A Disco Ball in Space

October 9, 2001: Normally when a spacecraft blasts off for Earth orbit, ground controllers hope it will stay up for a long time. But last month NASA launched something a little different: a satellite whose mission is to fall from the sky. Returning to Earth out of control and with no hope of rescue is exactly what it's supposed to do!

Its name is Starshine 3, and it looks as curious as its mission sounds. The 200-lb satellite, carried aloft from Alaska on Sept. 29th by a Kodiak Star rocket, is a meter-wide sphere studded with 1500 student-built mirrors. Sky watchers can easily see it as it glides overhead, spinning and glittering like an oversized disco ball.

Right: Mechanical engineer John Vasquez inspects the Starshine 3 satellite at the U.S. Naval Research Laboratory. Image credit: Michael A.Savell and Gayle R. Fullerton. [more]

"Starshine 3 is on a mission to explore the outer reaches of Earth's atmosphere and to discover what happens to satellites there," explains Prof. Gil Moore, the director of Project Starshine -- a unique program that combines cutting edge research with educational outreach.

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"Project Starshine is a consortium of volunteers -- universities, government agencies, private corporations -- led by the US Naval Research Laboratory and the Space Grant Program," he says. The most important participants, though, are students. Approximately 40,000 of them from all parts of the world helped polish Starshine 3's distinctive mirrors. And now that the "disco ball" is in space, students are going to help again by monitoring Starshine 3 as it falls .... at first slowly, then later with greater haste ... back to Earth.

Starshine 3 is falling because the atmosphere is dragging it down.

The satellite is orbiting 470 km above Earth in a region scientists call the thermosphere. The air at that altitude is very thin -- about 10¹² times less dense than the air at sea level. Indeed, it seems more like space than a part of Earth. The thermosphere is where the International Space Station (ISS), the space shuttle, and many other satellites orbit.

Even though the thermosphere is practically vacuum-thin, it's still dense enough to sap orbital energy from satellites by means of aerodynamic drag. Soon after the Kodiak Star disgorged it, Starshine 3 began to lose altitude -- dropping a few meters during each 90 minute orbit around our planet. The same thing happens to the ISS, which requires periodic re-boosts.
 

Above: Starshine 1, a beachball-sized cousin of Starshine 3, vaporized in Earth's atmosphere last year. These data, courtesy of NRL's Judith Lean, show how the satellite fell slowly at first, then with greater rapidity as it descended into lower (and denser) layers of Earth's atmosphere.

"Starshine 3 will slowly descend during the next 4 years," says Moore. As it sinks into ever denser layers of the atmosphere, its rate of orbital decay will accelerate. Eventually, when it sinks below the stratosphere, Starshine 3 will burn up completely. "The end will be spectacular," he added. If re-entry happens at night, the dazzling fireball would cast enough light for onlookers to read a newspaper!

There's no danger to anyone on the ground, Moore notes. "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 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.

No one knows exactly when or where Starshine 3 will return because the thermosphere is unpredictable. Earth's uppermost atmosphere "breathes" in and out in response to changing solar activity. During years (like 2000 and 2001) around the peak of the 11-year sunspot cycle, our star is a powerful and sometimes fitful source of extreme ultraviolet (EUV) radiation. Waves of EUV heat the thermosphere and cause it to "puff up," hastening the descent of falling satellites. Exploring the cause and effect relationship between solar activity and orbit decay is a primary goal of the mission.

Right: Magnetic fields loop above a sunspot group where hot gas glows brightly at extreme ultraviolet wavelengths. Such radiation heats our planet's thermosphere and increases its drag on Earth-orbiting satellites. Credit: TRACE.

"We have very few direct measurements of the density of the upper atmosphere. That's why Starshine 3 is so important," explains solar physicist Judith Lean of the Naval Research Laboratory (NRL) in Washington, D.C. "The satellite's well-defined spherical geometry allows us to estimate its ballistic coefficient with a reasonable degree of certainty. As a result, by monitoring Starshine's orbit and studying how that decays we can calculate the density of the gas that's dragging it down."

