The Black Cloud

by Fred Hoyle; Fred Hoyle

  ‘That’s very difficult to say without more information,’ said Marlowe, puffing smoke. ‘At the moment we don’t know whether the cloud is just a tiny fellow quite close to us or whether it’s a biggish cloud farther away. And we’ve got no idea at all of the density of the material inside it.’

  ‘If we could get the velocity of the cloud, then we should know how big it is and how far away,’ remarked Weichart.

  ‘Yes, I’ve been thinking about that,’ went on Marlowe. ‘The Australian radio boys could get the information for us. It’s very likely that the cloud consists mainly of hydrogen, and it should be possible to get a Doppler shift on the 21 cm line.’

  ‘That’s a very good point,’ said Barnett. ‘The obvious man is Leicester in Sydney. We ought to get a cable off to him right away.’

  ‘I don’t think that’s quite our job, Bill,’ Herrick explained. ‘Let’s stick to what we can do ourselves. When we’ve sent in our report, it’ll be Washington’s job to contact the Australians about radio measurements.’

  ‘But surely we ought to make a recommendation about getting Leicester’s group on to the problem?’

  ‘Certainly we can do that, and I think we ought to. What I meant was that we ought not to initiate action of this sort. The whole business is likely to have serious political implications, and I feel that we ought to keep away from such things.’

  ‘Right enough,’ broke in Marlowe; ‘politics is the last thing I want to get involved in. But obviously we need the radio boys to get the velocity. The mass of the cloud is more difficult. As far as I can see the best way, perhaps the only way, would be from planetary perturbations.’

  ‘That’s pretty archaic stuff, isn’t it?’ asked Barnett. ‘Who do it? The British, I suppose.’

  ‘Yes, h’m,’ murmured Herrick, ‘perhaps we’d better not emphasize that aspect of the matter. But the Astronomer Royal probably would be the best person to approach. I’ll make a point of it in the report, which I ought to start on as soon as possible. I think we’re agreed on the main points. Does anyone want to bring up anything further?’

  ‘No, we’ve gone over the ground pretty thoroughly, as far as we can go, that’s to say,’ answered Marlowe. ‘I think I’ll be getting back to one or two jobs that I’ve rather neglected during the last few days. I expect you’ll want to get that report finished. Glad I don’t have to write it.’

  And so they filed out of Herrick’s office, leaving him to get down to his writing, which he did forthwith. Barnett and Weichart drove back to Caltech. Marlowe went to his own office. But he found it impossible to work, so he strolled along to the library where there were several of his colleagues. A lively conversation of the colour-magnitude diagram of the stars of the galactic nucleus contrived to pass the time until it was generally agreed that the lunch hour had arrived.

  When Marlowe returned from lunch the Secretary sought him out. ‘Cablegram for you, Dr Marlowe.’

  The words on the piece of paper seemed to swell to a gigantic size:

  PLEASE INFORM WHETHER UNUSUAL OBJECT EXISTS AT RIGHT ASCENSION FIVE HOURS FORTY-SIX MINUTES, DECLINATION MINUS THIRTY DEGREES TWELVE MINUTES. MASS OF OBJECT TWO-THIRDS JUPITER, VELOCITY SEVENTY KILOMETRES PER SECOND DIRECTLY TOWARDS EARTH. HELIOCENTRIC DISTANCE 21.3 ASTRONOMICAL UNITS.

  With a startled cry Marlowe raced along to Herrick’s office, and burst in without the formality of a knock.

  ‘I’ve got it here,’ he shouted. ‘All the things we wanted to know.’

  Herrick studied the cablegram. Then he smiled somewhat wryly and said:

  ‘This alters things quite a bit. It looks as though we shall have to consult with Kingsley and the Astronomer Royal.’

  Marlowe was still excited.

  ‘It’s easy to diagnose the situation. The Astronomer Royal has supplied observational material on the planetary motions and Kingsley has done the calculations. If I know those two fellows there isn’t much chance of a mistake there.’

  ‘Well, it’s easy enough to do a quick check. If the object is 21.3 astronomical units distant and it’s moving towards us at seventy kilometres per second, then we can soon work out how long it should take to reach us, and we can compare the answer with Weichart’s estimate of about eighteen months.’

