Tropical regions north of the equator, including such areas as the Sahel in Africa which borders the Sahara desert, have already begun to get even drier and will continue to do so, the data show. Regions in the far north, including Canada, Northern Europe and Russia, will get wetter, as will the southern tropics.

Detecting the effects of climate change on rainfall patterns has proved much more elusive than temperature changes because of the much greater natural variability of precipitation.

The key was to take results from 92 computer simulations, using 14 different global circulation models, and to compare the average of these with actual rainfall data over wide bands of latitude around the world.

The results show a clear agreement with the observed trends in global rainfall data over the past century. In fact, although they agree in direction, the observed changes were much stronger than the predictions.

“Over the 20th century, we now detect the signal [in rainfall changes] that is predicted by climate models,” says Francis Zwiers, one of the research team. “If you’re able to reproduce the past, you also have greater confidence for predictions of the future.”

Dry zone

Zwiers, of Environment Canada in Toronto, says that the pattern shows a substantial drying of the region from the equator up to 30° north. This band encompasses all of north Africa, as well as India, southeast Asia, Mexico and northern South America. Some of these regions, such as the Sahara and Sahel in Africa, are already among the world’s driest.

Regions further to the south, including the rainforest regions of central Africa and South America, have begun to get increased rainfall and will continue to get wetter.

The findings are important, Zwiers says, because “as humans, our activities are much more constrained by limits of water than by temperature. In places where agriculture is marginal, it will become more marginal in the future”.

Precipitation extremes

Richard Seager, a climatologist at the Lamont Doherty Earth Observatory in New York, US, says this detection of 20th century rainfall changes seems “barely discernible from the noise right now”, but he agrees that the projected trends for the coming century shown by the combined climate models are a highly convincing prediction.

Seager’s own research has shown that, in addition to the trends shown by Zwiers’ team, there will also be a , including the US southwest and the Mediterranean.

But aside from the overall trends, Zwiers says an important message from the combined models is that they consistently show that, for all regions, there will be a significant increase in extremes of precipitation – both floods and droughts. Thus, even desert areas that will undergo serious drying could simultaneously suffer greater risks of flash flooding.

“More or less uniformly across all the models, these extreme events will become more intense just about everywhere,” he says.

Journal reference: (DOI: 10.1038/nature06025)

Update: The researchers have retracted their claim about the possibility of standing water on Mars after readers pointed out the terrain lies on the sloped wall of a crater – see our blog explaining what happened.

A new analysis of pictures taken by the exploration rover Opportunity reveals what appear to be small ponds of liquid water on the surface of Mars.

The report identifies specific spots that appear to have contained liquid water two years ago, when Opportunity was exploring a crater called Endurance. It is a highly controversial claim, as many scientists believe that liquid water cannot exist on the surface of Mars today because of the planet’s thin atmosphere.

If confirmed, the existence of such ponds would significantly boost the odds that living organisms could survive on or near the surface of Mars, says physicist Ron Levin, the report’s lead author, who works in advanced image processing at the aerospace company Lockheed Martin in Arizona.

Along with fellow Lockheed engineer Daniel Lyddy, Levin used images from the Jet Propulsion Laboratory’s website. The resulting stereoscopic reconstructions, made from paired images from the Opportunity rover’s twin cameras, show bluish features that look perfectly flat. The surfaces are so smooth that the computer could not find any surface details within those areas to match up between the two images.

The imaging shows that the areas occupy the lowest parts of the terrain. They also appear transparent: some features, which Levin says may be submerged rocks or pebbles, can be seen below the plane of the smooth surface.

Smooth surface

The smoothness and transparency of the features could suggest either water or very clear ice, Levin says.

“The surface is incredibly smooth, and the edges are in a plane and all at the same altitude,” he says. “If they were ice or some other material, they’d show wear and tear over the surface, there would be rubble or sand or something.”

His report was presented at a conference of the Institute of Electrical and Electronics Engineers, and will be published later this year in the institute’s proceedings.

No signs of liquid water have been observed directly from cameras on the surface before. Reports last year pointed to the existence of gullies on crater walls where water appears to have flowed in the last few years, as shown in images taken from orbit, but those are short-lived flows, which are thought to have frozen over almost immediately.

Speedy evaporation?

Levin and other reasearchers, including JPL’s Michael Hecht, have published calculations showing the possibility of “micro-environments” where water could linger, but the idea remains controversial.

“The temperatures get plenty warm enough, but the Mars atmosphere is essentially a vacuum,” says Phil Christensen of Arizona State University, developer of the Mars rovers’ mini-Thermal Emission Spectrometers. That means any water or ice exposed on the surface evaporates or sublimes away almost instantly, he says.

But, he adds, “it is theoretically possible to get liquid water within soil, or under other very special conditions”. The question is just how special those conditions need to be, and whether they ever really are found on Mars today.

If there were absolutely no wind, says Christensen, you might build up a stagnant layer of vapour above a liquid surface, preventing it from evaporating too fast. “The problem is, there are winds on MarsÂ… In the real world, I think it’s virtually impossible,” he told żěè¶ĚĘÓƵ.

Simple test

Levin disagrees. He says his analysis shows that there can be wind-free environments at certain times of day in certain protected locations. He thinks that could apply to these small depressions inside the sheltered bowl of Endurance crater, at midday in the Martian summer.

