During this upcoming week, skywatchers across most of the U.S. and southern Canada will get an opportunity to view the two largest space vehicles now in orbit around the Earth within minutes of each other.
They are the International Space Station (ISS) and China’s space station, Tiangong. If you are up during the predawn hours, you’ll probably be able to make a sighting of both within less than a half hour of each other.
And on the morning of July 5, it may be possible to see both in the sky at the same time.
Night lights
If you go out and carefully study the sky near dusk or dawn, the odds are that you should not have to wait more than a few minutes before you see one of the more than 30,700 satellites now in orbit around Earth.
Most of these are just “space junk” ranging in size from as large as 30 feet (9 meters) down to about the size of a softball, including defunct satellites, rocket bodies, and fragments from collisions. In addition to the tracked objects, there are an estimated 130 million pieces of debris too small to be tracked. U.S. Space Command, headquartered at Peterson Space Force Base in Colorado Springs, keeps a constant watch on all orbiting space debris.
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Most satellites are too faint to be seen with the unaided eye. But depending on who’s counting, about 500 can be seen with the unaided eye. These are the satellites that are large enough  (over 20 feet, or 6 m) in length) and low enough  (100 to 400 miles, or 160 to 640 kilometers) above Earth to be most readily seen. In his book Observing Earth Satellites, the distinguished British scientist Desmond King-Hele perhaps said it best: “A satellite looks like a star that has taken leave of its senses and decided to move off to another part of the sky.”
The International Space Station is by far the biggest and brightest of all the man-made objects orbiting Earth. More than four times as large as the defunct Russian Mir space station, the ISS has a total mass of approximately 925,000 pounds (420,000 kilograms), and measures 357 feet (109 m) across and 240 feet (73 m) long. This makes it almost the equivalent to the length of a football field, including the end zones, according to NASA.
The ISS draws power from four pairs of solar arrays, which are each 112 feet (34 m) long and 39 feet (12 m wide) — longer than a Boeing 777’s wingspan. Additionally, the ISS is being upgraded with six new, smaller Roll-Out Solar Arrays (iROSAs). These are being installed over portions of the existing arrays. Each new iROSA measures 63 feet (19 m) long by 20 feet (6 m) wide. Despite being smaller, they are more efficient due to technological advancements.
Presently circling Earth at an average altitude of 259 miles (416 km) and at a speed of 17,900 mph (28,800 kph), the ISS takes 93 minutes to orbit Earth and completes roughly 15.5 orbits per day. It can appear to move as fast as a high-flying jet airliner, sometimes taking about four to five minutes to cross the sky. Because of its size and the configuration of its highly reflective solar panels, the space station is now, by far, the brightest man-made object currently in orbit around Earth.
On its most favorable passes, it can appear some 2.5 times brighter than Venus and nearly 40 times brighter than Sirius, the brightest star in the night sky. And, as a bonus, sunlight glinting directly off the solar panels can sometimes make the ISS appear to briefly “flare” in brilliance to as bright as magnitude -8, some 19 times brighter than Venus!
China’s orbiting space station
Besides the ISS, there is another space station orbiting Earth. That’s Tiangong (Chinese for “Heavenly Palace”), a permanently crewed space station operated by China’s Manned Space Agency (CMSA). The chief purpose of Tiangong, according to the CMSA, is to develop and gain experience in spacecraft rendezvous technology and to serve the platform for the next-generation orbit transportation vehicles and technology for future deep-space missions.
Tiangong is smaller than the ISS, in overall dimensions measuring about one-fifth as large. Its total mass is approximately 77 tons (69,900 kg). It measures approximately 180 feet (55 m) in length and contains three modules (the ISS has 16 modules). It orbits Earth at an average altitude of 244 miles (393 km), somewhat similar to the ISS.
And, like the ISS, it is a prominent object when moving across the nighttime sky. On its most favorable passes, it can appear as bright as Venus (magnitude -4.1), though usually it tends to appear closer to magnitude -2 (as bright as Jupiter).
Currently, taking into account both the ISS and Tiangong, there are 14 humans in space.
Viewing windows of opportunity
From now through the final days of July, North Americans and Europeans will have many opportunities to see both the ISS and Tiangong flying over their homes, due chiefly to a seasonal circumstance. Nights are now the shortest, and the time that a satellite in low Earth orbit (like the ISS and Tiangong) can remain illuminated by the sun can extend throughout the night, a situation that can never be attained during other times of the year.
Because both space stations circle Earth about every 90 minutes on average, it’s possible to see them not just on one singular pass, but for several consecutive passes.
And from now through July 8, the ISS and Tiangong will be making morning passes — one to two hours before sunrise — within a half hour or less of each other. And on the morning of July 5, for parts of the northeastern U.S. and eastern Canada, it may be possible to see both in the sky at the same time.
This doesn’t happen too often, due to the fact that their orbital altitudes differ slightly, along with the fact that both space stations circle the Earth at different orbital inclinations: 51.63 degrees for the ISS, compared to 41.46 degrees for Tiangong.
Where and when should you look?
So, what is the viewing schedule for your particular hometown? You can easily find out by visiting one of these two popular web sites: Chris Peat’s Heavens Above or NASA’s Spot the Station.
With Heavens Above, you’ll be able to generate accurate sighting data for both the ISS and Tiangong (as well as other bright naked-eye satellites). You’ll simply need to input your location based on latitude and longitude.
NASA’s Spot the Station app is a great tool for quickly finding upcoming ISS viewing opportunities. Simply pop in the location you wish to know for ISS sighting opportunities and let the widget work its magic. It will tell you the time of the ISS flyover along with how long it is visible, the maximum height it will reach in the sky and which direction it will appear and disappear from your field of view.
Predictions computed a few days ahead of time are usually accurate within a few minutes. However, they can change due to the slow decay of the space station’s orbit and periodic re-boosts to higher altitudes. Check frequently for updates.
Joe Rao serves as an instructor and guest lecturer at New York’s Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope and other publications.
