A Piece of the Rock


Asteroid on Course for Earth in 2880

I would rather be ashes than dust!
I would rather be a superb meteor, every atom of me in magnificent glow, than a sleepy and permanent planet.

- Jack London

by Lisa Murray and Richard Macey

The world will be bracing itself for a collision with a kilometre-wide asteroid in anout 875 years.  There is a 1-in-300 chance of a direct hit, better odds than having the same birthday as your best friend, and it is expected to come closer to earth than any other object of its size.  If it does hit, it will obliterate a land mass the size of Britain or Japan and kill about 60 million people.

Bill McGuire, an expert in natural catastrophes, uses the asteroid example to get people's attention.  But as the head of the Benfield Hazard Research Centre at the University College London, he is more concerned about events that could take place within 70 years.  Professor McGuire told a reinsurance conference in Sydney on Tuesday that there was a 35% chance of an earthquake of similar size and impact of the great Tokyo earthquake that struck in 1923.  That earthquake cost today's equivalent of $US50 billion and killed or injured more than 150,000 people.  If it occurred today, the damages bill would be more than $US3 trillion.  Professor McGuire is also concerned about the 35 to 70% chance that another devastating tsunami will hit somewhere in the globe in the next 70 years.  McGuire is a member of the Natural Hazard Working Group set up by the British Government in January after the Asian tsunami, which left just under 300,000 people dead or missing.  The group is looking to form a multi-government panel to assess global threats and plan a response to them.  "The Indian Ocean countries knew they had a tsunami threat, but because it only happens every 100 to 200 years they decided not to spend money on it.  That was a ridiculous decision."

In terms of the asteroid that was expected to hit earth on 16 March 2880, he said scientists would have devised a way to divert it by then.  But Professor McGuire is not alone in recognising the threat.  Astronomers Gordon Garradd and Rob McNaught at Siding Springs Observatory, near Coonabarabran, have discovered "near-earth asteroids" at the rate of about one a week.  In December Mr Garradd spotted a 320-metre wide asteroid, which had one in 6,250 chance of hitting the earth by 2055, putting it at the top of the asteroid risk for this century.  If it struck earth, the asteroid would create a blast equal to an 8.7-megatonne nuclear bomb.

Mr Garradd suspects the biggest threat may come from much smaller asteroids.  A meteorite or asteroid 50-metres wide fell over Siberia in 1908, flattening 2000 square kilometres of forest.  "I am sure 50-metre objects pass closer than the moon many times a year.  Most pass by unseen," Mr Garradd said.

Source: stuff.co.nz 17 March 2005

A Lucky Break on Friday the 13th, 2029: Giant Asteroid Will Narrowly Miss Earth

by Steve Connor

A giant asteroid the size of 3 football pitches will make the closest flyby of earth in recorded history for an object of its size, scientists said yesterday.  It will pass between the earth and the moon and will even come closer than the orbit of many telecommunications satellites, although astronomers insisted that there was little chance of a collision with the massive rock.  Anxious earthlings need not worry too much for another 24 years, however, because asteroid 2004 MN4 is not due to make its closest approach to earth until about 10pm London time on Friday 13 April 2029.

The latest calculations of the rock's orbit suggest that it will come so close that it will probably be visible to the naked eye from Britain.  It will shine in the sky as a dim, fast-moving star - the first asteroid in modern times to be clearly visible from Earth without the aid of a telescope or binoculars.  The asteroid was first discovered in June 2004 and calculations of its orbit made by astronomers last Christmas eve suggested that there was a one in 60 chance of it colliding with the earth.  However, within a week this was revised down to virtually zero probability of a collision.  If it did collide it would cause an explosion equivalent to about 20 hydrogen bombs being detonated simultaneously, turning vast areas of land into desert or generating a giant tsunami if it landed in the ocean.

The latest revisions of the calculations have refined the asteroid's orbital path to suggest that it will pass our planet by the relative whisper of 36,000km (22,600 miles) - well within the orbit of geostationary satellites and about a tenth of the distance to the moon.  This is by far the largest of the top 10 closest asteroids recorded by astronomers.  Only two others have come closer and both were much smaller objects - tens of yards wide instead of the 350 yards of asteroid 2004 MN4.