He adds that highly briny water, as is probably found on Mars, could be stable even at much lower temperatures.

Although the rover is now miles away from this site, Levin proposes a simple test that would prove the presence of liquid if similar features are found: use the rover’s drill on the surface of the flat area. If it is ice, or any solid material, the drill will leave unmistakable markings, but if it is liquid there should be no trace of the drill’s activity.

Levin’s father Gilbert was principal investigator of an experiment on the Viking Mars lander, which found evidence for life on the planet, although negative results from a separate test for organic materials led most scientists to doubt the evidence for biology.

Journal reference: R. L. Levin and Daniel Lyddy, Investigation of possible liquid water ponds on the Martian surface (2007 IEEE Aerospace Applications Conference Proceedings, paper #1376, to be published in )

According to , the rise in carbon dioxide will usher in a new golden age where food production will be higher than ever before and most plants and animals will thrive as never before. If it sounds too good to be true, that’s because it is.

CO₂ is the source of the carbon that plants turn into organic compounds, and it is well established that higher CO₂ levels can have a fertilising effect on many plants, boosting growth by as much as a third.

However, some plants already have mechanisms for concentrating CO₂ in their tissues, known as C₄ photosynthesis, so higher CO₂ will not boost the growth of C₄ plants.

Where water is a limiting factor, all plants could benefit. Plants lose water through the pores in leaves that let CO₂ enter. Higher CO₂ levels mean they do not need to open these pores as much, reducing water loss.

However, it is extremely difficult to generalise about the overall impact of the fertilisation effect on plant growth. Numerous groups around the world have been conducting experiments in which are supplied with enhanced CO₂, while comparable nearby plots remain at normal levels.

These experiments suggest that higher CO₂ levels could boost the yields of non-C₄ crops by .

Limiting factors

However, while experiments on natural ecosystems have also found initial elevations in the rate of plant growth, these have tended to level off within a few years. In most cases this has been found to be the result of some other limiting factor, such as the availability of nitrogen or water.

The regional climate changes that higher CO₂ will bring, and their effect on these limiting factors on plant growth, such as water, also have to be taken into account. These indirect effects are likely to have a much larger impact than CO₂ fertilisation.

For instance, while higher temperatures will boost plant growth in cooler regions, in the tropics they may actually impede growth. A two-decade study of rainforest plots in Panama and Malaysia recently concluded that local temperature rises of more than 1ºC have reduced tree growth by 50 per cent (see Don’t count on the trees).

Another complicating factor is ground level ozone due to air pollution, which damages plants. This is expected to rise in many regions over the coming decades and could reduce or even negate the beneficial effects of higher CO₂ (see Climate change warning over food production).

In the oceans, increased CO₂ is causing acidification of water. Recent research has shown that the expected doubling of CO₂ concentrations could inhibit the development of some calcium-shelled organisms, including phytoplankton, which are at the base of a large and complex marine ecosystem (see Ocean acidification: the other CO2 problem). That may also result in significant loss of biodiversity, possibly including important food species.

Levelling off

Some have suggested that the increase in plant growth due to CO₂ will be so great that it soaks up much of the extra CO₂ from the burning of fossil fuels, significantly slowing climate change. But higher plant growth will only lock away CO₂ if there is an accumulation of organic matter.

Studies of past climate changes suggest the land and oceans start releasing more CO₂ than they absorb as the planet warms. The latest concludes that the terrestrial biosphere will become a source rather than a sink of carbon before the end of the century.

What’s more, even if plant growth does rise overall, the direct and indirect effects of higher CO₂ levels will be disastrous for biodiversity. Between 20 to 30% of plant and animal species face extinction by the end of the century, according to the .

As for food crops, the factors are more complex. The crops most widely used in the world for food in many cases depend on particular combinations of soil type, climate, moisture, weather patterns and the infrastructure of equipment, experience and distribution systems. If the climate warms so much that crops no longer thrive in their traditional settings, farming of some crops may be able to shift to adjacent areas, but others may not. Rich farmers and countries will be able to adapt more easily than poorer ones.

Predicting the world’s overall changes in food production in response to elevated CO₂ is virtually impossible. Global production is expected to rise until the increase in local average temperatures exceeds 3°C, but then start to fall. In tropical and dry regions increases of just 1 to 2°C are expected to lead to falls in production. In marginal lands where water is the greatest constraint, which includes much of the developing world but also regions such as the western US, the losses may greatly exceed the gains.

It is a feat millions of times more impressive than finding a needle in a haystack. The new Mars Reconnaissance Orbiter has spotted about a dozen spacecraft on the Martian surface and, incredibly, taken pictures of such sharpness that scientists have been able to identify individual rocks that were first photographed by the Viking landers in 1976.

The new series of pictures released late on Monday show both of the Viking landers, never spotted from orbit before, as well as their nearby heat shields and backshells. These are the top and bottom covers of the capsules in which the rovers decended through the Martian atmosphere to land.

The MRO has also found the Mars rover Spirit , the pyramid-shaped structure in which it landed, its backshell and parachute. The satellite probe had already found the rover Opportunity and its landing structure, sending back images within its first week of operations in October 2006.

Picturing the Viking landers from orbit is quite a coup for MRO. Tim Parker, a planetary geologist at NASA’s Jet Propulsion Laboratory in Pasadena, told żěè¶ĚĘÓƵ that it was possible to clearly identify them by analysing the Martian landscape, even though the images show little detail.

