EUROPA-OUTSIDE THE GOLDILOCKS ZONE

There is also another tempting target for our probes within our solar system: Europa. For decades, it was believed that life in the solar system can exist only in the "Goldilocks zone" around the sun, where planets are not too hot or too cold to sustain life. The earth is blessed with liquid water because it orbits at the right distance from the sun. Liquid water will boil on a planet like Mercury, which is too close to the sun, and will freeze on a planet like Jupiter, which is too far. Since liquid water is probably the fluid in which DNA and proteins were first formed, it was long believed that life in the solar system can exist only on earth, or perhaps Mars.

But astronomers were wrong. After the Voyager Voyager s.p.a.cecraft sailed past the moons of Jupiter, it became apparent that there was another place for life to flourish: under the ice cover of the moons of Jupiter. Europa, one of the moons of Jupiter discovered by Galileo in 1610, soon caught the attention of astronomers. Although its surface is permanently covered with ice, beneath that ice there is a liquid ocean. Because the ocean is much deeper on Europa than on earth, the total volume of the Europan ocean is estimated to be twice the volume of earth"s oceans. s.p.a.cecraft sailed past the moons of Jupiter, it became apparent that there was another place for life to flourish: under the ice cover of the moons of Jupiter. Europa, one of the moons of Jupiter discovered by Galileo in 1610, soon caught the attention of astronomers. Although its surface is permanently covered with ice, beneath that ice there is a liquid ocean. Because the ocean is much deeper on Europa than on earth, the total volume of the Europan ocean is estimated to be twice the volume of earth"s oceans.

This was a bit of a shock, realizing that there is an abundant energy source in the solar system other than the sun. Underneath the ice, the surface of Europa is continually heated by tidal forces. As Europa tumbles in its...o...b..t around Jupiter, that ma.s.sive planet"s gravity squeezes the moon in different directions, creating friction deep within its core. This friction creates heat, which in turn melts the ice and creates a stable ocean of liquid water.

This discovery means that perhaps the moons of distant gas giants are more interesting than the planets themselves. (This is probably one reason James Cameron chose a moon of a Jupiter-size planet for the site of his 2009 movie, Avatar. Avatar.) Life, which was once thought to be quite rare, might actually flourish in the blackness of s.p.a.ce on the moons of distant gas giants. Suddenly, the number of places where life might flourish has exploded by many times.

As a consequence of this remarkable discovery, the Europa Jupiter System Mission (EJSM) is tentatively scheduled for launch in 2020. It is designed to orbit Europa and possibly land on it. Beyond that, scientists have dreamed of exploring Europa by sending even more sophisticated machinery. Scientists have considered a variety of methods to search for life under the ice. One possibility is the Europa Ice Clipper Mission, which would drop spheres on the icy surface. The plume and debris cloud emerging from the impact site would then be carefully a.n.a.lyzed by a s.p.a.cecraft flying through it. An even more ambitious program is to put a remote-control hydrobot submarine beneath the ice.

Interest in Europa has also been stoked by new developments under the ocean here on earth. Until the 1970s, most scientists believed that the sun was the only energy source that could make life possible. But in 1977, the Alvin Alvin submarine found evidence of new life-forms flourishing where no one suspected before. Probing the Galapagos Rift, it found giant tube worms, mussels, crustaceans, clams, and other life-forms using the heat energy from volcano vents to survive. Where there is energy, there might be life; and these undersea volcano vents have provided a new source of energy in the inky blackness of the sea floor. In fact, some scientists have suggested that the first DNA was formed not in some tide pool on the earth"s coast but deep undersea near a volcano vent. Some of the most primitive forms of DNA (and perhaps the most ancient) have been found on the bottom of the ocean. If so, then perhaps volcano vents on Europa can provide the energy to get something like DNA off the ground. submarine found evidence of new life-forms flourishing where no one suspected before. Probing the Galapagos Rift, it found giant tube worms, mussels, crustaceans, clams, and other life-forms using the heat energy from volcano vents to survive. Where there is energy, there might be life; and these undersea volcano vents have provided a new source of energy in the inky blackness of the sea floor. In fact, some scientists have suggested that the first DNA was formed not in some tide pool on the earth"s coast but deep undersea near a volcano vent. Some of the most primitive forms of DNA (and perhaps the most ancient) have been found on the bottom of the ocean. If so, then perhaps volcano vents on Europa can provide the energy to get something like DNA off the ground.