Lean and her colleagues, led by NRL's Mike Picone, are already using data from an earlier mission (Starshine 1, which orbited Earth for 8 months before it disintegrated in February, 2000) to predict the decaying trajectories of satellites and bits of orbiting space debris. Their calculations are of keen interest to ISS mission planners and shuttle pilots who fly through the swarm of Earth-orbiting objects. "We're hoping that Starshine 3 will improve those predictions even more," says Gil Moore.

Before that can happen, though, someone must track the glittering satellite. Moore and his colleagues need to record about 800 sightings per day to define Starshine 3's changing path.

It's the perfect job for a legion of young scientists.

"We need students, teachers -- sky watchers of all kinds -- to go outside and spot Starshine 3 among the stars," says Moore. "The procedures for recording and reporting the data are straightforward. Anyone can do it. It's a great science project and we hope many classrooms will sign up to participate."

Above: Prof. Gil Moore holds a full-scale mockup of the Starshine 1 and 2 satellites. Sunlight is reflected from a single mirror in the image on the right. Photo by Kerry Kirkland.

Moore added: "Starshine 3 usually looks as bright as a 1st magnitude star when it passes overhead." During a typical transit the rotating satellite will seem to flash every few seconds as sunlight glints off one mirror after another. The trail of bright dots and dark dashes across the sky is unmistakable.

"It's beautiful," says Sharon Simon, a teacher at the Davis Creek Elementary School who saw the satellite glide over her town in West Virginia on October 2nd. "The flashes came in three quick segments that were clearly visible. [Each flash] was about as bright as Mars." (Visit Heavens-Above.com to find out when Starshine 3 will glide over your town, suggests Moore.)

Simon's students helped polish Starshine 3's mirrors, and their handwritten signatures are orbiting Earth on board the satellite. Indeed, the signature of every student who worked on the mirrors is up there -- all 40,000 of them laboriously digitized by Moore's wife Phyllis. "That was hard work for Phyllis," recalls Moore. "But these youngsters are important to us -- they are a crucial part of what we're doing."

Above: A student at Davis Creek Elementary School in Barboursville, West Virginia, grinds a mirror for Starshine 3.

Moore hopes that thousands more students will join the program. "We need more spotters," he says -- not only to monitor Starshine 3, but also to track Starshine 2, another "disco ball" slated for launch aboard space shuttle Endeavour in December 2001. Starshine 4 and Starshine 5 are on the drawing board, too, and they will need mirrors as well. Schools should soon be able to request mirror-building kits for those satellites at the Project Starshine web site.

"Our goal," says Moore, "is to monitor the thermosphere during all phases of the 11-year solar cycle using Starshine satellites launched every year or so." If his plan succeeds, watching disco balls glide across the heavens might soon become a regular part of K-12 science classes. Who knows ... perhaps disco isn't dead after all.
 

Heavens-Above.com -- Visit this website to find out when Starshine 3 will glide over your town.

NASA's J-Pass -- Another online resource to help you spot Starshine 3 from your own back yard.

Protecting Starshine -- All of Starshine 3's student-polished mirrors were coated with a layer of silicon dioxide. "The silicon dioxide coating adds no reflectivity," says Vince Huegele, "but it does protect the mirrors from corrosion by atomic oxygen [a chemical constituent of the thermosphere]." Huegele works at the Marshall Space Flight Center's Space Optics Manufacturing Technology Center where the essential coating was applied.

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.

Right: Layers of the Earth's atmosphere. The troposphere is the first layer above Earth's surface; it contains half of our planet's atmosphere. Weather occurs in the troposphere. Many jet aircraft fly in the stratosphere because it is very stable. The stratosphere contains the ozone layer. Meteors burn up in the mesosphere. Auroras glow in the lower thermosphere; satellites like Starshine 3 and spacecraft like the Space Shuttle orbit in the upper thermosphere.

Starshine Returns to Cinder - (Science@NASA) Last year, Starshine 1, a beachball-sized cousin of Starshine 3, vaporized in Earth's atmosphere after a successful 8-month mission exploring the thermosphere.