Life in the Universe

Legendary physicist Stephen Hawking made headlines this week when he suggested that "if aliens ever visit us...the outcome would be much as when Christopher Columbus first landed in America, which didn't turn out very well for the Native Americans." Most people are surprised to learn that many astronomers actually believe in extraterrestrial life, though few scientists tend to speculate about how the aliens might relate to our civilization. But what we know about the size and composition of the universe -- and what we've learned about the stars in our own Galaxy over the past 20 years -- make it unthinkable that we could possibly be alone.

The scientific argument for extraterrestrial life can be traced back to the 1960's, when astronomer Frank Drake first formulated what has come to be known as the "Drake Equation". Essentially, this is just a simple method to estimate the number of intelligent civilizations in our Galaxy. It starts with the number of stars (a very big number) and then multiplies by reasonable estimates of the fraction of stars that have planets, the fraction of those that are habitable, that form life, where the life becomes intelligent, that develop the technology to emit signals into space (like radio communication), and finally how long those civilizations exist before becoming extinct or destroying themselves. The main conclusion is that even if you make very pessimistic assumptions for all of these fractions, the number of stars is so large that there will still be a handful of intelligent civilizations like ours in the Galaxy. In Hawking's words, "To my mathematical brain, the numbers alone make thinking about aliens perfectly rational."

It was just 15 years ago that the first planet outside of our solar system was discovered around a star like the Sun. As of this week, more than 450 "exoplanets" have been discovered around other stars. Many of these planets do not resemble those in our own solar system at all. The methods that have been used to discover them are biased towards large planets like Jupiter that orbit relatively close to their suns like Mercury (and in most cases even closer), but a few of the known exoplanet systems include up to five planets. The diversity of planetary systems that astronomers are finding around other stars suggests that planets are much more common than we originally thought, and it's a first step towards actually measuring the fractions in Drake's equation.

Despite our progress in detecting planets around other stars, we still don't know of any habitable planets other than the Earth. However, within the next few years NASA's Kepler mission will tell us exactly how common habitable planets are in our Galaxy. Launched just over a year ago, the Kepler space telescope is performing a detailed census of planets around stars in our Galactic neighborhood. Its enormous digital camera has the sensitivity to detect the tiny eclipse of a planet the size of the Earth passing in front of its host star, and the mission is monitoring more than 150,000 stars like the Sun for such signals. Since a habitable planet like the Earth takes a full year to complete an orbit, Kepler needs to search for several of these "transits" over the course of a few years to identify the distant Earths reliably. If such planets are as common as we believe, Kepler should find a dozen or so within the first few years -- but in any case, it will measure another fraction in Drake's equation.

Throughout human history whenever we have believed ourselves to be special in some way, we turned out to be wrong. We now know that the Earth is just one planet among many in our Galaxy, and we will soon know how common planets are that might also be habitable. With a short list of distant Earths, we can begin to search for alien radio signals. It may take some time, but the conclusion is inevitable -- we are almost certainly not alone.

Comet Crash

Last week, astronomers watched in awe as several anonymous comets plunged into the Sun. Many people are surprised to learn that similar events happen almost every day, but the icy fragments are typically too small to be seen. A string of comet fragments struck Jupiter in 1994, causing quite a splash. Should we worry that the Earth might be next?

Comets are generally believed to be leftover debris from the formation of our solar system more than 4 billion years ago. A distant icy halo surrounding the Sun, known as the "Oort cloud", is the source of nearly all of the comets that have been observed throughout history. Some of these comets return regularly (like Halley's comet in its 75.3 year orbit), while others pass through once and head back to the cloud, never to be seen again. Astronomers believe that comets are like "dirty snowballs", containing a loosely packed mixture of ice and dust. As they approach the Sun they warm up and begin to evaporate, spawning a long bright tail that generally makes them much more visible.

