The Minor Planets

The International Astronomical Union has decided that these small bodies must be called minor planets and not asteroids as they have nothing to do with stars.

When William Herschel discovered Uranus, the first new planet to be discovered in modern times, astronomers were fired UD to search for new planets.  Nine years before the discovery of Uranus, Johann Bode fixed attention to a "rule" which Johann Titius had annotated in a book by Karl Bonnet in which the relative distances of the planets from the Sun figured..  The "rule" begins with zero, then  3, followed by 6, 12...doubling each time.  To each term 4 was added and then each term was divided by 10.

The values of the “rules” are close to the actual relative distances of the planets from the Sun.  Earth and Jupiter are dead exact.  But there is no planet corresponding to the value 2,8.  So it was concluded that there must be a planet at this distance of 2,8 AU but that it was missing.  So the astronomers fine combed the strip of sky covered by the Signs of the Zodiac, astride the ecliptic.  And great was the excitement when G Piazzi announced on 1 January 1801 that he had discovered a planet which was in such an orbit that It would take  4,61 years to go around the Sun.  The square of 4,61, i.e. T² is 21,521, and the cube root of this value is 2.77 and this must be the value of the distance R from the Sun because T² must equal R³ according to Kepler's Third Law.  This value 2.77 was very close to 2,8 of the "rule" The new planet was called 1 CERES.  Its orbit was found to have an inclination to the ecliptic of 10° 36' 28" but nobody took exception-to that.

On 28 March 1802 H W H Olbers discovered a second planet whose distance also worked out at 2,77 AU.  The inclination of its orbit was found to be no less than 34° 43' 09" and it was called 2 PALLAS.  And on 1 September 1804 K L Harding discovered a third planet at an average distance from the Sun of 2,67 AU.  It was called 3 JUNO.

The cup overflowed when Olbers discovered his second planet on 22 March 1807 with an inclination of only 7° 7' 55,7" to the ecliptic.  But its average distance from the Sun worked out at 2,36 AU, now deviating considerably from the "rule".  It was called 4 VESTA.

By the end of 1850  another nine minor planets had been discovered; another 21 before 1Q55 and another 24 before 1350.  So the total of 58 had been reached by 1360.  Today the number exceeds 3000 of which the orbits have been calculated and tested  for perihelion distances on at least three passages through perihelion..  And there are many thousands more that have since been discovered.

1 Ceres, the first discovered, is the largest, being 1000 km in diameter.  Second largest is 2 Pallas, 508 km in diameter; then 4 Vesta 538 km; Higea 450 km; Euphrosine 370 km; Interamnia 350 km; Davida 323 km and Cibeline 309 km.  There are 18 with diameters between 300 and 200 km; 41 between 200 and 100 km and the rest are smaller than 100 km.  The best way of measuring these small diameters is to time the occultation of a star by the minor planet.  Such an occultation was monitored by Johannesburg amateur astronomers T Cooper, B Fraser, H Lund, the late Danie Overbeek and J Smit.  They measured the times of vanishing and reappearance of the minor planet 248 Lameia.  The times of occultation varied from 2 to 5,8 seconds.  From these values it was found that the diameter had values of 52 and 55 km, the planet being oval in shape.

The diameter of the largest, Ceres is only 0,29 that of the Moon.  The total mass of all the thousands of planetoids has been calculated to be only 1/3000 that of the Earth and 1/37 that of the Moon.  The smallest so far identified are as large as very big houses.  Many meteorites that have fallen on the Earth are minor planets.  If a house-sized body struck a built-up area, the damage would be frightful.

The greatest brightness is displayed by Vesta which has a magnitude of only 6,5, only just beyond visibility by the naked eye.  Ceres must have a darker surface because its magnitude is 7,4 despite the fact that it is much larger than Vesta.  All the rest are very much dimmer.

Many Minor Planets have orbits that cut across that of the Earth.  They are called Earth Grazers.  It was probably an Earth Grazer that collided with the Earth 65 million years ago, which collision led to the wiping out of 90% of the life forms then existing.  All that survived were small mammals such as rats, mice, squirrels and hares,  from these all the present day life forms evolved, including you and me!

On 30 October 1937 the minor planet called Hermes came to within 890 000 km of the Earth.  After that it was never seen again.  It has probably crashed into Venus or Mercury or most probably into the Sun.  Apollo and Adonis have perihelions within the orbit of Venus.  Icarus comes closest to the Sun, 0,187 AU and thus nearer than the perihelion of Mercury.  Today an extensive watch is being conducted to find Earth Grazers.  Some have been found that come closer than the Moon which is only 384 000 km distant..  The best thing that we can do is to have a rocket perpetually at the ready to be launched at a moment's notice to go and push the Grazer into a safe orbit around the Earth, where we will be able to go and mine its minerals!  We cant say what minerals may be found there.

How did the Minor planets come about?  Some astronomers hold the view that a smallish planet collided with another body of similar size which had formed in orbit between Mars and Jupiter and this collision broke both planets up into thousands of fragments which still exist today.  This theory cannot explain how it is that Ceres is very nearly spherical and how so many other Minor Planets are very nearly spherical.  For a solid body to be spherical it must by accretion be compacted together.  This happens when very small particles overtake other small particles moving in orbits only slightly further from the Sun.  The inner particle will be moving faster and will thus tend to overtake the outer particle and in doing so the two particles will stick together.  When these planetesimals cleave together they become bigger and bigger and tend to become spherical when they have diameters of 600 or more kilometres.  The moons of the planets are also subject to this limitation.  Pallas which is 608 km wide is still rather oval. 

The Minor Planet Gaspra which was photographed by the space probe Galileo is covered by craters and its surface seems covered by dust which must have been formed by micrometeorites pulverising the surface.

At the distance where most minor planets revolve the gravitational influence of Jupiter must have had an influence when the planetesimals accreted together from the primeval nebula from which the Sun and all the other planets and their moons formed.  This seems to be a better theory than the crash theory.  Crashes in the universe are very rare.  Think of Saturn's rings which were formed by Saturn's gravity breaking up one of the planet's icy moons.  The fragments just go on revolving round the planet because the particles are in orbits which are ellipses of very low eccentricity.  A collision would have sent the fragments flying in very elliptical orbits.  If not in parabolas, never to return to the scene of the crash.  Crashes indeed!

Jan Eben van Zyl