  ‘Right you are,’ said Marlowe. He then jotted the following remarks and figures on a sheet of paper:

  Distance 21.3 astr. units = 3 × 1014 cm approximately.

  Time required to travel this distance at a speed of 70 km per sec.

  ‘Perfect agreement,’ exclaimed Marlowe. ‘And what’s more, the position they give is almost dead on our position. It all fits together.’

  ‘This makes my report a much more difficult matter,’ Herrick said with a frown. ‘It really should be written in consultation with the Astronomer Royal. I think we ought to get both him and Kingsley over here as soon as possible.’

  ‘Absolutely right,’ agreed Marlowe. ‘Get the Secretary on to it right away. It should be possible to get ’em over in about thirty-six hours, the morning after tomorrow. Better still, let your friends in Washington make the arrangements. And about the report, wouldn’t it be a good idea to write it in three parts? Part one could deal with our discoveries here at the Observatory. Part two would be contributed by Kingsley and the Astronomer Royal. And part three would be an account of our conclusions, especially the conclusions we reach when the British get here.’

  ‘There’s a great deal in what you say, Geoff. I can get part one finished by the time our friends arrive. We can leave part two to them, and lastly we can thrash out our conclusions.’

  ‘Excellent. I reckon you’ll probably get through by tomorrow. How about bringing Alison over for dinner tomorrow night?’

  ‘I’d be glad to, delighted to, if I can get through by tomorrow afternoon. Can I leave it until then?’

  ‘Sure, that’s fine. Just let me know tomorrow,’ said Marlowe getting up.

  As Marlowe was leaving, Herrick said:

  ‘It’s pretty serious, isn’t it?’

  ‘It certainly is. I had a sort of premonition when I first saw Knut Jensen’s pictures. I didn’t realize how bad it was until this cable arrived. The density works out in the region of 10–9 to 10–10gm. per cm3. That means it’ll block out the Sun’s light entirely.’

  Kingsley and the Astronomer Royal arrived in Los Angeles early on the morning of 20 January. Marlowe was waiting to meet them at the airport. After a quick breakfast in a drug store they hit the freeway system to Pasadena.

  ‘Goodness me, what a difference from Cambridge,’ grunted Kingsley. ‘Sixty miles an hour instead of fifteen, blue skies instead of endless rain and drizzle, temperature in the sixties even as early in the day as this.’

  He was very weary after the long flight, first across the Atlantic, then a few hours’ waiting in New York – too short to be able to do anything interesting, yet long enough to be tiresome, the epitome of air travel, and lastly the trip across the U.S.A. during the night. Still it was a great deal better than a year at sea getting round the Horn, which is what men had to do a century ago. He would have liked a long sleep, but if the Astronomer Royal was willing to go straight to the Observatory, he supposed he ought to go along too.

  After Kingsley and the Astronomer Royal had been introduced to those members of the Observatory that they had not previously met, and after greetings with old friends, the meeting started in the library. With the addition of the British visitors it was the same company that had met to discuss Jensen’s discovery the previous week.

  Marlowe gave a succinct account of this discovery, of his own observations, and of Weichart’s argument and startling conclusion.

  ‘And so you see,’ he concluded, ‘why we were so interested to receive your cablegram.’

  ‘We do indeed,’ answered the Astronomer Royal. ‘These photographs are most remarkable. You give the position of the centre of the cloud as Right Ascension 5 hours 49 minutes, Declination minus 30 degrees 16 minutes.
That seems to be in excellent agreement with Kingsley’s calculations.’

  ‘Now would you two care to give us a short account of your investigations?’ said Herrick. ‘Perhaps the Astronomer Royal could tell us about the observational side and then Dr Kingsley could say a little about his calculations.’

  The Astronomer Royal gave a description of the displacements that had been discovered in the positions of the planets, particularly of the outer planets. He discussed how the observations had been carefully checked to make sure that they contained no errors. He did not fail to give credit to the work of Mr George Green.

  ‘Heavens, he’s at it again,’ thought Kingsley.

  The rest of the company heard the Astronomer Royal out with interest, however.

  ‘And so,’ he concluded, ‘I’ll hand over to Dr Kingsley, and let him outline the basis of his calculations.’

