Blast from the Past

More than 400 years ago, a nearby star in our Galaxy exploded with a flash of light so bright that we can still see echoes of the event from the surrounding dust clouds. The event is known as "Tycho's supernova", after the Danish astronomer who published detailed observations of it in 1572. In today's issue of the journal Nature, astronomers announced new observations that have for the first time established the exact cause of the explosion.

When a new star appeared in the sky in 1572 -- bright enough to be visible in full daylight -- it instantly attracted public attention. At the time the "sphere of the stars" was believed to be unchanging and eternal, so the event challenged the prevailing view of the universe. Tycho Brahe was one of the first astronomers to make precise observations of the positions of stars and planets, before he famously died of a urinary infection after straining his bladder at a royal banquet as a courtesy to the host. Tycho's observations of the 1572 event were published in a book titled "Stella Nova" (Latin for "new star"), which demonstrated that the object was far beyond the orbit of the moon.

Modern analysis of the historical record suggests that the event was actually an exploding star, now commonly referred to as a "supernova". There are several types of supernova explosions, arising either from the detonation of a white dwarf star (the ember that is left at the end of the life of a regular star like our Sun), or from the gravitational collapse of a very massive star at the end of its life. The available evidence suggested that Tycho's supernova was probably the first type, but it could not distinguish between two possible scenarios that can trigger such an event. In one scenario the white dwarf is in a close orbit with a regular star -- so close that its gravity slowly consumes the mass from its companion until the white dwarf exceeds a critical size and collapses under its own weight. In the other scenario the companion is also a white dwarf, and the two white dwarfs slowly draw closer together until they merge and similarly collapse in a spectacular explosion.

The usual way of deciding what triggered the supernova explosion requires evidence that was not available in 1572. The light from the event is passed through a prism to separate it into its constituent colors (a spectrum), where dark lines appear at specific colors that reveal what the object is made of and how it is moving. For the observations that were announced today, astronomers used the interstellar dust that surrounds Tycho's supernova as a sort of time machine to record the spectrum of the event. Here's how it works: the explosion created a sudden flash that moved out in all directions at the speed of light and reached the Earth in 1572. Now imagine that there was a cloud of dust 200 light years behind the supernova that reflects some of the light of the original flash back toward the Earth. This dust cloud would not be lit up until 200 years after the explosion, and the reflected light would then have to travel another 200 years just to get back to the supernova itself -- so the flash from the dust cloud would arrive 400 years after the original explosion. Using this basic idea, astronomers recorded the spectrum of an illuminated dust cloud near Tycho's supernova and discovered that it looked exactly like an explosion caused by the first scenario -- from a white dwarf consuming a regular star.

It's a wonderful piece of detective work, and by repeating the experiment on dust clouds in every direction around the remnant of Tycho's supernova, scientists hope to eventually measure the three dimensional shape of a 400 year old explosion. Tycho would surely be impressed by our ability to study this blast from the past.

Science under Obama

The decisive presidential victory of Barack Obama last night has given millions of ordinary people hope for the future, but it also represents a symbolic end to the "war on science" that has been waged by the current administration. Scientists across the country now share the hope that the newly-elected administration will not only reverse the politicization of science that has taken place over the past eight years, but also demonstrate a renewed commitment to maintaining the competitiveness of the United States in science and technology by expanding our investment in basic research.

The abuses of the Bush administration -- distorting and suppressing scientific research for political purposes -- has been well documented within several federal agencies. Much of this information has come from a series of surveys conducted by the Union of Concerned Scientists over the past few years, and their efforts to restore scientific integrity now represent one of the top priorities of the organization. Scientists are consistently ranked by the public as more credible than politicians or even journalists, so it's important that their research is free from government interference and that their conclusions are released to the taxpayer-sponsors without political censorship. One indication that Barack Obama intends to use science to craft policy -- rather than the other way around -- can be seen in his proposal for addressing climate change, which aims to "reduce greenhouse gas emissions 80 percent by 2050". This target is not arbitrary, but was taken directly from the recommendations of the UN Intergovernmental Panel on Climate Change, representing the consensus of thousands of scientists from around the world.

