Dying comets probe the Sun

22/01/2012

Updating Magic Universe

Debris traces the solar magnetic field

What started as a bonanza for comet spotters becomes a new tool for exploring levels in the Sun’s atmosphere that have been hard to see up to now. The SOHO spacecraft (Solar and Heliospheric Observatory) has identified more than 1400 small “sungrazing” comets that fly close to the Sun and evaporate. In July last year, the comet observers using SOHO’s Large Angle and Spectrometric Coronagraph (LASCO) team alerted colleagues operating the newer SDO (Solar Dynamics Observatory) to a larger-than-usual sungrazer heading for its doom.

As he reports in the current issue of Science magazine, Karel Schrijver from the Lockheed Martin Advanced Technology Center in California tracked Comet 2011 N3 SOHO by extreme ultraviolet light with his Atmospheric Imaging Assembly on SDO, which observes highly ionized atoms. What he learned about the comet and about the Sun I’ll tell below as a concise update for Magic Universe. Meanwhile the word is that SDO also observed Comet Lovejoy last month, when it survived a close encounter with the Sun, passing behind it and reappearing on the other side.

Here are a few relevant paragraphs from my story about Comets and Asteroids in Magic Universe.

The big comet count came from another instrument on SOHO, called LASCO, developed under US leadership. Masking the direct rays of the Sun, it kept a constant watch on a huge volume of space around it, looking out primarily for solar eruptions. But it also saw comets when they crossed the Earth-Sun line, or flew very close to the Sun.

A charming feature of the SOHO comet watch was that amateur astronomers all around the world could discover new comets, not by shivering all night in their gardens but by checking the latest images from LASCO. These were freely available on the Internet. And there were hundreds to be found, most of them small ‘sungrazing’ comets, all coming from the same direction. They perished in encounters with the solar atmosphere, but they were related to larger objects on similar orbits that did survive, including the Great September Comet (1882) and Comet Ikeya-Seki (1965).

SOHO is seeing fragments from the gradual break-up of a great comet, perhaps the one that the Greek astronomer Ephorus saw in 372 BC,’ explained Brian Marsden of the Center for Astrophysics in Cambridge, Massachusetts. ‘Ephorus reported that the comet split in two. This fits with my calculation that two comets on similar orbits revisited the Sun around AD 1100. They split again and again, producing the sungrazer family, all still coming from the same direction.’

The progenitor of the sungrazers must have been enormous, perhaps 100 kilometres in diameter or a thousand times more massive than Halley’s Comet. Not an object you’d want the Earth to tangle with. Yet its most numerous offspring, the SOHO-LASCO comets, are estimated to be typically only about 10 metres in diameter.

Update January 2012

In July 2011 a larger than usual sungrazer spotted by SOHO was tracked across the face of the Sun by a newer spacecraft, the Solar Dynamics Observatory, SDO. Named as Comet 2011 N3 SOHO, it evaporated to the point of invisibility after 20 minutes, but not before the event had transformed the game from comet-spotting fun to highly productive cometary and solar physics.

Led by Karel Schrijver from the Lockheed Martin Advanced Technology Center in California, the SDO team was able to gauge the size of the comet. Initially it was up to 50 metres wide. This opened the way to investigating the sungrazers in much more detail. It should become possible to learn more about the composition of these comets, according to how they boil and rupture in the intense heat.

As for solar physics, the miniature tail of the dying comet lit up magnetic field lines at altitudes high in the solar atmosphere that otherwise are almost impossible to detect. Seeing the lines traced by sungrazers at different heights above the Sun will make it possible to trace more accurately the links between the magnetism near the visible surface and the vast field that reaches out into space and influences the Earth.

References

Karel Schrijver et al., Science 20 January 2012, vol. 335, pp. 324-328 DOI: 10.1126/science.1211688

NB: Movies are available at http://www.sciencemag.org/content/335/6066/324/suppl/


Target comet spotted

08/06/2011

Updating Comets

Rosetta spies Comet Churyumov-Gerasimenko

The European Space Agency’s Rosetta spacecraft, launched in 2004, has just gone into hibernation until 2014, as it continues to cruise towards its far-flung rendezvous with a comet. Once there it will drop a lander on the nucleus and then accompany the comet as it orbits towards the Sun. Before Rosetta went to sleep, its camera OSIRIS was able to pick out its target, Comet Churyumov-Gerasimenko,160 million kilometres away among the background stars. This is the picture released today from the Max Planck Institute for Solar System Research (MPS).