One can only speculate about the possible life-forms that might form under Europa"s ice. If they exist at all, they probably will be swimming creatures that use sonar, rather than light, for navigational purposes, so their view of the universe will be limited to living under the "sky" of ice.

LISA-BEFORE THE BIG BANG

Yet another s.p.a.ce satellite that could create an upheaval in scientific knowledge is the Laser Interferometer s.p.a.ce Antenna (LISA) and its successors. These probes may be able to do the impossible: reveal what happened before the big bang.

Currently, we have been able to measure the rate at which the distant galaxies are moving away from us. (This is due to the Doppler shift, where light is distorted if the star moves toward or away from you.) This gives us the expansion rate of the universe. Then we "run the videotape backward," and calculate when the original explosion took place. This is very similar to the way you can a.n.a.lyze the fiery debris emanating from an explosion to determine when the explosion took place. That is how we determined that the big bang took place 13.7 billion years ago. What is frustrating, however, is that the current s.p.a.ce satellite, the WMAP (Wilkinson Microwave Anisotropy Probe), can peer back only to less than 400,000 years after the original explosion. Therefore, our satellites can tell us only that there was a bang, but cannot tell us why it banged, what banged, and what caused the bang.

That is why LISA is creating such excitement. LISA will measure an entirely new type of radiation: gravity waves from the instant of the big bang itself.

Every time a new form of radiation was harnessed, it changed our worldview. When optical telescopes were first used by Galileo to map the planets and stars, they opened up the science of astronomy. When radio telescopes were perfected soon after World War II, they revealed a universe of exploding stars and black holes. And now the third generation of telescopes, which can detect gravitational waves, may open up an even more breathtaking vista, the world of colliding black holes, higher dimensions, and even a multiverse.

Tentatively, the launch date is being set for between 2018 and 2020. LISA consists of three satellites that will form a gigantic triangle 3 million miles across, connected by three laser beams. It will be the largest s.p.a.ce instrument ever sent into orbit. Any gravity wave from the big bang still reverberating around the universe will jiggle the satellites a bit. This disturbance will change the laser beams, and then sensors will record the frequency and characteristics of the disturbance. In this way, scientists should be able to get within a trillionth of a second after the original big bang. (According to Einstein, s.p.a.ce-time is like a fabric that can be curved and stretched. If there is a big disturbance, like colliding black holes or the big bang, then ripples can form and travel on this fabric. These ripples, or gravity waves, are too small to detect using ordinary instruments, but LISA is sensitive and large enough to detect vibrations caused by these gravity waves.) Not only will LISA be able to detect radiation from colliding black holes, it might also be able to peer into the prebig bang era, which was once thought to be impossible.

At present, there are several theories of the prebig bang era coming from string theory, which is my specialty. In one scenario, our universe is a huge bubble of some sort that is continually expanding. We live on the skin of this gigantic bubble (we are stuck on the bubble like flies on flypaper). But our bubble universe coexists in an ocean of other bubble universes, making up the multiverse of universes, like a bubble bath. Occasionally, these bubbles might collide (giving us what is called the big splat theory) or they may fission into smaller bubbles and then expand (giving us what is called eternal inflation). Each of these prebig bang theories predicts how the universe should release gravity radiation moments after the initial explosion. LISA can then measure the gravity radiation emitted after the big bang and compare it with the various predictions of string theory. In this way, LISA might be able to rule out or in some of these theories.

But even if LISA is not sensitive enough to perform this delicate task, perhaps the next generation of detectors beyond LISA (such as the Big Bang Observer) may be up to the task.