With the rampant international aggression and reckless fiscal policies of the Bush administration, funding for non-military programs has stagnated even as deficit spending has soared and the national debt has more than doubled. The Bush strategy of using irresponsible tax cuts for the ultra-wealthy coupled with massive increases in military spending to justify a shrinking domestic budget has been disastrous for the basic government services that millions of people depend on. But it has also had a corrosive effect on our nation's scientific research facilities -- leaving some of the most promising technologies undeveloped, and stunting the bold new discoveries that attract the brightest young people to pursue careers in science. The newly-elected administration understands that a federal commitment to basic research provides the raw material for future economic growth and prosperity, and ensures our nation’s competitive position in the world. That's why Barack Obama co-sponsored legislation in the Senate in 2007 that tried to boost support for NASA and double the funding for basic research through the National Science Foundation by 2015. Though it sounds ambitious, the annual research budget of the NSF could be doubled for the current cost of about one week of military operations in Iraq.

The global financial crisis has created some uncertainty about what sort of economy the next administration will inherit from George W. Bush, and this has understandably led to a reflection on the nation's priorities. With the right leadership -- one that uses sound science to craft effective policy, and that understands the importance of an investment in basic research -- scientists can help turn this economy around. With Barack Obama at the helm, it just might happen.

Climate Conservatives

Over the weekend, I attended a high school reunion with my wife. One of the great things about reunions is that you get to talk with people from all walks of life, who typically have a wider variety of opinions than your co-workers or neighbors. One of my wife's classmates, after hearing that I work at a federal laboratory that does climate change research, asked whether or not I believed in global warming. The question really took me by surprise because, to me, global warming is like gravity: it doesn't really matter whether you "believe in" it -- it just is.

I responded that her question made me wonder whether or not she believed in climate change. She explained that the issue made her very upset because of how political the debate had been, and that she didn't believe in global warming at all. She said she didn't trust Al Gore, and she thought he was a hypocrite for making so much money from his film and for flying his private jet between stops on his "climate crisis" tour. If the facts in the film had been presented by a scientist, she said, it probably would have done more to persuade her about the problem. The central issue seemed to be about credibility, not about facts. I tried to imagine my own response to claims about a crisis coming from a source that I didn't trust.

In situations like these, it's important to try to communicate with people using arguments that will not instantly be rejected by their conservative viewpoint. I tried to explain that when our parents were our age there were still significant uncertainties about climate change, so we couldn't really blame them for not acting. While scientists were establishing the root causes of global warming, the greenhouse gas emissions of humanity had exceeded the capacity of the natural world to absorb them -- and there was so much surplus carbon-dioxide in the atmosphere that even if we stopped the emissions now, the planet would still continue to warm for nearly the rest of our lives. So the debate about climate change was really about what (if anything) we should do to influence the kind of world our children and grandchildren would live in.

I also pointed out that politicians were no longer arguing about the reality of climate change. The real debate was about how much we should do to mitigate the problem. Look at the proposals of the two major presidential candidates in this election year: the Democratic candidate is proposing to cut greenhouse gas emissions 80% by 2050 (a target drawn directly from the scientific recommendations of the UN Intergovernmental Panel on Climate Change) while the Republican candidate is proposing a 60% cut. Furthermore, only the Republican candidate for vice-president has expressed skepticism that climate change is primarily caused by human activity. Everyone else agrees with the scientific consensus.

I may not have convinced my wife's classmate to reconsider her position on global warming, but I'm fairly sure that I presented the issue in a manner that she could identify with. If conservatives continue to resist efforts to mitigate climate change, they are the ones who will have to explain to their grandchildren why they failed to take action even after the science was clear.