Caption: Seen in the second enlargement from the crowded starfield of the Scorpius constellation, the comet became visible as a single point of light to the 10-cm OSIRIS telescope on Rosetta, thanks to exposures totalling 13 hours. Credits: ESA 2011 MPS for OSIRIS-Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA.

We had not expected to be able to create first images from so far away,” says the lead investigator for OSIRIS, Holger Sierks of MPS.

The press release from MPS is here http://www.mps.mpg.de/en/aktuelles/pressenotizen/pressenotiz_20110608.html

And from ESA here http://www.esa.int/esaSC/SEM38RJ4LOG_index_0.html

By the way, next week (15 June) I’ll be taking part in an ESA TV programme about Rosetta and its predecessor Giotto, at ESOC, ESA’s mission control in Darmstadt.

Added 12 June: The event will start in the afternoon at 16:30 CEST. (15:30 BST) and will be webstreamed live at www.esa.int

 

 

 


Comets from sister stars

09/07/2010

Upating Comets

Did our comets come from sister stars?

How did the Solar System acquire its never-ending supply of comets to keep startling us? An explanation comes in today’s Science magazine, from Harold Levison and David Kaufmann of the Southwest Research Institute in Boulder, Colorado, working with Canadian and French colleagues.

The presumed source of supply is a very distant cloud of 100 billion or more comets, loosely bound to the Sun, called the Oort Cloud. The new report suggests that, in the tight cluster of stars in which the Sun was born, comets were scattered hither and yon in close encounters between stars, and many of our comets were captured from the Sun’s sisters.

In this extract from Comets I am at pains to stress that Jan Oort wasn’t the inventor of the distant comet cloud.

Ernst Öpik is an Estonian astronomer and musician who has recently been running the Armagh Observatory in Northern Ireland. For most of his long life he has adopted the role of cosmic garbage-sorter, concerning himself with the stray material of the Solar System. In 1932 he calculated that an invisible cloud of comets and meteors, surrounding the Sun at enormous distances, could survive throughout the long lifetime of the Solar System. In 1950 the doyen of Dutch astronomers, Jan Oort of Leiden, who is better known for classic work on the nature of galaxies, reworked Öpik’s idea. He emphasised a different aspect of it, namely that passing stars would cause a few of the objects to fall out of the cloud and into the heart of the Solar System, to become observable as ‘new’ comets.

Thus was the fabulous Öpik-Oort Cloud conceived, as the source of the comets. I abridge the name to the Öoo Cloud and defend this coinage on grounds of sight and sound. It looks like an untidy collection of roughly round objects of various sizes, and it is pronounced ‘Er, oh!’ – just what a neophyte comet lover is liable to utter when he is first told that there are many billions of the things out there.

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Comets and life 5

05/06/2010

Updating Comets and Magic Universe

Did comets spark life on Earth?

Part 5: Summary

It’s in the nature of blogs that developing stories come out backwards. So it may be helpful to summarize the key updates that have arisen.

Comets and life 1 – Recent news: ultra-clean snow melted and sieved for meteoritic particles, at the French-Italian CONCORDIA station in Antarctica, yielded UCAMMs [ultra-carbonaceous Antarctic micrometeorites] with very high carbon contents. Larry Nittler says they “may well have profound implications for the original delivery of organic molecules to the early Earth”.

Comets and life 2 – I did not err in betting heavily in Magic Universe on the magic of polycyclic aromatic hydrocarbons, PAHs. Pascale Ehrenfreund says, “We suggest the aromatic material can be used as a container, as a metabolic unit, and as a genetic information carrier.”

Comets and life 3 – Both Jochen Kissel as instigator and I as reporter remain persuaded that the Kissel-Krueger hypothesis, that chemically active materials from outer space interacted with water to create the first living cells, is still the most promising explanation for the origin of life on the Earth.

Comets and life 4 – I now accept Hoyle & Wickramasinghe’s idea, about pre-existing bacteria in space being rehydrated by the Earth’s water, as another promising hypothesis. And here I repeat what I said at the very end of part 4.

