by Paul Murdin
Asteroids
Remnants of the early Solar System
The discovery of a new dish does more for the happiness of the human race than the discovery of a star.
Gastronome and amateur scientist Jean Anthelme Brillat-Savarin, The Physiology of Taste: Or, Meditations on Transcendental Gastronomy,1825
In the sixteenth century, the German astronomer Johannes Kepler noticed that there was a rather large gap in the arrangement of the planets in the Solar System. This mysterious gap between Mars and Jupiter led to the discovery of asteroids. Some asteroids are very small planets, as thought when they were first identified in the nineteenth century, and others fragments from the collisions of larger asteroids, but many are actually scrap material left over from the formation of the planets in the early Solar System (plate VII). Some are spherical, but most are random in size and shape. Most follow orbits in a belt between Mars and Jupiter.
Setting the Sun–Earth distance at 10 units, the distances from the Sun to the known planets were as follows:
Sun–Mercury
3.9 units
Sun–Venus
7.2 units
Sun–Earth
10 units
Sun–Mars
15 units
Sun–Jupiter
52 units
Sun–Saturn
96 units
Kepler saw that the distances between the planets roughly doubled at each step. Yet Jupiter was nearly four times the distance of Mars from the Sun – twice the expected amount. Kepler mused whether there might be an undiscovered planet in the gap. Isaac Newton noticed the same phenomenon and suggested that perhaps Providence had put Jupiter and Saturn at an extra-large distance from the Sun to minimize their disruptive effects on the inner planets of the Solar System.
In 1766 Johann Daniel Titius, a professor of physics at the University of Wittenberg, discovered a more accurate formula for estimating the distances of the planets from the Sun. The formula was popularized by a German astronomer, Johann Bode, and consequently became known as Bode’s Law (more correctly, but infrequently, called the Titius–Bode Law). The formula is still regarded as an interesting fact, although in the intervening 200 years no one has been able to explain why the planets in the Solar System are spaced apart so regularly.
For all planets except Mercury, the formula for Bode’s Law works like this:
A = 4 + 3 × 2ⁿ
A represents the distance from the planet in question to the Sun (measured in units equivalent to 1⁄10 the distance between the Earth and the Sun). Mercury has a fixed value of A = 4. n represents the consecutive order of the planets after Mercury in the Solar System, beginning with Venus at 0: 0, 1, 2, 3…
Applying Bode’s Law to all the known planets gave the following results:
Actual distance (as measured)
n
Distance according to Bode’s Law
Sun–Mercury
3.9 units
4 + 0 = 4 units
Sun–Venus
7.2
0
4 + 3 = 7
Sun–Earth
10
1
4 + 6 = 10
Sun–Mars
15
2
4 + 12 = 16
?
?
3
4 + 24 = 28
Sun–Jupiter
52
4
4 + 48 = 52
Sun–Saturn
96
5
4 + 96 = 100
Sun–Uranus
192
6
4 + 192 = 196
If you compare the ‘actual’ distance of the planets from the Sun with the figures calculated by Bode’s law, you can see that the law produces a good (but not exact) estimate for all the planets known to astronomers up to the eighteenth century (Mercury–Saturn).
There are, however, some problems. First of all, there is the funny way that the distance to Mercury is calculated. The value for Mercury is really just put in to make the formula look like it works. Secondly, there is an obvious gap at n = 3 between Mars and Jupiter. Like Kepler, Bode thought that there must be an undiscovered planet in the gap: ‘Can one believe that the Creator of the Universe has left this position empty? Certainly not!’ When William Herschel discovered the planet Uranus in 1781, belief in the validity of Bode’s Law strengthened, because it fitted the formula so well. But the gap became even more significant. It was an obvious challenge to discover the unseen planet.
