Astro Bob: Spot the space station at dusk, Tiangong at dawn
The International Space Station begins a new round of easy-to-see passes, while early risers can catch China's Tiangong space station. We also take a whirlwind tour of Earth's atmosphere.
Hear ye! Hear ye! The International Space Station has returned to the evening sky. Now through early October it makes one to two passes every night over most northern hemisphere locations. Since the station travels with the rotation of the Earth, we always see it rise in the west and "set" in the east, the same direction the planet spins, from west to east.
And since the ISS is the biggest thing in orbit, it shines the brightest, looking like the planet Jupiter or Venus speeding across the heavens.
Although it orbits about 257 miles (414 km) high — well above the 62-mile-limit (100 km) where outer space is considered to begin — the station nonetheless travels through the Earth's atmosphere. Granted, 99 percent of the gases that make up the atmosphere are found in the first 20 miles (32 km) above the surface, but there's still air up there, thin as it may be.
The space station sails through the thermosphere, which starts at 56 miles (90 km) up and reaches to about 620 miles (1,000 km). The layer gets it name from the temperature, which climbs steeply from -184° (-120° C) at its base to as high as 3,600° F (2,000° C) near the top.
Scant molecules of oxygen and nitrogen, the main ingredients of the air we breathe, absorb ultraviolet (UV) light and X-rays from the sun and heat up. Paradoxically, despite the increasing temperature as we make our way up the thermosphere, there are so few molecules pinging around at these altitudes you wouldn't feel the heat. An ordinary thermometer would still register temperatures well below freezing.
Scientists measure temperature by how fast molecules are moving. If they're zipping around really, really quickly like the ones in the UV-baked thermosphere, they have a high temperature. We only sense heat if enough of them are pinging our skin, something that happens all the time here on the ground, where the atmosphere is thickest, but not up high where there's next to nothing.
Air in the lower part of the atmosphere is mostly composed of a familiar mix of nitrogen and oxygen molecules. In the thermosphere, solar radiation breaks apart the molecules into individual atoms of oxygen and nitrogen. Beyond the thermosphere we reach the even more rarefied exosphere, which ultimately merges with the vacuum of space.
During periods of high solar activity, when the sun crackles with sunspots and explosive flares, that extra energy heats and puffs up the thermosphere. This increases the drag (friction) on orbiting satellites and slowly lowers their altitudes.
Atmospheric drag can cause the ISS to drop as much as 330 feet (100 meters) a day, requiring boosts as often as once a month. Thrusters on Russian spacecraft attached to the ISS provide those boosts. If Russia were to pull out of our partnership, cargo ships carrying extra fuel would serve as backup.
While the astronauts do science and eat tacos, their unique, orbital perspective on the Earth helps to broaden our understanding of familiar on-the-ground phenomena.
For example, take the beautiful rays of light we sometimes seen streaming from the sun near sunset and sunrise. They're called crepuscular rays and appear to fan out in a great halo centered on the sun. The bright streamers are sunlight shining between gaps in the clouds; the dark gaps are where the sun's rays are blocked by the clouds.
While they may appear to radiate from the sun or moon, the beams are actually all parallel to each other. We see them come to a point the same way two parallel railroad tracks appear to come to a point in the distance. The photo from orbit reveals the true nature of the phenomenon by seeing it anew from a different perspective.
While the ISS works the evening shift, China's Tiangong space station, presently home to three astronauts, plies the sky at dawn. It's considerably smaller than the ISS (but growing), so it's a couple magnitudes fainter. On good passes, it's can rival Vega or Saturn.
And while the steeply inclined orbit of the ISS (51.6°) makes it an easy catch from higher latitudes, the lower 41.5° inclination of Tiangong means it doesn't climb as high in the sky for skywatchers in the northern U.S.
To find out when and where to see either the ISS or Tiangong, go to heavens-above.com and select your city by clicking on the blue Change your observing location and other settings link. Then return to the home page and click on either the blue ISS or Tiangong link to see a 10-day table of passes that includes time, direction, brightness and altitude. Ten degrees (10°) of altitude is equal to one fist held at arm's length against the sky.
The higher the negative number in the brightness column, the brighter the pass. Click on any pass time and a map will appear showing the station's path across the sky. All times shown are local times on the 24-hour clock for your location, so 18:30 = 6:30 p.m. local time and 2:15 = 2:15 a.m.
You can also get a list of customized passes and alerts by downloading the free ISS Spotter app for iPhone and ISS Detector for Android devices. Or you can sign up for alerts at spotthestation.nasa.gov .