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Phobos as seen by Mars Express from 200 kilometers away in 2004. Tomorrow the spacecraft will get three times closer.

Given all the angst recently about NASA astronauts needing a new destination, it’s good to step back and review the options. There aren’t many. There’s the moon, of course, and Mars. A near-Earth asteroid. And one more possibility, often forgotten—the Martian moons Phobos and Deimos.

Tomorrow at 3:55 p.m. Eastern time, Europe’s Mars Express spacecraft will make the nearest pass ever of Phobos, coming within 67 kilometers (41 miles) of its surface, close enough for scientists to learn about the moon’s gravity (and density) from the way the spacecraft’s radio signal is distorted during the swingby.

People have been thinking about human missions to Phobos for more than 30 years. The first proposal I remember hearing about, in the early 1980s, was from Fred Singer, better known today as a climate change skeptic. He called his mission “Ph-D,” for Phobos-Deimos.

Since then, most ideas have focused on robotic probes, including the Russian Phobos-Grunt mission currently targeted for a launch in 2011/2012. But the notion of a human mission to Phobos has never gone away, and even got a plug in last year’s Augustine report on the future of human space exploration:

A Phobos-based teleoperated exploration of the Martian
surface, returning with samples from that surface, would
likely precede a crewed Mars landing mission, and would
provide dramatically more responsive remote control than
with the communication delays incurred between Mars and
Earth. The use of Phobos- and/or Mars-produced in situ
propellant could likely reduce the flight cost of a crewed
Mars landing expedition by a factor of two to three.

A human Phobos mission has a lot of appeal. It’s far easier than a Mars landing, it would force us to solve the problems of long-term living in space, and it’s not much of a stretch from a near-Earth asteroid mission. To those who say it would be unsatisfying, and that it’s foolish to think about traveling all the way to Mars without landing there, I have just two words: Apollo 8.

It’s nice when an expensive new machine works as advertised—nicer still when that machine has the ability to revolutionize a whole field of science.

At this week’s meeting of the American Astronomical Society in Washington, scientists couldn’t stop gushing about the exquisite performance of NASA’s Kepler telescope, which was launched last March and is now staring at a field of 150,000 stars in the constellation Cygnus, looking for tiny (less than a hundredth of a percent) dips in brightness that signal the presence of an eclipsing planet.

The First Five

“We are doing photometry at a level never seen before,” said David Latham of the Harvard-Smithsonian Center for Astrophysics. And as the high quality of the data becomes apparent, “We’re all burning the midnight oil seven days a week,” to keep up, said planet hunter and Kepler team member Geoff Marcy of the University of California at Berkeley.

Kepler scientists announced the discovery of five new planets at the meeting, including one oddball with the density of Styrofoam. But perhaps more interesting were some of the objects that show unusual light curves but have yet to be identified. Until they’re confirmed as planets or something else, the project simply calls these “Kepler Objects of Interest,” or KOIs. KOI #74 and #81 are especially strange—only four times the size of Earth and the size of Jupiter respectively, they’re both hotter than the stars they orbit. “Does anybody know what they are?” Latham asked an auditorium full of astronomers. Nobody ventured a confident answer.

It will take three years for Kepler scientists to confirm Earth-size planets circling stars at Earth-like distances (such orbits take a year, and astronomers need a couple of orbits before making a positive ID). But it’s possible that small planets orbiting closer to their stars will turn up sooner in the data.

The real bottleneck will be follow-up observations from the ground. Just because a star’s light dims periodically, it doesn’t mean there’s a planet. It might be another star in a binary system regularly crossing in front of its companion. The best way to tell the difference is with ground-based spectroscopy.

In the first 43 days of observing, Kepler turned up 175 KOIs—light curves that could indicate the presence of planets. Examination of the first 50 yielded 5 confirmed planets, but the other 125 in the queue are waiting to be followed up, and Kepler has been looking for several months since then.

We’re going to have to be patient.

