The Sky at Night Phần 8 pps

A long-known comet, Holmes, surprised us all by suddenly flaring up to a million times its normal brightness, and so I thought that this would be a good moment to talk about cosmic debri

P Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_30,

© Springer Science+Business Media, LLC 2010

The year 2008 began in a rather unexpected way A long-known comet, Holmes, surprised us all by suddenly flaring up to a million times its normal brightness, and so I thought that this would be a good moment to talk about cosmic debris – the “bits and pieces” of the Solar System I was joined by two leading experts,

Dr Simon Green and Dr Richard Greenfield.

Chapter 30

Cosmic Debris

Encke’s Comet (Credit: Spitzer)

118 30 Cosmic Debris The Sun’s family is dominated by the eight planets, from Mercury out to Neptune, together with their satellites But we must not forget that there are other members too; comets and asteroids, for example There is also a vast amount of cosmic dust We have just seen a comet take us by surprise, so let us begin with these strange, wraithlike objects, which can sometimes look so much more impor-tant than they really are

A comet is not a solid, substantial body similar to a planet The only fairly substan-tial part is in fact the nucleus, seldom more than a few miles across, and made up

of rocky particles and ice; the mass is very small compared with that of a planet or even a satellite, such as the Moon When the comet moves in towards the Sun and

is heated, it may develop a tail of either gas or dust; some comets may produce both Tails always point away from the Sun Gas-tails do this because the particles are driven out by what is called the solar wind and dust-tails because of the pressure

of sunlight (and light does produce a pressure, albeit a very weak one) Most comets move in very eccentric orbits, and there are many with short periods, so that we always know when and where to expect them Encke’s Comet has a period of only just over 3 years, so that it is an old friend Like most short-period comets, it seldom becomes bright enough to be seen with the naked eye The only bright comet which returns reasonably often is Halley’s, whose period is 76 years; it was last at perihelion

in 1986 and is next due in 2061 Comets are generally named after their discoverers, but occasionally after the mathematicians who first computed their orbits, as with both Halley and Encke Really spectacular comets have much longer periods, of centuries, millennia, or even millions of years, so they cannot be predicted Thus, Hyakutake’s Comet of 1997 will be with us again about the year 14,000 ad

I wonder whether any astronomer of that time will remember to search for it as it draws inward?

As a comet moves along, it leaves a “dusty” trail behind it If Earth passes through such a trail, the tiny particles dash into the upper air, moving at relative speeds of up to 45 miles per second; they become heated by friction against the air-particles and burn away, producing what are termed meteors – more commonly called shooting-stars We plough through a number of trails each year, and so there are a number of meteor showers, some rich and others sparse; there are also “sporadic” meteors, not connected with any particular showers and which may come from any direction at any moment Because the meteors in a shower are travelling in parallel paths, they will all seem to come from one definite point in the sky, known as the radiant Thus, the August meteors have their radiant in the constellation of Perseus, and we call them Perseids

Meteors are of sand-grain size, and burn out when they are at least forty miles above the ground, but we also encounter larger bodies, which are too big to burn away and land intact; these are called meteorites Note that meteorites are not connected with comets, and have absolutely nothing to do with meteors; they come from the belt of asteroids, which are miniature planets moving between the orbits

of Mars and Jupiter Meteorites may produce craters, and they are fascinating places; the famous Meteor Crater in Arizona is a well-known tourist attraction

It was formed about 50,000 years ago, long before the area was inhabited – had the

30 Cosmic Debris

meteorite fallen upon a city, the death-roll would have been colossal Many other impact craters are known, but there is no well-authenticated case of any human casualty, though several people have had narrow escapes

Could the Earth be hit by a missiles massive enough to do really serious damage

to the world? The answer is: “Yes” There is strong evidence that a major impact occurred about 65 million years ago, and resulted in a climate change which proved fatal to the dinosaurs A similar impact may happen at any time – we can only hope that if so, we will cope with the situation better than the dinosaurs did! The chances

of such a disaster in the near future are slight, but are not nil Our Moon is crowded with impact craters, and so is the planet Mercury; both these worlds are airless, and

so the craters remain, whereas most of Earth’s craters have been eroded away by the action of wind and water

Comets, then? The tails are so rarified that they are harmless, but it is true that a hit from an icy nucleus, several miles across, would cause wide devastation Ancient people were terrified of comets, and believed them to be prophets of doom, which is not surprising – they can be remarkably spectacular Great comets have been rare in recent years, but several were seen during the nineteenth century, and the comets of

1811, 1843 and 1882 were brilliant enough to cast shadows Astronomers would give

a warm welcome to a visitor of this kind, provided that it did not come too close

On June 30, 1908, there was a kind of explosion in the Tunguska region of Siberia which blew pine trees flat over a wide area Nobody was hurt because the area was unpopulated, but if the explosions had happened at a slightly different era,

it might have devastated the city of St Petersburg What was the body responsible?