Grassroots Astronomy

A catalog of stars that a NASA satellite will search for distant solar systems over the next few years is scheduled to be released today through the Space Telescope Science Institute. In an unconventional approach to funding their research on the project, a team of astronomers is working with a non-profit organization to put the stars up for adoption on the Internet.

"NASA's budget is heavily earmarked, and research funding has been stagnant in recent years," said astronomer Travis Metcalfe, who leads the fundraising project. "We decided to engage the public directly for some grassroots financial support of this crucial element of the program."

The scientists are preparing for the launch of NASA's Kepler satellite, which will monitor more than 100,000 stars beginning in April of next year. To bring the catalog to life, the likely target stars were marked in Google Sky, where potential sponsors can browse them through the project website before selecting a star to adopt for a modest $10 donation. All contributions are used to support the analysis that will determine the physical sizes of any planets discovered by the mission.

The adopt-a-star service, named the "Pale Blue Dot" project after a phrase coined by popular astronomer Carl Sagan, has already attracted donations from hundreds of people who signed up early to get their first choice of stars. Donors receive a certificate of adoption by email, and updates when any planets are discovered around their adopted stars. Each star is tagged with the name of the sponsor, both in Google Sky and in a text version of the catalog, so no two people can adopt the same star. The final targets will not be decided until later this fall, but if an adopted star does not end up on the list, the associated donor will make a new selection.

"This is a creative way of involving the community," said Danish professor Joergen Christensen-Dalsgaard, who organized the international team that is conducting the research. "The present list of adopters is already fairly impressive, and of course the funding that it brings in is useful."

The outreach and fundraising effort is being coordinated by White Dwarf Research Corporation, a non-profit located in Boulder, Colorado. Dedicated to scientific research and public education, the organization hosts the project web pages and accepts donations on behalf of the research team free of charge. If most of the Kepler target stars are ultimately adopted, the resulting endowment is expected to provide significant support to the research project throughout the lifetime of the mission.

"This program educates the public about the Kepler satellite, but it also makes them stakeholders in its success by allowing anyone to adopt a star that NASA will search for planets," said Metcalfe. "How cool is that?"

Arctic Meltdown

This week, scientists at the National Snow and Ice Data Center (NSIDC) announced that the amount of unmelted sea ice in the Arctic has reached the second lowest level since satellite monitoring began 30 years ago, and it may be on a trajectory in the coming weeks to break the record melting that occurred just last year. The revelation comes on the heels of reports earlier this month of polar bears swimming in the open waters off the northwest coast of Alaska. It has long been known that the effects of global warming appear first near the north and south poles, so this news provides a glimpse of what's in store for the rest of the world if we don't begin to change course soon.

On the summer solstice this year, I was on an airplane returning from a two week visit to Denmark -- I purchased carbon offsets for the trip, but the irony of the situation is still tangible. In the middle of the day, I peered out the window to discover that we were flying over the east coast of Greenland. I was encouraged to see that there still seemed to be plenty of sea ice floating around, but the land was littered with cobalt blue melt ponds and the dramatic retreat of the surrounding glaciers was readily apparent from above. Little did I know at the time that only about a third of that sea ice would still be around by the annual low point in mid-September.

Through most of the melting season this year, the total area covered by sea ice in the Arctic has followed roughly the same pattern as observed in 2005, which was previously the second greatest melt in recorded history. But in early August this year, the melting seemed to accelerate -- maybe from all of those flights to China for the Olympics. "We could very well be in that quick slide downwards in terms of passing a tipping point," said Mark Serreze, a senior scientist at the Colorado-based NSIDC. Within the next few weeks as the melting bottoms out, we'll know whether this year's melt breaks the all time record from last year.