  ‘There is not a great deal to be said,’ began Kingsley. ‘Granting the accuracy of the observations that the Astronomer Royal has just told us about – and I must admit to having been somewhat reluctant at first to concede this – it was clear that the planets were being disturbed by the gravitational influence of some body, or material, intruding into the solar system. The problem was to use the observed disturbances to calculate the position, mass, and velocity of the intruding material.’

  ‘Did you work on the basis that the material acted as a point mass?’ asked Weichart.

  ‘Yes, that seemed to be the best thing to do, at any rate to begin with. The Astronomer Royal did mention the possibility of an extended cloud. But I must confess that psychologically I’ve been thinking in terms of a condensed body of comparatively small size. I’ve only just begun to assimilate the cloud idea, now that I’ve seen these photographs.’

  ‘How far do you think your wrong assumption affected the calculations?’ Kingsley was asked.

  ‘Hardly at all. So far as producing planetary disturbances is concerned, the difference between your cloud and a much more condensed body would be quite small. Perhaps the slight differences between my results and your observations arise from this cause.’

  ‘Yes, that’s quite clear,’ broke in Marlowe amid aniseed smoke. ‘How much information did you need to get your results? Did you use the disturbances of all the planets?’

  ‘One planet was enough. I used the observations of Saturn to make the calculations about the Cloud – if I may call it that. Then having determined the position, mass, etc., of the Cloud, I inverted the calculation for the other planets and so worked out what the disturbances of Jupiter, Mars, Uranus, and Neptune ought to be.’

  ‘Then you could compare your results with the observations?’

  ‘Exactly so. The comparison is given in these tables that I’ve got here. I’ll hand them round. You can see that the agreement is pretty good. That’s why we felt reasonably confident about our deductions, and why we felt justified in sending our cable.’

  ‘Now I’d like just to know how your estimates compare with mine,’ asked Weichart. ‘It seemed to me that the Cloud would take about eighteen months to reach the Earth. What answer do you get?’

  ‘I’ve already checked that, Dave,’ remarked Marlowe. ‘It agrees very well. Dr Kingsley’s values give about seventeen months.’

  ‘Perhaps a little less than that,’ observed Kingsley. ‘You get seventeen months if you don’t allow for the acceleration of the Cloud as it approaches the Sun. It’s moving at about seventy kilometres per second at the moment, but by the time it reaches the Earth it’ll have speeded to about eighty. The time required for the Cloud to reach the Earth works out at nearly sixteen months.’

  Herrick quietly took charge of the discussion.

  ‘Well, now that we understand each other’s point of view, what conclusions can we reach? It seems to me that we have both been under some misapprehension. For our part we thought of a much larger cloud lying considerably outside the solar system, while, as Dr Kingsley says, he thought of a condensed body within the solar system. The truth lies somewhere between these views. We have to do with a rather small cloud that is already within the solar system. What can we say about it?’

  ‘Quite a bit,’ answered Marlowe. ‘Our measurement of the angular diameter of the Cloud as about two and a half degrees, combined with Dr Kingsley’s distance of about 21 astronomical units, shows that the Cloud has a diameter about equal to the distance from the Sun to the Earth.’

  ‘Yes, and with this size we can immediately get an estimate of the density of the material in the Cloud,’ went on Kingsley. ‘It looks to me as though the volume of the cloud is roughly 1040 c.c. Its mass is about 1·3 × 1030 gm., which gives a density of 1·3 × 10–10 gm. per cm3.’

  A silence fell on the little company. It was broken by Emerson.

  ‘That’s an awful high density. If the gas comes between us and the Sun it’ll block out the Sun’s light completely. It looks to me as if it’s going to get almighty cold here on Earth!’

  ‘That doesn’t necessarily follow,’ broke in Barnett. ‘The gas itself may get hot, and heat may flow through it.’

  ‘That depends on how much energy is required to heat the Cloud,’ remarked Weichart.

  ‘And on its opacity, and a hundred and one other factors,’ added Kingsley. ‘I must say it seems very unlikely to me that much heat will get through the gas. Let’s work out the energy required to heat it to an ordinary sort of temperature.’

  He went out to the blackboard, and wrote:

  Mass of Cloud 1.3 × 1030 grams.