If successful, these s.p.a.ce probes may answer the question that has defied explanation for centuries: Where did the universe originally come from? So in the near term, unveiling the origin of the big bang may be a distinct possibility.

MANNED MISSIONS TO s.p.a.cE.

While robotic missions will continue to open new vistas for s.p.a.ce exploration, the manned missions will face much greater hurdles. This is because, compared to manned missions, robotic missions are cheap and versatile; can explore dangerous environments; don"t require costly life support; and most important, don"t have to come back.

Back in 1969, it seemed as if our astronauts were poised to explore the solar system. Neil Armstrong and Buzz Aldrin had just walked on the moon, and already people were dreaming about going to Mars and beyond. It seemed as if we were on the threshold of the stars. A new age was dawning for humanity.

Then the dream collapsed.

As science fiction writer Isaac Asimov has written, we scored the touchdown, took our football, and then went home. Today, the old Saturn booster rockets are idling in museums or rotting in junkyards. An entire generation of top rocket scientists was allowed to dissipate. The momentum of the s.p.a.ce race slowly dissipated. Today, you can find reference to the famous moon walk only in dusty history books.

What happened? Many things, including the Vietnam War, the Watergate scandal, etc. But, when everything is boiled down, it reduces to just one word: cost.

We sometimes forget that s.p.a.ce travel is expensive, very expensive. It costs $10,000 to put a pound of anything just into near-earth orbit. Imagine John Glenn made of solid gold, and you can grasp the cost of s.p.a.ce travel. To reach the moon would require about $100,000 per pound. And to reach Mars would require about $1,000,000 per pound (roughly your weight in diamonds).

All this, however, was covered up by the excitement and drama of competing with the Russians. Spectacular s.p.a.ce stunts by brave astronauts hid the true cost of s.p.a.ce travel from view, since nations were willing to pay dearly if their national honor was at stake. But even superpowers cannot sustain such costs over many decades.

Sadly, it has been over 300 years since Sir Isaac Newton first wrote down the laws of motion, and we are still dogged by a simple calculation. To hurl an object into near-earth orbit, you have to send it 18,000 miles per hour. And to send it into deep s.p.a.ce, beyond the gravity field of the earth, you have to propel it 25,000 miles per hour. (And to reach this magic number of 25,000 miles per hour, we have to use Newton"s third law of motion: for every action, there is an equal and opposite reaction. This means that the rocket can go rapidly forward because it spews out hot gases in the opposite direction, in the same way that a balloon flies around a room when you inflate it and then let it go.) So it is a simple step from Newton"s laws to calculating the cost of s.p.a.ce travel. There is no law of engineering or physics that prevents us from exploring the solar system; it"s a matter of cost.

Worse, the rocket must carry its own fuel, which adds to its weight. Airplanes partially get around this problem because they can scoop oxygen from the air outside and then burn it in their engines. But since there is no air in s.p.a.ce, the rocket must carry its own tanks of oxygen and hydrogen.

Not only is this the reason s.p.a.ce travel is so expensive, it is also the reason we don"t have jet packs and flying cars. Science fiction writers (not real scientists) glamorized the day when we would all put on jet packs and fly to work, or go on a Sunday day trip blasting off in our family flying car. Many people became disillusioned by futurists because these predictions never came to pa.s.s. (That is why we see a rash of articles and books with cynical t.i.tles like "Where"s My Jetpack?" "Where"s My Jetpack?") But a quick calculation shows the reason. Jet packs already exist; in fact, the n.a.z.is used them briefly during World War II. But hydrogen peroxide, the common fuel used in jet packs, quickly runs out, so a typical flight in a jet pack lasts only a few minutes. Also, flying cars that use helicopter blades burn up an enormous amount of fuel, making them far too costly for the average suburban commuter.

CANCELING THE MOON PROGRAM.

Because of the enormous cost of s.p.a.ce travel, currently the future of the manned exploration of s.p.a.ce is in flux. Former president George W. Bush presented a clear but ambitious plan for the s.p.a.ce program. First, the s.p.a.ce shuttle would be retired in 2010 and replaced in 2015 by a new rocket system called Constellation. Second, astronauts would return to the moon by 2020, eventually setting up a permanent manned base there. Third, this would pave the way for an eventual manned mission to Mars.

