Bill Tracy, a flight dynamics officer in the Mission Control Center in Houston said: "Sightings of a wide variety of satellites are fairly common. Usually, however, sightings of the Station are limited to some parts of the country on some days, and other parts of the country on other days."
Image to right: Backdropped by the blackness of space and Earth's horizon, this full view of the International Space Station was photographed by a crewmember onboard the Space Shuttle Endeavour. Credit: NASA
"This week, the conditions all came together that will provide most of the continental United States with at least one sighting opportunity of the ISS on Wednesday or Thursday evenings -- with Thursday being the better of the two days," Tracy said.
NASA's Human Space Flight Web site provides lists of sighting opportunity times for hundreds of cities in the United States and the world. If a person cannot find a nearby location on the city list, the SkyWatch application allows a person to get ISS sighting times by providing some information. It will also provide sighting opportunity times for other satellites orbiting the Earth.
Because of the Station's orbit and the rotation of the Earth, the number of sighting opportunities and times will vary from location to location. For example on Thursday, Ruston, La., will have two 5-minute windows at 5:37 a.m. and 8:28 p.m. CDT. Salt Lake City will have one 6-minute opportunity at 4:32 a.m. MDT on Thursday and two evening opportunities -- at 9:03 p.m. for 4 minutes and at 10:37 p.m. for 5 minutes.
The Space Station will appear as a bright, slow, but steady moving star. Tracy said several conditions must occur for the sighting of the ISS or a satellite to take place in particular locations.
"First, obviously, the satellite must be above the horizon at the observer's location. That's easy enough," he said. "The second requirement is that the observer must be in darkness, when the sun is more than 6 degrees below the horizon. It may not be totally dark, but past experience shows that 6 degrees is about right.
"Third, the satellite itself must be lit by the sun. This means that the sun must be above the satellite's horizon. With the observer in darkness, sightings generally occur near sunrise and/or sunset at the observer's location. Finally, the lit side of the satellite must be facing roughly in the direction of the observer. Even if all other conditions are met, if the lit side of the spacecraft is facing away from the observer, then a sighting cannot take place."
Also, the ISS will appear in different parts of the sky from location to location. For example, Thursday evening Station gazers in Philadelphia should look for the ISS to appear at 9:32 p.m. EDT 10 degrees above the southwestern horizon and sail directly overhead before it disappears 11 degrees above the northeastern horizon.
If a location misses out on the ISS sighting opportunities this week, Tracy said there would be another excellent opportunity this summer. "There will be a few evening opportunities over the next two weeks," he said, "but the best evening sighting opportunities of ISS around the country will not occur again until mid-July of this year."