I share Wickramasinghe’s concern about the “cultural barrier”. Science became over-specialized in the creation of university departments in the 19th Century. Since then, in several major advances, earth scientists and biologists offered strong and prolonged resistance when astronomers and astronomically-minded folk said:

  • extinctions due to impacting comets and asteroids have redirected the course of evolution (Halley, about a comet)
  • the Earth’s wobbly orbit around the Sun sets the timing of coolings and warmings in the ice ages (Milankovitch)
  • to explain the origin of life on Earth we have to consider possible cosmic sources (Arrhenius)
  • to understand climate change we have to look to the role of cosmic rays in cloud formation (Svensmark)

Plus ça change, plus c’est la même chose.

You can see all parts of the story (in reverse order) just by clicking on 1c) Comets in the Categories menu.

Or individually:

Part 1 http://calderup.wordpress.com/2010/05/10/comets-and-life/

Part 2 http://calderup.wordpress.com/2010/06/01/comets-and-life-2-2/

Part 3 http://calderup.wordpress.com/2010/06/04/comets-and-life-3/

Part 4 http://calderup.wordpress.com/2010/06/05/comets-and-life-4/


Comets and life 4

05/06/2010

Updating Comets and Magic Universe

Did comets spark life on Earth?

Part 4: Life footloose in space

In Part 1 of this series, I mentioned that in Comets, written 30 years ago, I made fun of propositions from the astrophysicists Fred Hoyle and Chandra Wickramasinghe about viable entities living in comets and being delivered ready-made to the Earth, scattered from the comets’ tails. What follows fulfils a promise to look at Chandra’s present ideas — I hope with an open mind.

To back-track a little, there’s a 100-year history of eminent scientists, driven by despair about explaining the very improbable chemistry of life by home cooking on the Earth, suggesting that life came from elsewhere. Of course, their scenarios didn’t explain the origin of life, they merely transferred it somewhere else. Out of sight, out of mind, perhaps.

  • 1907 Svante Arrhenius (yes, the CO2 warming pioneer) suggested that bacterial spores escaped from an alien planet, were driven through interstellar space by the pressure of sunlight, and revived when they reached the Earth.
  • 1971 Francis Crick (yes, of DNA fame) with Leslie Orgel proposed that intelligent beings in another part of the Galaxy spotted the Earth as a suitably wet planet and sent bacteria in a spaceship to seed it.
  • 1979 Fred Hoyle (yes, celebrated for the origin of the elements) with Chandra Wickramasinghe said that life on Earth began in comets, and diseases still come from them.

It was hard not to chuckle over their book Diseases from Space, because Hoyle and Wickramasinghe’s account filled the sky with germs, in a distant echo of the old superstition that comets portended plagues.

John Gadbury (1665) linked comets and catastrophes -- "Famine, Plague & Warrs" In actuality, bubonic plague afflicted London following the depicted 1664-5 comet. Obtained from the Royal Astronomical Society, this is an illustration in N. Calder, Comets: Speculation and Discovery.

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Comets and life 3

04/06/2010

Updating Comets and Magic Universe

Did comets spark life on Earth?

Part 3 Initiating biochemical action

Pascale Ehrenfreund rides again (as in Part 2) in the story in Magic Universe called “Life’s origin: will the answer to the riddle come from outer space?”. But please focus first on Wlodzimierz Lugowsky.

I can trace my ancestry back to a protoplasmal primordial atomic globule,’ boasts Pooh-Bah in The Mikado. When Gilbert and Sullivan wrote their comic opera in 1885 they were au courant with science as well as snobbery. A century later, molecular biologists had traced the genetic mutations, and constructed a single family tree for all the world’s organisms that stretched back 4 billion years ago, to when life on Earth probably began. But they were scarcely wiser than Pooh-Bah about the precise nature of the primordial protoplasm.

In 1995 Wlodzimierz Lugowsky of Poland’s Institute of Philosophy and Sociology wrote about ‘the philosophical foundations of protobiology’. He listed nearly 150 scenarios then on offer for the origin of life and, with a possible single exception to be mentioned later, he judged none of them to be satisfactory. Here is one of the top conundrums for 21st Century science. The origin of life ranks with the question of what initiated the Big Bang, as an embarrassing lacuna in the attempt by scientists to explain our existence in the cosmos.

After discussing possible “home cooking” of life by hypercycles, RNA catalysis or lipid catalysis, and touching on the possibility of false starts, the tale turns back to the sky in pursuit of the only hypothesis acceptable to Lugowsky.

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Comets and life 2

01/06/2010

Updating Comets and Magic Universe

Did comets spark life on Earth?