There is a population of comets known as the "Sun grazers", which appear to be the leftover fragments of a much larger comet that probably broke up more than 2000 years ago. The orbits of these comets are extremely elongated, and in some cases shoot them directly into the Sun, a fiery demise that is barely noticed by the enormous boiling mass. Most of the fragments are so small that they are nearly invisible until they begin their plunge, where satellites that are always watching the Sun for signs of hazardous space weather can finally see them brighten and abruptly disappear.

In the summer of 1994, the planet Jupiter had a similar encounter with a train of comet fragments known as "Shoemaker-Levy 9". About two years earlier a large comet passed so close to Jupiter that it was broken into many pieces by tidal forces and thrust into an orbit that would strike the giant planet like a string of pearls on the next pass. I was an undergraduate student at the time, working with a group known as SpaceWatch at the University of Arizona. Over the months leading up to the collision, we carefully measured the positions of each fragment to calculate the individual orbits and make predictions of the precise impact times. For the nine largest pieces, we calculated times that were correct to within a few minutes.

As for something similar happening to the Earth -- don't worry! Jupiter is a huge target with significant gravitational pull, and the Sun is over 1000 times more massive. Nothing as large as a comet has struck the Earth since the disappearance of the dinosaurs, 65 million years ago.

Red Giant Groove

Earlier this month I attended a scientific conference on a small Spanish island called Lanzarote, just off the coast of Morocco. The meeting was for scientists who study the insides of the Sun and other stars using a technique similar to seismology. Without a doubt, the coolest result presented at the meeting came from Italian astronomer Andrea Miglio and his colleagues in Belgium. For the first time in any star other than the Sun, they measured an abrupt variation in the internal structure -- like the boundary between the crust and the core of the Earth, but in a red giant star more than 350 light years away.

A red giant star is what our Sun will become when it begins to run out of hydrogen fuel, about 5 billion years in the future. As the hydrogen begins to burn faster in a shell around the helium core, our star will slowly bloat and cool -- eventually engulfing the inner planets, including the Earth. Like the Sun, the surface of a red giant seems to boil as convection brings heat from below and radiates it into the coldness of outer space. The boiling churns more slowly in a red giant, but the turbulent motions still create sound waves that travel down through the star and then back toward the surface. Some of these sounds have just the right pitch, a million times lower than we can hear with the human ear, and they set up standing waves that cause the entire star to slowly change its brightness over hours and days.

Like the standing waves on a vibrating string, the brightness changes that we can see in a red giant have very specific frequencies. The fundamental frequency of a vibrating string is sort of like a jump rope, with the entire length moving up and down together. If the people at the ends of the rope move their hands up and down at twice the fundamental frequency, the standing wave will have two lobes with a stationary point in the middle -- one side will move up while the other moves down. At three times the fundamental frequency you get three lobes, and so on to create a whole series of evenly spaced frequencies. Now imagine that somewhere along the jump rope you attach a small lump of clay -- the extra weight at this position will change the way that waves travel along the rope, and the frequencies of the standing waves will change slightly from the evenly spaced pattern. Here's the cool part -- by looking at the deviations from uniform spacing, you can actually figure out where along the rope you have attached the clay!

This is essentially what Andrea and his friends did for the red giant. Using measurements taken over 5 months from a French space telescope called COROT, they identified a series of almost evenly spaced frequencies of brightness variation in a red giant known as "HR 7349". By looking at how the frequencies they observed differed from perfectly uniform spacing, they determined the position of a boundary layer on the inside of the star. Comparing this prediction with theoretical models of the star, they identified this layer as the depth below the surface where the temperature was just high enough to strip both of the electrons from every helium atom. From a distance of more than 350 light years, the team had pinpointed the position of a subtle change in the composition and density of the star -- just by measuring its regular changes in brightness over time.

This beautiful result promises to be just the first demonstration of the types of measurements that are possible using stellar seismology. The same basic techniques will allow us to measure the size of the boiling layer on the surface of a star like the Sun, providing new ways to test our understanding of how stars are actually built.

