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Introduction Preparing for the Unexpected Heidi Hammel is an explorer. As a planetary astronomer she is not actually walking on the surface of other planets, but she is explor- ing them through images. For Heidi the two giant planets Neptune and Uranus hold a special fascination. Whether she is observing these "ice giants" from one of the world's great observatories in Hawaii or analyzing images that have been collected from a space telescope, she is forever on the lookout for a discovery. By opening her eyes to small but interesting details, Heidi is always prepared for the unexpected. And when the unexpected comes--even in the far reaches of the solar system--Heidi turns it into an adventure. Heidi and other scientists once found a giant storm on Neptune that they expected to rage for decades. But only five years later, Heidi discovered it had vanished. Another time Heidi led a team of Hubble Space Telescope scientists who took photos of a battered planet Jupiter as pieces of a fragmented comet crashed into it every day for a week. Heidi's enthusiasm and down-to-earth descriptions of the event made her one of the celebrities of what became known as "the Great Comet Crash." These discoveries have brought Heidi acclaim, but none of the attention has changed her approach to life in the least. She still looks to the future and sees adventure around every corner, and she wants everyone to come along for the ride. ix

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Heidi turns the wheel and starts the climb. Far ahead is the summit and one of the world's great observatories.

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1 A DATE WITH TWO PLANETS H eidi Hammel sits behind the wheel of her rental car, preparing to turn off the Saddle Road, which crosses Hawaii's Big Island between two huge volcanoes. She grips the steering wheel a little tighter, anticipating the The IRTF (opposite) steep, narrow, curving road that climbs the dormant Mauna Kea appears to be sitting on top of the world. [MOW-nah KAY-uh]. The volcano's name means "white mountain," It's one of several so-called because its peak is often snow covered. world-class telescopes that share the Mauna Heidi turns the wheel and starts the climb. Far ahead is the Kea summit. Views of summit and one of the world's great observatories--the Infrared Uranus and Neptune Telescope Facility (IRTF), part of the National Aeronautics and (above) are shown in false color. Space Administration (NASA). If all goes according to plan this week in August 2003, she will spend three nights here observing the giant planets Uranus and Neptune. But first she will make a stop at the Onizuka Center for International Astronomy, located 9,300 feet above sea level. The center includes a comfortable dormitory, which will be Heidi's home for the next four days. The higher Heidi drives, the thinner the air becomes. At an altitude of nearly 14,000 feet, where the Mauna Kea Observatories are located, the air pressure is low enough to activate a passenger airplane's emergency oxygen system. At this pressure, the body isn't getting the oxygen it needs to function properly. 1

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Experienced observers like Heidi always rest for a few hours at the center to give their bodies time to adapt before going higher. After checking into her room, Heidi calls her husband, Tim. JUPITER Heidi's children in 2003 She'd love to say hello to their three children--Beatrix, Tobias, (from left): Tobias and Lucas--but it's nearly 11 P.M. back in Connecticut. By now (age 3), Beatrix (age 5), they're all sound asleep. Though Heidi has been awake for 18 and Lucas (age 1). BEYOND hours and has traveled 5,000 miles, she wants to hear the details of Tim's Sunday with the children. The conversation is short because Tim has to get up early to go to work the next morning. But as usual, he leaves Heidi smiling. Then it's time to get down to business. Heidi freshens up and goes to the dining room for a meal with other members of her observing team. Her day is still not over. As the Sun sets, she heads up the nine-mile hill to the IRTF--this time in a four- wheel-drive vehicle. All but the last mile or two is unpaved, with switchback curves. Until recently, it was barely wide enough for two vehicles to pass. Near the top, the road is paved with asphalt to avoid stirring up dust that could damage the observatories' mirrors, lenses, and sensitive equipment. Heidi flicks off her headlights because their glare can interfere with observing. She peers carefully into the darkness as she approaches the facility. Inside, telescope operator Paul Sears is already hard at work. Tonight Heidi won't be doing any work herself. Her main mission is to give her body time to adjust to the thin air. She's glad Paul is on duty. A former tour guide, he entertains her with stories of the geology of the mountain, the history of the Big Island, and his many nights operating the telescope. Even with 2

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good conversation, Heidi's long day finally catches up with her. Stars of the Milky She heads for her vehicle and looks up at the splash of the Milky Way shine above Mauna Kea. Against Way overhead. Mars, glowing deep red, dominates the eastern moonlit clouds stands sky. Uranus and Neptune are in the same general direction but are an "ahu hoku," or too faint to be seen by her unaided eyes. Tomorrow night the star altar, built of rocks and topped by IRTF Telescope will make them visible. white coral. Heidi drives back down the dark, winding road to the dormitory. It's 10 P.M. when she gets there. She has been awake for 23 hours! As she drifts off to sleep, Heidi thinks about the observations to A DATE WITH TWO PLANETS 3

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come. Neptune is always interesting, but it's a particularly fascinating time on Uranus, the planet with the most extreme seasons in the solar system. Uranus is approaching an equinox--the transition JUPITER into spring in one hemisphere and autumn in the other. The last time Uranus had similar weather was 42 years earlier in 1961, BEYOND The Strange Seasons of Uranus Uranus takes 84 Earth years to circle the spins nearly on its side. Its north pole Sun, so its seasons each last 21 Earth years. actually points slightly south of its When Heidi was born, an equinox on orbital path! Uranus was only five years away. In Earth This makes for unearthly Uranian seasons. terms, it was like the beginning of March, The planet's north pole points nearly the time when early signs of spring appear directly at the Sun during midsummer. in the northern hemisphere. Forty-two years later, at the depth of Seasonal changes on Uranus are the most northern winter, its south pole points extreme of any planet because its axis--the at the Sun (right). imaginary line through its north and south With such dramatic sunlight shifts, poles--has a crazy tilt. astronomers like Heidi suspect that an Every planet's axis is angled at least slightly equinox on Uranus must be a time of toward the planet's orbital path around amazing seasonal change. And because the Sun (below). Earth's tilt is fairly large. it occurs only once every 42 Earth years, Its axis is inclined about one-quarter of the it's a once-in-a-lifetime opportunity to way toward its orbital path. But Uranus discover something interesting. 4

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when the young astronomer-to-be was only a year old. No one could have guessed that Heidi would grow up to be an expert on Uranus and Neptune. But then no one has been able to predict most of what has happened in the life of Heidi Hammel--not even Heidi herself. A DATE WITH TWO PLANETS 5