62 16 The Problem of Plutocertainly moving far beyond Neptune, and was thought to be considerably larger than the Earth.. A satellite, Charon, was found in 1977; its diameter was more th
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In August, there was a meeting of the Nomenclature Commission of the International Astronomical Union, held in Prague I could not go – particularly disappointing because I was, for many years, a member of that Commission, and enjoyed working with it John Mason did attend, and on return told us what had been decided.
In 1930 Clyde Tombaugh, at the Lowell Observatory in Arizona, was carrying out a systematic search for a planet moving beyond the orbit of Neptune using a telescope which had been obtained specially for the purpose The existence of
“Planet X” had been predicted by Percival Lowell, founder of the Observatory, from slight irregularities in the movements of Neptune and (particularly) Uranus It was not long before Tombaugh found a body not far from the position given by Lowell It was
Chapter 16
The Problem of Pluto
Clyde Tombaugh at the blink comparator (Credit: Lowell)
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certainly moving far beyond Neptune, and was thought to be considerably larger than the Earth Naturally, it was assumed to be a planet, and it was named after the God of the Underworld (Conveniently, the symbol, PL, also fitted in with Percival Lowell’s name.)
From the outset Pluto was an enigma Its orbit was much more eccentric than those of the other planets, and was also more highly inclined (17°) Its orbital period was almost 248 years, and at perihelion it moved closer-in than Neptune; the last perihelion fell in 1989, and between 1979 and 1999 its distance from the Sun was less than that of Neptune, though its orbital inclination meant that there could
be no chance of collision More worrying was the revelation that it was not only smaller than the Earth but even smaller than our Moon and Triton, the main satellite
of Neptune With a diameter of only 1,444 miles, it simply did not fit in with the general pattern of the Solar System
A satellite, Charon, was found in 1977; its diameter was more than half that of Pluto, and its orbital period, 6.3 days, was the same as Pluto’s axial rotation period, so that the two were tidally locked To an observer standing on the surface of Pluto, Charon would remain stationary in the sky Could the pair be regarded as a double planet – and
in any case, could the diminutive Pluto really deserve full planetary status? There was
no general agreement, but the situation changed in 1992 when another planetary object was found moving further-out than Neptune For some reason or other it has never been given a proper name, and is still known by its catalogue listing, 1992 QB1 It proved to
be the first of many By now over a thousand Trans-Neptunians are known
Much earlier, the existence of a swarm of asteroid-sized bodies in this remote part of the Solar System had been mooted by the Dutch astronomer Gerard Kuiper, and today we refer to the Kuiper Belt (In fact, a less positive suggestion had been made previously by Kenneth Edgeworth, in Ireland, and we still sometimes hear it called the Edgeworth-Kuiper Belt.) Pluto is the brightest KBO (Kuiper Belt Object) but it is not the largest That distinction, so far as we know, belongs to Eris, which
is around 1,500 miles across, while others such as Quaoar and Varuna are compa-rable If Pluto is to be ranked as a planet, then so must Eris, Quaoar and the rest This seems to make no sense at all
I wish I had been at that IAU meeting – I would have had a great deal to say! The first official proposal was illogical; Pluto was to be retained as a planet and to add three more: Charon and Eris together with Ceres, the largest of the main-belt asteroids, even though Ceres is a mere 600 miles in diameter and Charon is the satellite of Pluto (the excuse here, that the centre of gravity of the Pluto-Charon system lies between the two bodies, was surely irrelevant; after all, the centre of gravity of the Jupiter-Sun system lies above the solar surface) It was fairly clear that the idea of keeping Pluto as a bona fide planet was due to sentiment and tradition The proposal was put to a general vote, and was defeated The Commission then came up with a new recommendation: A planet would be a body moving round the Sun, massive enough to assume a spherical form, and to have cleared other bodies out of its orbit A dwarf planet would be in orbit round the Sun and to have assumed
a spherical form, but without clearly its orbit All others would be lumped together
as Small Solar System bodies
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This meant that the only accepted planets would be the familiar ones, from Mercury to Neptune; nobody was likely to quarrel with that The dwarf planets would be Eris, Pluto and Ceres The Committee’s proposal was accepted, but to me
it seems to muddy the waters Why should Ceres be a dwarf planet, and Pallas, the second main-belt asteroid, simply an SSSB? My suggestion would have been to class all the minor bodies orbiting the Sun as “planetoids” But the IAU is the controlling body of world astronomy, and the die is cast
Despite this, there is still considerable resentment about the demotion of Pluto
It is now known to have three satellites, though the two new discoveries, Nix and Hydra, are very small, and a certain amount of surface detail has been made out with the Hubble Space Telescope; we will know much more in 2015 when, if all goes well, the New Horizons space-craft will swoop past it But insofar as its status
is concerned, we have to be logical rather than sentimental, and relegate it from the Premiership of the Solar System A planetoid, certainly; A remote asteroid, possibly;
A KBO, undoubtedly; But a planet it isn’t
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Earth and Venus have often been regarded as twins In so far as size and mass are concerned this is true enough, but they are certainly not identical Drs Fred Taylor and David Rothery joined me to talk about the results from the latest mission there, Venus Express.