Kepler's First Planets

This month the Kepler mission announced its first batch of extrasolar planet discoveries. Many people were surprised that only five new planets were announced, since the mission is monitoring more than 150,000 stars. But there are very sensible reasons why the initial catch was limited.

The first thing to understand about the newly discovered planets is that all of them were found in just the first 43 days of Kepler observations. The satellite was launched in early March, but the first two months were spent largely on engineering to ensure that the instrument was operating as expected. This was followed by a 10-day "commissioning" run in early May, and then a 33-day initial science run before the spacecraft made its first quarterly roll in mid-June to keep the solar panels facing the Sun. Because the search method requires a minimum of three transits, only planets with orbital periods shorter than about 14 days were detectable from the initial 43 days of observations. In fact, all of the planets that were announced had orbital periods between 3.2 and 4.9 days.

Fair enough, but shouldn't Kepler have discovered hundreds of such planets during the first 43 days? It probably did, but the mission requires detailed follow-up observations from ground-based telescopes to confirm each planet discovery. This is necessary to weed out "false positives" -- stars that look like they may host planets, but are actually something else. A simple example is an eclipsing binary star, where one star periodically passes in front of the other just like a transit. If there is a bright star close to the binary, or just along the line of sight, it dilutes the eclipses so they look like they are caused by a smaller object like a planet. The follow-up observations involve measuring the wobble of the host star caused by the orbiting object -- the traditional method of detecting planets around other stars. If the orbiting object is a star, the wobble is enormous; If it's a planet, the wobble is tiny.

Kepler is searching for planets in a large patch of the sky in the constellations Cygnus and Lyra. This area of the sky is well placed for ground-based telescopes during the summer months, but by the time the early data from Kepler was processed it was already August. The season for these follow-up observations was very short, so the team could only confirm a limited number of the planet candidates. In the end, they decided to announce only five of the more than 70 candidates that emerged from the initial analysis. By next summer, the mission will have many more candidates in longer orbits -- but they will also have a full summer season to confirm the discoveries.

By next January, expect to hear about hundreds of new planets. It will still be too early for the Earth-like planets, since that requires three 1-year orbits. But planets as tiny as the Earth may be found in faster orbits, and if they are hosted by smaller stars they might even be habitable. Stay tuned.

Climate and Altruism

Writing from the floor of the climate summit in Copenhagen this week, Danish economist Bjorn Lomborg suggested that a singular focus on reducing greenhouse gas emissions was misguided. Such cuts, he argues, would be "breathtakingly expensive and woefully ineffective" and the money would be better spent on more immediate problems like global health and education in developing countries. Is his suggestion realistic, or just a diversion?

In his 1970 book "The Possibility of Altruism", philosopher Thomas Nagel tried to understand why people make sacrifices -- sometimes even give their lives -- to save others. He argued that the notion of altruism is similar to the notion of prudence. Just as a prudent individual must extend their concept of self to another time, an altruistic individual must extend their concept of self to a different space, in another person. Thus, if we believe it is important to plan for our own future (prudence), it is possible to act on an analogous belief in the value of other people's lives (altruism). It was an assertion of the commutability of space and time that would have made Einstein proud. This is relevant to Lomborg's suggestion because the arguments for taking action on climate change invoke both prudence and altruism: we should reduce our carbon emissions to secure our own future, but also to improve the future of our children and the citizens of developing nations who will be most affected.

Richard Dawkins promotes a different view of altruism in his 1976 book "The Selfish Gene". He suggests that it is useful to think of altruism from the perspective of the individual genes inside of a person. Instead of attaching a survival instinct to the individual, Dawkins ascribes this trait to the genes themselves. In this view, a gene does not care where it survives -- only that it survives. As a consequence, if I have two children who are in danger of being eaten by a lion it is rational from the perspective of my genes to risk my life to save them. Since each of my children shares half of my genes, my complete DNA may have a better chance of continuing to survive in the two kids than in me. Brothers and sisters are even more likely to share my genes, while more distant relatives share less and citizens of developing countries share almost none. According to Dawkins, any act of altruism must be justified by this sort of detailed calculus. This naturally explains why the average person feels very little obligation to ease the suffering of people in distant lands, whether caused by climate change or more immediate problems.

