![]() Yet Neptune sported many bright and dark cloud features - a far more interesting assortment than the blandness found 3½ years earlier at Uranus. The giant planet, hued blue due to methane in its upper atmosphere, is bathed in sunlight just 1⁄ 900 the intensity that we enjoy here. The dark spot in the middle is a cyclonic storm as wide as Earth.įor example, mission scientists found themselves peering into the Great Dark Spot, a giant cyclonic storm in Neptune's atmosphere as large as Earth. Methane gas, which preferentially absorbs red light, causes the blue hue. Voyager 2 imaged Neptune when it flew by in August 1989. For Sky & Telescope's February 1990 issue, the first with detailed results, devoted 20 pages to the flyby's images and scientific findings. That trickle of data carried a torrent of amazing discoveries. Despite having an oversized radio dish 12 feet across, the spacecraft could only relay 21,600 bits of data to Earth per second in a faint electronic whisper of just 10 –16 watt. I recall all these details vividly because on that day I was sitting anxiously at the Jet Propulsion Laboratory awaiting the outcome of events taking place 3 billion miles away. The spacecraft swept by the blue-and-white clouds capping Neptune's north pole from just 3,050 miles (4,900 km) away - a nearness made necessary so that its trajectory could be angled downward sharply to skim past the big moon Triton just 5¼ hours later. That's almost 24 hours' difference, but it's close enough, I suppose.) Billions of miles from home, the Voyager spacecraft (one illustrated here) are still collecting data.Īnd "close" is what flight engineers had in mind when they planned Voyager 2's Neptune encounter was its fourth and final planetary flyby. Eastern Daylight Time, whereas Voyager 2's flyby came at 3:57 Universal Time, which is 11:57 p.m. Flight engineers calculate that New Horizons crossed Neptune's orbit at 10:04 p.m. (Technically, the coincidence isn't exact. ![]() The difference - and it's a big one - is that Neptune was nowhere in sight when New Horizons swept over the threshold about an hour ago, but it was very near when Voyager 2 made its historic crossing exactly 25 years ago. Here's a remarkable coincidence: August 25th will be remembered as the day when two NASA spacecraft passed the orbit of Neptune. NASA's Voyager 2 spacecraft swept past Neptune exactly 25 years ago, revealing that this distant blue-hued planet is far more dynamic and unique that expected. ![]()
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