Parker carefully matched rocks and other topographic features seen in the orbital views with those seen images the landers took on the ground. “I found a much better location match” than had been made from earlier orbital pictures, he says. It turns out Viking 1 is about 6 kilometres (3.8 miles) away from the spot identified as the landing site during the Viking mission.

“Ground truth”

Even individual boulders, just a metre across, seen by the Viking landers can be identified in the new images, Parker says. “The biggest surprise is that you can still see what appears to be the parachute [of a Viking lander] after 30 years,” Parker says. Such observations could help scientists determine the rate at which dust accumulates on the surface.

The complete series of MRO images help future Mars landings avoid potentially dangerous rocky outcrops. The Phoenix mission, scheduled for 2007, will be the first to benefit from the new topographical information.

By comparing the rockiness of the surface seen with MRO’s High Resolution Imaging Science Experiment (HiRISE) camera with the views already seen from the ground, the Phoenix team will be able to make a much better assessment of which regions will be safe enough to land on.

This calibration will in turn allow the team to interpret images from other regions on Mars more accurately, Alfred McEwen, leader of the HiRISE camera team, told żěè¶ĚĘÓƵ.

Earlier MRO images showed the landing site originally picked for Phoenix was too dangerous. But with the help of the new matches between the orbital and ground-level views, the team has now found “some regions that should provide safe landing sites” while still being geologically and biologically interesting, Parker says.

Ironically, McEwen says, the new MRO images show the area where Viking 2 came down is so rocky that “they wouldn’t let us land there now”. The new images of Spirit are already being put to use by its team to plan that rover’s next movements.

Comets have been known, and often feared, since ancient times. Their spectacular appearance in the night sky, with wide bright tails, has inspired awe and even been considered an omen.

Asteroids, comets and other cosmic debris have also had a fundamental impact on the development of planet Earth and the life on it, by bombarding it constantly at first, and periodically since. Future impacts pose a significant threat to human civilisation.

Comets are incredibly numerous – astronomers predict as many as one trillion could exist in the outer reaches of our solar system. Bright comets, however, appear in our skies just once a decade on average. The brightest recent ones were comets Hale-Bopp in 1996 and Hyakutake in 1997. Perhaps the most interesting recent comet was Shoemaker-Levy 9. This broke apart into dozens of pieces which then crashed into Jupiter in 1994.

The most famous comet of all, Halley’s Comet, was the first one recognised to reappear in the sky at regular intervals. Sir Edmond Halley studied records of past appearances and suggested that one comet followed a similar track through the sky roughly every 76 years, appearing in 1531, 1607, and 1682. He then predicted that it should pass by again in 1759. Though Halley did not live to see the reappearance, the successful prediction proved these bodies orbited the Sun, and were not atmospheric phenomena.

Leftover debris

Comets and asteroids are now understood to be leftover debris from the formation of the solar system. Astronomers currently think the solar system’s planets and minor bodies – including asteroids, comets and moons – all formed from the same cloud of dust and gas that initially condensed to form the Sun.

As the Sun grew, its gravitational field condensed the remaining matter into a large, swirling, flattened disc. Particles of dust collided to form bigger particles, and soon, until they built up into large chunks of rock and ice called planetesimals.

The small bodies that formed far from the Sun were rich in ice and other volatile materials, and billions of them orbit our star in a distant halo called the Oort Cloud. This is the source of “long-period” comets, those with orbits longer than 200 years.

In the inner solar system, many of these icy planetesimals coalesced to form the giant planets, while others were ejected out into the Kuiper Belt, beyond the orbit of Pluto but closer than the Oort Cloud. Pluto has been considered the ninth planet but some a stronomers believe it should really be considered a Kuiper Belt Object. Objects whose orbits are disturbed from the Kuiper Belt form the short-period comets.

The leftover planetesimals that remained within the inner solar system were largely contained in a region between the orbits of Mars and Jupiter, known as the asteroid belt.

Though the distinction can be fuzzy, asteroids are smaller than comets, are made up of less ice and volatile compounds, and have more regular orbits. Unlike comets, asteroids can usually only be observed with telescopes, so the first one, Ceres, was not discovered until 1801, by Italian astronomer Giovanni Piazza. It orbits in the asteroid belt and remains the largest known asteroid, at 930 kilometres (580 miles) across, about the size of Texas.

Other similar objects were found in rapid succession – Pallas in 1802, Juno in 1804 and Vesta in 1807 – leading to the realisation that there was a whole family of objects forming the asteroid belt.

Catastrophic impacts

It was not until 1979, however, that scientists began to realise that comets and asteroids can catastrophically impact the Earth.

The theory was initially proposed by physicist Luis Alvarez and others. It was based on evidence of a massive impact at the time of one of Earth’s greatest mass extinctions of life, when the last dinosaurs became extinct. The 65-million-year-old evidence included anomalous amounts of the element iridium – common in meteorites – in sediments deposited at the Cretaceous/Tertiary boundary.

Since then, astronomers have become increasingly a ware of the risk of future impacts, and major efforts are underway to discover and calculate the orbits for all asteroids larger than 1 km wide. A scale for comparing the potential risks of newly-found asteroids, the Torino scale, was proposed in 1999.

