By Duncan Lunan

When we look at a scale map of the Solar System, it’s immediately obvious that there’s a disproportionately large gap between the orbits of Mars and Jupiter.  After he had deduced the relative sizes of the planetary orbits, Johannes Kepler modestly wrote, ‘Between Mars and Jupiter I have put a planet’.  In the late 18th century an international group of astronomers started a search, calling themselves ‘the Celestial Police’, and on January 1st 1801 Piazzi discovered Ceres, the first of the asteroids.  It was immediately obvious that it wasn’t large enough to be classed as a true planet, so the search continued.  Even though Ceres is the largest of the asteroids, the surface gravity can’t be much more than 0.03 g.   

Wilhelm Olbers discovered Pallas, the second known asteroid, on March 28 1802,  and Harding of Lilienthal discovered Juno, the third asteroid, on September 2^nd 1804.  Vesta, the 4^th asteroid to be discovered in the Main Belt, was found by Olbers on March 29^th 1807, and since then many thousands more have been found.  Vesta is visually the brightest, at the limit of naked-eye visibility when it’s overtaken by the Earth.  About the size of Arizona, in the southern hemisphere it has a huge crater 465 km across and 12 km deep, as wide as Vesta itself;  the equivalent on Earth would be the size of the Pacific basin.  More than 50 smaller asteroids with similar compositions, ‘Vestoids’, were formed in the collision and many fragments blasted off it make up a family of asteroids.   (‘Hubble Images of Asteroids Help Astronomers Prepare for Spacecraft Visit’, Hubble News Release STScI-2007-27, June 20^th 2007.)   Some of them reach Earth as meteorites.

In 2010 a close flyby of Mars’s inner moon, Phobos, by Europe’s Mars Express probe, detected hydrated rocks of types which had already been located on the Martian surface.  It was already known that there were void spaces within Phobos, accounting for the low density which was measured by the Viking Orbiters in the 1970s.  The startling conclusion was that Phobos and presumably Deimos were formed from material blasted off the surface of Mars in big impacts – like the formation of Earth’s Moon, on a smaller scale.  It ruled out the idea that the moons of Mars are captured asteroids, which until then had been widely believed.

But the Viking flybys found another oddity.   The surface of Phobos is strewn with deep grooves, mostly at right angles to its long axis  (which is gravitationally stabilised towards Mars), looking so regular from some angles that they look like sedimentary strata – though on the other side of Phobos they break up into crater chains.  Nothing like them had been seen elsewhere.  But just when the information about Phobos’s composition came out, the Rosetta space probe passed by the asteroid Lutetia and saw similar parallel grooves, but curved.  It now appears that the Phobos grooves are evidence of multiple breakups and reassembly due to tidal forces, while the grooves on Vesta and presumably Lutetia are due to impact stress.

While we were still scratching our heads over that, Dawn went on to reach Vesta, and after flying in  formation with the asteroid for a short time it entered orbit.  

We knew that Vesta had a large crater on it, believed to be the source of many meteorites which fall on Earth, and we’d been looking forward to seeing the interior of it.  

But even the first images from orbit have opened up a new sensation.   Hubble Space Telescope images of Vesta show a marked point at the south pole, and that was thought to be a ridge between craters.   But instead it’s a mountain, planked on the pole for all the world as if by a gentle impact.   Curving around it is a long, scalloped line, looking at first sight like overlapping crater rings but in closer views, it appears to be something we have seen elsewhere – a line of columnar cliffs.   And on the other side, possibly explaining it all, there’s another familiar-looking feature:  roughly parallel grooves, in the shape of a chevron.

Similar markings are found on Ganymede, the largest moon of Jupiter, and they indicate distortion of the crust by tidal forces.  But the most prominent example of a chevron is on Miranda, the innermost of the five large moons of Uranus.  That, and the scalloped cliffs, the mismatches in the composition  of the surfaced and the general non-spherical lumpiness of the moon, all indicate that Miranda has been blown apart by a huge impact and the fragments have been reassembled in the wrong order, like pieces of a jigsaw forced into places where they don’t belong.  More recent analysis indicates that Vesta has an iron core, which may well have cracked into two pieces due to the big impact.  (Gordon McKay, lecture, ‘Controversies in Astronomy, Part 1’, Clydesdale Astronomcial Society Zoom meeting, 13^th September 2021.)  

After over a year taking photographs from orbit at various heights, and examining the crater in depth, Dawn moved on to Ceres, which had a very intriguing feature reflecting sunlight as if it’s liquid water. 