However, the economics of s.p.a.ce travel have changed significantly since then, especially because the great recession has drained funds for future s.p.a.ce missions. The Augustine Commission report, given to President Barack Obama in 2009, concluded that the earlier plan was unsustainable given current funding levels. In 2010, President Obama endorsed the findings of the Augustine report, canceling the s.p.a.ce shuttle and its replacement that was to set the groundwork for returning to the moon. In the near term, without the rockets to send our astronauts into s.p.a.ce, NASA will be forced to rely on the Russians. In the meantime, this provides an opportunity for private companies to create the rockets necessary to continue the manned s.p.a.ce program. In a sharp departure from the past, NASA will no longer be building the rockets for the manned s.p.a.ce program. Proponents of the plan say it will usher in a new age of s.p.a.ce travel, when private enterprise takes over. Critics say the plan will reduce NASA to "an agency to nowhere."

LANDING ON AN ASTEROID.

The Augustine report laid out what it called the flexible path, containing several modest objectives that did not require so much rocket fuel; for example, traveling to a nearby asteroid that happened to be floating by or traveling to the moons of Mars. Such an asteroid, it was pointed out, may not even be on our sky charts yet; it might be a wandering asteroid that might be discovered in the near future.

The problem, the Augustine report said, is that the rocket fuel for the landing and return mission from the moon, or especially from Mars, would be prohibitively expensive. But since asteroids and the moons of Mars have very low gravitational fields, these missions would not require so much rocket fuel. The Augustine report also mentioned the possibility of visiting the Lagrange points, which are the places in outer s.p.a.ce where the gravitational pull of the earth and moon cancel each other out. (These points might serve as a cosmic dump, where ancient pieces of debris from the early solar system have collected, so by visiting them astronauts may find interesting rocks dating back to the formation of the earth-moon system.) Landing on an asteroid would certainly be a low-cost mission, since asteroids have very weak gravitational fields. (This is also the reason asteroids are irregularly shaped, rather than round. In the universe, large objects-such as stars, planets, and moons-are all round because gravity pulls evenly. Any irregularity in the shape of a planet gradually disappears as gravity compresses the crust. But the gravity field of an asteroid is so weak that it cannot compress the asteroid into a sphere.) One possibility is the asteroid Apophis, which will make an uncomfortably close pa.s.s in 2029. Apophis is about 1,000 feet across, the size of a large football stadium, and will come so close to the earth that it will actually pa.s.s beneath some of our satellites. Depending on how the orbit of the asteroid is distorted by this close pa.s.s, it may swing back to the earth in 2036, where there is a tiny chance (1 out of 100,000) that it might hit the earth. If this were to happen, it would hit with the force of 100,000 Hiroshima bombs, sufficient to destroy an area as large as France with firestorms, shock waves, and fiery debris. (By comparison, a much smaller object, probably the size of an apartment building, slammed into Tunguska, Siberia, in 1908, with the force of about 1,000 Hiroshima bombs, wiping out 1,000 square miles of forest and creating a shock wave felt thousands of miles away. It also created a strange glow seen over Asia and Europe, so that people in London could read the newspapers at night.) Visiting Apophis would not strain the NASA budget, since the asteroid is coming near earth anyway, but landing on the asteroid might pose a problem. Since it has a weak gravity field, one would actually dock with the asteroid, rather than land on it in the traditional sense. Also, the asteroid is probably spinning irregularly, so precise measurements have to be made before landing. It would be interesting to test to see how solid the asteroid is. Some believe that an asteroid may be a collection of rock loosely held together by a weak gravity field. Others believe that it may be solid. Determining the consistency of an asteroid may be important one day, if we have to use nuclear weapons to blow one up. An asteroid, instead of being pulverized into a fine powder, might instead break up into several large pieces. If so, then the danger from these pieces might be greater than the original threat. A better idea may be to nudge the asteroid out of the way before it comes close to earth.