Lomborg is a former Greenpeace member, and author of the 2001 book "The Skeptical Environmentalist" where he applies the economic principle of cost-benefit analysis to world problems. Whatever your understanding of altruism, his suggestion that the world should address the more immediate problems in developing countries rather than fight climate change is a false dichotomy. If the international community had the will to improve human health and education in the third world, it would have done so already. On the other hand, climate change is a problem that will affect the entire planet -- not just developing nations -- so the motivation to avoid the worst consequences is a personal imperative whether Nagel or Dawkins is right about altruism. Whether we cap our emissions because we recognize the humanity of others or because we simply want our genes to survive in our children, the outcome will be the same.

The worst thing that could happen at the climate summit in Copenhagen is nothing. If the delegates fail to reach a consensus on carbon reductions, there is at least one good idea promoted by Lomborg. "Instead of making far-fetched promises about greenhouse gases," he writes, "how about a concrete commitment to green energy research and development?" Now there's something we can all support.

Stimulus for Science

This week the $787 billion in economic stimulus funding approved earlier this year came under intense scrutiny, as reporters and citizens examined preliminary data posted on a government website. The primary focus in the news has been whether the estimate of 640,000 "jobs saved or created" is accurate, and on a few typographical errors in the database such as non-existent congressional districts that supposedly received funding from the stimulus. I looked at the data to see how the stimulus was supporting astronomy research, and how well the funding agencies were doing at getting that money into the economy.

Although NASA's larger budget normally provides much more support for astronomy research than the National Science Foundation (NSF), the opposite was true for stimulus funding. The NSF has so far received more than $2.4 billion dollars from the American Recovery & Reinvestment Act that was signed into law last February. By the end of October the agency reported spending only $50 million of these stimulus funds. Most of the allocated funds (about $2 billion) are for "Research & Related Activities", which supports individual researchers through various grants and fellowship programs. Typically, the proposals to such programs require 6-9 months for review prior to being awarded -- so maybe the low level of spending so far is not surprising. Another $254 million has been allocated to support "Major Research Equipment and Facilities Construction" including the Advanced Technology Solar Telescope, which is scheduled to begin construction in 2010. None of these funds have yet been spent.

NASA has done a better job of pumping stimulus funds into the economy, having already spent $68 million of the $572 million it has received. In fact, most of this spending (about $66 million) has been to support NASA's "Astronomy & Astrophysics Research Recovery" plan, a $212 million program that primarily supports development of the James Webb Space Telescope, which will supersede the Hubble Space Telescope in 2014. NASA has actually been promised more than $1 billion in stimulus funding, so part of the delay in spending seems to arise from the slow allocation of funds by the government. The NSF has received a larger fraction of the funding it was promised under the stimulus ($2.4 billion allocated out of $3 billion total), but it has spent a much lower fraction of what it has received. It was not immediately obvious how many jobs were "saved or created" due to the funding at either agency.

Overall, the data posted to recovery.gov represents a valiant attempt by the Obama administration to ensure the transparency of stimulus spending. The deployment hasn't been flawless, but it's a step in the right direction in terms of trying to establish a more open government. The relatively slow spending at the agencies that support astronomy research reflects the careful consideration that must go into their funding decisions. This is certainly preferable to the alternative -- more rapid funding of projects with questionable or unknown merit. Hopefully, we can expect more of this research funding to enter the economy soon.

NASA at the Crossroads

When I was in college, many of my astronomy classmates joined a group called "Students for the Exploration and Development of Space". Although I strongly support exploration, I didn't join the group because I felt uneasy about the "development" of space -- to some people this means glowing billboards in low-earth orbit, or inflatable space hotels for wealthy clients. In an era of dwindling support for the space program, NASA is wrestling with similar questions about the future.