Nothing seems to bring this crisis into clearer focus for most people than the plight of the polar bears. Last week an aerial survey of wildlife off the northwest coast of Alaska reported observing 9 polar bears swimming in open waters between 15 and 65 miles offshore. Ironically, the survey was being conducted to prepare for future offshore oil development. Satellite data from the time of the survey showed that the nearest pack ice was about 100 miles from the coast. Polar bears are good swimmers, but they typically only swim distances of 10 to 15 miles. "We have some observations of bears swimming into shore when the sea ice was not visible on the horizon. In some of these cases, the bears arrive so spent energetically, that they literally don't move for a couple days after hitting shore" said Steven Amstrup, senior polar bear scientist for the U.S. Geological Survey in Anchorage. If the changes in Arctic melting were happening more gradually, the polar bears might have time to adapt over many generations. But the unprecedented pace of Arctic warming makes the outlook fairly grim.

The inconvenient truth of the matter is that these events give us a sneak preview of the types of changes we are likely to see at lower latitudes in the coming decades. Only it won't be the polar bears who are struggling to adapt to the new circumstances -- it will be us.

Secrets of the Aurora

Anyone who has ever witnessed the northern or southern lights would certainly attest to their beauty, but few would imagine that the mysteries of the aurora are still being uncovered. Last week, scientists announced a fundamental discovery about the physical mechanism that converts a stream of charged particles from the Sun into a stunning display of color and light in the Earth's upper atmosphere. The work represents a significant step forward in efforts to predict such events, which can damage satellites in orbit and disrupt power grids on the ground.

The basic mechanism that drives the aurora has been known for a long time. Our Sun is constantly shedding a "wind" of charged particles, primarily electrons and protons, which slams into the Earth's magnetic field. Since charged particles in motion create their own magnetism (the underlying concept of an electromagnet), the solar wind spirals towards the Earth's north and south magnetic poles. There, it slams into the upper atmosphere and interacts with the oxygen and nitrogen atoms to produce shimmering curtains of green and red light in the sky.

The researchers used observations of the aurora from detectors on the ground, as well as measurements from a network of five NASA satellites that orbit at different heights within the Earth's magnetic field. By combining the data from these different sources, the scientists determined the detailed series of electro-magnetic events that precede and follow an auroral display. During times of relatively mild solar wind, the Earth's magnetic field works a bit like a capacitor -- accumulating charge for several hours and then quickly releasing it. The process is accompanied by a sudden disruption of the current in the upper atmosphere (something like blowing an electric fuse), as well as a snapping of the Earth's magnetic field lines far above (a process known as magnetic reconnection). The standard model for aurora suggested that the magnetic reconnection caused a disruption in the current, which then produced the aurora. The new observations show that the aurora are produced directly by the snapping field lines, and the disruption of the current occurs later.

Although the discovery is unlikely to change the way most people view the northern and southern lights, it may improve scientist's ability to predict the potentially harmful consequences of the storms. This could ultimately allow us to avoid regional electric blackouts and service outages from satellites that provide navigation services and communications -- the dark side of the auroral lights.

Communicating with Mars

In a landmark discovery this week, NASA's Phoenix lander dug a trench in the Martian soil and watched as small chunks of water ice slowly evaporated. Scientists have known about the existence of water ice in the polar cap of Mars for a long time, but it was only in 2002 that the Odyssey spacecraft found evidence of much more ice just below the surface. This is what motivated the experiment on Phoenix that directly confirmed this vast reservoir of frozen sub-surface water. It's not easy to control an experiment from a hundred million miles away. Have you ever wondered how NASA actually does it?

Even when the Earth and Mars are as close as they can come to each other in their orbits they are separated by more than 30 million miles, and sometimes they are nearly 250 million miles apart. How do we communicate across such enormous distances? It's similar to how we do it on Earth: we encode our signals into radio waves and send them from one antenna to another. Radio waves travel at the speed of light, more than 186000 miles or eight times around the Earth in a single second. This is very practical for communicating across the planet, but it's fairly slow to communicate between planets. When it is closest, a radio signal to the Mars lander takes several minutes to get there -- and its response takes several minutes to return to Earth. More typically, the trip takes 10 minutes each way. Imagine playing a video game where any move you make with the controller doesn't show up on the screen for 20 minutes!

