Spotting the space station

Q: It seems that the space station, with its newly installed solar arrays should be visible to the naked eye.

· Is it visible to the naked eye?

· How bright, in comparison to the brightest planets Venus, Mars and Jupiter, is the space station?

· What is the celestial path of the space station and how can we calculate the next possible observation of it?

· When will the next sighting be possible from the northern hemisphere?

A: Yes, it is visible to the naked eye and best seen near dawn or dusk, when the Sun lights the International Space Station (ISS) and you are in near darkness.

The Station is brighter sometimes than others, depending on its illumination and distance from us. It has a maximum brightness of -3.0, when it's fully lit and nearest (about 230 miles, i.e., 370 kilometers) to Earth. That's a little brighter than Jupiter (-2.5). When the Station is only halfway lit and 620 miles (1,000 km) distant, it's brightness is only +1.5 - about as bright as a dim Mars.

Mars varies in brightness from -3.0 (outshining Jupiter) to +1.6. Venus has a magnitude of -4.4, which is the brightest object in the night sky, except for the full moon (-12.7). We measure star brightness with a logarithmic scale, similar to the way we measure earthquake magnitudes.

The Station's celestial path is an oval-shaped path - an elliptical orbit, which varies from 218 miles (351 km) to 221 miles (356 km) from Earth. Going about 17,200 miles per hour, the Station circles Earth 16 times a day at an inclination (a measure of the tilt of the ISS' orbital plane) of 51.6 degrees to the equator. The Station eventually crosses almost every point on Earth: flying over 85 percent of the globe and 95 percent of its population.

Actually, "the height of the ISS varies quite a bit over time due to air drag and corrective reboosts," says Chris Peat of Heavens Above. He built a chart so we can see this effect. Over last year, the height varied between about 205 to 214 miles (330 to 344 km) — repeatedly jumping up with a reboost and decaying back down due to the drag of the atmosphere. The decay rate, however, is not constant due to changes in the density of the tenuous outer atmosphere, caused mainly by solar activity.

You can get tracking software to calculate the next sighting (see Further Reading) but it's easier to surf the web to any of several tracking sites. I like the Heavens Above site because it not only gives you the next sightings but it also grades the sightings, telling which is brightest.

You just enter the name of your city (or its latitude and longitude) and... Presto! The program displays screens giving several days of sighting times, how to locate the Station each time it passes, and how visible the Station is during the pass.

For example, I live in Albuquerque, New Mexico. I just had a good sighting on the 26th of March at about 19:55 MDT (7:55 pm). I looked to the south-southwest (203 compass degrees) to see the Station rise above the horizon at 19:54:50. I followed the rising satellite to the north where it reached its maximum elevation (42 degrees up from the horizon — where 90 degrees is straight up, overhead) for this pass at 19:57:33, and then watched it set in the south at 20:00:16. The program provides all this information so I can tell where and when to look.

The next good one in Albuquerque will be on the 11th of April about 21:05 (9:05 am), rising in the northwest. I can hardly wait.

Further Reading:

Aerospaceweb.org: Emergency ISS-Shuttle Rendezvous

Discovery Channel: Inside the space station

Heavens Above, tracking site

NASA orbit trajectories and tracking software

Austin Astronomical Society: How to estimate angles in the sky

ESA: Space station and useful terminology

NASA: International Space Station press kit

NASA: International Space Station

(Answered July 29, 2005, updated April 2, 2008)