This week NASA is scheduled to launch the new Ares I-X, an experimental version of the rocket that is supposed to replace the space shuttle. Unfortunately the Ares is unlikely to be ready by the time the space shuttles are retired at the end of 2010, leaving at least a 5-year gap in the ability of NASA to send astronauts to the International Space Station (ISS). NASA is hesitant to extend the life of the shuttle program, since the accidents in 1986 and 2003 raised fundamental questions about safety. The other immediate option is to hitch a ride to the ISS with the Russians for a few years until the Ares rocket is finished. But a committee appointed by the Obama administration earlier this year would like to see a different solution. Led by a former aerospace executive, the panel concluded that NASA should turn over the business of putting astronauts in orbit to private companies.

The notion of a "public-private" partnership for space exploration involves large public subsidies to aerospace companies, who will then take over the business when it is mature and collect the profits. Of course the risks would continue to be insured by the government, not the private sector. Former NASA administrator Michael Griffin is skeptical about the safety of commercial space travel, commenting that the plan "will work right up until there is the first accident." But Elon Musk, founder of the private launch company SpaceX, disagrees. "It's incredibly bad business to kill your customers", he said. There are other private launch companies with a longer track record, but none have any experience putting people into orbit. So the problem remains.

Perhaps the most sensible thing for NASA to do is once again rethink its long-term objectives. The "Vision for Space Exploration" set forth by the Bush administration will only funnel more money to aerospace companies -- fueling the "development" of space at the expense of "exploration" that would provide real scientific advancement.

Personalizing Climate Impacts

A report released today by the World Bank includes a simple observation about the failure of humanity to act against the dangers of a warming planet. "The slow pace of climate change as well as the delayed, intangible and statistical natures of its risks simply do not move us." Scientists must strike a careful balance between communicating the seriousness of the problems we face, without using scare tactics. How can we bring the future impacts home to the citizens of the world?

A few weeks ago I had a conversation with a young woman at the birthday party of a mutual friend. When she learned that I work at the National Center for Atmospheric Research, she asked, "is climate change real, or is it just a scam to get more funding from the government?" I was floored. I explained that when our parents were our age there was still some uncertainty about the exact causes of global warming. There have always been natural changes in climate caused by a slight wobble in the Earth's orbit around the Sun, leading to ice ages and warm periods that alternate over tens of thousands of years. During the past century the changes have been much faster than ever before, and less than a quarter of the recent warming can be explained by natural cycles. The rest is from heat-trapping gases released by human activities. Whatever we do now, the globe will continue to warm for the next several decades as the Earth slowly absorbs the excesses of earlier generations. We can't blame them, because they didn't know what they were doing. But now we know, and our actions will determine the kind of world our children will live in. I told her that the necessary changes wouldn't be as dramatic as everyone imagines. If we all adopt a lifestyle more like our parents in the 1960's, with smaller houses and one car per family, it would go a long way toward solving the problem. At the same time it will improve our real quality of life, allowing us to spend less time working and commuting with more time for the things that truly matter.

The evidence for climate change is all around us. Here in Colorado, recent warming has expanded the population of parasitic pine beetles that have decimated our national forests. One campground in Rocky Mountain National Park now resembles a clear-cut logging operation, with all of the "beetle kill" removed for the safety of visitors and the surrounding trees that are still healthy. As dramatic as it seems, the connection to climate change is not obvious to the casual observer. The park staff do not distribute pamphlets that explain the cause of the beetle problem, and there are no signs to proclaim "global warming in action". But the Nature Conservancy recently announced a website that tries to convey the impacts of climate change on a local level. Their Climate Wizard is a science-based website that allows anyone to select their state or country and see the temperature and precipitation projections over the next 50 to 100 years from the most recent report of the UN Intergovernmental Panel on Climate Change. Users can zoom in on their state and then switch between alternate futures with high, medium or low heat-trapping emissions. The default is to view the temperature changes over the next 100 years across the U.S., since the next 50 years are dominated by the emissions of the past. The main lesson is that the future is typically hotter and drier, but some regions are bigger losers than others -- like the area of the U.S. where most of the food is produced.

