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New telescope technology improves view of 'blandest' Uranus

Elizabeth Howell, News Writer
Jan 11, 2013, 8:00 UTC

Sen—At one time, we only knew Uranus as a small point of light in a telescope. Then, 205 years after its discovery in March 1781, scientists excitedly geared up for a close-up view with the Voyager 2 spacecraft.

The ship sped by the distant planet, taking a flurry of images with its visual-light camera. It revealed a practically featureless blue world sheathed in methane. Unfortunately for Uranus scientists, its moons appeared more interesting. Two, Oberon and the largest moon Titania, were discovered on this day in 1787 by William Herschel.

"Uranus got the reputation as the blandest planet in the solar system. As far as Voyager was concerned, that was certainly justified," recalled Larry Sromovsky, a University of Wisconsin-Madison planetary scientist who was on the Voyager team.

That impression was wrong.

Space missions take decades to plan. After the featureless ball of Uranus made its way into newspapers and television broadcasts, public attention turned to other planets - such as Mars. 

But Sromovsky, who will probably face Earth-bound observations of Uranus for the rest of his scientific career, takes comfort in how much telescope technology has improved. 

The first breakthrough came after the Hubble Space Telescope turned a near-infrared camera towards Uranus in the 1990s. The telescope, first launched in 1990, sits above Earth's planet-blurring atmosphere.

It turned out that looking at Uranus in other wavelengths was the key. The University of Arizona's Erich Karkoschka and his collaborators found several cloud features on Uranus, and through tracking them estimated the blistering wind speed found on the planet. Today, scientists believe winds can reach as high as 900 kilometers an hour.

"It's looking at salt on a black surface, instead of a white surface. You can see things," Sromovsky said.

But even Earthly telescopes have better resolution than astronomers dreamed of centuries ago. Through an advance called adaptive optics, telescopes can autonomously adjust their mirrors to compensate for the constantly shifting atmosphere.

Hubble has a relatively small mirror, at 2.4 metres across. Sromovsky's instrument of choice today is the 10-metre W.M. Keck Observatory in Hawaii. Although it is best known for exoplanet hunting, the telescope's resolution allows it to see large features on Uranus.

Keck stands apart from other "adaptive optics" systems because the system can calibrate itself on a planet, which looks relatively large in a telescope. Most other telescopes require a point of light, such as a star.

Because Keck can calibrate on Uranus, it is easier to perform observations on the planet because the calibration and observations are on an object of the same brightness, Sromovsky pointed out. Other telescopes require calibration using one of Uranus' moons.

Just recently, Sromovsky put the telescope to good use. Sromovsky and his colleagues examined the clouds of Uranus to see what sort of weather the planet has. They found cloud features on the planet that have never been seen before at the south pole.

"It's insanely more detailed," Sromovsky said.

While most future Keck projects are on hold due to uncertainty in U.S. federal astronomy funding, Sromovsky does have hours of recent Hubble observations of Uranus in the can. "It will tell us something about the distribution of methane in the atmosphere of Uranus," he said, adding this follows on from puzzling 2002 observations showing depletion of methane at high latitudes.

But why the methane is depleted is still a mystery. It's just one reason Sromovsky is eager to return to Uranus with a spacecraft. "I'm not real optimistic about anything significant happening in that area," he added. "[American] planetary budgets seem to be shrinking rather than growing."

Analysis: Scientists seek a return mission to Uranus