During the early years of planetary space research Venus was regarded as a prime target, because it did not seem to be really hostile – probably more welcom-ing than Mars Without gowelcom-ing back 80 years to the ideas of Svante Arrhenius, who believed Venus to be in a state similar to that of the Earth during the Carboniferous
Chapter 17
Non-identical Twins
Venus South polar from Venus express (Credit: NASA)
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Period, when the coal measures were being laid down and the lands were covered with lush tropical vegetation, there seemed no reason to doubt that there might be oceans, and that the climate was no more than tolerably hot The probes of the 1960s and 1970s showed that this attractive picture was very far from the truth; the atmosphere was made up chiefly of carbon dioxide, the surface pressure was around 100 times that of the Earth’s air at sea-level, and the temperature was far too high for advanced life-forms of our kind Moreover, the clouds were rich in sul-phuric acid The U.S Magellan orbiter surveyed the whole surface in detail, and as
a potential colony, Venus was ruled out; the main attention swung back to Mars Venus is a world dominated by vulcanism There are lava-flows everywhere, and there are craters together with deep valleys – though impact craters are rare; there is overwhelming evidence that the whole landscape is “young”, and has been re-surfaced
in what we may call the relatively recent past Whether the volcanoes are active now is a matter for debate, but most people believe they are Astronauts may well
be able to survey Venus from a safe distance, but certainly not yet awhile, and a manned landing there is obviously quite out of the question Venus may have been named after the Goddess of Beauty and is a glorious sight when shining down in the evening or morning sky, but conditions there are much more akin to the conven-tional idea of the Inferno
Now we have a new probe, Venus Express, which was launched on 9 November
2005 from the Baikonur Cosmodrome in Kazakhstan, by a Soyuz-Fregat rocket It reached Venus on 11 April 2006, after a journey lasting 150 days, and was put into closed orbit round the planet The orbit is eccentric; the distance from Venus ranges between 156 and 41,000 miles (250–66,000 km) and is polar, with a period
of 24 h Transmissions began immediately, and it was clear that the mission was a complete success It was scheduled to operate until May 2009, but was still working perfectly in 2010 There is no lander; this is purely an atmospheric space-craft
In addition to analysing the atmosphere, and measuring its temperature, Venus Express carries a camera to operate in the visible, ultra-violet and near-infra-red regions of the spectrum One early surprise was that the polar vortex, already known to exist, is like a hurricane with two “eyes” instead of one; nothing of this kind has ever been known on Earth There is also a magnetometer This may seem rather strange in view of the fact that Venus (unlike Mercury) has no magnetic field strong enough to be detected, but the magnetometer should be able to study the interactions between the solar wind and the uppermost part of Venus’ carbon-dioxide “air” An observer standing on the surface would be able to see the Sun dimly through the clouds, and this leads on to a new theory about the Ashen Light – that is to say the faint visibility of the night side of Venus, seen from the Earth during the crescent stage Its reality is not in doubt, but its origin has led to many explanations, some plausible and others bizarre In the nineteenth century Franz von Paula Gruithuisen maintained that they were illuminations lit by the Venusians
to celebrate the accession of a new Emperor, while others believed that there were electrical storms in the upper atmosphere, similar in nature to our aurorae but much stronger Of course, the “Earthshine” on the non-sunlit side of the Moon is familiar
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enough, but Venus has no satellite It is now believed that the cause is simply the glow from the fiercely hot surface passing through the atmosphere
Why are Venus and Earth so different? Surely it must be due to Venus’ lesser distance from the Sun, 67 million miles against our 93 million When the planets were formed from the solar nebula, the two worlds may well have been similar, starting to evolve along similar lines The Sun then was not as powerful as it is now, and probably both Venus and Earth developed seas, pleasantly warm but no more But as time went by, the Sun’s luminosity increased Earth was at a safe distance; Venus was not The oceans boiled away and the carbonates were driven out of the rocks, so that the atmosphere became thick with carbon dioxide In a very short time, astronomically speaking Venus was transformed into the inferno