With such slow communication, scientists do not control the Phoenix lander the same way you might play a video game. Many of the functions of the experiment need to be automated and programmed ahead of time. So they simply tell the lander to "dig a trench" then "take a picture" and wait for the image to be sent back. This led to the discovery earlier this week of some "white stuff" beneath the soil. To test whether it might be water ice they waited for a day, asked the lander to take another picture, and compared the second picture to the first one. This showed that some of the crumbs from the digging actually disappeared in the second picture -- suggesting that they were made from ice and had evaporated.

If you think playing a video game with a 20 minute lag time might be frustrating, imagine what it would be like with people on both ends of the line. This is one of the many challenges of sending astronauts to Mars, but it's something that most people don't think about.

WorldWide Worthless

This morning I received an email from my mother-in-law with the subject line: "Travel to the Stars!' Naturally, I was intrigued. She wanted to know whether I had heard about the new free program from Microsoft called WorldWide Telescope, just released yesterday. I had actually seen some of the headlines and my impression was: oh, Microsoft is trying to compete with Google Sky, which came out last August. But to be fair, I decided to download the new software and try it out.

My first complaint is that WorldWide Telescope only runs under Windows (surprise!). Google Sky also runs on Mac and Linux machines. I decided to install it on a laptop I use for presentations, where I had successfully installed Google Sky several months ago. I went to the website at WorldWideTelescope.org, which looked very nice, and downloaded the 20 MB install application. No problem. When I ran this application, it downloaded an additional 90 MB of Microsoft software that WorldWide Telescope depends on. Everything seemed to install just fine, so I clicked the new shortcut that had been placed on my desktop. The program loaded and a dialog box appeared with instructions about what to do with my mouse to control the interface. Looks a lot like Google Sky. So I started to pan around the sky -- and within a few seconds the program crashed. I rebooted the machine for a fresh start, launched the program and tried to pan again -- another crash. And a third. And a fourth.

At this point, I decided to uninstall the software and everything that came with it. I also decided to check that my copy of Google Sky was still working -- and thankfully, it was. In the process I also discovered that Google now has a version of Sky that runs in the maps interface through a web browser. If you go to the WorldWide Telescope website you can watch flash movies of children's reactions as they explore the sky through the software. Microsoft is great at public relations, but not so great at writing software that actually works. As for me, I'm sticking with Google Sky.

Ozone and Climate

Many people confuse the issue of climate change (or global warming) with the other major influence of humanity on the Earth's atmosphere: the hole in the ozone layer. These phenomena are actually caused by two very different aspects of atmospheric chemistry, and they have distinct consequences for the environment. But this week, climate scientists announced a surprising connection between the ozone layer and climate change: if we adopt one approach to cooling the planet, it could make the ozone problem worse.

Ozone is just another form of oxygen. The oxygen that we breath is really a molecule consisting of two oxygen atoms bound together. Ozone is similar, but it has three oxygen atoms. High in the Earth's atmosphere there is a relatively thin layer of ozone, and because of its specific structure it acts like sunblock -- filtering out much of the harmful ultraviolet radiation coming from the Sun before it ever reaches the ground. Back in the 1980's, scientists discovered that large holes were developing in the ozone layer, particularly over the polar regions of the planet. Further study linked this depletion to widespread use of chlorofluorocarbons (CFCs), particularly as refrigerants and as propellants in spray cans for consumer products. Alternative gases were quickly identified for these uses, and CFCs were phased out in most industrialized countries. The results were dramatic -- further monitoring of the ozone holes showed them begin to recover as the widespread use of CFCs was discontinued. It was a landmark achievement for climate science and its influence on public policy.

