Apr 25, 2002

Starshine 2 Return




A glittering satellite named Starshine 2 will disintegrate in Earth's atmosphere on April 26th.




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April 25, 2002: For some months now sky watchers have occasionally looked up and seen a curious flashing light in the sky. Pulsing as brightly as a first magnitude star, it would appear, race across the heavens, and be gone in a matter of moments. A high-powered telescope -- if one could be made to track the fast-moving thing -- would reveal a half-meter wide sphere, spinning and studded with sunlight-reflecting mirrors. A disco ball in space.


In fact, it's a satellite. Scientists and the students who helped them build it call the orb "Starshine 2." It was carried to Earth-orbit by NASA's space shuttle Endeavour in Dec. 2001. But now, only five months later, Starshine 2 is falling back to Earth. Experts say it will disintegrate in the atmosphere like a vivid meteor on April 26th.




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Above: Starshine 2 returning to Earth will resemble a slow-moving Leonid fireball, like the one pictured here. [ ]

"I'm thrilled," says Gil Moore, the director of Project Starshine. "This is exactly what Starshine 2 is supposed to do."

Project Starshine is a consortium of volunteers supported by the US Naval Research Laboratory, the Space Grant Program, and NASA. Together, they work to involve school-age kids in cutting-edge research. Starshine 2 is one of their many successes.

The satellite, which is dotted with mirrors ground by thousands of students, is on a mission to explore the outermost reaches of our planet -- a super-thin layer of the atmosphere called the thermosphere. The thermosphere is so thin (1012 times less dense than air at sea level) it seems more like a part of space than a part of Earth. Indeed, many Earth-orbiting satellites are inside it.


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Left: Students have monitored Starshine 2 and its orbit by tracking flashes of sunlight reflected from the satellite's mirrors.

"There's a lot we don't about the thermosphere -- particularly how it responds to solar activity," says Moore. "Starshine 2 is helping us understand that by falling back to Earth."

He explains: "As satellites skim through the upper atmosphere, sparse molecules of air nip away at their kinetic energy little by little." This "aerodynamic friction" causes all spacecraft in low-Earth orbit to fall. Even the International Space Station sinks, albeit slowly, and must occasionally be reboosted to counteract the drag.

If the thermosphere were steady, figuring out the decay rates of Earth-orbiting spacecraft would be easy. But it's not. The thermosphere expands and contracts in response to solar activity.

Last week, for example, explosions on the Sun hurled three coronal mass ejections (or "CMEs") into space and toward Earth. When the electrified clouds swept past our planet they warmed the thermosphere and caused it to expand. (They also triggered beautiful Northern Lights.) The puffed-up atmosphere exerted more drag than usual on Earth-orbiting satellites.


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Right: On April 23, 2002, these auroras appeared over Canada just after a coronal mass ejection swept past our planet. Credit and copyright Yuichi Takasaka. [more]


"By monitoring the orbit of Starshine 2, we can see how the thermosphere responds to such events," says Moore.

When Starshine 2 was launched, Moore and colleagues expected it to return to Earth in June. It's returning early because the Sun has been extra-active during the first months of 2002. "The ongoing solar maximum is double-peaked," explains Moore. "The first peak happened in 2000. The second peak, which no one expected, coincided more or less with the launch of Starshine 2 -- it was perfect timing!"

On April 26th Starshine 2 will sink below the stratosphere and disintegrate. "The end will be spectacular," predicts Moore. If re-entry happens at night, sky watchers will see a brilliant fireball -- much like a bright Leonid meteor, only slower moving. No one will know exactly where the fireball will appear until a few hours before the end; orbital decay is that unpredictable. Click here for the latest re-entry predictions.

There's no danger to anyone on the ground, Moore assures. "We designed the satellite so that it will be 100% consumed about 80 km up." Except for a few small steel screws, the body of the spacecraft (including the mirrors) is made entirely of aluminum -- a substance that will vaporize during the fiery descent. "We had no choice," quips Moore. "Otherwise I was going to have to buy a 100 million dollar insurance policy with a $50,000 premium." The aluminum was cheaper.


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Above: Kids of all ages work on Project Starshine. (Left) An elementary-school student grinds a mirror for Starshine 3. (Middle) Prof. Gil Moore holds a full-scale mockup of Starshine 2. (Right) Moore's wife Phyllis inside Endeavour's cargo bay gives Starshine 2 a thumbs-up.

The end of Starshine 2, welcomed as it is by researchers, might seem a sad occasion for the many school kids who helped grind and polish its glittering mirrors and who have tracked it faithfully for months. But it's not the end of Project Starshine. Indeed, Starshine 2's larger cousin Starshine 3 (launched aboard a Kodiak Star rocket in Sept. 2001) remains in orbit around our planet. It won't likely descend until October or November 2002. Meanwhile, Starshine 4 and Starshine 5 are slated to launch on board a NASA shuttle early next year.

The truth is, many kids, including kids-at-heart like Gil Moore, love the idea that they helped make a fireball. They'll gladly do it again and again (indeed, Moore has plans for Starshine 6 and beyond). And if scientists can learn something by it, so much the better.



Editor's Note: This story mentions CMEs as one form of solar activity that can warm the thermosphere. There are others. For example, great magnetic loops above sunspots harbor million-degree gas. Extreme ultraviolet radiation and X-rays emitted by such gas-filled "magnetic bottles" warm the thermosphere from afar. EUV radiation from sunspots is, indeed, the most important source of long-term thermospheric warming. Solar flares (which are not the same thing as coronal mass ejections) can also bathe the thermosphere with warming radiation.

more information

Project Starshine -- home page

Starshine 2 re-entry predictions -- check this page for frequent updates


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Above: Starshine 2's orbital decay curve. Courtesy

The Resurgent Sun -- (Science@NASA) Starshine 2 orbited Earth during the second peak of an ongoing double-peaked solar maximum.

Solar S'Mores -- (Science@NASA) As a result of the solar maximum, Earth's atmosphere is "puffed up" like a marshmallow over a campfire leading to extra drag on Earth-orbiting satellites.

A Disco Ball in Space -- (Science@NASA) Students and scientists team up for a far-out experiment to monitor the outer reaches of Earth`s atmosphere.

Starshine, Too -- (Science@NASA) On Dec. 16, 2002, shuttle astronauts deployed the eye-catching Starshine 2 satellite. Now there are two "disco balls in space" for students to track and study.


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