LANDING ON A MOON OF MARS.

Although the Augustine report did not support a manned mission to Mars, one intriguing possibility is to send astronauts to visit the moons of Mars, Phobos and Deimos. These moons are much smaller than earth"s moon and hence have a very low gravitational field. There are several advantages to landing on the moons of Mars, in addition to saving on cost.

1.First, these moons could be used as s.p.a.ce stations. They would provide a cheap way of a.n.a.lyzing the planet from s.p.a.ce without visiting it.

2.Second, they could eventually provide an easy way to access Mars. Phobos is less than 6,000 miles from the center of Mars, so a quick journey to the Red Planet can be made within a matter of hours.

3.These moons would probably have caves that could be used for a permanent manned base to protect against meteors and radiation. Phobos, in particular, has the huge Stickney crater on its side, indicating that the moon was probably hit by a huge meteor and almost blown apart. However, gravity slowly brought back the pieces and rea.s.sembled the moon. There are probably plenty of caves and gaps left over from this ancient collision.

BACK TO THE MOON.

The Augustine report also mentioned a Moon First program, where we would go back to the moon, but only if more funding were available-at least $30 billion over ten years. Since that is unlikely, the moon program, in effect, is canceled, at least for the coming years.

The canceled moon mission was called the Constellation Program, which consisted of several major components. First was the booster rocket, the Ares, the first major U.S. booster rocket since the old Saturn rocket was mothballed back in the 1970s. On top of the Ares sat the Orion module, which could carry six astronauts to the s.p.a.ce station or four astronauts to the moon. Then there was the Altair lander, which was supposed to actually land on the moon.

The old s.p.a.ce shuttle, where the shuttle rocket was placed on the side of the booster rocket, had a number of design flaws, including the tendency of the rocket to shed pieces of foam. This had disastrous consequences for the s.p.a.ce Shuttle Columbia, Columbia, which broke up on reentry in 2003, killing seven brave astronauts, because a piece of foam from the booster rocket hit the shuttle and made a hole in its wing during takeoff. Upon reentry, hot gases penetrated the hull of the which broke up on reentry in 2003, killing seven brave astronauts, because a piece of foam from the booster rocket hit the shuttle and made a hole in its wing during takeoff. Upon reentry, hot gases penetrated the hull of the Columbia, Columbia, killing everyone inside and causing the ship to break up. In the Constellation, with the crew module placed directly on top of the booster rocket, this would no longer be a problem. killing everyone inside and causing the ship to break up. In the Constellation, with the crew module placed directly on top of the booster rocket, this would no longer be a problem.

The Constellation program had been called "an Apollo program on steroids" by the press, since it looked very much like the moon rocket program of the 1970s. The Ares I booster was to be 325 feet tall, comparable to the 363-foot Saturn V rocket. It was supposed to carry the Orion module into s.p.a.ce, replacing the old s.p.a.ce shuttle. But for very heavy lifting, NASA was to use the Ares V rocket, which was 381 feet tall and capable of taking 207 tons of payload into s.p.a.ce. The Ares V rocket would have been the backbone of any mission to the moon or Mars. (Although the Ares has been canceled, there is talk of perhaps salvaging some of these components for future missions.) PERMANENT MOON BASE.

Although the Constellation Program was canceled by President Obama, he left open several options. The Orion module, which was to have taken our astronauts back to the moon, is now being considered as an escape pod for the International s.p.a.ce Station. At some point in the future, when the economy recovers, another administration may want to set its sights on the moon again, including a moon base.

The task of establishing a permanent presence on the moon faces many obstacles. The first is micrometeorites. Because the moon is airless, rocks from s.p.a.ce frequently hit it. We can see this by viewing its surface, pockmarked by meteorite collisions, some dating back billions of years.