Since then, a number of asteroids have presented a future risk of impact, but in most cases those were quickly ruled out by further observations which refined the calculation of their orbits. The main exceptions so far have been 1950 DA, which has a possible collision course with Earth in the year 2880, and 2004 MN4, now renamed Apophis, which has a slight possibility of impact in 2036. An earlier possible impact path for Apophis, in 2029, has been ruled out, but will be the closest known miss ever predicted for a large asteroid, and will be visible to the naked eye from some parts of the Earth.

While no large impacts have been recorded in historical times, a mysterious blast in the skies over the Tunguska region of Siberia in 1908 has now been identified as a comet that exploded at high altitude as it entered Earth’s atmosphere.

Comet flyby

Rather than only waiting for comets and asteroids to arrive on Earth, astronomers have also been sending probes into space to chase down the objects. The first close-up views of a comet’s nucleus were obtained in 1986 by ESAâ€ČŮ Giotto mission, which passed within 20,000 km of the nucleus of Halley’s Comet. Flying through its tail of dust and gas – the coma – it obtained good images of the irregular, potato-shaped nucleus. The USSR’s twin spacecraft Vega 1 and 2 also made close flybys of Halley’s nucleus in 1986.

The next close encounter came in 2001, when NASA’s Deep Space 1 spacecraft flew by comet Borrelly, coming within 2200 km of the nucleus.

Then, on 2 January 2004, NASA’s Stardust mission flew by Comet Wild 2. It survived and obtained high-resolution images that showed large, deep steep-sided circular depressions, which were initially interpreted as craters. Later analysis showed they are more likely collapsed vents caused by jets of gas bursting out from deeper layers of ice. Stardust returned to Earth in January 2006 with precious samples comet dust.

The most spectacular comet encounter came on 4 July 2005, when NASA’s Deep Impact craft deliberately struck Comet Tempel 1, creating a controlled impact that was studied by cameras and spectrometers on a separate section of the spacecraft, and also by hundreds of telescopes on Earth and in orbit.

The next cometary encounter will be the European Space Agency’s Rosetta mission, which will go into orbit around comet Churyumov-Gerasimenko in 2014.

Collision course

Asteroids were first observed close-up by the Galileo probe on its way to Jupiter, which passed Gaspra in 1991 and Ida in 1993, discovering its tiny moon Dactyl. Then came the first dedicated asteroid mission, NEAR Shoemaker, which visited Mathilde in 1998 and then went into orbit around Eros in 2001 and eventually crash-landed on it.

The Japanese Hayabusa probe visited the asteroid Itokawa in 2005, hoping to scoop up some samples to be returned to Earth, but the sampling probe is thought to have failed in this goal and will probably return to Earth empty in 10 years.

Most attention today is focused on the Near-Earth Asteroids (NEAs), which are the ones that may someday collide with Earth. As an attempt to prove methods that might someday be used to deflect an asteroid from a collision course, a private group called the B612 Foundation hopes to change a non-threatening asteroid’s course slightly, using rocket motors or perhaps the gravitational pull of a spacecraft close to it. Giant mirrors and “cosmic airbags” have even been suggested.

Perhaps the most ambitious asteroid mission yet, NASA’s Dawn, was supposed to be launched in 2006, but has now been delayed for at least a year.

4.5 billion years ago

A solar nebula condenses to form the planets, moons, asteroids and comets of our solar system. The planets’ gravity propels comets outward to form the distant Oort cloud, and funnels most asteroids into a belt between Mars and Jupiter

3.8 billion years ago

The Late Heavy Bombardment: many massive impacts from debris left over after planet building, batter the Earth and Moon, and the inner planets, Mercury, Venus and Mars. The impacts are large enough to keep much of their surfaces molten

65 million years ago

An object – probably an asteroid about 10 kilometres across – crashes into what is now Mexico’s Yucatan peninsula, forming a 300-km-wide crater called Chixculub. The resulting global hail of red-hot impact debris sets fires everywhere. The soot and dust of the impact and fires creates an “impact winter” lasting for many years, leading to the death of all dinosaurs and many other forms of life

50,000 years ago

A 50-metre-wide, nickel-iron meteorite slams into what is now Arizona, forming Meteor Crater – Earth’s best-preserved large impact crater, 4 km across

613 BC

China’s Annals record a great comet, now thought to have been the first known sighting of Halley’s Comet and perhaps the earliest record of any comet

350 BC

Aristotle suggests that comets exist in the upper atmosphere, a view that held sway for nearly two millennia

312 AD

Roman Emperor Constantine claims to see a vision of a cross in the sky, causing his conversion to Christianity. Some think this was a comet, others that it was the flash of a meteorite impact

1066

A widely seen, spectacular appearance of Halley’s comet, is recorded on the Bayeux Tapestry, which depicts the Norman conquest of England

1577

Tycho Brahe measures the position of a great comet and compares it with sightings from other places on Earth, proving it is at least four times further away than the Moon

1705

Edmond Halley makes the first prediction of the reappearance of a comet, proving that they orbit the Sun

1801

The first asteroid, Ceres is discovered, by Italian astronomer Giovanni Piazza

1908

An airburst over Tunguska, Siberia, flattens trees over hundreds of square miles, and the boom is heard thousands of miles away. It is later thought to be a comet that exploded high in the atmosphere. The blast released 500 kilotons of energy – the equivalent of a hydrogen bomb