Professor Mark Bailey, director of the Armagh Observatory, said that there was little danger from the asteroid even though it would come close enough for its orbit to be directly affected by the earth's gravity - causing the path of the space rock to swing away.  "I think everyone is saying that it's going to miss.  It'll pass so close though that you'll be able to see it with a small telescope and even with the naked eye," Professor Bailey said.  "It's like being on a train station platform and watching an express train go by 3 feet away.  You're close, but it's not dangerous," he said.

Large asteroids have frequently collided with the earth in the past and some of the larger ones have caused massive devastation on a global scale.  They can send huge plumes of dust and debris into the atmosphere, blocking out sunlight for several years and causing the environmental equivalent of a "nuclear winter".  Last autumn a much bigger asteroid called Toutatis, which is about 2.9 miles long and 1.5 miles wide, made its closest flyby to earth but its distance was still 4 times greater than that separating the moon and the earth.  Unlike asteroid 2004 MN4 and despite its size, Toutatis was not visible to the naked eye.  Steve Chesley, of NASA's Jet Propulsion Laboratory in Pasadena, California, said that asteroid 2004 MN4 was unusual because so much was known about its orbit before it makes its closest approach in 24 years' time.  "All the others in the top 10 were discovered during the close approach, whereas for 2004 MN4 the close approach is predicted well in advance," Dr Chesley said.

Scientists estimate that on average an asteroid of this size would be expected to pass this close to earth about once in every 1,300 years.  Asteroid 2004 MN4 circles the sun, but unlike most asteroids that reside in a belt between Mars and Jupiter, the orbit of this one lies mostly within the orbit of the earth, making further encounters likely.  "However, our current risk analysis for 2004 MN4 indicates that no subsequent earth encounters for the 21st century are of concern," said Dr Chesley and his colleagues at the Jet Propulsion Laboratory.

Source: news.independent.co.uk 16 February 2005

But not everyone is so sanguine...

Mission: to Destroy It before It Gets Us

by Jonathan Leake

NASA is drawing up a shortlist of ideas to be unveiled early next year for diverting a 40 million-ton asteroid that is on course to pass dangerously close to earth.  The schemes will be presented and discussed at the annual meeting of the American Association for the Advancement of Science.  Fears that the planet may be in danger from asteroids were heightened by the discovery of one orbiting the sun that, on its present path, will pass within 22,000 miles - a hair’s breadth in astronomical terms - in April 2029.

NASA’s idea is to engineer a minor shift in its trajectory that would make the asteroid miss earth by a wider margin on this and all subsequent passes.  Under one possible plan, a robotic craft would be sent to the asteroid to attempt to alter its course.  One option might be to install a propulsion system on the surface to nudge it onto a new course.  The studies follow the discovery of hundreds of small asteroids orbiting the sun that repeatedly cross earth’s orbit, raising the possibility of a devastating collision.  The one causing most concern is a rock of more than 1,000 feet called Apophis, the Greek name for the Egyptian god Apep, known as "the Destroyer".  It will come so close that it will pass under many satellites and may destroy some.

Astronomers fear that, although 2029 should pass without incident, coming so close to earth might change Apophis’s 323-day orbit around the sun - during which it crosses the planet’s path twice - creating an even bigger risk in the future.  A second close encounter is predicted for 2036.  Since Apophis was discovered NASA scientists have been drawing up proposals for diverting it or any other asteroid that might present a threat.  NASA estimates that if it hit earth it would release energy equivalent to the detonation of 880 megatons of TNT. The 1883 eruption of Krakatoa was the equivalent of roughly 200 megatons.

One option, to be proposed by a former astronaut, Edward Lu, of NASA’s Johnson Space Center in Houston, Texas, would involve building a heavy, nuclear-powered spacecraft to act as a gravitational tug.  The spacecraft would hover over the surface of Apophis, using the asteroid’s gravitational pull to stay in orbit.  Lu calculates that a 20-ton craft gently firing its thrusters could safely deflect a typical 650-foot asteroid in about a year, assuming there was 20 years of warning to launch and get into position.  Lu’s approach is far more cautious than that proposed by Hollywood in films such as Deep Impact or Armageddon.  In the latter, the character played by Bruce Willis dies leading a team of astronauts who drill into an earth-bound asteroid to plant a nuclear weapon that destroys it, and him along with it.  Lu and others say that such an approach would increase the threat by turning a single piece of rock into smaller chunks that could bombard the planet.