The court astronomer of the Duchy of Saxe-Gotha, Germany, Baron Franz Xaver von Zach, took up the search for the new planet in September 1800 and organized a team of two dozen astronomers to share the work. They became known as the ‘Celestial Police’. The first day of the new century (properly reckoned), 1 January 1801, brought success even before the celestial police could get to work. A member of the team (who did not even know that his efforts had been volunteered!) discovered the new planet.
The successful astronomer was a Sicilian monk, Father Giuseppe Piazzi, who came across a moving object as he constructed a star catalogue with a telescope in Palermo. As his observations progressed, it became clear that the new object was not a comet. It had a nearly circular orbit in the right zone between Mars and Jupiter, with a distance of 28 units as measured by Bode’s Law. Piazzi named the planet Ceres, after the Roman goddess of the harvest and the patron goddess of Sicily, but subsequently lost sight of the object, the sequence of his observations interrupted by illness and the passage of the asteroid out of sight behind the Sun. However, the German mathematician Carl Friedrich Gauss was able to compute its orbit, which enabled Piazzi’s planet to be relocated.
When William Herschel examined the new planet with his large telescope, he could see no disc. The planet must be small – a ‘minor planet’ – and Herschel used a new word to describe it: ‘asteroid’, meaning ‘an almost star-like object’. (The word was suggested by Herschel’s son.) But then, to everyone’s surprise, another minor planet was discovered in 1802 by the German Heinrich Wilhelm Matthias Olbers, who was a doctor in Bremen by day, and an amateur astronomer by night. Olbers located a second asteroid in 1807. A fellow German astronomer, Karl Ludwig Harding, had discovered yet another asteroid in 1804. Like Ceres, these three asteroids were named after classical goddesses: Pallas, Vesta and Juno. A further hundred new asteroids had been discovered by 1868, two hundred by 1879 and three hundred by 1890 – as astrophotography became more sensitive, so many asteroids began to spoil photographs of the stars that by the late nineteenth century they were dismissed as ‘vermin of the skies’.
The discovery of asteroids went through four phases. The first phase consisted of targeted searches by astronomers who had reason to believe there was something in the gap between Mars and Jupiter; these searches produced Ceres, Pallas, Vesta and Juno. After it became clear that there were many such asteroids, there were systematic searches by eye and by photography to sample the population. This produced thousands more. Most astronomers got bored, and searches were taken up by amateur enthusiasts caught up in the romance of discovering and naming a new world, however minor. Then it became clear that there was the potential for an asteroid to hit the Earth and cause damage, and that it might be possible to mitigate against this possibility by some sort of space intervention – pushing an asteroid found to be a threat onto a new course so it misses us, or blowing it up. NASA got involved and led searches using modern imaging techniques and computing power to process the data. In this most recent, fourth phase, still in progress, asteroids are being found by the thousands, discovering the smaller members of the population. NASA flags up the asteroids that have orbits such that they have the potential to collide with Earth, so-called ‘Near-Earth Asteroids’.
Compared to the planets of the Solar System, most asteroids are very small. It is estimated that there are 200 asteroids over 100 kilometres across, 1,000 over 30 kilometres, and perhaps 25 million over 100 metre
s. Presumably there are even bigger numbers of asteroids in the size range that extends down to 1 metre. Below that size, one could say that there are no asteroids, because they are then called meteoroids. Out of the nearly 1 million known asteroids, 500,000 have precisely determined orbits. Most lie in what is known as the ‘asteroid belt’ between Mars and Jupiter (21–33 units away from the Sun, as measured by Bode’s Law). The first asteroids to be discovered were the larger ones. Ceres is the largest at 950 kilometres in diameter. It is regarded as a ‘dwarf planet’ and looks like Mercury or the Moon. The next largest is Vesta, just over 500 kilometres in size, with a large hole in its south pole area. A meteor impact on Vesta in the past ejected a number of fragments which became a family of small, co-orbiting asteroids and meteoroids.