1950

Fred Lawrence Whipple proposes the “dirty snowball” model of cometary nuclei, which still prevails

Jan van Oort proposes that comets originate from a shell of billions of such objects thousands of times farther from the Sun than the Earth is. The region is now known as the Oort Cloud

1960

Geologist Eugene Shoemaker proves that Meteor Crater, Arizona really was produced by an impact. It was the first impact crater identified on Earth

1979

Luis Alvarez and colleagues publish the first paper linking the final dinosaur extinction to a massive asteroid impact

1980

The Spacewatch survey begins, at the University of Arizona, led by Tom Gehrels. This is the first systematic search for asteroids that might impact the Earth

1990

The Chixculub impact crater in Mexico, now buried deep below ground, is finally identified and tied to the impact that killed the dinosaurs

1992

US Congress-appointed panel proposes Spaceguard program to search for all near-Earth asteroids larger than 1 kilometre across within 10 years

1993

Comet Shoemaker-Levy 9 discovered. Resembling a string of pearls, it is the first known discovery of a comet shattered by a gravitational encounter with Jupiter. The dozens of pieces of the comet crash into Jupiter over a few hours in 1994

1995

The NEAT (Near-Earth Asteroid Tracking) project begins at an air-force tracking telescope in Hawaii. The rate of near-Earth asteroid (NEA) discovery increases more than tenfold

1996

The first discovery is made of an asteroid with an orbit computed to have a possibility of striking Earth

1999

The Torino Scale is published by astronomer Rick Binzel. It is an attempt to quell public fears about future near-Earth asteroid discoveries that have some small chance of striking Earth. It classifies all possible impacts on a scale with zero being no risk and 10 being virtually certain to cause global devastation.

2001

An asteroid is discovered that still has highest known possibility of impact. 1950 DA has 1-in-600 chance of striking Earth in 2880

2002

It is found that the Yarkovsky Effect can significantly alter asteroid orbits and therefore could be used to move one out of harm’s way, just by spray-painting it white. The Yarkovsky Effect causes thrust, because of imbalances between solar energy reaching the object and heat being radiated out

2004

NASA’s Stardust spacecraft flies by Comet Wild 2 on 2 January, gathering comet dust and taking close-up pictures

The highest number yet on the Torino Scale (see above) is reported in December 2004 – it is 4. The asteroid is now called Apophis and improved orbital calculations have reduced the threat level 1

NASA’s Deep Impact spacecraft becomes first object deliberately sent into collision with a comet, called Tempel 1, on 4 July

A method is proposed to move an asteroid from a collision course without touching it, using gravity alone

2005

The US Congress directs NASA to extend search for NEAs to all objects larger than 150-metres-wide, rather than those wider than 1 km

2006

NASA’s Stardust spacecraft returns to Earth on 15 January, bearing samples of comet dust

2029

Apophis will make the closest approach to Earth ever predicted for a sizable asteroid on 13 April. It will come close enough to be visible to the naked eye and the Earth’s gravity will change its orbit

2036

Apophis may impact Earth on 13 March – but only if it passes through a particular “keyhole” in 2029. The chances now stand at 1 in 6250

2082

Asteroid 2006 CS has a mere 1-in-77 million chance of striking Earth on 12 March – but it is 2.1 km across. An impact of that size could kill off most life on Earth. 2006 CS is by far the largest on the current list of objects with a small risk of impact

2102

Possible impact by 2004 VD17 on 2 May. This asteroid has only a 1-in-2777 chance of impact, but it is 580 m across and capable of continent-scale devastation

2880

Asteroid 1950DA has the greatest known probability of a major impact, on 16 March. The present odds are 1 in 600, and at about 1.1 km across, the asteroid is large enough to devastate most life on the planet

The report, by the Government Accountability Office (GAO), specifically criticises NASA’s stated plan to award a contract in September 2006 for the design, testing, fabrication and maintenance of the Crew Exploration Vehicle. The CEV is an all-purpose crew capsule designed to carry astronauts to the International Space Station and later, in a modified form, to the Moon and Mars.

The problem, says the report, is that fulfilling these goals could cost $230 billion over the next 20 years, yet NASA wants to award the single-source CEV contract, that could run to 2019, before it has developed “well-defined requirements, a preliminary design, mature technology, and firm cost estimates”.

That increases the risk of “significant cost overruns, schedule delays, and decreased capability”, says the GAO. It recommends Congress put NASA on a short leash, restricting its budget appropriations to the bare year-by-year minimum needed to sustain the programme through a readiness review that would allow for firmer cost and schedule estimates.

“Legitimate concerns”

Science committee chairman Sherwood Boehlert says the new report “raises legitimate concerns about NASA’s approach” and underscores the need for Congress to “keep a close eye on CEV costs to make sure they do not hamper the agency’s other activities”. Already, the report says, the agency’s own estimates show it will fall short of the money needed to carry out its stated plans by more than $1 billion per year from 2008 to 2011.

However, NASA seems to be unfazed by the report, and intends to carry on as planned. Deputy Administrator Shana Dale wrote in typical NASA jargon that the agency “nonconcurs with the GAO’s recommendation” that it modify its contract plans.

One private advocacy group has suggested an alternative solution to the problem. In a report issued on Tuesday, a day before the GAO’s, the Space Frontier Foundation says NASA could achieve all its plans and more by letting the competitive marketplace come to the rescue.

Taxi or truck?