In reality the chances of a sizeable asteroid striking earth in any given year are extremely small.  Such impacts are thought to happen every 1,000 years or so, with the really big strikes - those that have a global impact - being separated by millions of years.  A near-miss by an asteroid codenamed QW7 in September 2000 prompted the Liberal Democrat MP Lembit Opik to declare: "It’s not a case of if will we be hit, it is a question of when.  Each of us is 750 times more likely to be killed by an asteroid than to win this weekend’s lottery."

Source: timesonline.co.uk

Ending Time

Even if an asteroid NEVER strikes the earth, I read where, because the sun is expanding and getting hotter, the oceans will boil away in less than a billion years anyway.

How much less than a billion years?

Asteroid Gives the Earth a Close Shave

Asteroids are rocky objects - some partly metallic and many containing organic molecules - that orbit the sun and are considered minor planets by astronomers.  Most of them lie in a belt between the orbits of Mars and Jupiter, although some are orbiting as far out as Saturn.  They range in size from one with a diameter of about 650 miles, named Ceres, down to many thousands that are only the size of pebbles.  They have been bombarding the earth and moon since each was "born."

An asteroid big enough to wipe out a major country gave the Earth a close shave yesterday, passing less than twice the distance of the Moon from our planet, astronomers reported.  The space rock, designated 2001 YB5, measures between 220 and 490 metres (roughly 1,000 feet) and at its closest point, at 6:37pm, Sydney time, was about 627,000km (roughly 400,000 miles) from the Earth, according to varying estimates on US and European specialist websites (almost twice the distance between the earth and the moon).  Had it arrived four hours earlier on its journey around the Sun it would have scored a direct hit.

2001 YB5 was spotted a month ago by a Near-Earth Asteroid Tracking (NEAT) survey telescope on Mount Palomar in California, NASA said on its Near-Earth Object (NEO) Program website.  Although there was no danger of collision from the asteroid, experts said the distance was a whisker in cosmic terms.  They warned that the asteroid was one of up to 400,000 small NEOs up to 1000m wide that could strike Earth with little or no warning because of the absence of an adequate early warning system.  These present an unknown risk to the planet.

"Such an object would literally wipe out a medium-sized country and lead to a global economic meltdown, unless we were extremely fortunate and it hit somewhere remote," Benny Peiser, an asteroid expert at Liverpool John Moores University, said.  Only one other identified asteroid, a rock called 1999 AN10, will come closer, making a flyby on 7 August 2027.

An object 220-490 metres across would release energy equivalent to hundreds of atomic bombs if it hit the Earth.  If it hit London, it would destroy everything within a 150km radius and cause severe damage for a further 800km - wiping out the United Kingdom, the Low Countries and much of France.  If it hit the San Francisco Peninsula, it would make a crater out of the Bay Area and take out most of California and probably Nevada as well.

The idea of a catastrophic asteroid strike has long been a source of morbid fascination, most recently manifesting itself in Hollywood disaster movies such as Armageddon and Deep Impact.  The potential consequences are indeed apocalyptical.  In the more likely event of a similarly-sized NEO landing in the sea (70% of the Earth's surface is covered by water), it would trigger a series of tsunamis that would devastate coastal regions.

What limited resources exist for tracking asteroids are dedicated to spotting the 700 to 1,200 which are more than a kilometre in diameter which, if they hit the Earth, could wipe out the planet.

A large object, believed to be up to 10km long, smashed into Mexico's Yucatan peninsula 65 million years ago, triggering a firestorm and a dust cloud that wiped out the dinosaurs and nearly half of all other forms of life, scientists believe.  In 1908, an asteroid or comet about 60 metres long exploded over Siberia with the force of 600 times the Hiroshima bomb, reducing a 40km wide patch of forest to matchwood.