I got a personal look at this danger when I was a graduate student at the University of California at Berkeley. Moon rocks brought back from s.p.a.ce in the early 1970s were creating a sensation in the scientific community. I was invited into a laboratory that was a.n.a.lyzing moon rock under a microscope. The rock I saw looked ordinary, since moon rock very closely resembles earth rock, but under the microscope I got quite a shock. I saw tiny meteor craters in the rock, and inside them I saw even tinier craters. Craters inside craters inside craters, something I had never seen before. I immediately realized that without an atmosphere, even the tiniest microscopic piece of dirt, hitting you at 40,000 miles per hour, could easily kill you or at least penetrate your s.p.a.ce suit. (Scientists understand the enormous damage created by these micrometeorites because they can simulate these impacts, and they have created huge gun barrels in their labs that can fire metal pellets to study these meteor impacts.) One possible solution is to build an underground lunar base. Because of the moon"s ancient volcanic activity, there is a chance our astronauts can find a lava tube that extends deep into the moon"s interior. (Lava tubes are created by ancient lava flows that have carved out cavelike structures and tunnels underground.) In 2009, astronomers found a lava tube about the size of a skysc.r.a.per that might serve as a permanent base on the moon.

This natural cave could provide cheap protection for our astronauts against radiation from cosmic rays and solar flares. Even taking a transcontinental flight from New York to Los Angeles exposes us to a millirem of radiation per hour (equivalent to getting a dental X-ray). For our astronauts on the moon, the radiation might be so intense that they might need to live in underground bases. Without an atmosphere, a deadly rain of solar flares and cosmic rays would pose an immediate risk to astronauts, causing premature aging and even cancer.

Weightlessness is also a problem, especially for long missions in s.p.a.ce. I had a chance to visit the NASA training center in Cleveland, Ohio, where extensive tests are done on our astronauts. In one test I observed, the subject was suspended in a harness so that his body was parallel to the ground. Then he began to run on a treadmill, whose tracks were vertical. By running on this treadmill, NASA scientists could simulate weightlessness while testing the endurance of the subject.

When I spoke to the NASA doctors, I learned that weightlessness was more damaging than I had previously thought. One doctor explained to me that after several decades of subjecting American and Russian astronauts to prolonged weightlessness, scientists now realize that the body undergoes significant changes: degradation occurs in the muscles, bones, and cardiovascular system. Our bodies evolved over millions of years while living in the earth"s gravitational field. When placed in a weaker gravitational field for long periods of time, all our biological processes are thrown into disarray.

Russian astronauts who have spent about a year in s.p.a.ce are so weak when they come back to earth that they can barely crawl. Even if they exercise daily in s.p.a.ce, their muscles atrophy, their bones lose calcium, and their cardiovascular systems begin to weaken. Some of the astronauts take months to recover from this damage, some of which may be permanent. A trip to Mars, which might take two years, may drain the strength of our astronauts so they cannot perform their mission when they arrive. (One solution to this problem is to spin the s.p.a.cecraft, which creates artificial gravity inside the ship. This is the same reason that you can spin a pail of water over your head without the water spilling out. But this is prohibitively expensive because of the heavy machinery necessary to spin the craft. Every pound of extra weight adds $10,000 to the cost of the mission.) WATER ON THE MOON.

One game changer has been the discovery of ancient ice on the moon, probably left over from ancient comet impacts. In 2009, NASA"s lunar crater observation and sensing satellite (LCROSS) probe and its Centaur booster rocket slammed into the moon"s south polar region. They hit the moon at 5,600 miles per hour, creating a plume almost a mile high, and a crater about 60 feet across. Although TV audiences were disappointed that the LCROSS impact did not create a spectacular explosion as predicted, it yielded a wealth of scientific data. About 24 gallons of water were found in that plume. Then, in 2010, scientists made the shocking announcement that 5 percent of the debris contained water, so the moon was actually wetter than parts of the Sahara desert.

This could be significant, because it might mean that future astronauts can harvest underground ice deposits for rocket fuel (by extracting the hydrogen in the water), for breathing (by extracting the oxygen), for shielding (since water can absorb radiation), and for drinking once it is purified. So this discovery could shave hundreds of millions of dollars off any mission to the moon.