Part 2: Cosmic carbon compounds

An earlier post, Part 1 under this heading, commented enthusiastically but briefly on a French team’s find of extraterrestrial dust grains rich in carbon in the snow of Antarctica. http://calderup.wordpress.com/2010/05/10/comets-and-life/

I promised more to come, and here it begins. Part 2 deals with cosmic carbon compounds. Later, Part 3 will reconsider the initiation of biochemical action, and Part 4 will look at suggestions of natural life footloose in space.

There were far more comets around when the Solar System was young, in the “heavy bombardment” phase of Earth history lasting until 3.8 billion years ago. Water is abundant in interstellar space and available to build the icy nuclei of comets. Comets may have delivered most of the Earth’s surface water, essential for life.

Carbon compounds are the other main ingredient for life. Comets’ tails consist mainly of small dust grains released from the nuclei, including grains laden with carbon compounds that may have contributed to the origin of life on the Earth. Here’s a general impression of important “prebiotic” molecules made in the vicinity of dying stars and newborn stars and available for incorporation into comets.

PAHs, polyaromatic hydrocarbons observable in interstellar space, could be ancestral to the aromatic compounds that have the very smell of life. Illustration from Pascale Ehrenfreund & Steven Charnley, 2000 – see reference. Graphic art: ©2000 R. Ruiterkamp


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Comets and life

10/05/2010

Updating Comets and Magic Universe

Did comets spark life on Earth?

Part 1

A French team’s find of extraterrestrial dust grains rich in carbon in the snow of Antarctica is thrilling for any of us who have pondered the cosmic ancestry of life. A scarcity of carbonaceous material in the samples that NASA’s Stardust mission brought home from Comet Wild 2 in 2006 was a little disappointing – especially after the high hopes raised by molecular results radioed from ESA’s Giotto and the Soviet Vega spacecraft that intercepted Halley’s Comet in 1986, and by organic materials seen when NASA’s Deep Impact probe hit Comet Tempel 1 in 2005.

Collecting ultra-clean snow to be melted and sieved for meteoritic particles, in a trench at the French-Italian Concordia station in Antarctica. Photo Jean Duprat/CSNSM-CNRS.

But the Antarctic report by Jean Duprat and his colleagues, in the current issue of Science (7 May) puts us back on track to look for credible links between the complex carbon compounds seen interstellar space and the first living things to appear in the waters of the early Earth. That’s not just my opinion. Larry Nittler of the Carnegie Institution says in closing a commentary on the Duprat report:

The very high carbon contents of UCAMMs [ultra-carbonaceous Antarctic micrometeorites] may well have profound implications for the original delivery of organic molecules to the early Earth, with possible consequences for the earliest prebiotic chemistry.

It’s is a huge subject, stretching from the chemistry of dying stars and the search for extraterrestrial life to issues about chemical thermodynamics, the climate of the young Earth, and what surviving genes may tell us about the earliest viable entities. It’s peppered with hypotheses that I discuss in Giotto to the Comets as well as in Comets and Magic Universe. To deal adequately with this Update will take some time, so “Part 1” is a signal that there’ll be more to come.

Let me just mention that in Comets, written 30 years ago, I made fun of propositions from Fred Hoyle and Chandra Wickramasinghe about viable entities living in comets and being delivered ready-made to the Earth, scattered from the comets’ tails. Fred is dead, but Chandra thrives in Cardiff and has refreshed his ideas in “The origin of life in comets” (2007, Napier ref. below) and just last month in “The astrobiological case for our cosmic ancestry”. I’ll comment on those too, in the follow-up.

References

J. Duprat et al., Science, Vol. 328, pp. 742-5, 2010

L.R. Nittler, Science, Vol. 328, pp. 698-9, 2010

W.M. Napier, J.T. Wickramasinghe and N.C. Wickramasinghe, International Journal of Astrobiology, Vol. 6, pp. 321-323, 2007

C. Wickramasinghe, International Journal of Astrobiology, Vol.9, pp. 119-129, April 2010


Fragmented comet impact

01/05/2010

Updating Comets

A fragmented comet impact

Cosmic traffic accidents were the main news story when The Comet is Coming! first appeared in 1980. The collision with a comet or asteroid that wiped out the dinosaurs – then newly indicated – was certainly not the only event of its kind, as my book stressed at the time. Now Prof. Bill Napier at Cardiff University’s Astrobiology Centre suggests that 12,900 years ago the Earth strayed into a trail of fragments from a very large but disintegrating comet. And that encounter, he proposes, caused a drastic change of climate.