2001 YB5 has been categorised by NEAT as a "potentially hazardous" asteroid.  Although it poses no danger at all to the Earth at the moment, that could theoretically change in the future if its orbit around the Sun is deflected by the gravitational pull of a nearby planet.  By the time it was spotted last month, there was nothing that could have been done to avert a catastrophe.

Its trajectory crosses the orbits of Mars, Earth, Venus, and Mercury, NEAT said.  Large meteorites rock earth every 100,000 years.  The NEAT system has spotted 322 asteroids at least 1km in size flying in orbits that bring them near earth.  At least 2-3 others probably come just as close every year.  (There is always some uncertainty in the orbit calculation.)  Deflecting one of these would be a huge problem requiring monster rockets fired directly at it in order to change its orbit.

Modest-sized asteroids hit the earth every century or so.

NASA's Ames Research Center maintains an updated asteroid and comet site at impact.arc.nasa.gov and the NEAT site is neat.jpl.nasa.gov.

Source: smh.com.au/news Tuesday 8 January 2002, The Independent 9 January 2002, and the San Francisco Chronicle 9 January 2002

Odds Are a Million-to-One, but an Asteroid Cometh

by William J Broad New York Times service

Astronomers try to calculate Orbit of Big Rock

For the fifth time in two years, astronomers have discovered an asteroid hurtling through space that might collide with the Earth.  The likelihood of collision is considered slim - one chance in a million.  While the projected date of any impact is 2022, astronomers say additional observations, if acquired, would allow them to better calculate the orbit of the asteroid and perhaps rule out a collision.

The asteroid, named 2000 BF19, is about half a mile (0.8 kilometre) wide, relatively small by cosmic standards, and it would threaten a regional disaster on Earth rather than planetwide destruction.  Its discovery was made by astronomers using the Spacewatch Telescope at the Steward Observatory on Kitt Peak in Arizona.  They spotted the object on 28 Jan and tracked it until 3 Feb at which point an announcement of its existence was made for the International Astronomical Union by the Minor Planet Centre of Smithsonian Astrophysical Observatory in Cambridge Massachusetts.

The announcement did not mention the possibility of a collision but suggested that the asteroid was potentially dangerous and worth watching.  On Monday, the possibility of impact was raised by Benny Peiser, who runs the Cambridge Conference Network, or CCNET, an Internet newsletter about astronomy.  "The current impact probability of one in a million can easily and quickly go up or down," Mr Peiser wrote.  In all likelihood, the asteroid will turn out to be no risk, he said.  "Only further observations can provide us with the information to assess the potential danger," he added.

Mr Peiser reported that the calculation of the current risk factor and projected impact date had been done by Andrea Milani, an astronomer at Pisa University in Italy.  He quoted Mr MIlani as saying that the object should raise no alarms.  "Nevertheless, shame on the astronomical community if we lose this dangerous fellow, which is unfortunately quite dim and fading," Milani added.

The asteroid is expected to pass near the Earth every 11 years, next in 2011 and again in 2022.

"There are a wide variety of orbits that can fit the observations," said Gareth Williams, an astronomer at the Smithsonian Astrophysical Observatory, who wrote the asteroid's official birth announcement.  "The only people who should be concerned are astronomers, who should be trying to get additional observations.  Since the arc of observation is so short, it is highly possible that any additional observations will be sufficient to reject the 2022 impact solution.  However, it is possible that another possible impact scenario will turn up for a later date."

Mr Williams said that in other instances follow-up observations have let astronomers rule out collisions with the Earth.  In one case observations were too limited to let them do so before the object disappeared from the world's telescopes.  For 2000 BF19, he said, with the asteroid fading fast, a clarification might take years rather than weeks or days.  "It's a hefty object," Mr Williams said.  "We would not like an object of that size to hit us."

Source: One of the two Wellington newspapers, about March 2000

Looks like space flight may be the answer.  Although...

The necessity of sex in space was emphasised by Jerry Brown of the US Space Foundation.  "If it's going to take 10,000 years for us to get to the next place, we need to have children in space," he said in remarks quoted in a Montana newspaper.