Like these small bits from Vesta, many asteroids are irregular fragments from the collision of one asteroid with another. Most asteroids are potato shaped, and thought to be planetesimals (small proto-planets); most planetesimals gravitated into larger masses and formed the major planets, but the strong gravity of Jupiter affected those planetesimals in the asteroid belt and prevented them from settling together and congealing into a single planet.
Many asteroids have been kicked out of the asteroid belt by near-miss collisions and encounters with large planets, some of them having been ejected into interstellar space. Some fall in towards the Sun and, if they come near the Earth, they become a natural hazard both for astronauts and for life on the surface of our planet. A few small asteroids have impacted the Earth and burnt up in the atmosphere. Some asteroids pass close enough to the Earth that they can be imaged by radar, although none of these has come close enough to pose a danger to the Earth’s inhabitants.
A few asteroids have been visited and imaged at close range by spacecraft, in recognition of their scientific importance as remnants of the early Solar System. On its way to Jupiter, the Galileo spacecraft visited Gaspra in October 1992 and Ida in August 1993. It discovered that Ida had a satellite, which was later named Dactyl – the first asteroid satellite discovered. Other spacecraft have been programmed to fly close to asteroids to take a good look in pursuit of their main missions: Deep Space 1 visited asteroid Braille in 1999; Stardust visited Annefrank in 2002; Rosetta visited Šteins in 2008 and Lutetia in 2010; China’s Chang’e 2 flew past Toutatis.
The first mission dedicated to an individual asteroid was the Near Earth Asteroid Rendezvous (NEAR) mission, which targeted Mathilde in June 1997, and then Eros, which the spacecraft orbited for a year until it was skilfully landed onto Eros’s surface on 14 February 2001, the touch down interrupting the last picture frame being transmitted back to Earth. The surface of Eros is rubble-strewn with boulders that come mostly from the meteor impact that caused its largest crater. In 2005, the Japanese Hayabusa probe studied asteroid Itokawa, and returned samples of its surface to Earth. NASA’s Dawn spacecraft inspected Vesta for a year starting in July 2011 and then went on to Ceres, entering into orbit around it in 2015. Finally, in 2019 the Japanese Hayabusa2 probe visited Ryugu, landing probes on it to gather and return samples to Earth. Returned samples of asteroids are important because they could be the same as the material from which the Solar System formed.
Pluto
A planet deliberately sought – but not a planet, and discovered by accident
Then felt I like some watcher of the skies
When a new planet swims into his ken;
Or like stout Cortez when with eagle eyes he star’d at the Pacific.
John Keats, ‘On First Looking into Chapman’s Homer’, 1817
The discovery of Uranus, the asteroids and Neptune made scientists suspect that there might be even more planets in the Solar System. This suggestion was reinforced by the fact that, by the end of the nineteenth century, it was apparent that Neptune was drifting off its calculated orbit. One speculation, offered by several scientists, was that a large planet could be hiding in the darkest and most distant reaches of the Solar System.
Mindful of Le Verrier’s and Adams’s discovery of Neptune as the source of a gravitational attraction that pulled Uranus off course, the American astronomer William Pickering calculated where the undiscovered planet might be. Pickering unsuccessfully searched the photographic archive of the Mount Wilson Observatory, California, for images that might show the mysterious new planet. Explaining his lack of success, in 1911 the Indian astronomer Venkatesh Ketakar published a calculation that posited details of the orbits of two hypothetical planets beyond Neptune, a model based on the assumption that the gravity generated by each of the three bodies affected the orbit of the others.
The American businessman Percival Lowell took a more pragmatic approach. Lowell was a member of a wealthy, influential Boston family, who had trained at Harvard as a mathematician. From the age of thirty-eight he devoted himself to astronomy, moving in 1894 to Flagstaff, Arizona, where the clear skies were favourable for astronomical observation. He built a private observatory to study the planet Mars and, starting in 1906, to search for the trans-Neptunian planet, which he termed Planet X. He devoted ten years to his search, concentrating on the regions indicated by Pickering.
