Beaming Success for Station Fans
Did you ever use a flashlight to send a Morse code message to your neighbor at night as a kid? People like to say hello using lights and it's no different for space aficionados who want to twinkle a greeting from the Earth to the International Space Station during a sighting
as it passes overhead -- except that it is a whole lot more complicated.
Although the space station has been in orbit for more than a decade, the first successful flashing of a beam of light to the laboratory happened only recently. On March 3, 2012, the San Antonio Astronomical Association
met to attempt to shine a signal to the station. Aboard the orbiting lab, astronaut Don Pettit was watching and waiting.
"It sounds deceptively easy," said Pettit in a related blog entry
. "But like so many other tasks, it becomes much more involved in the execution than in the planning."
The ground group used a one-watt blue laser and a white spotlight to track the station as it flew overhead. Pettit worked via e-mail with the association members to run complicated engineering calculations to ensure they were accurately tracking the station. Considerations included the diameter of the light beam, the intensity of the laser, and the fact that the station is a moving target, as Pettit pointed out in another blog post
on the difficulty of Earth photography from space.
"From my orbital perspective, I am sitting still and Earth is moving," said Pettit. "I sit above the grandest of all globes spinning below my feet, and watch the world speed by at an amazing eight kilometers per second [approximately 17,880 miles per hour]."
Pettit had additional complications to address to capture an image of the beam of light from the Texas fans of the space station. Even with a shutter speed of 1/1000 of a second, the camera he used on station was not fast enough to photograph the Earth below, which also is moving. To compensate for this, Pettit used precise manual tracking -- a technique of moving the camera along the same path as the object being photographed -- a skill perfected on orbit while working on Crew Earth Observations
While photographing the Earth may provide an entertaining pastime for the crew, there also are important research goals and benefits
for those of us on the ground. It can take up to a month, according to Pettit, for astronauts to become proficient at taking this kind of planned image. The crew's photographic efforts can provide orbital perspectives of natural disasters and man-made alterations of the planet, which aid in relief and environmental efforts.
Preparing to capture the laser flash provided practice for Pettit in planning and tracking a specific Earth target. With the station circling the Earth every 90 minutes, you might think there is ample opportunity, but the circumstances of the pass had to align. Pettit and the team in San Antonio had to choose their timing carefully, selecting a "dark pass" when the station could see the ground, but those on the ground could not see the station.
"Ironically, when earthlings can see us, we cannot see them," said Pettit. "The glare from the full sun effectively turns our windows into mirrors that return our own ghostly reflection. This often plays out when friends want to flash space station from the ground as it travels overhead."
Planning took weeks for this particular event, between calculations
and timing. That morning Pettit was excitedly waiting, camera in hand, for the precise moment. When the instant came, he was able to see not only the flash of light from San Antonio, but to capture a digital image showing the beam of light from his Texas fans.
"I was ready with cameras for the early morning San Antonio pass," said Pettit. "And [I] can report that it was a flashing success."
by Jessica Nimon
International Space Station Program Science Office
NASA's Johnson Space Center