This discovery may mean that it will be possible for our astronauts to live off the land, harvesting ice and minerals on the moon to create and supply a permanent base.

MISSION TO MARS.

President Obama, when he journeyed to Florida in 2010 to announce the cancellation of the moon program, held out the prospects of a mission to Mars instead. He supported funding for a yet-unspecified heavy booster rocket that may one day send astronauts into deep s.p.a.ce beyond the moon. He mused that he might see the day, perhaps in the mid-2030s, when our astronauts would walk on Mars. Some astronauts, like Buzz Aldrin, have been enthusiastic supporters of the Obama plan, because it would skip the moon. Aldrin once told me that the United States has already been to the moon, and hence the real adventure lies in going to Mars.

Of all the planets in the solar system, only Mars seems to resemble earth enough to harbor some form of life. (Mercury, which is scorched by the sun, is probably too hostile to have life as we know it. And the gas giants-Jupiter, Saturn, Ura.n.u.s, and Neptune-are too cold to support life. Venus is a twin of the earth, but a runaway greenhouse effect has created a h.e.l.lhole: temperatures soar to 900F, its mostly carbon dioxide atmosphere is 100 times denser than ours, and it rains sulfuric acid. Walking on the Venusian surface, you would suffocate, be crushed to death, and your remains would be incinerated by the heat and dissolved by the sulfuric acid.) Mars, on the other hand, was once a wet planet, like earth, with oceans and riverbeds that have long since vanished. Today, it is a frozen desert, devoid of life. Perhaps microbial life once flourished there billions of years ago or may still live underground in hot springs.

Once our nation has made a firm commitment to go to Mars, it may take another twenty to thirty years to actually complete the mission. But getting to Mars will be much more difficult than reaching the moon. In contrast to the moon, Mars represents a quantum leap in difficulty. It takes only three days to reach the moon. It takes six months to a year to reach Mars.

In July 2009, NASA scientists gave a rare look at what a realistic Mars mission might look like. Astronauts would take approximately six months or more to reach Mars, then spend eighteen months on the planet, then take another six months for the return voyage.

Altogether, about 1.5 million pounds of equipment would need to be sent to Mars, more than the amount needed for the $100 billion s.p.a.ce station. To save on food and water, the astronauts would have to purify their own waste and then use it to fertilize plants during the trip and while on Mars. With no air, soil, or water, everything must be brought from earth. It will be impossible to live off the land, since there is no oxygen, liquid water, animals, or plants on Mars. The atmosphere is almost pure carbon dioxide, with an atmospheric pressure only 1 percent that of earth. Any rip in a s.p.a.ce suit would create rapid depressurization and death.

The mission would be so complex that it would have to be broken down into several steps. Since carrying rocket fuel for the return mission back to earth would be costly, a separate rocket might be sent to Mars ahead of time carrying rocket fuel to be used for refueling the s.p.a.cecraft. (Or, if enough oxygen and hydrogen could be extracted from the ice on Mars, this might be used for rocket fuel as well.) Once on Mars, it might take weeks for the astronauts to get accustomed to living on another planet. The day/night cycle is about the same as on earth (a day on Mars is 24.6 hours). But a year is almost twice as long. The temperature on Mars never goes above the melting point of ice. The dust storms on Mars are ferocious. The sand of Mars has the consistency of talc.u.m powder, and dust storms that engulf the entire planet are common.

TERRAFORM MARS?.

a.s.suming that astronauts visit Mars by midcentury and establish a primitive Martian outpost, there is the possibility that astronauts might consider terraforming Mars, that is, transforming the planet to make it more hospitable for life. This would begin late in the twenty-first century, at the earliest, or more likely early in the twenty-second.

Scientists have a.n.a.lyzed several ways in which Mars might be terraformed. Perhaps the simplest way would be to inject methane gas or other greenhouse gases into the atmosphere. Since methane gas is an even more potent greenhouse gas than carbon dioxide, the methane gas might be able to trap sunlight, raising the surface temperature of Mars to above the melting point of ice. In addition to methane, other greenhouse gases have been a.n.a.lyzed for possible terraforming experiments, such as ammonia and chlorofluorocarbons.