  Composition of Cloud probably hydrogen gas, for the most part in neutral form.

  Energy required to lift temperature of gas by T degrees is 1·5 × 1·3 × 1030 RT ergs

  where R is the gas constant. Writing L for the total energy emitted by the Sun, the time required to raise the temperature is 1·5 × 1·3 × 1030 RT/L seconds

  Put R = 8·3 × 107, T = 300, L = 4 × 1033 ergs per second gives a time of about 1·2 × 107 seconds, i.e. about 5 months.

  ‘That looks sound enough,’ commented Weichart. ‘And I’d say that what you’ve got is very much a minimum estimate.’

  ‘That’s so,’ nodded Kingsley. ‘And my minimum is already very much longer than it will take the Cloud to pass us by. At a speed of 80 kilometres per second it’ll sweep across the Earth’s orbit in about a month. So it looks to me pretty certain that if the Cloud does come between us and the Sun it’ll cut out the heat from the Sun quite completely.’

  ‘You say if the Cloud comes between us and the Sun. Do you think there’s a chance it may miss us?’ asked Herrick.

  ‘There’s certainly a chance, quite a chance I’d say. Look here.’

  Kingsley moved again to the blackboard.

  ‘Here’s the Earth’s orbit round the Sun. We’re here at the moment. And the Cloud, to draw it to scale, is over here. If it’s moving like this, dead set for the Sun, then it’ll certainly block the Sun. But if it’s moving this second way, then it could well miss us altogether.’

  KINGSLEY’S DRAWING OF PRESENT SITUATION

  KINGSLEY’S DRAWING OF SITUATION IN SIXTEEN MONTHS’ TIME

  ‘It looks to me as if we’re rather lucky,’ Barnett laughed uneasily. ‘Because of the Earth’s motion round the Sun, the Earth will be on the far side of the Sun sixteen months hence when the Cloud arrives.’

  ‘That only means that the Cloud will reach the Sun before it reaches the Earth. It won’t stop the sunlight being blocked out if the Sun gets covered, as in Kingsley’s case (a),’ Marlowe remarked.

  ‘The point about your cases (a) and (b),’ said Weichart, ‘is that you only get case (a) if the Cloud has almost exactly zero angular momentum about the Sun. It only needs a very slight angular momentum and we have case (b).’

  ‘That’s exactly it. Of course my case (b) was only one example. The Cloud could equally well sweep past the Sun and the Earth on the other side, like this:’

  ‘Do we have anything to say about whether the Cloud is co
ming dead at the Sun or not?’ asked Herrick.

  ‘Not on the observational side,’ answered Marlowe. ‘Look at Kingsley’s drawing of the present situation. Only a very slight difference of velocity makes a big difference, all the difference between the Cloud hitting and missing. We can’t say yet which it’s to be, but we can find out as the Cloud comes in nearer.’

  ‘So that’s one of the important things to be done,’ concluded Herrick.

  ‘Can you say anything more from the theory?’

  ‘No, I don’t think we can; the calculations aren’t accurate enough.’

  ‘Astonishing to hear you distrusting calculations, Kingsley,’ remarked the Astronomer Royal.

  ‘My calculations were based on your observations, A.R.! Anyway I agree with Marlowe. The thing to do is to keep a close watch on the Cloud. It should be possible to see whether we’re going to have a hit or a miss without too much trouble. A month or two should settle it, I suppose.’

  ‘Right!’ answered Marlowe. ‘You can rely on us to watch this fellow from now on as carefully as if it was made of gold.’

  After lunch Marlowe, Kingsley, and the Astronomer Royal were sitting in Herrick’s office. Herrick had explained the plan of writing a joint report.

  ‘And I think our conclusions are very clear. May I just outline them for you?

  1. A cloud of gas has invaded the solar system from outer space.

  2. It is moving more or less directly towards us.

  3. It will arrive in the vicinity of the Earth about sixteen months from now.

  4. It will remain in our vicinity for a time of about a month.

  ‘So if the material of the Cloud interposes itself between the Sun and the Earth, the Earth will be plunged into darkness. Observations are not yet sufficiently definitive to decide whether or not this will occur, but further observations should be capable of deciding this question.’