Hubble image (2006) of a broken chunk of Comet Schwassmann-Wachmann 3 dividing into dozens of “house-sized” fragments. NASA, ESA, H. Weaver (JHU/APL), M. Mutchler and Z. Levay (STScI)

Comets certainly can break up, and Napier interprets meteor streams of the Taurid Complex as traces of a large comet that entered the inner Solar System system 20,000-30,000 years ago. His idea is that the Earth later ran into a dense trail of its material over the course of an hour, with North America bearing the brunt and experiencing thousands of fireball explosions each comparable with a nuclear bomb.

That would explain a layer of soot found at many sites in the USA, telling of continental-scale wildfires, together with microscopic diamonds of the type produced by meteoritic shocks. And 35 genera of North American mammals were wiped out. All this is persuasive.

Napier also suggests that the impacts provoked a major cooling that lasted 1300 years. The event is well-known to palaeoclimatologists as the Younger Dryas, which takes its name from a flower, Dryas octopetala or mountain avens, which likes cool conditions. The climatic downturn occurred just as the world was emerging from the most recent Ice Age. It was a miniature ice age of its own, with temperatures falling by perhaps 8 deg. C and glaciers re-advancing. Drastic regional changes of climate drove people in the Middle East to invent agriculture, just to survive.

Napier’s multi-impact event coincided with the start of the Younger Dryas. Although you can imagine that impact debris in the stratosphere and smoke from the wildfires would have caused a dimming of the Sun for months or perhaps years, it’s not at all clear why the effects should have persisted for 13 centuries. Napier doesn’t discuss the climatology in his report for the Monthly Notices of the Royal Astronomical Society.

Some climate theorists will offer to explain the Younger Dryas by melt-water from the receding ice sheets stifling the North Atlantic circulation. For me, it’s no coincidence that the Younger Dryas was a time of intense cosmic radiation associated with a long period of weak solar activity. But having faced a lot of flak myself, back in 1980, for “buying” the dinosaur impact story, I’ll not scorn Napier’s latest idea, which is astronomically convincing. His impacts might well have helped to trigger, accelerate and/or intensify the Younger Dryas, even if the depth and persistence of the cooling may need more explaining.

By the way, it’s amusing to note that not until March 2010, 30 years after The Comet is Coming!, did an international panel of 41 experts solemnly pronounce in Science (the Schulte reference below) that a cosmic impact really did do for the dinosaurs. They disposed of the only surviving geophysical hypothesis, namely that massive flood-basalt volcanism in India (the Deccan traps) caused the mass extinction 65.5 million years ago. The main Deccan eruption phase was half a million years too soon. That finding also rules out a proposed compromise between the hypotheses, namely that shock waves from the impact might have triggered the flood-basalt event.

Schulte et al. said without discussion that the impactor was an asteroid. The celebrated astrogeologist Eugene Shoemaker would have begged to differ. He estimated that whatever made the Chicxulub crater in Mexico was 15 kilometres wide and he noted that no asteroids as large as that are known in the Earth’s vicinity. As reported in Magic Universe:

Not impossible that it was an asteroid,” Shoemaker said. “But if you’re a betting man like I am, you put your chips on a comet. My own hunch is that we will find that the impact of large objects, probably chiefly comets, has profoundly influenced the evolution of life.” That was Shoemaker’s last word on the subject. Soon after recording it for television, in 1997, he lost his life in a car smash in Australia while prospecting other impact craters.

References

W.M. Napier, “Palaeolithic extinctions and the Taurid Complex”, Mon. Not. R. Astron. Soc., in press, released online 1 April 2010.

Peter Schulte, et al., “The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary”, Science, Vol. 327, pp. 1214 ff. 2010

Nigel Calder, “Impacts” in Magic Universe, OUP 2003


About Comets

01/05/2010

Occasional postings will comment on news of comets and asteroids. This introduction explains my long-standing interest.

About Comets: Speculation and Discovery Dover Classic re-issue of The Comet is Coming!

Comet Ikeya-Seki at dawn, 1965. NOAO/ R. Lynds.