Although it's hard to get people motivated to make lifestyle changes now that will affect the temperature of the world inhabited by their children and grandchildren, it's helpful to frame it as a moral issue. Many citizens are concerned about passing trillions of dollars in national debt to the future, and climate change is really just another kind of debt. Somebody will eventually have to pay. The sooner we address the problem, the smaller the burden will be for future generations.

Waiting for Colbert

This week I finally succeeded at getting some media attention for a fundraising project that I have volunteered for during the past two years. The basic idea is to take the 100,000 stars that NASA's Kepler satellite will search for planets, and allow anyone to adopt one of them for a $10 donation. Donors receive a certificate of adoption by email, and updates when any planets are discovered around the star they adopted. Unlike the many "name a star" scams on the Internet, no two people can select the same star and all of the proceeds go to support scientific research on the target stars. The program hasn't been without its challenges, and the exposure this week has certainly pushed it forward -- but to meet our fundraising goal, we need a Colbert bump.

After all of the news about the first science results from Kepler two weeks ago, I decided to try and ride the tail of the wave of coverage by issuing my own press release. I had tried press releases before without success, but this time it sparked the interest of reporters at both Space.com and New Scientist. From there, the story was picked up and translated for articles in Russia, China, and Brazil. By the end of the week the coverage had driven more than 4000 visitors to our website, and inspired star adoptions by more than 250 new donors. It generated as much funding in a few days as the website normally attracts in 5 months! But we are still far from attracting the millions of visitors that we need to adopt thousands of stars.

Along the road, there have been additional challenges aside from the difficulty of getting our message heard. Last summer, after the first successful fundraising from a short post to slashdot, NASA became aware of the program. They expressed some concern that donors might mistakenly believe the project was sponsored by NASA, or maybe they would think that the donor name would be officially assigned to the Kepler target stars. To appease NASA, we added a disclaimer at the bottom of every page. After the news coverage this week we were contacted by the estate of Carl Sagan, who believed that calling our adopt-a-star program the "Pale Blue Dot" project constituted unauthorized use of their copyright from his 1994 book. They were concerned that the name might lead donors to assume some kind of endorsement by Carl Sagan. Whatever the legal status of their copyright, our non-profit educational use of the phrase clearly falls within the "fair use" exception -- but to alleviate their concerns we added "the estate of Carl Sagan" to our disclaimer. Who knew there could be such a disconnect between the good intentions of scientists and the nervous deliberations of managers and lawyers?

The biggest lesson of the week is that we will need exposure to a much larger audience than we can possibly reach on the web or in print. We need television, and who would be better than Comedy Central host Stephen Colbert? He loves outer space, and his character is obsessed with having things named after him. Although our program doesn't actually name the stars, we did reserve the few stars with previously known planets just for such an opportunity. So we adopted a planet-hosting star for Stephen Colbert, and we even set up a special page for his fans. He's our first genuine pale blue dot! Best of all, one of our early adopters has a connection at the show, and offered to pitch the idea for us. Now we're just waiting for Colbert. Will he invite me to come on the show and present him with a Certificate of Adoption? If so, I'm certain that the "Pale Blue Dot" project will finally reach its goal.

Cap and Trade

Last month the U.S. House of Representatives narrowly passed the "American Clean Energy and Security" (ACES) Act, which would regulate the pollution that is responsible for global warming. The centerpiece of the bill is the establishment of a "cap and trade" system -- a market-based approach that was successfully used during the last two decades to reduce acid rain. Of course all markets have winners and losers, and how the rules are written determines which side you end up on.