How would you find a distant planet? The stars are fixed in position relative to each other, but planets are in orbit and change position quickly. Lowell and his assistants repeatedly photographed the sky in the search regions to check for star-like images that moved. Using this method, Lowell discovered 515 asteroids, but no Planet X.
Lowell Observatory remained in operation after Lowell’s death in 1916 and recommenced the search for Planet X in 1927 under its new director, Vesto Melvin Slipher. In December 1929, he hired an amateur astronomer, Clyde Tombaugh, as an assistant to take pairs of photographs two weeks apart. The pairs were put side by side in a viewing device called a ‘blink comparator’. Its operator rapidly shifts a mirror back and forth to view each photograph alternately. Any image that has moved between the exposures leaps from one position to another, and is readily identifiable. At first Slipher and his brother worked the comparator, but Tombaugh produced pictures faster than the Sliphers could process them and the brothers became bored. They delegated the task of inspecting the pictures to Tombaugh, who discovered Planet X in February 1930, at the age of twenty-four.
Tombaugh and the Sliphers received lots of advice on the name of the new planet. Lowell’s widow suggested that the planet should be named after herself. The astronomers at the Lowell Observatory took a dim view of this, given that she had been trying for ten years to get her hands on Lowell’s endowment for the observatory. The name Pluto was proposed by Venetia Burney, an eleven-year-old schoolgirl living in Oxford who was interested in classical mythology, and suggested the name of the Roman god of the underworld because Planet X was presumably dark and cold. She made the suggestion to her grandfather, a librarian at the university, after he read a newspaper article to her about the planet that raised the issue of its name. He passed the suggestion to the Oxford astronomer Herbert Hall Turner, who passed it on to his American colleagues. The name found unanimous favour in a vote of the Lowell Observatory astronomers and was announced in May 1930. A strong point in its favour was that the name started with Percival Lowell’s initials, and neatly got around the historical disinclination of astronomers to name planets after people. It left the way open, however, for Pickering to aggrandize himself with the claim that PL stood for Pickering-Lowell.
There are pre-discovery photographs of Pluto dating back to 1914, including two images taken before Lowell’s death. Pluto was much fainter than had been expected and was therefore overlooked in these early pictures. Ironically, it seems that Pluto is not massive enough to have caused the deviation of Neptune from its predicted orbit – the discrepancies were the result of errors in estimating the masses of the other planets, and using modern, more accurate values it has been shown that there is no actual deviation in Neptune’s orbit. Thus, the reason that Tombaugh discovered Pluto was not that he searched where the planet w
as calculated to be, but that he diligently searched at all.
In powerful telescopes Pluto shows an unusually large and strangely broadened image, and it looks big – but it is a lot smaller than it looks. Astronomer Jim Christy of the US Naval Observatory in Washington, DC, discovered the reason for Pluto’s odd appearance in 1978, noticing that in especially clear photographs of Pluto taken at regular intervals, the elongation seemed to rotate around Pluto. This turned out to be because Pluto has a moon that is nearly as large and as bright as the planet itself (the moon is half of Pluto’s diameter). The moon is distinctly separate from Pluto but still unusually close for a satellite, at a distance of only seventeen times its radius. The moon was named Charon, after the mythological ferryman to the Underworld, Hades, Pluto being the Roman lord of that domain.
The existence of Charon was confirmed between 1985 and 1990 when the orbital plane of Pluto and Charon became visible edge-on from Earth, producing the expected series of mutual eclipses as Pluto and Charon passed in front of each other in turn. Since the right alignment occurs for only two 5-year intervals in Pluto’s 248-year orbit, it was lucky that this happened so soon after Charon’s discovery. Any lingering doubts about the existence of the moon were removed when the Hubble Space Telescope (HST) imaged Pluto and Charon side by side in 1994. Between 2005 and 2012 the HST discovered a further four, smaller moons orbiting around Pluto. They were given names that are appropriately connected to the domain of Pluto: Styx, Nix, Kerberos and Hydra.