No fragments were found, and the body may have been a fragment of a comet which exploded well above ground Another fall in Siberia occurred on 12 February 1947,

in the Sikhote-Alin region This was certainly a meteorite, because fragments of it were found, plus many small craters

Comets, meteors and meteorites are the stray members of the Solar System They may be junior members of the Sun’s family – but there are few sights in nature equal to that of a great comet, with a gleaming head and a tail sweeping gracefully across the sky

P Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_31,

© Springer Science+Business Media, LLC 2010

It was some time since “Sky at Night” had been devoted to the Sun, and it was the plan for April, but I could not ignore a vital news item Mr Gordon Brown’s Labour Government, always the enemy of anything that could not either be taxed or else turned to its own political advantage, was plotting to cut off funds to Jodrell Bank, our great radio astronomy observatory, and cripple al the projects associated with

it The loss would be irreparable, and something had to be done.

There were protests not only from scientists, but also from the general public Ever since it had been opened, in 1957, Jodrell Bank had led the way – and Bernard Lovell, who had created it, was still as active as ever I wanted to play my part in the clamor, and I proposed going to Cambridge and talking to the Astronomer Royal Permission was refused, “Sky at Night” was to remain apolitical.

“All right In this case, I will at once retire from the programme”.

Reply: “Then someone else can go in your place”.

“No; either I do it, or I stand down” I meant it To my immense relief, permission was then granted.

Chapter 31

Nearest Star

Stereo Mission Sun (Credit: NASA)

122 31 Nearest Star

This was the first time I had put my foot down so firmly, and I imagine that it will also be the last I went to Cambridge and talked to Lord Rees about theout-standing Astronomer Royal who has done so much for British and world science Well, Jodrell has been reprieved Other important astronomical projects are threat-ened, but one battle has been won I am not conceited enough to think that I could make any real contribution, but Martin Rees is a key figure, and at least “The Sky

at Night” nailed its colors to the mast.

The interview took up the first part of the program, after which we returned to the Sun My guests were two leading experts, Dr Tim Horbury and Dr Chris Davis.

There are two good reasons for choosing this month to discuss the Sun First, the last solar cycle has come to its end, and the new cycle has just started Second, there

is news of the latest solar probes Ulysses is finishing its immensely successful career; Stereo is starting a career, which we can hope will be equally fruitful Ulysses is a box-shaped probe, 11 ft high and spin-stabilized, carrying a variety

of instruments It was designed to study the poles of the Sun – which is not possible from Earth because we always view the globe reasonably broadside-on Rather surprisingly, putting a space craft into a path, which takes it well away from the plane of the ecliptic, is not easy Ulysses was launched with the Shuttle Discovery

on December 6, 1990, and went out as far as Jupiter, so that it could use the gravi-tational pull of the Giant Planet to throw it into the required orbit It functioned well until the late summer of 2008 – a total of more than 17 years, which is longer than

a solar cycle It surveyed the Sun’s north polar regions in 1994–1995, and the south pole in 2000–2001 Unexpectedly, it was found that the two poles are not alike, at least magnetically The Sun is not a magnetic monopole, because the south magnetic pole is not a clear-cut point; it is decidedly diffuse

The average duration of a solar cycle is 11 years old At maximum, the disk is very active with many sunspot-groups and faculae; near minimum there may be no spots at all for many successive days The last maximum fell in late 2000, so that Ulysses was able to follow the Sun in all its moods It also found that dust entering the Solar System from outer space is about 30 miles more time abundant than had been previously believed Ulysses was able to make useful observations of Jupiter, and even found time to examine several comets

The two Stereo probes were launched from Cape Canaveral, on a Delta II rocket

on October 26, 2006, and were fully operational by 2007 They were almost identical, but have different orbital periods, 347 days for the first vehicle (A) and 387 for the second (B); (A) moves in a heliocentric orbit inside that of Earth, while the orbit of (B) is outside ours Because (A) is moving faster than (B), the two are separating

at a rate of half a million miles per year, but the main observational programs were scheduled to last for a minimum of 2 years Working together, the probes can provide stereoscopic results concerning the Sun – something which has never been previously achieved Valuable data were soon received, notable with regard to the solar wind and to the violent outbursts known as Coronal Mass Ejections (CMEs) From the viewpoint of manned-space flight, in particular, we need to find out as