Once the temperature starts to rise, the underground permafrost may begin to thaw out, for the first time in billions of years. As the permafrost melts, riverbeds would begin to fill up with water. Eventually, lakes and even oceans might form again on Mars as the atmosphere thickens. This would release more carbon dioxide, setting off a positive feedback loop.

In 2009 it was discovered that methane gas naturally escapes from the Martian surface. The source of this gas is still a mystery. On earth, most of the methane gas is due to the decay of organic materials. But on Mars, the methane gas may be a by-product of geologic processes. If one can locate the source of this methane gas, then it might be possible to increase its output and hence alter the atmosphere.

Another possibility is to deflect a comet into the Martian atmosphere. If one can intercept a comet far enough away, then even a small nudge by a rocket engine, an impact with a probe, or even the tug of the gravity of a s.p.a.ceship might be enough to deflect it. Comets are made mainly of water ice and periodically race through our solar system. (Halley"s comet, for example, consists of a core-resembling a peanut-that is roughly twenty miles across, made mainly of ice and rock.) As the comet gradually gets closer to the surface of Mars, it would encounter friction from the atmosphere, causing the comet to slowly disintegrate, releasing water into the atmosphere in the form of steam.

If comets are not available, it could also be possible to deflect one of the ice moons of Jupiter or perhaps an asteroid that contains ice, such as Ceres, which is believed to be 20 percent water. (These moons and asteroids would be harder to deflect, since they are usually in stable orbits.) Instead of having the comet, moon, or asteroid slowly decay in its...o...b..t around Mars, releasing water vapor, another choice would be to maneuver them into a controlled impact on the Martian ice caps. The polar regions of Mars are made of frozen carbon dioxide, which disappears during the summer months, and ice, which makes up the permanent part of the ice caps. If the comet, moon, or asteroid hits the ice caps, they can release a tremendous amount of heat and vaporize the dry ice. Since carbon dioxide is a greenhouse gas, this would thicken the atmosphere and help to accelerate global warming on Mars. It might also create a positive feedback loop. The more carbon dioxide is released from the ice caps, the warmer the planet becomes, which in turn releases even more carbon dioxide.

Another suggestion is to detonate nuclear bombs directly on the ice caps. The drawback is that the resulting liquid water might contain radioactive fallout. Or we could try to create a fusion reactor that can melt the polar ice caps. Fusion plants use water as a basic fuel, and there is plenty of frozen water on Mars.

Once the temperature of Mars rises to the melting point of ice, pools of water may form, and certain forms of algae that thrive on earth in the Antarctic may be introduced to Mars. They might actually thrive in the atmosphere of Mars, which is 95 percent carbon dioxide. They could also be genetically modified to maximize their growth on Mars. These algae pools could accelerate terraforming in several ways. First, they could convert carbon dioxide into oxygen. Second, they would darken the surface color of Mars, so that it absorbs more heat from the sun. Third, since they grow by themselves without any prompting from the outside, it would be a relatively cheap way to change the environment of the planet. Fourth, the algae can be harvested for food. Eventually these algae lakes would create soil and nutrients that may be suitable for plants, which in turn would accelerate the production of oxygen.

Scientists have also looked into the possibility of building solar satellites surrounding the planet, reflecting sunlight onto Mars. Solar satellites by themselves might be able to heat the Martian surface above freezing. Once this happens and the permafrost begins to melt, the planet would naturally continue to warm on its own.

ECONOMIC BENEFIT?.

One should have no illusions that we will benefit immediately from an economic bonanza by colonizing the moon and Mars. When Columbus sailed to the New World in 1492, he opened the door to a historic economic windfall. Soon, the conquistadors were sending back huge quant.i.ties of gold that they plundered from Native Americans, and settlers were sending valuable raw materials and crops back to the Old World. The cost of sending expeditions to the New World was more than offset by the fabulous fortunes that could be made.