In 1968 the BBC producer Philip Daly and I visited a teacher of classical guitar called Tsutomu Seki in Kochi City, Japan. From his small rooftop observatory, he had co-discovered the spectacular Comet Ikeya-Seki 1965 when it was just a faint smudge in the sky. For the first BBC-TV science blockbuster “The Violent Universe”(1969) about quasars, pulsars and cosmic microwaves, Seki’s patient searching provided some light relief. As professional telescopes grew bigger, comet-hunting became a job mainly for the amateur astronomers.

Ahead of the competition, the BBC anticipated the public interest in the impending return of Halley’s Comet in 1985 and in 1980 published my book The Comet is Coming!: The Feverish Legacy of Mr Halley. In the following year BBC-TV broadcast a 60-minute TV documentary also called “The Comet is Coming!” produced by Martin Freeth. We labelled the programme “a prank” because the comic actor Tim Brooke-Taylor played the ghost of Edmond Halley fated to ride in limbo on his eponymous comet, voiced by Leo McKern.

Both the book and the programme climaxed with important new science, the first strong evidence that the impact of a comet or an asteroid 65 million years ago wiped out the dinosaurs. My inquiries took me to a gorge near Gubbio in Italy, where the thin red clay layer marking the end of the Mesozoic Era had proved to be doped with extraterrestrial atoms of iridium. As so often with my science reporting, the story was highly controversial at the time (“Unbelievable arrogance” one dinosaur expert called the impact theory) but now it’s standard stuff.

At mission control in Darmstadt in 1986 I was present for the tumultuous interception of Halley’s Comet by the European Space Agency’s Giotto spacecraft. Although badly damaged in the high-speed encounter with the comet’s dusty head, Giotto not only returned excellent data but survived to intercept Comet Grigg-Skjellerup in July 1992. Then my book Giotto to the Comets was published very quickly by Presswork, because most of it was drafted in advance of the second encounter.

When The Comet is Coming! re-appeared from Dover in 1994 under its time-adjusted title Comets: Speculation and Discovery, a preface briefly summarized the results from March 1986, when five spacecraft, the Soviet Vega 1 and Vega 2, the Japanese Sakigake and Suisei, and Europe’s Giotto, all intercepted Halley’s Comet.

In Magic Universe (2003) my story about “Comets and asteroids: snowy dirtballs and their rocky cousins” stressed that the distinction between the two kinds of micro-planets had become fuzzy. It also looked forward to NASA’s Deep Impact encounter with Comet Tempel 1 in 2005. By then political interest had been engaged in the search for wayward objects that threaten to collide with the Earth, under the tag Spaceguard. Cross-links to “Impacts” and “Extinctions” led readers to the re-interpretation of key events in evolution on the Earth as consequences of traffic accidents like the one that killed the dinosaurs.

Rosetta and its lander. ESA.

Writing for the European Space Agency kept me up to speed with the discovery of vast numbers of sungrazing comets, spotted by the ESA-NASA SOHO spacecraft, and with ESA’s ambitious Rosetta mission, launched in 2004 to rendezvous with Comet Churyumov-Gerasimenko in 2014. Rosetta will put a small lander onto the icy nucleus, and spend the next two years orbiting the comet as it heads towards the Sun and swings around it, spewing out its tail. Rosetta is the de luxe mission that comet investigators have waited for ever since the Space Age began.

Comment on The Comet is Coming!

As we would expect from Calder, Britain’s most entertaining science writer, The Comet is Coming! is factual, easy to understand, and great fun. Timothy Ferris, New York Magazine

Comment on Giotto to the Comets

Nigel Calder has risen to the occasion. A most interesting book about a most interesting and important mission. Patrick Moore

You can buy Comets: Speculation and Discovery at

UK: http://www.amazon.co.uk/gp/search?index=books&linkCode=qs&keywords=0486278794

US: http://www.amazon.com/Comets-Speculation-Discovery-Nigel-Calder/dp/0486278794

Or see a few pages on Google Books at: http://books.google.co.uk/books?id=Mx0xEDvj7WUC&printsec=frontcover&dq=Comets:+Speculation+and+Discovery&source=bl&ots=UqFLmuK_oG&sig=AihAYk83G2e6KRQFPNgObCMGdW8&hl=en&ei=Mw7CS6XmHpOi0gSU9-mbCQ&sa=X&oi=book_result&ct=result&resnum=1&ved=0CAYQ6AEwAA#v=onepage&q&f=false


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