31 Nearest Star

much as we can about “space weather” Travel beyond the atmosphere at the time

of a major CME is emphatically not to be recommended

Comets have come under scrutiny, and a very bright visitor Comet McNaught, and its entry at just the right moment Stereo also saw the tail of a familiar periodical comet, Encke’s, being literally torn off by the onslaught of a CME Fortunately, the comet was quite unfazed, and promptly produced a new tail!

Equipment carried by the Stereos makes it possible to study the region close to the Sun, between the Sun and the Earth Here, we might well find “Vulcanoids”, asteroids moving in stable orbits The name comes from Vulcan, the planet once thought to move in stable orbits Vulcan does not exist, but tiny Vulcanoids may, and if so Stereo could well locate them If so, I hereby promise that “The Sky at Night” will give a suitable prize to the discoverer of the first Vulcanoid!

Time will tell

P Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_32,

© Springer Science+Business Media, LLC 2010

This was “a programme with a difference” NASA’s Phoenix probe was scheduled

to land on Mars on May 25, and clearly our followers expected us to cover it In olden days, we would have put on a special programme, but this is impossible now Also, our producer, Jane Fletcher, was on holiday somewhere in Scotland, and could not be contacted We did not want to let our faithful viewers down, so we decided to do the programme ourselves and broadcast it on our web.

Chapter 32

The Flight of the Phoenix

Phoenix Lander on Mars (Credit: NASA)

126 32 The Flight of the Phoenix

I must stress that Chris Lintott was the guiding spirit, acting as planner, organiser and figurehead We enlisted the aid of Pete Lawrence (who else?) and Peter Grindrod, research Fellow at U.C.I., and converted my dining-room into a studio, which worked quite well We had no producer (which did not matter), and no director (which mattered even less), but we had only one fixed camera; we took turns in “shooting”, and we could certainly have done with a mobile camera and an operator Of course, NASA was doing a live programme and we were able to use it whenever we liked.

We did what publicity we could, and at the appointed time we went on the air, heralded by the usual “Sky at Night” Sibelius music Obviously, we were limited, but I think I can say that all went well – including our links with NASA The programme was watched by many thousands of people, and reactions were very favourable

So the improvised programme was well worth doing, and we did not let our viewers down.

We were all very aware of the importance of the missions Phoenix was a new

type of probe By the start of 2000, there were several space craft studying the red planet, including the highly successful Mars Reconnaissance Orbitor (MRO) and

the two amazing rovers, Spirit and Opportunity, which showed no signs of flagging

even though they had been active for so much longer than their planners had dared

to hope Phoenix would not move around after arrival, and neither would it depend

on airbags to cushion its landing – it was too massive for that Instead it would use parachute braking and then retro-pockets, finally touching down gently in the Vastitas Borealis – the Martian Arctic At the time of landing, that region was in constant sunlight; not until the following September would Phoenix see its first sunset It could not expect to survive through the long, bitterly cold night, so that its active lifetime was bound to be limited It would not last for more than 90 sols (92 Earth days, so the planners thought)

The whole journey would cover over 420 million miles, between August 4,

2007, and May 25, 2008, but the last 7 min would be the most nerve-racking of all Phoenix would plunge into the Martian atmosphere, and reduce its speed from 13,000 mph to virtually zero; for that vital 7 min all contact would be cut off We

at home would simply have to wait Remember too, that at time it took radio signal

15 min to travel from Mars to Earth, so that we could not possibly receive a signal until a quarter of an hour after the actual touch-down I think we were all tense as

we assembled in the dining room – partly also because we had no real idea how successful our broadcast would be We had announced it as part of the usual “Sky

at Night” series, but it was completely unofficial We hoped for the best

Chris opened – as I had been doing for over 50 years: “Good evening” He intro-duced Peter Grindrod and me (it was Peter’s first appearance on “Sky at Night”, though it certainly will not be the last) We all made comments, and then we linked

up to NASA Phoenix was speeding towards Mars When it entered the planet’s atmosphere, there would be an agonising 7 min of silence

NASA admitted that there was only a 50–50 chance of a safe landing There was

so much that could go wrong This time there were no air bags, and the touch-down