It is a similar message to that given by Presidential Commission in 2004 – that NASA should leave private companies to design, own and operate the rockets needed to take people and cargo to low-Earth orbit. Instead, NASA should concentrate its resources on tasks such as lunar exploration – a task that really requires government programmes.

Keith Cowing, a former NASA engineer who edits the website NASA Watch, told żěè¶ĚĘÓƵ that the fact these two reports reached similar negative conclusions about NASA’s plans should be a wakeup call.

“NASA’s plans are not well thought out, and the funding is not in place,” he said. “Maybe it’s time to step back and take a look at what’s going on.”

And while the foundation’s suggestion that all work towards a new vehicle should go to untested companies may be extreme, Cowing says, the underlying criticism of a lack of flexibility is valid. NASA’s plan requires “the little taxi to go to the space station to look like the truck that’s going to go to the Moon”, he says, and that is not an efficient plan. “NASA is being not just rigid but dictatorial” in setting its designs, he adds.

A newly discovered asteroid is now the biggest thing known with a possibility of hitting the Earth in this century – and it is also the one that could hit the soonest.

But the odds of impact currently stand at just one in six million, reducing the fear factor somewhat, and these odds should further diminish with additional observations. This latest addition to NASA-JPL’s list of potentially hazardous asteroids was discovered on 27 April 2006.

The asteroid, called 2006 HZ51, has an estimated diameter of about 800 metres and is the one of the largest objects ever to make the list. An object of that size would cause widespread devastation if it did strike the Earth.

HZ51 also has one of the shortest lead-times to a potential impact of any such object yet found, and the shortest of any potential Earth-impactor currently on the list. The earliest of its 165 possible impact dates is just over two years away, on 21 June 2008.

Hollywood movies

Dan Durda, an asteroid expert and president of the B612 Foundation – which aims to anticipate and prevent such impacts – thinks the discovery of HZ51 highlights that at present there are no good options when faced with so little time to prepare. “There really isn’t a whole lot we could do,” he told żěè¶ĚĘÓƵ. “Most of the options that don’t resemble a Hollywood movie involve deflection techniques that require many years or decades.”

Other than stockpiling food and supplies and evacuating the regions most likely to be affected, he said, we would have to “hunker down and take the impact”.

But this is an unusual case, statistically speaking. It is far more likely that Earth’s nations would benefit from a much greater lead time before a potential impact, allowing more time for planning.

For example, the second-most imminent threat now on the list is the asteroid Apophis, which has about a 1-in-6000 chance of hitting Earth in 2036 – plenty of time to prevent it.

Altering orbits

The B612 Foundation has been pushing for a mission to place a tracking device on Apophis sometime in the next decade, so that the possibility of impact can be definitively proved or ruled out. The foundation also wants to send a mission to test ways of altering the orbit of a non-threatening asteroid, to test the viability of such methods.

But the chance of an impact by Apophis might be ruled out as early as this weekend, which will be the last chance until 2013 to observe it by radar, from the Arecibo radio telescope in Puerto Rico.

As for the newfound 2006 HZ51, the orbit calculations so far are based on just over 24 hours of observations, and so are likely to change quickly and should not be seen as a serious concern. As Clark Chapman of the Southwest Research Institute in Boulder, Colorado, US, explains: “Almost certainly, observations from one or two more nights will put this to bed as a zero probability.”

Asteroids, comets and other cosmic debris have also had a fundamental impact on the development of planet Earth and the life on it, by bombarding it constantly at first, and periodically since. Future impacts pose a significant threat to human civilisation.

Comets are incredibly numerous – astronomers predict as many as one trillion could exist in the outer reaches of our solar system. Bright comets, however, appear in our skies just once a decade on average. The brightest recent ones were comets Hale-Bopp in 1996 and Hyakutake in 1997. Perhaps the most interesting recent comet was Shoemaker-Levy 9. This broke apart into dozens of pieces which then crashed into Jupiter in 1994.

The most famous comet of all, Halley’s Comet, was the first one recognised to reappear in the sky at regular intervals. Sir Edmond Halley studied records of past appearances and suggested that one comet followed a similar track through the sky roughly every 76 years, appearing in 1531, 1607, and 1682. He then predicted that it should pass by again in 1759. Though Halley did not live to see the reappearance, the successful prediction proved these bodies orbited the Sun, and were not atmospheric phenomena.

Leftover debris

Comets and asteroids are now understood to be leftover debris from the formation of the solar system. Astronomers currently think the solar system’s planets and minor bodies – including asteroids, comets and moons – all formed from the same cloud of dust and gas that initially condensed to form the Sun.

As the Sun grew, its gravitational field condensed the remaining matter into a large, swirling, flattened disc. Particles of dust collided to form bigger particles, and soon, until they built up into large chunks of rock and ice called planetesimals.

The small bodies that formed far from the Sun were rich in ice and other volatile materials, and billions of them orbit our star in a distant halo called the Oort Cloud. This is the source of “long-period” comets, those with orbits longer than 200 years.

In the inner solar system, many of these icy planetesimals coalesced to form the giant planets, while others were ejected out into the Kuiper Belt, beyond the orbit of Pluto but closer than the Oort Cloud. Pluto has been considered the ninth planet but some a stronomers believe it should really be considered a Kuiper Belt Object. Objects whose orbits are disturbed from the Kuiper Belt form the short-period comets.