More recently it has become clear that the global climate is getting warmer and that, like ozone depletion, a substantial fraction of the effect can be attributed to human activities. In this case the underlying cause is the release of so-called greenhouse gases -- molecules like carbon dioxide (CO2) and methane (CH4) that trap the Sun's infrared radiation or heat from being released back into outer space. Large volumes of these gases are produced from burning fossil fuels like coal in power plants or gasoline in automobiles -- and as humanity releases more greenhouse gases than the environment can naturally absorb, it builds up in the atmosphere, traps more heat, and gradually warms the planet. This has been happening dramatically since the industrial revolution in the late 1800's, and unless we find suitable alternatives for our energy needs we risk a major disruption of our climate system later in this century.

Since this is a much bigger problem than finding substitute refrigerants and propellants -- and because the consequences are potentially much more severe -- some climate scientists are considering how to cool the planet without reducing greenhouse gas emissions. In the long term climate record that has been reconstructed from ice cores drilled in Antarctica, scientists noticed that historical periods of global cooling often follow major volcanic eruptions. The cooling was traced to large quantities of sulfates that were hurled into the Earth's upper atmosphere during the eruptions. The sulfates block some of the incoming sunlight from ever reaching the surface, which can counteract the warming effects of increased greenhouse gas concentrations (though other problems like ocean acidification still remain). So if global warming gets really out of control, the scientists reasoned that we could manually inject sulfates into the upper atmosphere like an artificial volcanic eruption to cool the planet.

Working from this premise, the study released this week examined the other possible consequences of engineering the climate with massive injections of sulfates -- and this is where ozone returns to the picture. It turns out that sulfates also deplete the ozone layer, and if enough of them were injected into the upper atmosphere to offset the expected warming in the coming decades it would destroy between 1/4 and 3/4 of the Arctic ozone layer and delay the expected recovery of the ozone hole over the Antarctic by between 30 and 70 years. So if we want a reasonable climate and an ozone layer, sulfates might not be the best answer.

Personal Space Telescope

The beautiful images produced by the Hubble Space Telescope have probably done more to capture the public imagination than any other NASA project since the Apollo moon missions. As you might expect, not just anyone can use Hubble. The allocation of observing time is extremely competitive, and even most professional astronomers have never used it. Few people realize that Canada also has a space telescope -- and now, a team of Canadian astronomers wants to help any Canadian citizen use it for their own projects.

In June 2003, Canada launched its first space telescope called "MOST" for Microvariability & Oscillations of STars (or Microvariabilité & Oscillations STellaires for those in Quebec). It's quite a bit smaller than Hubble, roughly the size of a large suitcase, earning it the nickname "Humble Space Telescope". Because of its modest size, MOST was lifted into orbit on a decommissioned nuclear missile from Russia. Under an international peace treaty the missile was required to be destroyed anyway -- so they decided to use it for science! Inside MOST there is a 6-inch diameter reflecting telescope and a high quality digital camera. Scientists use the camera to take a series of images of pulsating stars, and record the amount of light emitted by the stars over time. They use this information to learn about the insides of the stars -- a cutting-edge technique known as asteroseismology.

After four years of very successful science operations, MOST had reached the end of its expected life, but it was still running. Rather than keeping the telescope for themselves, the MOST science team decided to do something unprecedented: they announced that any Canadian citizen could submit a proposal to use it. Young, old, scientist, amateur -- it didn't matter. The process was open to all. They called this new phase of the project "MOST: My Own Space Telescope". No word yet on whether any citizen projects have been selected, or announcements of their results. But the concept certainly reflects the best of the Canadian spirit.

Lunar Eclipse Demystified

Tomorrow night the Moon will once again plunge into the shadow of the Earth, marking the third lunar eclipse in the past 12 months. Despite the recent frequency of such events, your next chance to see a total lunar eclipse won't come for nearly three years, in December 2010. While not as spectacular (or rare) as a solar eclipse, watching the Moon turn dark red (but not invisible) has its own rewards. And this time, it will be joined by Saturn and the bright star Regulus -- both within a few degrees on the sky.