The basic idea of a cap and trade system is simple. First you determine how much pollution can be allowed, and you limit or "cap" total emissions at that level. The cap is initially set slightly below current levels, and it is lowered slowly over time to the final goal. Polluting is no longer free -- every polluter must have permits to cover their emissions. Those who figure out how to reduce their emissions can sell or "trade" their permits to polluters who have more trouble meeting the new requirements. As the number of permits is reduced over time the market price goes up, creating an incentive to reduce your emissions and sell the permits for a profit. In 1990 a cap and trade system was established for sulfur, a common pollutant from coal burning that produces acid rain. Congress mandated a 50% cut in sulfur emissions over 20 years, but the cap and trade system worked so well that it only took 15 years to achieve the goal. Initial worries about the possible economic impact were revealed to be entirely wrong.

The ACES Act would establish a similar system for carbon, which arises primarily from burning fossil fuels and contributes to global warming. The bill requires carbon emissions to be 17% below 2005 levels by 2020, and 83% below 2005 levels by 2050. The long-term goal is based on a scientific assessment of what will be required to avoid the worst consequences of climate change. The short-term goal is somewhat disappointing, since emissions cuts are similar to compound interest -- the more we do early, the greater the long-term impact. The other important feature is how pollution rights are distributed -- the bill initially gives away 85% of the emission permits to energy companies and electric utilities for free! This fraction is gradually reduced to only 30% by 2030, but it still means huge potential profits for these businesses -- even though some of the revenue is required to be given to customers as rebates. The remaining permits are sold in an auction with most of the proceeds divided equally among all citizens and a small fraction reserved just for low-income families. The idea is to help offset the potential increase in energy prices.

An alternative vision for the carbon cap and trade system was outlined in the 2001 book "Who Owns the Sky?" by Peter Barnes. He believed that every American should get an equal share of the emission permits, and the energy companies should be required to buy them from us! The extra costs that the energy companies would inevitably pass on to consumers would be perfectly offset for the average customer by the extra income from selling the permits. Customers who found ways to decrease their energy consumption would end up making money -- providing a strong personal incentive for conservation and efficiency, which is one of the least expensive ways to reduce global warming emissions quickly.

This bill isn't perfect, but it's a step in the right direction. The Senate is expected to consider similar legislation sometime this fall. With luck a compromise bill can be signed into law before the international climate negotiations in Copenhagen this December.

Earth as an Exoplanet

Last week a group of Spanish scientists published some unique observations of the Earth's atmosphere, as it might be seen by an alien civilization looking for signs of life. The idea was to see what they could learn about our home from the type of data that may soon be available for distant Earth-like planets orbiting other stars. The results were impressive. The researchers identified several of the dominant molecules in the Earth's atmosphere -- and they could even tell that the sunsets are red, and the skies are blue.

More than 350 planets have now been discovered around other stars, with nearly 60 of them passing directly in front of their Suns from our vantage point on Earth. These "transiting" planets provide a special opportunity to study the atmospheres of alien worlds. As they pass in front of their host star, they block some of the starlight from reaching us. Just from the amount of missing light, we can determine the relative size of the planet compared to the star. So far, most of the planets we know about from such measurements are big -- like the outer planets in our solar system: Jupiter, Saturn, Uranus and Neptune. But with more precise measurements from satellites, we will soon be able to find smaller ones. During the transit a small fraction of the starlight passes through the planet's atmosphere, and we can use this light to learn about its composition and other properties.

Something similar happens during a lunar eclipse. As our Moon passes into the shadow of the Earth, the only sunlight that reaches it passes through our planet's atmosphere. By measuring the light on the Moon during an eclipse, we can see what a transit of the Earth would look like to a distant observer. The Spanish scientists measured a lunar eclipse last August, passing the moonlight through a prism to separate it into its spectrum of colors. Individual molecules in the Earth's atmosphere absorb specific colors of light, creating dark lines in the spectrum like a fingerprint that identifies the molecule. The lunar eclipse spectrum revealed signatures of oxygen, water vapor, carbon dioxide and methane -- as well as traces of many other molecules. But the most dramatic feature of the spectrum was immediately obvious -- nearly all of the blue light was missing.

