Asteroids, a history and overview        

                                                                        By   J.Bakkelund

 

On February 14th, 2000 The NEAR spacecraft went into orbit around 433 Eros, a 33 km X 13 km asteroid . That was the genesis for this article in which I will talk about the history of asteroid discovery and what we have learned about them in the past 200 years.

   

After Uranus was discovered on March 13, 1781, astronomers noted that it fit neatly into the Titus-Bode rule.  To use this rule you write down 0, 0.3, 0.6, 1.2, doubling each time until 38.4.  then you add 0.4 to each number.  This gives the approximate mean distance of each planet from the sun in astronomical units. ( An astronomical unit is 149,000,000 km., the distance from the earth to the Sun. )

Uranus’s predicted distance was 19.6 AU and its actual distance is 19.19 AU; very close.  (Neptune was not found until September 23, 1846.  It and Pluto do not follow Bode’s rule.)

In the meantime, astronomers noted that there was a large gap between Mars and Jupiter and determined that there should be a planet at 2.8 AU.  In 1800, six German astronomers organized a search for the missing planet.  The plan was to divide the zodiac - that band around the sky centered on the ecliptic, and through which the known planets’ orbits lie- into 24 sections and to assign each section to a different European astronomer to search.  One of these was the Sicilian astronomer Giuseppe Piazzi.

Piazzi had not yet been informed of his role in the search, however.  On January 1, 1801, while working on another project, he spotted an intruder “star” in Taurus.  The next night he observed it again and thought it had moved.  On the third night he was sure that it had moved.  Piazzi thought he had found a new comet. He continued to watch it for a few weeks and relayed the news to astronomers in Germany.  The news was greeted  enthusiastically, because everyone thought the missing planet had been discovered!  But by the time astronomers were aware of the discovery, the new object was too close to the sun to be observed and it would be September before it should be visible.  By summer 1801, they realized the new planet was lost.  Piazzi had observed it for 6 weeks, too short a time to calculate its orbit by techniques then available.  Fortunately, the discovery was saved by a 23 year old genius named Karl Friedrich Gauss (A unit of magnetism is named after Gauss).  He put his other work aside and worked on the problem for 2 months to devise a new method of orbit calculation that would work with observations made over only a short arc of total orbit.  He finished his work in November, but bad weather prevented confirmation until December 31, one year after its initial discovery.  It was in the constellation Virgo almost precisely where Gauss had predicted it would be.  Piazzi, being Sicilian, named the object Ceres, after the goddess of agriculture and protector of Sicily.  It was very small for a planet though having a diameter of only about 1000km and it’s computed distance fit perfectly into the Titus-Bode rule at 2.7 A.U.. It is unlikely that the Titus-Bode rule has any real meaning, it appears to be a mathematical coincidence. The important thing is that it encouraged people to look .

Having found the missing planet astronomers were surprised when second planet was discovered by the German astronomer W. Olbers a year later at the same distance.  This one was named  Pallas.  After this a search began in earnest and  Juno was discovered in 1804 by K.L. Harding . Olbers then put forward the hypothesis that the three objects were the remnants of a single planet which had somehow been destroyed, and he suggested that there could be other fragments of it moving in nearby orbits. He himself found one, called  Vesta in 1807.  No more asteroids were discovered for 38 years until  Astraea in 1845.  Subsequently, new ones were found until by 1890, more than 300 were known, mostly in orbits between Mars and Jupiter. Unfortunately for Olbers theory the mass of all the known asteroids is much less than that of our moon so the answer to the formation of the asteroids is a bit more complicated. More on this later.

 

 Sir William Herschel called them asteroids after the Greek word aster meaning star, because of there starlike appearance in most telescopes. When observed under exceptionally favorable conditions the biggest asteroids sometimes appear non-starlike. E.E. Barnard made a high power examination with the Lick 36 inch refractor of Ceres, Pallas and Vesta and ranked them correctly in relative size.