"Sex is very difficult," said one unidentified astronaut who had travelled in space with his significant other.  "Maybe it's impossible.  It's hard to keep in one place, and you keep floating off in different directions."

A harness invented in 1985 has been patented but not tried out in space, said Brown.  Furthermore, the different daily cycle - in space, the sun rises and sets every 45 minutes - destroys the natural rhythm of testosterone surges.  The lack of gravity means that eggs don't drop and sperm don't swim up, nor is a baby likely to drop through the birth canal.

Astronauts typically develop "fat face chicken leg" syndrome, as a result of body fluids being pumped upwards.

"If we go to space," asked Brown, "will we remain the same species?  If we go and stay, the children will be born in microgravity, and we better figure out what that would look like.  They may not look human, they may not survive and they may not be able to return to earth."


by Jeff Hecht

The moon was forged in a giant collision that ripped the iron from its heart

When an asteroid 10 kilometres wide slammed into the earth 65 million years ago, it punched out a crater as big as Belgium, darkened the atmosphere with debris and wiped out the dinosaurs.  Yet on a planetary scale that collision was a mere love-peck.  For a real cataclysm, you must go back 4.5 billion years, to the splash that made the moon.

Strictly speaking, the earth wasn't the earth then.  It was a "protoearth", half its present mass and growing rapidly as it gobbled up rubble that had condensed from the dust cloud swirling round the young sun.  The protoearth had a rival: another body about a third its size, swinging round the sun in a perilously similar orbit.  Both were hot, violent places, pummelled by smaller objects and sporting molten rock mantles wrapped around dense hot cores of liquid iron.

One day, the two collided...

For what happened next, we must turn to computer models.  The modeller-in-chief is Al Cameron, a planetary scientist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.  Twenty-five years ago Cameron and three others (his Harvard colleague William Ward and, independently, William Hartmann and Donald Davis of the Planetary Science Institute in Tucson, Arizona) proposed the "big impact" scenario for the birth of the moon.

Until then, scientists had toyed with other ideas - that the moon had condensed from stuff spewed out by a madly whirling earth or that the two bodies had coalesced side by side in space.  But each notion had at least one fatal flaw.  One sticking point was iron.  Iron makes up more than 30% of the earth but only about 2% of the mass of the moon.  Such a difference would be hard to explain if, for example, the earth and moon had formed as a pair.

But Cameron showed that the iron difference could be accounted for by a collision.  If two bodies with rocky outsides and iron centres smacked together in the right way, the iron "yolks" would run together like drops of mercury while part of the molten "white" would splatter out into space.  Once in orbit, the splattered pieces would be pulled together by gravity until they forged a protomoon.

Since the 1980s, Cameron has bolstered his case with scores of computer models, in which virtual bodies of different sizes and characteristics bash, splat and congeal into a variety of earth-moon systems.  His latest models portray the big splash as a one-two punch.  After ploughing into the protoearth, the smaller object skidded off and bounced back into space.  Then, badly stretched and distorted, it swooped round behind the protoearth and struck it again.  This time the bodies stuck.  The iron core of the attacker sank into the protoearth, while part of its lighter molten rock layer spiralled out in a long arc, finishing up in orbit.

The double whammy took only a matter of hours.  Within a few weeks, most of the orbiting debris had clumped together into one big lump.  The result was an earth with about 2/3 its present mass, whirling on its axis every three hours or so, while about 20,000 kilometres away - a 20th its present distance - a similarly scaled-down moon swung round in a nine-hour "month".

It took another 50 million years before smaller collisions had bulked up the earth and moon to roughly their present sizes.  Tidal interactions slowed the earth's rotation and pushed the moon farther out, giving us the 24-hour day and 28-day month we now enjoy.

Source: newscientist.com

Phases of the Moon

Source: strak.com

Until I saw the animated .gif above, I hadn't really realised just how much the moon wobbled.  Interesting...

See also:

bulletGiving Her the Moon - for an interesting picture of a total eclipse taken from the Mir spacestation.
bulletThe page immediately before and immediately after this one (press the "Back" or "Next" buttons respectively located at the bottom of the page) for some of my favourite astronomical photos, including (going forward) the most breathtaking picture of the sun I've ever seen.