As sunlight passes through the Earth's atmosphere, some of the light that isn't absorbed by the individual molecules is scattered by them instead. The typical size of air molecules makes this process, known as "Rayleigh scattering", much more effective for blue light than it is for red light. As a consequence mostly the red light makes it through the atmosphere to the Moon during a lunar eclipse, while the blue light is scattered and makes the sky appear blue. This is also why sunsets look red, since the sunlight passes through much more atmosphere when it is near the horizon than when it is high in the sky.

The most amazing thing about these new observations is that nobody had ever thought to look at the Earth in this way before. In the near future, after NASA has discovered transiting Earth-like planets around other stars, satellites such as the James Webb Space Telescope will be able to obtain spectra if their atmospheres. And now we know what it might see... blue skies.

Lifeline for Hubble

A long drama finally came to a close this week, as the crew of the space shuttle Atlantis released the Hubble Space Telescope back into orbit after 8 days of grueling repairs. Headlines have understandably been dominated by the overwhelming success of the servicing mission -- but behind the scenes there was another story, with a more human dimension, that most people never knew about.

After the tragic loss of the space shuttle Columbia and its crew in February 2003, everything in orbit that relied on the shuttle program came to a grinding halt. The future of the final Hubble servicing mission was suddenly very uncertain. It took nearly 29 months before the shuttle returned to the skies in July 2005, and another year before it continued regular operations in July 2006. During these three and a half years, the primary concern was whether NASA could ensure the safety of shuttle crews -- so in August 2004, administrator Sean O'Keefe commissioned a study to determine the viability of a robotic servicing mission to Hubble. This idea was ultimately rejected in April 2005 by the next administrator, Michael Griffin, who finally approved the servicing mission in October 2006. In the meantime, Hubble was gradually losing many of its scientific functions.

Throughout this period, the life of Hubble was extended again and again by the heroic efforts of the scientists and operators at the Space Telescope Science Institute in Baltimore. Every time a component of Hubble failed, these men and women figured out a way to continue limited science operations on whatever was left. For many, their jobs literally depended on it -- without Hubble they would have nothing to do until the launch of the James Webb Space Telescope in June 2013 (at the earliest). So they were determined to keep Hubble working, and they did. The servicing mission was finally scheduled for October 2008, but Hubble's on-board computer system failed just two weeks before launch. It took weeks for engineers to design a short-term fix, and months for astronauts to incorporate the replacement computer into their long-rehearsed training routine.

Which brings us to this week -- when all of that training was finally put to good use, and the long-anticipated repair mission has come to a successful conclusion. The main digital camera on Hubble was replaced with a newer model, and the old corrective lenses it received in 1993 to fix its fuzzy vision were replaced by a new light-splitting spectrograph that will peer to the very edge of the observable universe. Two of the other scientific instruments, STIS and ACS which have been dead or crippled since power supply failures in 2004 and 2007, have now been restored. These science upgrades were complemented by a host of routine repairs including: the installation of new gyroscopes for the telescope's pointing system, new batteries which are charged by Hubble's solar panels for 60 minutes of each orbit and supply all of the power during the remaining 36 minutes on the dark side of the Earth, and a new insulating outer blanket that helps protect the satellite from harmful radiation and space junk. With the installation of a replacement on-board computer system and an external dock to support the future robotic capture of the telescope for disposal at the end of its life, the servicing mission was complete.

Hubble is now expected to continue its marathon of ground-breaking discoveries well into the next decade -- and most importantly, until the next space telescope is operational. The dozens of people at the Space Telescope Science Institute whose jobs depend on this mission can now breath a collective sigh of relief.