That wasn’t possible at that distance from the Sun, still less in vacuum – unless it was under glass?  Was it the galactic lido?  In the event, the bright patches turned out to be salt deposits from water outbreaks.  It turns out that most of the crust of Ceres is in the form of water-rich clathrate  (good for building very large space habitats or starships), and a 3-mile high volcano, Ahuna Mons, may be composed mostly of frozen mud.

At first it was thought that there had originally been a planet between Mars and Jupiter, where the Asteroid Belt is now.  Instead, we now know that Jupiter’s gravitational pull caused the protoplanets in that area of the Solar System to collide with too much violence for their fragments to coalesce.  Some of those protoplanets were large enough to have been heated internally by radioactive decay, causing them to be gravitationally differentiated, with crusts, mantles and cores.   Those in the outer region also had significant concentrations of water and possibly organic compounds.  So in the multiple collisions asteroids of many different types were formed, and although there is a general difference in composition between the inner and outer belts, all kinds are represented, as far as we know, in the population of asteroids whose orbits have been perturbed sufficiently to pass near the Earth and occasionally collide with it.  Generally, carbonaceous asteroids come from the outer edge of the Belt, stony ones from the middle and metallic ones from the inner edge.  Some of them had undergone virtually no evolution from the primal material of the Solar System, and they contain the decay products of radioactive transuranic elements which were formed in the supernovae whose shockwaves caused the original Solar System nebula to collapse.  

Writers, artists and film producers  (even today)  like to portray dense asteroid fields.  But although the pulls of the planets have separated the Main Belt into three main streams, separated by ‘Kirkwood gaps’, and although occasional collisions between asteroids produce ‘jetstreams’ of asteroids in near-identical orbits, there’s so much space between them that normally one could spend a lifetime on an asteroid without ever having another come within naked-eye range.  If the Asteroid Belt were really as dense as portrayed, maybe it would lend weight  (literally)  to the idea that the Belt is the debris of a shattered planet.  In fact the total mass of the Belt is less than 10% of Earth’s, possibly much less.   

But when the Galileo space probe photographed the asteroid Ida, it proved to have an unexpected satellite which was named Dactyl.  Until the Ida flyby, professional astronomers insisted that asteroids were too small to have satellites.   Amateur observers and meteoriticists were less surprised because there had been a number of occasions when asteroids passed in front of stars and amateurs reported double occultations. 

In Canada there’s a matched pair of impact craters called the Clearwater Lakes, and a number of the asteroids were found to have dumb-bell shapes, though nobody could explain how such small bodies might bump together gently enough to stick. 

But perhaps it does happen, because Ida and Dactyl are quite different in composition, so Dactyl isn’t a fragment detached from Ida by a collision, although capture of one asteroid by another is even less likely.  Now it’s thought that dumb-bells have been formed by mergers.  Just such a valley was found by the Galileo probe in its flyby of Ida, and a second one on Eros was found by the NEAR-Shoemaker probe which orbited and eventually set down on it, though contact was lost because it hadn’t been designed for landing.  

Along the plane of the Ecliptic, to either side of the Sun, there are cones of diffused light called the Zodiacal Light, and directly opposite the Sun there’s a faint patch called the Gegenschein  (Counterglow). 

The Zodiacal Light is faint and the Gegenschein much fainter;  one of the more remarkable things about the George Pal/Chesley Bonestell film The Conquest of Space  (Paramount, 1955), is that you can see the solar disc and the Zodiacal Light at the same time!  Until the space age it wasn’t known whether the two effects were truly in interplanetary space or generated by a dust cloud surrounding the Earth, but the Pioneer 10 space probe found that both effects persisted as far out as the Asteroid Belt.  The dust comes partly from there, generated by collisions, and part of it is released by comets passing through the inner Solar System.   It spirals towards the Sun due to the Poynting-Robertson effect, in which light from the Sun exercises a slight but significant braking on the dust.  

Thule is the Greek and Roman word for the north, as in ‘Ultima Thule’ = furthest north, and it was allocated to an asteroid at 4.3 Astronomical Units which was thought to mark the outer limit of the Asteroid Belt, until the Trojan asteroids were found sharing the orbit of Jupiter.  The Trojan asteroids are in the same orbit around the Sun as Jupiter itself, equidistant from the planet and the Sun.  They are in the fourth and fifth position calculated by Lagrange as special solutions to the problem of gravitational attraction between three bodies, which has no general solution even today.   The L4 and L5 points are also known as the Jupiter Equilaterals.  Originally, the asteroids in one point were to be Trojans and the ones in the other were to be Greeks, all named after characters in the Iliad, but not all astronomers have full classical educations and the two groups quickly became mixed up.  NASA has an upcoming mission, Lucy, intended to launch later this year and explore Eurybates and five other Trojan asteroids.

Coming up:  Asteroids and Impacts

See this month’s Star Map: The Sky Above You – October 2021