Freeze, Fry or Dry: How Long Has the Earth Got?

by Robert Roy Britt

When a bunch of leading scientists got together to discuss the latest in big thinking, there was no shortage of doomsday predictions.  In particular, earth's fate was painted in three shades of grim.  Sometime in the next few billion years, according to new studies presented at the annual meeting of the American Association for the Advancement of Science, the third rock from the sun will either freeze or fry.  Unless things simply dry up much sooner.

While the earth's fate is not entirely sealed, predictions of the death of the sun are widely accepted.  The life-giving, ageing star we orbit is using up its fuel supply and will collapse within 7 billion years.  Before that, though, there will be an agonising period of repeated swelling, as the sun grows into a red giant.  How giant?  "Earth will end up in the sun, vapourising and blending its material with that of the sun," said Iowa State University's Lee Anne Willson.  "That part of the sun then blows away into space, so one might say earth is cremated and the ashes are scattered into interstellar space."  Willson and her colleague George Bowen studied other red giants, medium-sized stars like our sun that are near death, and used their findings to calculate the fate of the earth.  As the sun burns its core of hydrogen, gravity will force a collapse.  When compacted, the sun will heat up and burn the small amount of hydrogen that remains in a shell wrapped around the star's core.  This will force the sun to expand into a red giant.  Eventually, the core will heat up enough to burn stored helium and the sun will fluctuate in size before collapsing into a white dwarf.  "Earth will get scorched as part of the process the sun will go through as it transforms from being a red giant into a white dwarf," Willson said.

There are two possible paths to salvation, though both involve a frigid end.  "If the sun loses mass before it gets too big, then earth moves into a larger orbit and escapes," Willson said.  "The sun would need to lose 20% of its mass earlier in its evolution, and this is not what we expect to happen."

Fred Adams, a University of Michigan physicist, has for a few years been modelling the fate of the entire universe.  He said his work agrees with Willson's.  "If the earth stays in its present orbit, its fate is to be fried," Adams said in a telephone interview.  "That is the most likely fate."  Meanwhile, Adams has modelled a second possible method of escape.  Other scientists have learned that planets around other stars often follow odd-shaped orbits, indicating their paths might have been disrupted by the gravity of a passing star.  Adams and a colleague got to wondering whether some future passing star or star system might, in similar fashion, kick earth into the cosmic hinterlands.  So he and Gregory Laughlin, of NASA's Ames Research Laboratory, simulated many possible encounters with passing stars over the next 3.5 billion years - assuming earth would support life at least that long.  The odds of the planet being ejected from the solar system, they determined, are one-in-100,000.  "Life on earth would actually continue longer if earth is sent out of the solar system than if it stays.  These aren't real good odds," Adams points out, "but they're greater than the odds of winning the lottery, so they're worth considering."  (A report on the work was published in the journal Icarus.)  Adams figures if earth is sent off into some cold cosmic corner, the oceans would freeze solid after about a million years.  But some forms of life, supported by hydrothermal vents or other internal energy sources, might continue for up to 30 billion years, he estimates.

Before the earth's oceans ever have a chance to freeze or fry, they might have already dried up and evaporated into space, said James Kasting, a Penn State professor of meteorology and geosciences.  Kasting estimates his version of the end is a mere 1 billion years away.  "The sun ... is getting brighter with time and that affects the earth's climate," Kasting said.  "Eventually temperatures will become high enough so that the oceans evaporate."  And, Kasting said, a cataclysmic finale may come even sooner.  As earth becomes a global desert, CO2 levels are expected to drop.  At a certain level, which he and his colleagues say might be achieved in half a billion years, there would not be enough CO2 to support photosynthesis, and most plants would die.  Remaining plants would not be sufficient to support a biosphere, Kasting contends.  So while the entire planet might incinerated in a few billion years, or cast off into a deep freeze, it's possible that life on earth is already in the sunset years.  "If we calculated correctly, earth has been habitable for 4.5 billion years and only has a half-billion years left," Kasting said.