But except for a couple of the largest ones, which have diameters of less than one second of arc, asteroids do indeed appear starlike. The only thing that distinguishes them from stars is their motion. Today about 6000 asteroids have been found with many more being discovered every year. The largest, Ceres, is less than 1/3 the diameter of the moon and only about 1/5 the diameter of Mercury, one of the smallest planets. Pallas and Vesta are just over 500km . About 33 have diameters of more than 200km and an estimated 1,150

have a diameter larger than 30km. There are many thousands and thousands of them in the one to 10 Km. size and many hundreds of thousands in the 1 km and smaller sizes. The numbers increase dramatically as the sizes get smaller. There is no lower limit but the smallest ones detected in close approaches to Earth have been in the 100m to 200m size. Anything smaller than this is called a meteoroid and undoubtedly there are millions of them because they hit earth every day.

Orbits

The asteroids all revolve around the sun in the same direction as the earth and  most in the same plane although a few have inclinations of 25* and one called 2102 Tantalus is inclined 64* to the ecliptic. Most  belt asteroids are found between 2 A.U. and 4 A.U. and have orbits of 3 to 6 years but there are exceptions.  Ceres for instance is smack in the middle of the asteroid belt and has an orbital period of  4.6 years. The asteroid belt has thousands of meteors but it is not like the science fiction movies where the spaceship has to dodge them as it travels through. In reality the space between asteroids is so large that an astronaut traveling through the asteroid belt might only see a couple and they would be distant and appear starlike.Interestingly however, when Pioneer 10 passed through the Belt in 1973 it had a detector that measured an increase in particle collisions in 0.1mm to 1.0mm size. At that time there was some uncertainty whether or not it would survive passage through the Belt because no one knew the density of the small particles.

The structure of the belt is not uniform, that is there are orbital distances that are clear areas or gaps.They are called zones of avoidance or Kirkwood gaps after Daniel Kirkwood who interpreted them as resonance phenomena associated with Jupiters gravitational field. The best way to picture this is to think of the rings of Saturn. I’m sure you’ve seen photographs showing the gaps in the rings. they are caused by small shepherd moons within the rings that perturb the small particles that make up the rings. The asteroid belt is similar, within certain zones the gravity of Jupiter will eject any asteroids there into different orbits. this may be the mechanism that is responsible for several classes of asteroidal orbit types that are outside of the belt.

Trojans - asteroids that are in similar orbits to Jupiter

             an example is 624 Hector, 60* ahead of Jupiter

Atens - (for 2062 Aten) have orbits that cross the orbit of Earth, but lie wholly within the orbit of Mars. The first was only discovered in 1976                                      

Apollos - (for 1862 Apollo) have orbits that cross both the orbits of the earth and Mars. Famous members include 1566 Icarus, and 1620 Geographos and Phaeton. First found in 1932

Amors - (for 1221 Amor) have orbits that cross the orbit of Mars but do not come as close as the earth’s orbit. Eros is one of these

 

Theory of the formation of asteroids

At this point I’ll pause and ask the question- whats so interesting about these chunks of rock flying around in space ? Most of them are just points of light and once you’ve figured out it’s orbit, there is not much else to interest anyone, is there ? This was pretty much the state of thinking at the turn of the century when one astronomer referred to them as the “vermin of the sky” because they messed up his photographs, kind of like satellites do today.

Interest in asteroids has intensified in this century because astronomers now believe that they (and comets) are like fossils, and so they preserve a record of the formation of the solar system 4.5 billion years ago.On earth this record has been erased because of weathering and volcanism.

The other reason that they have generated more interest is we have been able to learn more about their through a variety of new methods of study and improved technology.  Here are some of them :

- albedo and light curves studies with CCD cameras and occultation studies have revealed their size and shape

- radar imaging of close approaching ones like Geographos, Toutatis and Castelia have also revealed their size and shape

- Hubble space telescope images can resolve some detail on large asteroids like Vesta

- Space probes- Galileo was the first spacecraft to give a close up view of an asteroid. It photographed Gaspra( it revealed it as a battered potato shaped world) in 1991 and Ida in 1993 (it turned out to have a satellite orbiting it, now named Dactyl). It also showed evidence of a blanket of loose fragmental material, or regolith, covering the asteroids surface.

- reflectance spectrophotometry (in both visible and infrared wavelengths), This is the study of reflected sunlight to reveal chemical composition. I’ll go into a little bit of detail about this because this is the method by which we’ve learned about the different asteroid types which I’ll be talking about in a minute. The basis of this approach is that most minerals reflect light at some wavelengths and absorb it at others. reflectance spectra has been measured for about 100 asteroids and compared to the spectra of recovered meteorites and provided a link between the two. It seems that almost all meteorites are pieces of Apollo or Amor asteroids that have hit the earth.

