a solid (i.e. ice) to a vapour (i.e. gas) There are now more than 140 catalogued Centaurs.

The existence of the Kuiper Belt was suggested in the late 1940s as a belt of icy bodies beyond Neptune left over from the formation of the Solar System. The first KBO was discovered in 1992 and the orbits of about 800 have since been catalogued. We’ll learn more about KBOs in the Activity Pluto, Charon and the Plutons. (iv) Kuiper Belt Objects Kuiper Belt Objects (KBOs) are icy bodies found in the outer reaches of the Solar System, generally past the orbit of Neptune, between 30 to 50 AU and beyond. (They’re also know as trans-Neptunian objects or “TNOs”.)

(e) Properties - Collisions A common image of the asteroid belt often portrayed in science fiction is of a “minefield” of asteroids through which spacecraft must navigate. However the average distance between an asteroid in the asteroid belt and its nearest neighbour is thousands of times larger than the distance between the Earth and the Moon. In fact the spacecraft Galileo had to go out of its way to approach close enough to asteroids to provide us with the images shown throughout this Activity. Nevertheless asteroids have occasionally collided, as evidenced by their cratered surfaces.

* Differentiation is explained in the Activity on “Planetary Evolution” This series of images from the Galileo spacecraft show the belt asteroid Ida pass through one full rotation. Note its irregular shape and cratered surface. Only Ceres, Pallas and Vesta are large enough for differentiation* to have taken place and given them spherical shapes.

Tidal Effects The shape of asteroids is also affected by tidal forces *. Let us examine the Apollo asteroid Geographos. As Geographos orbits close to the Earth, the far side of the asteroid “pulls” out while the side nearest the Earth “pulls” in. Thus Geographos has gained its elongated shape aligned roughly toward Earth. Geographos * To review Tidal Forces refer to the Activity “Time and Tide” Tidal Forces

“S type” S type are the brightest asteroids, are reddish in colour, and dominate the inner belt region. These asteroids are composed of silicates & metals, largely iron. Their spectra also indicates the presence of the mineral olivine. Asteroids are classified into 3 categories depending on their surface colours and the spectra of light which they reflect. Chemical composition

“M type” M type asteroids are also bright but are not red. They make up 10% of the total population and are mostly in the inner belt region. Their spectra indicates that they are composed of iron & nickel alloys. “C type” About 75% of asteroids are C type which are very dark. They inhabit the main belt’s outer regions. These asteroids have a relatively high content of carbon in the form of organic compounds.

In this Activity it has become clear that Jupiter’s gravity plays an important role in the continuing evolution of the Solar System, and in particular in the distribution of asteroids. In the next Module we will explore the Jovian Gas Giants in general and Jupiter in particular.

Image Credits NASA: Ida and Dactyl Welcome to Planet Earth Full Moon Gaspra Ida montage Geographos Asteroid distribution

Now return to the Module 14 home page, and read more about the Asteroid Belt in the Textbook Readings. Hit the Esc key (escape) to return to the Module 14 Home Page

which worked ok, but seemed to indicate that there should be a planet at 2.8 AU where the clearly wasn’t one: Bode’s “Law” The rule given by Bode was: Bodes law, more formally known as the “Titius-Bode law”, is a mathematical formula that describes the distances of the planets from the Sun – or at least it did in 1766 when only the inner 6 planets were known. MercuryVenusEarthMars??JupiterSaturn Predicted distance 0.4 AU0.7 AU1.0 AU1.6 AU2.8 AU5.2 AU10.0 AU Actual distance 0.4 AU0.7 AU1.0 AU1.5 AU??5.2 AU9.5 AU for N = 0, 3, 6, 12, 24, 48, 96

No-one paid much attention to Bode’s law until the discovery of Uranus in Bode’s law “predicted” a planet at 19.6 AU, just 2% further than Uranus’ distance of AU. This encouraged Bode to urge people to search for the missing 5 th planet at 2.8 AU, which lead to Pizzini’s discovery of Ceres at 2.77 AU. Neptune’s discovery in 1846, however, threw the law in doubt as it didn’t fit with Bode’s law at all. Bode’s 9 th planet should be at 38.8 AU, which fits Pluto (at AU) better than it does Neptune which is AU from the Sun. Further, there is no physical basis whatsoever to Bode’s law - it is purely a mathematical relation (and with 3 free parameters, it is quite easy to fit the data). That said, Bode’s law works remarkably well out to Uranus.

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