Source: space.com 25 February 2000

Impact "Showered Debris over Britain"
World Looked Different 214 Million Years Ago

by Helen Briggs

Evidence has emerged of how Britain's history was shaped by an asteroid collision 214 million years ago.  Rock blasted out of the ground by an asteroid hitting the Earth has been found for the first time in the southwest of England.  Canada still bears the scars of the explosion, which splattered hot rock and dust across the British Isles.  The space rock hit what is now Manicouagan, Quebec, opening up a 100-kilometre-wide (62-mile-wide) crater that can be seen by astronauts from space (see photo below).  The Atlantic Ocean had not appeared at the time, so the two land masses (Europe and North America) were much closer than they are today.  The rock was found near Bristol by geologists at the University of Aberdeen.

Dr Gordon Walkden said: "We have found evidence of a massive shockwave carrying molten rock and dust that has left a thin layer of glass beads and shattered mineral grains across the ancient British land surface."  The material has the distinctive hallmark of an asteroid slamming into the planet.  The space body, about 5 km (3 miles) wide, generated a shockwave 40 million times larger than the Hiroshima blast.  The impact material was found among sediments of the late Triassic Period.  Molten rock and debris were hurled high into the earth's atmosphere, some of it falling on to Britain.

The landscape 214 million years ago was very different to the rolling green fields of today.  It was largely arid desert, sparsely populated by ferns, reptiles, lizards, and small mammals.  "Anything standing would have been flattened by the blast," said Dr Walkden, who discovered the rock in the 1980s.  He knew at once there was something unusual about the sample, which he describes as tiny green balls embedded in pinky-coloured rock, but its ancient origins have only just come to light.  "It was very clearly something strange that I wasn't able to identify at the time," the senior geology lecturer said.  "It sat in a drawer for a long time."

It was to be 20 years before he realised its scientific value, when he saw similar deposits from a crater in Mexico, where the asteroid blamed for killing off the dinosaurs landed.  According to Dr Simon Kelley, of the Open University in Milton Keynes, who dated the Bristol sample, it is the first recorded example of such an impact deposit in Britain.  "If you had been in Britain at the time, the sun would have been blotted out by the dust and gases from the impact," he said.  "First you would have seen a big flash over the horizon.  Then the skies would have gone dark; then it would have rained hot dust and rocks.  The effects would have lasted for years."  The shock wave from the asteroid would have carried molten rock and dust thousands of kilometres.

Scientists are now searching for other remnants of the blast to study what happened to biodiversity on earth.  The Canadian impact does not seem to have had as devastating an effect as the impactor that signalled the downfall of the dinosaurs.  This is thought to be because it hit normal rock, rather than salt deposits capable of releasing poisonous gas.  "It may well be that we might get devastating meteorite impacts in the future but it is unlikely that there will be a repeat of the impact that saw the end of the dinosaurs," said Dr Kelley.

A piece of the material discovered by Dr Walkden will be on display at the Darwin Centre at the Natural History Museum, London, on 15 November.  Specimens will also be on display at the National Space Centre in Leicester, the National Museum of Scotland, Edinburgh, and at the University of Aberdeen.

The research is published in the journal Science.

Helen Briggs is the BBC News Online science reporter

Source: news.bbc.co.uk Friday 15 November 2002

Manicouagan is one of the largest known terrestrial impact craters.  It is 65 kilometers (40 miles) in diameter, but it is worth remembering that this is small compared with some of the larger lunar structures that measure more than 600 km (375 miles) across.  Produced by an asteroid impact some 200 million years ago, the concentric structure of the Manicouagan Crater results from the shock waves transmitted by the impact event.  These resemble somewhat the rings that result when a pebble is dropped into water.