People often wonder why lunar eclipses are so much more common than solar eclipses, and why the Moon doesn't pass through the shadow of the Earth every month. Since both events involve the alignment of the Sun, Earth, and Moon it seems like they would both occur with similar frequency. But the dark shadow of the Earth (the umbra) is roughly three times the diameter of the full Moon, so the alignment doesn't have to be perfect as with a solar eclipse -- where the Sun and Moon appear almost the same size in the sky. This also makes lunar eclipses last much longer (about an hour, compared to just a few minutes) and allows half of the planet to see them each time (since you just need to be on the night side of the planet when they happen, not a special location where the alignment is perfect). Despite these advantages, the Moon doesn't pass through the Earth's shadow every month because its orbit is actually inclined by 11 degrees relative to the plane of the Earth's orbit around the Sun. So we only see a lunar eclipse when the full Moon occurs near the intersection of these two orbital planes (the so-called "line of nodes").

One of the interesting features of a lunar eclipse is that the Moon doesn't go completely black. Instead, it usually turns some shade of red when it passes through the darkest part of the Earth's shadow. We know that the Moon normally just reflects the light of the Sun -- so where does this red light come from? During a lunar eclipse, the light that reaches the Moon is just sunlight that has been diverted towards the Moon through the Earth's atmosphere. Why is it red? The answer turns out to be the same as "why is the sky blue?". The molecules in the Earth's atmosphere are similar in size to the wavelength of blue light, so they interact and scatter the blue light (making the sky blue) while letting most of the red light through (making the Moon look red). This is also why sunsets often look red.

If you've never seen Saturn through a telescope, this event will offer a good chance, since the ringed planet will appear within a few degrees (5 or 6 Moon diameters) of the lunar eclipse. The bright blue star Regulus (in the constellation Leo) will also appear nearby -- but it should be easy to distinguish from yellowish Saturn. Viewed with high power binoculars or a small telescope, the rings of Saturn are fairly easy to see, even for a pea-sized image.

Mars Asteroid Uncertainty

Earlier today, an asteroid as wide as a football field cruised past Mars at a distance just over three times the diameter of the red planet. By cosmic standards, this was a very near miss. The asteroid was discovered in late November, and in mid-December astronomers placed the odds of an impact at 1-in-75. Additional observations just before the new year increased these odds to nearly 1-in-25, but within two weeks the probability of an impact sank dramatically to just 1-in-10,000. All of the attention given to the developing story led a satire website to release an article headlined: "NASA plans to blow up Mars if asteroid misses".

With such wildly oscillating predictions, it would be easy to imagine that scientists simply don't know what they're doing. But predicting the path of an asteroid in space is a little like predicting the path of a hurricane over the ocean -- natural uncertainties in the observations that are used for the calculations lead to a range of possible positions as we predict further into the future. In the case of an asteroid, astronomers use the observed positions of the space rock over time to define an orbit from Kepler's laws. With positions observed over a reasonable fraction of the asteroid's path around the Sun, the orbit can be calculated very accurately. But with data spanning only a few weeks, the orbit is harder to define and each new observation can lead to significant changes when estimating the future path of the asteroid.

This is exactly what happened in the case of the Martian asteroid, officially named "2007WD5". While routine calculations of asteroid trajectories typically lead to one-in-a-million odds for any planet impacts, the early observations of 2007WD5 gave a better than 1% chance of striking Mars. This got the attention of many astronomers, and new observations came pouring in from around the globe, leading to a better estimate of the orbit and a new calculation of the impact probability, by then up to 4%. This led to even more excitement, and many new observations -- defining the orbit so well that an impact was effectively ruled out.

As cool as it would have been to watch a big rock punch a fresh crater into Mars, we should be comforted by the fact that astronomers are conducting their census of the asteroid belt. After all, the next near miss could belong to the Earth...