Astro Bob: It's space station season again!
Both the International Space Station and China's Tiangong are making convenient evening flybys. Can you spot them simultaneously?
I don't mind getting up before dawn to observe the sky, but I'm more of an evening person. A friend of mine greets the day around 4 a.m. with quietude and a cup of coffee. Sounds like a really lovely way to begin. Maybe I'll get there someday. For now, I fall asleep around midnight and roll out of bed at 8.
If your day-night cycle is like mine we're going to see a lot of space station passes together in the next few weeks. And not just the ISS. The completed Chinese station Tiangong (Heavenly Palace) will also join the scene. Both of these science and observation platforms orbit at approximately the same altitude of 250 miles (400 km).
The ISS circles the Earth on a more steeply inclined orbit, so it's visible across more of the planet. For the same reason it can appear high in the sky even for observers living in the northern U.S. or southern Australia. Tiangong's orbit is less steep, so it doesn't reach as high an altitude. Here in Duluth, we only ever see it in the bottom third of the southern sky, but if you live in Jackson, Mississippi, it can pass nearly overhead.
Tiangong is also fainter because it's considerably smaller, measuring 180 by 128 feet (55 x 39 meters) or about half the size of the ISS. Being smaller it reflects less sunlight. The ISS is so enormous that its solar panels and other modules are clearly visible through a telescope magnifying 50x or more once you get a fix on it.
For observers in the northern states the Chinese satellite also appears fainter because lower passes mean there's more horizontal distance between the observer and the station. The more distant something is, the dimmer it appears. During a high pass in Jackson Tiangong shines as brightly as Jupiter. In Duluth it's about equal to Saturn.
Since the viewing "windows" for both space stations overlap, I thought it would be fun to see if they would cross the sky simultaneously. A quick check revealed a nice pair of passes for the Duluth region on Jan. 21. Tiangong will slide across the southern sky from 6:28-31 p.m., while the ISS crosses the northern sky from 6:29-32 p.m. That minute-plus overlap will make it possible to briefly see both at the same time by tilting one's head back and forth.
You can find out if there are overlapping passes for your location by going to the heavens-above.com website. Select your city by clicking the blue Change your observing location and other settings link. Then return to the home page and click the blue Tiangong link to open a 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. You can also click on the blue ISS link for a similar table.
The bigger the negative number in the brightness (magnitude) column the brighter the pass. The bigger the positive number the fainter. A magnitude of "0" sits on the continuum from brighter to fainter. For example, the star Vega has a magnitude of 0.
Click on any pass time and a map will appear showing the station's path across the sky. The 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.
Next, compare the Tiangong pass times with those for the ISS and check for overlaps or closely-spaced times when both will be visible. It's as easy as that.
You can also use astroviewer.net for a list of passes, times and directions to look for both the ISS and Tiangong. While the site doesn't provide maps it's extremely easy to use. Just key in your city's name (or the nearest larger city) and hit enter. The link above will take you to Tiangong. For an ISS list, click on the white Observation ISS link while you're there.