The significance of asteroid impacts on the history of the earth has only been fully recognised in recent years.  The extinction of whole groups of animals such as dinosaurs and ammonites at the end of the Cretaceous Period some 60 million years ago is now believed to have been caused by an asteroid impact.  The catastrophic collision raised a dust cloud that obscured the sun and lowered temperatures dramatically.  Fallout from the impact covered the entire world, and provided a key pointer to the impact event: the fallout is rich in iridium, an element abundant in meteoritic materials and rare in surface rocks on earth.  The presence of this element at the Cretaceous-Tertiary boundary in several important sections has convinced many scientists that asteroidal impact was responsible for the mass extinctions at the end of the Cretaceous, but others remain skeptical.  It is possible that the asteroid responsible may not have been a great deal larger than that which excavated the Manicouagan Crater.  No extinctions associated with the Manicouagan event have yet been demonstrated, however.

STS-9, November-December 1983.  Picture #9-48-3139.

Source: lpi.usra.edu

I highly recommend a visit to lpi.usra.edu for some dynamite views of the earth from satellites...

Earth's Satellites

P C-W Fu and A Hanson (Indiana U), P Frisch (U of Chicago),
NASA grant numbers NAG5-8163, NAG5-11999; Earthday graphics program

Satellites can operate in several types of Earth orbit.  The most common orbits for environmental satellites are geostationary and polar, but some instruments also fly in inclined orbits.  Other types of orbits are possible, such as the Molniya orbits commonly used for Soviet spacecraft.

Thousands of satellites orbit the Earth.  Costing billions of dollars, this swarm of high altitude robots is now vital to communication, orientation, and imaging both Earth and space.  A geostationary (geo = geosynchronous) orbit is one in which the satellite is always in the same position with respect to the rotating Earth.  The satellite will appear to hover above one point on Earth's equator because it orbits at the same rate and in the same direction as Earth.  Geostationary (deep space) orbits are very high up - 38,500km (over five times the radius of the Earth) - and possible only because the satellite orbital period is exactly one day (23 hrs, 56 mins, 4.09 secs).  Most geo-stationary satellites are communication satellites and there are more satellites in the northern hemisphere than the southern.  They provide a "big picture" view, enabling coverage of weather events.  This is especially useful for monitoring severe local storms and tropical cyclones.  Because a geostationary orbit must be in the same plane as the Earth's rotation, that is the equatorial plane, it provides distorted images of the polar regions with poor spatial resolution.

Polar-orbiting satellites provide a more global view of Earth, circling at near-polar inclination (the angle between the equatorial plane and the satellite orbital plane - a true polar orbit has an inclination of 90 degrees).  Orbiting at an altitude of 700 to 800km, these satellites cover best the parts of the world most difficult to cover in situ (on site).  For example, McMurdo, Antarctica, can be seen on 11 - 12 of the 14 daily NOAA polar-orbiter passes.

These satellites operate in a sun-synchronous orbit.  The satellite passes the equator and each latitude at the same local solar time each day, meaning the satellite passes overhead at essentially the same solar time throughout all seasons of the year.  This feature enables regular data collection at consistent times as well as long-term comparisons.  The orbital plane of a sun-synchronous orbit must also rotate approximately one degree per day to keep pace with the Earth's surface.

Inclined orbits fall between those above.  They have an inclination between 0 degrees (equatorial orbit) and 90 degrees (polar orbit).  These orbits may be determined by the region on Earth that is of most interest (for example, an instrument to study the tropics may be best put on a low inclination satellite), or by the latitude of the launch site.  The orbital altitude of these satellites is generally on the order of a few hundred km, so the orbital period is on the order of a few hours.  These satellites are not sun-synchronous, however, so they will view a place on Earth at varying times.

It is usually cheaper to place a satellite in low Earth orbit, about 300km to 1500km from Earth's surface, just high enough to avoid the effect of Earth's atmosphere.  The above animated sequence starts by showing the halo of Earth's satellites, including the ring at geostationary, and finishes by zooming in on the only one currently hosting humans: the International Space Station, which is also a low orbit object around the Earth.  The ISS is at about 389km over central United state at the chosen moment.

Data sources: Earth Satellites: celestrak.com; ISS space station model: spaceflight.nasa.gov with modifications by John H Huffman, Indiana University

Source: antwrp.gsfc.nasa.gov, Looking at Earth From Space, a Teacher's Guide with Activities for Earth and Space Science, 1994, available from NASA Educator Resource Centres, asd-ww.larc.nasa.gov

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