In the past (this was a raging argument up to the early sixties), there have been 2 extreme theories for the origin of asteroids. According to the first, the asteroids are fragments of a shattered planet. According to the second, they are pieces of a planet that never formed. The truth, as usual, seems to lie between the extremes. There is not enough mass  in all the asteroids to have formed a even a decent moon . On the other hand the mineral structure of some of the asteroids indicates that they are differentiated in a way that they must have formed in sizable bodies similar to the larger asteroids like Ceres. The presence of the metallic asteroids are evidence for this. They must have formed in a parent body large enough and hot enough for gravity to settle the nickel-iron core and have the lighter silicate bearing minerals float to the top. The prevailing theory now is that there may have been  a 100 or so larger asteroids that were large enough to separate like this.  The smaller asteroids did not have sufficient gravity to do this and so should still be in their original form, most of these would be of the carbonaceous type. Why these bodies did not accrete to form a single planet seems to have something to do with the presence of Jupiter, it’s gravity seems to have speeded up collisions between the asteroids which has broken many of them up and exposed their metallic cores.

 

BASIC ASTEROID TYPES

90% of asteroids can be divided into two groups based on albedo and color.

 

C - type -  75% of all asteroids, extremely dark (3.5% reflection)

              - carbonaceous, some contain water

              - other examples are Bamberga, Hygiea and Phobos & Deimos (moons of Mars)

              - RD - is a sub type, reddish-dark, common for bodies at the outer edge of the

               asteroid Belt including many Trojans, some have amino acids

              - considered to be basically unmodified

              - the recent Yukon meteorite is a carbonaceous type , very rare on earth

 

S - type - 16% of asteroids, moderate albedo (16%) and reddish colors

             - spectra indicates iron & magnesium silicates mixed with pure nickel -iron

               but proportions are uncertain

             - Eros is an example of this type

             - similar to the stony iron type of meteorite

             - most common type of meteorite

             - altered by heat

 

M - type - 5 % of asteroids, ambiguous spectrum and moderate albedos

              - no silica absorption bands so they may be metallic asteroids

              - perhaps remnant cores of larger differentiated precursor bodies

              - example - Psyche - largest metal meteorite is the 66 ton Hoba

                in Namibia, South Africa

              - a single 1 km. metal asteroid would contain 8 billion tons of metal

               and could supply the world with iron for 15 years, nickel for 1250 years ,

               copper for 10 years and cobalt for 3000 years

             - it would have a  (1988) value of 5 million billion dollars

 

The remaining  4 % are assorted rare types, some unique. Vesta is unique and seems to have a composition similar to terrestrial basalt (volcanic type rock) and it’s high albedo and size makes it’ the brightest of all asteroids.

With the NEAR spacecraft in orbit around Eros a new era in asteroid science will begin. It will gradually drop it’s orbit and study Eros in greater detail. It should be very interesting to hear it’s results in the coming year. Upcoming missions to asteroids may even include sample returns.

 

Observing Asteroids

For amateurs there is not much to see in observing asteroids. Basically there is the challenge of finding them and watching them move against the background stars. I have run across one amateur observation of Ceres, in which he observed the disk of Ceres at 540x in a 12.5 inch Dobsonian under superb seeing conditions. This should be possible because Ceres has a diameter of  0.7 seconds of arc and a 12.5 inch mirror has theoretical resolution of 0.36 seconds of arc.

Vesta reaches naked eye visibility at about magnitude 5.5, Ceres around magnitude 7, Pallas around 7.5. Occasionally astronomy magazines will have articles on observing asteroids, the March 2000 Issue of Sky & Tel. has one.

The bright asteroids (> 10th mag.) can be photographed by just a camera, a fast 50mm lens and a tripod. With the lens wide open hold the shutter open for about 30 seconds. Take another exposure later that night or in the next couple of days. A careful inspection should reveal a moving “star”. Piggy backing your camera on a telescope will allow longer exposures and longer focal length lenses.

A Main Belt asteroid will typically moves about 1 minute of arc per hour when it is at opposition. the following night it will have shifted nearly 1/2 degree and sometimes a asteroid  swings close to a bright star  making it s motion much more obvious.