of today; there was what may be called a “runaway greenhouse” effect If life had ever started there, it was ruthlessly snuffed out
This sequence of events may or may not be accurate, but it does seem plausible Venus Express will help us to solve some of the problems which still puzzle us, but
no further specialised probes there have been funded as yet, though in 2008 Messenger is due to make some observations as it flies by Venus on its way to Mercury We may not be able to go there, and perhaps this will never be possible, but we must surely be deeply interested in the Earth’s non-identical twin
Trang 10P Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_18,
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Some time ago we did present a “musical” programme, but that was conventional music on cosmic themes Our last programme of 2006 was different – the music of the stars themselves I was joined by two leaders in the field of astroseismology, Drs Don Kurtz and Yvonne Elsworth.
Chapter 18
The Sounds of the Stars
Coronal loops (Credit: NASA SOHO)
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The concept of the “Music of the Spheres” is far from new; it goes back to the time of Pythagoras, and survived until a surprisingly late stage in astronomical history Kepler, for example, took it very seriously indeed Today everyone knows about radio waves from space, and there are people who fondly imagine that you can fit
up a receiver and listen to the Pole Star Alas, this cannot be done Sound waves cannot travel in a vacuum, and there is very little air above a few tens of miles
On Mars, where the atmospheric pressure is very low (less than 10 mbar, against around 1,000 mbar at sea-level on Earth), even the most raucous auctioneer or football referee would struggle to make himself heard Sound-waves are pressure waves, and depend upon a medium of some sort The more rarefied the medium, the higher the frequency of sound waves within it Blow up a balloon filled with the light gas helium, inhale some helium from the balloon, and your voice will be very squeaky until you clear your lungs!
Helioseismology is the study of the propagation of acoustic (pressure) waves in the Sun; it has turned out to be immensely valuable The waves are generated by convection near the Sun’s surface – that is to say, in the convection zone – and some of the frequencies make the Sun “ring like a bell” They reach the bright surface or photo-sphere, and set up oscillations which we can detect by means of the familiar Doppler principle Even from close range, we would be unable to hear the solar music Our ears can detect sounds over only a limited range of frequencies For example, not many people can hear the squeaks of bats (I can, but even now, when I am over 80, I still have rather exceptional hearing.)
Explosive events at or near the Sun’s surface seem to trigger acoustic waves which penetrate to a certain depth and are then bounced back when they encounter gas-layers of different densities A sort of “loop” forms, and the Sun would be a noisy place – if we could hear it The acoustic waves which bounce between the ends of the loops produce a phenomenon known as a standing wave The loops – essentially magnetic phenomena – have been said to be analogous to a simple guitar string; pluck the string, and you will hear a musical note In the cosmic version, the sound waves generated travel in the Sun and are linked with vibrations, which can be tracked Among the results from helioseismology is the revelation that the Sun spins in a rather unexpected way Of course, it has long been known that we are dealing with differential rotation; at the equator, the rotation period is 26 days, while at the poles the period is 9 days longer – to measure this, all we have to do is to track sunspots
as they are carried across the disk Obviously, we cannot see the far side – the Sun
is not transparent! – but sound waves travel through the globe comparatively quickly, and by studying them we can actually find out what is happening there This is important, because outbursts such as solar flares can not only cause disruption of radio communications, but can also prove really dangerous to astronauts who are outside the protective shield of the Earth’s atmosphere If the new methods can locate an active area on the far side, we can easily tell when the rotation will bring
it on to the Earth-facing side – and astronauts can make sure that they are protected
It takes an acoustic wave only one and a half hours to travel right through the Sun, and the inner regions of the Sun rotate in the way that a solid sphere would do