The leftover planetesimals that remained within the inner solar system were largely contained in a region between the orbits of Mars and Jupiter, known as the asteroid belt.

Though the distinction can be fuzzy, asteroids are smaller than comets, are made up of less ice and volatile compounds, and have more regular orbits. Unlike comets, asteroids can usually only be observed with telescopes, so the first one, Ceres, was not discovered until 1801, by Italian astronomer Giovanni Piazza. It orbits in the asteroid belt and remains the largest known asteroid, at 930 kilometres (580 miles) across, about the size of Texas.

Other similar objects were found in rapid succession – Pallas in 1802, Juno in 1804 and Vesta in 1807 – leading to the realisation that there was a whole family of objects forming the asteroid belt.

Catastrophic impacts

It was not until 1979, however, that scientists began to realise that comets and asteroids can catastrophically impact the Earth.

The theory was initially proposed by physicist Luis Alvarez and others. It was based on evidence of a massive impact at the time of one of Earth’s greatest mass extinctions of life, when the last dinosaurs became extinct. The 65-million-year-old evidence included anomalous amounts of the element iridium – common in meteorites – in sediments deposited at the Cretaceous/Tertiary boundary.

Since then, astronomers have become increasingly a ware of the risk of future impacts, and major efforts are underway to discover and calculate the orbits for all asteroids larger than 1 km wide. A scale for comparing the potential risks of newly-found asteroids, the Torino scale, was proposed in 1999.

Since then, a number of asteroids have presented a future risk of impact, but in most cases those were quickly ruled out by further observations which refined the calculation of their orbits. The main exceptions so far have been 1950 DA, which has a possible collision course with Earth in the year 2880, and 2004 MN4, now renamed Apophis, which has a slight possibility of impact in 2036. An earlier possible impact path for Apophis, in 2029, has been ruled out, but will be the closest known miss ever predicted for a large asteroid, and will be visible to the naked eye from some parts of the Earth.

While no large impacts have been recorded in historical times, a mysterious blast in the skies over the Tunguska region of Siberia in 1908 has now been identified as a comet that exploded at high altitude as it entered Earth’s atmosphere.

Comet flyby

Rather than only waiting for comets and asteroids to arrive on Earth, astronomers have also been sending probes into space to chase down the objects. The first close-up views of a comet’s nucleus were obtained in 1986 by ESAâ€ČŮ Giotto mission, which passed within 20,000 km of the nucleus of Halley’s Comet. Flying through its tail of dust and gas – the coma – it obtained good images of the irregular, potato-shaped nucleus. The USSR’s twin spacecraft Vega 1 and 2 also made close flybys of Halley’s nucleus in 1986.

The next close encounter came in 2001, when NASA’s Deep Space 1 spacecraft flew by comet Borrelly, coming within 2200 km of the nucleus.

Then, on 2 January 2004, NASA’s Stardust mission flew by Comet Wild 2. It survived and obtained high-resolution images that showed large, deep steep-sided circular depressions, which were initially interpreted as craters. Later analysis showed they are more likely collapsed vents caused by jets of gas bursting out from deeper layers of ice. Stardust returned to Earth in January 2006 with precious samples comet dust.

The most spectacular comet encounter came on 4 July 2005, when NASA’s Deep Impact craft deliberately struck Comet Tempel 1, creating a controlled impact that was studied by cameras and spectrometers on a separate section of the spacecraft, and also by hundreds of telescopes on Earth and in orbit.

The next cometary encounter will be the European Space Agency’s Rosetta mission, which will go into orbit around comet Churyumov-Gerasimenko in 2014.

Collision course

Asteroids were first observed close-up by the Galileo probe on its way to Jupiter, which passed Gaspra in 1991 and Ida in 1993, discovering its tiny moon Dactyl. Then came the first dedicated asteroid mission, NEAR Shoemaker, which visited Mathilde in 1998 and then went into orbit around Eros in 2001 and eventually crash-landed on it.

The Japanese Hayabusa probe visited the asteroid Itokawa in 2005, hoping to scoop up some samples to be returned to Earth, but the sampling probe is thought to have failed in this goal and will probably return to Earth empty in 10 years.

Most attention today is focused on the Near-Earth Asteroids (NEAs), which are the ones that may someday collide with Earth. As an attempt to prove methods that might someday be used to deflect an asteroid from a collision course, a private group called the B612 Foundation hopes to change a non-threatening asteroid’s course slightly, using rocket motors or perhaps the gravitational pull of a spacecraft close to it. Giant mirrors and “cosmic airbags” have even been suggested.

Perhaps the most ambitious asteroid mission yet, NASA’s Dawn, was supposed to be launched in 2006, but has now been delayed for at least a year.