Avatar's Pandora (20th Century Fox)

In the three years since film director James Cameron wrote the script for his new blockbuster Avatar, a lot has changed in the field of exoplanet research (the study of planets around other stars). Nobody knows this better than one of its leading practitioners, Lisa Kaltenegger of the Harvard-Smithsonian Center for Astrophysics, who marvels herself at how fast the young science is progressing. “We can actually overtake a sci-fi movie,” she says with a laugh.

Cameron set his story on Pandora, a moon orbiting a giant gas planet circling Alpha Centauri A, one of the closest stars to our sun. Recent, real-life searches have all but ruled out that scenario: Jupiter-size worlds don’t appear to exist around Alpha Centauri. But a new paper by Kaltenegger offers something perhaps more exciting: a near-term hope of finding signs of life around “exomoons” like Pandora.

One method used to find exoplanets is to watch how the light of their host stars dims and brightens as the dark planet crosses, or “transits,” the star’s disk. Kaltenegger realized that if a transiting planet had a large enough moon, and if the moon’s orbit extended beyond the star’s diameter as seen from our vantage point, the moon would also briefly appear to cross the stellar disk just before or after the planet finished its own transit. In that short interval when the moon partly eclipses the star, astronomers could analyze the starlight filtering through the moon’s atmosphere (if it has one), looking for oxygen, water, carbon dioxide, and other chemicals suggestive of life.

NASA’s Kepler telescope should soon tell us whether the technique is viable. Any exomoons discovered by Kepler will be too far away to inspect for bio-signatures, but if the method works, it could be applied to nearby stars like Alpha Centauri. Better still, we don’t have to wait for some exotic new instrument to be built. The 6.5-meter James Webb Space Telescope, due to launch in 2014, should work nicely.

The hunt for habitable worlds beyond Earth just picked up pace.

The Cassini probe to Saturn and Titan is just one of those spacecraft that keeps returning very cool stuff, such as the beautiful view of Saturn during its equinox a few months ago.

Now, the mission has just released tantalizing footage of Saturn’s moon Janus, which is about 111 miles across, overtaking another of the planet’s larger moons, Rhea, about 949 miles across. The video is actually 12 frames snapped by Cassini over a 24-minute period on November 8, 2009.

Scientists later reformatted the series of images on a computer to smooth out the moons’ motions between the frames. Janus is the tiny white dot coming from the far left to the far right, about 1.4 million miles away from the spacecraft, while Rhea was about 1.2 million miles away from it. Janus orbits Saturn at about 36,000 miles per hour, double the speed of Rhea. For a better, slower version of the video, go to this page and click on “Flash 2.0 MB.”

“As yet another year in Saturn orbit draws to a close,” said Cassini imaging team leader Carolyn Porco, “these wondrous movies of an alien place clear across the solar system remind us how fortunate we are to be engaged in this magnificent exploratory expedition.”

Little telescopes, little planets: The MEarth array on Mt. Hopkins, Arizona (Photo: Dan Brocious, CfA)

Announcements of newly discovered planets come so frequently these days that it’s hard to tell which ones are significant. But GJ 1214b deserves its moment of fame.

Discovered by a team led by David Charbonneau of the Harvard-Smithsonian Center for Astrophysics, the planet is only 5.4 times the diameter of Earth, one of the smallest ever found. Its mass is just 6.5 times Earth’s, yielding a density about twice that of water, which leads to speculation that the planet has deep oceans. It orbits very close to its host star—a red dwarf called GJ 1214 located 40 light years away in the constellation Ophiuchus—so its surface should be as hot as Venus. And that’s actually pretty cool for such a close-in planet.

All of this adds up to what planet-hunter Geoff Marcy, writing in the current issue of Nature (which published Charbonneau’s paper), calls “the most watertight evidence so far for a planet that is something like our own Earth, outside our Solar System.”

Almost as impressive is how the planet was discovered—with an array of telescopes no bigger than what many serious amateurs have at home. The new MEarth (pronounced “mirth”) project hopes this is just the first Earthlike planet to be discovered around a small, nearby star—which is exciting news.