One of the biggest stories in 21st-century astronomy is the discovery of hundreds of planets around other stars. Thanks to NASA's Kepler planet-hunting satellite, it's only a matter of time (probably less than 3 years) before astronomers find an Earth-like world orbiting a Sun-like star. But you don't have to wait. If you own a telescope, you can look out into space and scrutinize an Earth-like planet every clear evening this season. It's called Mars.

Mars is only about half the diameter and a tenth the mass of Earth, and it has no oceans — though it may have had them in the remote past. Still, in a telescope, it shows numerous features reminiscent of our home planet, including polar ice caps, dark surface markings, bright clouds, and blowing dust. But observing Mars is practical only once every 2 years or so, when it and Earth are relatively close together on the same side of the Sun. The rest of the time, Mars is so far away that it appears as little more than a featureless dot.

Mars (arrowed) gleams with a distinctly ochre hue and outshines all the bright stars with which it shares the sky these evenings. The Red Planet was closest to Earth, shining brightest and appearing biggest in telescopes, in late January. Made with Stellarium.

That "every 2 years or so" has come around again, so Mars is now ideally placed for telescopic enjoyment. It was closest to Earth - about 61 million miles away - on January 27th. Two days later it was at opposition, directly opposite the Sun in our sky. That night it rose at sunset, got highest in the sky at midnight, and set at sunrise. It's easy to see why Mars is called the Red Planet. Go outside on any clear evening and look to the east. There you'll spot a rust-red "star" shining more brightly than any other point in that part of the sky. That, unmistakably, is Mars.

NASA's planet hunter is named for Johannes Kepler, who, in 1609, revealed that planets orbit the Sun in ellipses, not circles. Kepler made this landmark discovery by studying the orbit of Mars, which turns out to be one of the most eccentric, or "out of round," planetary orbits in the solar system. If we happen to catch up to Mars when it's in the part of its orbit closest to the Sun, the Red Planet appears especially bright and big in our telescopes. But this time we've caught up to Mars when it's in the part of its orbit farthest from the Sun, so the planet's disk is at best a disappointingly small 14 arcseconds across. Four oppositions from now, in July 2018, Mars will be much more impressive, spanning 24 arcseconds. (For comparison, Jupiter's disk ranges from about 33 to 50 arcseconds in angular diameter.)

Mars's surface features, clouds, and ice caps are best seen in telescopes of aperture 4 inches or larger. I find that inexperienced observers don't see much of anything on Mars in telescopes smaller than 8 inches and at magnifications less than about 200x. Discerning detail on the disk requires patience and diligence, because unlike Jupiter's main cloud belts and Saturn's rings, Mars's bright and dark markings are remarkably subtle.

What you see when you examine Mars in a telescope depends on when you look. Damian Peach captured these views of the planet's opposite hemispheres through Celestron Schmidt-Cassegrain telescopes. On January 4, 2010 (left), Mars showed its most distinctive dark marking, Syrtis Major, shaped somewhat like Africa. Two weeks earlier, on December 16, 2009 (right), the vast and largely featureless Amazonis desert faced Earth. South is up in both images. Courtesy Damian Peach.

No matter what your level of experience, what you see on Mars depends strongly on when you look. Virtually all of the most contrasty surface features are concentrated in one hemisphere; the other side of the planet is largely blank. Whenever Mars is in the evening sky, as it is now, astronomy magazines publish tables indicating which longitudes are visible at which times. You can then look up the numbers on a Mars map to determine which part of the planet is facing Earth during your observations. Sky & Telescope has a particularly nice utility on its website to make this task easier. S&T's Mars Profiler shows you a map of the side of Mars facing Earth for any date and time, and you can set it to display the map in whatever orientation suits your optics, for example, south up or mirror-reversed. I always have S&T's Mars Profiler open on my laptop when I'm observing the Red Planet. By knowing exactly what to look for and being able to name what I see, I find I can tease out subtle details that I'd otherwise miss.

An armada of robotic spacecraft have visited Mars since the 1960s, and several are operating there now, giving us unprecedented close-ups of the planet's surface. This view of Victoria Crater was snapped by NASA's Mars Reconnaissance Orbiter. Courtesy NASA/JPL-Caltech/University of Arizona.

We no longer have to content ourselves with distant views of Mars from earthbound telescopes, or even from the Earth-orbiting Hubble Space Telescope. The world's spacefaring nations have made the Red Planet a prime target for on-site exploration, mainly because of the slim chance that life may have once gotten a toehold there. There are currently three spacecraft orbiting Mars and mapping its surface in unprecedented detail: NASA's Mars Odyssey and Mars Reconnaissance Orbiter and the European Space Agency's Mars Express. Thanks to the Internet, anyone can browse the extensive image archives from these (and earlier) missions and explore Mars from a perspective just a few hundred miles above its surface.

And if that's not close enough for you, you can virtually descend to the ground itself thanks to the Mars Exploration Rovers Spirit and Opportunity, which have been ambling across the dusty surface and taking pictures for 6 full years now — much longer than anyone expected when the plucky robots were launched on 3-month missions. Someday, probably much farther in the future than space buffs would like, humans will stand on the Red Planet themselves and look back at Earth, the Blue Planet.

NASA's Opportunity rover rolled to the rim of Victoria Crater and captured this shot of the Cape Verde promontory. The dramatic cliff of layered rocks is about 50 meters (165 feet) away from the rover and about 6 meters (20 feet) tall. Courtesy NASA/JPL/Cornell.

While exploring Mars in the eyepiece, it's fun to think of all the spacecraft working there to reveal the secrets of the planet's past and paving the way for human exploration in the future. But it's even more fun, I think, to see Mars with your own eyes, from millions of miles away, and to get to know it not just as a light in the sky, but as another world with places you can name and identify on a map. Enjoy the view now, while you can, because in another few months Mars will recede into the distance, not to strut its stuff again until 2012.