David Chandler, 15 February 2006

• A solar nebula condenses to form the planets, moons, asteroids and comets of our solar system. The planets’ gravity propels comets outward to form the distant Oort cloud, and funnels most asteroids into a belt between Mars and Jupiter

3.8 billion years ago

• The Late Heavy Bombardment: many massive impacts from debris left over after planet building, batter the Earth and Moon, and the inner planets, Mercury, Venus and Mars. The impacts are large enough to keep much of their surfaces molten

65 million years ago

• An object – probably an asteroid about 10 kilometres across – crashes into what is now Mexico’s Yucatan peninsula, forming a 300-km-wide crater called Chixculub. The resulting global hail of red-hot impact debris sets fires everywhere. The soot and dust of the impact and fires creates an “impact winter” lasting for many years, leading to the death of all dinosaurs and many other forms of life

50,000 years ago

• A 50-metre-wide, nickel-iron meteorite slams into what is now Arizona, forming Meteor Crater – Earth’s best-preserved large impact crater, 4 km across

613 BC

• China’s Annals record a great comet, now thought to have been the first known sighting of Halley’s Comet and perhaps the earliest record of any comet

350 BC

• Aristotle suggests that comets exist in the upper atmosphere, a view that held sway for nearly two millennia

312 AD

• Roman Emperor Constantine claims to see a vision of a cross in the sky, causing his conversion to Christianity. Some think this was a comet, others that it was the flash of a meteorite impact

1066

• A widely seen, spectacular appearance of Halley’s comet, is recorded on the Bayeux Tapestry, which depicts the Norman conquest of England

1577

• Tycho Brahe measures the position of a great comet and compares it with sightings from other places on Earth, proving it is at least four times further away than the Moon

1705

• Edmond Halley makes the first prediction of the reappearance of a comet, proving that they orbit the Sun

1801

• The first asteroid, Ceres is discovered, by Italian astronomer Giovanni Piazza

1908

• An airburst over Tunguska, Siberia, flattens trees over hundreds of square miles, and the boom is heard thousands of miles away. It is later thought to be a comet that exploded high in the atmosphere. The blast released 500 kilotons of energy – the equivalent of a hydrogen bomb

1950

• Fred Lawrence Whipple proposes the “dirty snowball” model of cometary nuclei, which still prevails

• Jan van Oort proposes that comets originate from a shell of billions of such objects thousands of times farther from the Sun than the Earth is. The region is now known as the Oort Cloud

1960

• Geologist Eugene Shoemaker proves that Meteor Crater, Arizona really was produced by an impact. It was the first impact crater identified on Earth

1979

• Luis Alvarez and colleagues publish the first paper linking the final dinosaur extinction to a massive asteroid impact

1980

• The Spacewatch survey begins, at the University of Arizona, led by Tom Gehrels. This is the first systematic search for asteroids that might impact the Earth

1990

• The Chixculub impact crater in Mexico, now buried deep below ground, is finally identified and tied to the impact that killed the dinosaurs

1992

• US Congress-appointed panel proposes Spaceguard program to search for all near-Earth asteroids larger than 1 kilometre across within 10 years

1993

• Comet Shoemaker-Levy 9 discovered. Resembling a string of pearls, it is the first known discovery of a comet shattered by a gravitational encounter with Jupiter. The dozens of pieces of the comet crash into Jupiter over a few hours in 1994

1995

• The NEAT (Near-Earth Asteroid Tracking) project begins at an air-force tracking telescope in Hawaii. The rate of near-Earth asteroid (NEA) discovery increases more than tenfold

1996

• The first discovery is made of an asteroid with an orbit computed to have a possibility of striking Earth

1999

• The Torino Scale is published by astronomer Rick Binzel. It is an attempt to quell public fears about future near-Earth asteroid discoveries that have some small chance of striking Earth. It classifies all possible impacts on a scale with zero being no risk and 10 being virtually certain to cause global devastation.

2001

• An asteroid is discovered that still has highest known possibility of impact. 1950 DA has 1-in-600 chance of striking Earth in 2880

2002

• It is found that the Yarkovsky Effect can significantly alter asteroid orbits and therefore could be used to move one out of harm’s way, just by spray-painting it white. The Yarkovsky Effect causes thrust, because of imbalances between solar energy reaching the object and heat being radiated out

2004

• NASA’s Stardust spacecraft flies by Comet Wild 2 on 2 January, gathering comet dust and taking close-up pictures

• The highest number yet on the Torino Scale (see above) is reported in December 2004 – it is 4. The asteroid is now called Apophis and improved orbital calculations have reduced the threat level 1

• NASA’s Deep Impact spacecraft becomes first object deliberately sent into collision with a comet, called Tempel 1, on 4 July

• A method is proposed to move an asteroid from a collision course without touching it, using gravity alone

2005

• The US Congress directs NASA to extend search for NEAs to all objects larger than 150-metres-wide, rather than those wider than 1 km

2006

• NASA’s Stardust spacecraft returns to Earth on 15 January, bearing samples of comet dust

2029

• Apophis will make the closest approach to Earth ever predicted for a sizable asteroid on 13 April. It will come close enough to be visible to the naked eye and the Earth’s gravity will change its orbit

2036

• Apophis may impact Earth on 13 March – but only if it passes through a particular “keyhole” in 2029. The chances now stand at 1 in 6250

2082

• Asteroid 2006 CS has a mere 1-in-77 million chance of striking Earth on 12 March – but it is 2.1 km across. An impact of that size could kill off most life on Earth. 2006 CS is by far the largest on the current list of objects with a small risk of impact

2102

• Possible impact by 2004 VD17 on 2 May. This asteroid has only a 1-in-2777 chance of impact, but it is 580 m across and capable of continent-scale devastation

2880

• Asteroid 1950DA has the greatest known probability of a major impact, on 16 March. The present odds are 1 in 600, and at about 1.1 km across, the asteroid is large enough to devastate most life on the planet

David Chandler, 15 February 2006