If you watch the twilight sky from 45 to 90 minutes after sunset, or 45 to 90 minutes before sunrise, you may see a few "moving stars." They are most likely artificial satellites. The grandest of all is the international space station, and seasonal circumstances make now a great time for anyone to spot the orbiting outpost.
Satellites shine because they reflect sunlight. Their relative closeness to Earth dictates that some don't appear in our sky for long: A satellite entering Earth’s shadow immediately vanishes from view and pursues an unseen path until it again emerges into full sunlight, perhaps for some other skywatcher in another part of the world.
If you go out and carefully study the sky near dusk or dawn, you should not have to wait more than 15 minutes to see an Earth-orbiting satellite. And there are many.
The biggest satellite
About 5,000 payloads have been placed into Earth orbit. About 2,000 are still up there. Thousands of smaller pieces of junk — which you can't typically see — also circle the globe. Most are faint and rendered invisible. But depending on whose counting, anywhere from 100 to 300 satellites can be seen with the unaided eye. These are generally more than about 20 feet (6 meters) in length and anywhere from 100 to 400 miles (160 to 640 kilometers) above Earth, a region called low-Earth orbit.
The international space station is by far the biggest and brightest hunk of technology circling the planet. Its solar arrays span 240 feet (73 meters). The main modules are together 146 feet (45 meters) long. It stands as tall as a nine-story building and weighs 412,000 pounds (186,900 kilograms).
Traveling around the Earth at 18,000 mph (28,970 kilometers per hour) at an average altitude of 240 miles (386 kilometers), the station can appear to move as fast as a high-flying jet airliner, sometimes taking about four to five minutes to cross the sky. Appearing to the unaided eye as a point of light, it can easily be confused with aircraft lights.
The brightness of the station can be compared to stars and planets. Astronomers use an inverted scale. The dimmest objects visible to the human eye under perfect conditions are about magnitude 6.5. Most bright stars are in the range of 1st or 2nd magnitude. The brightest stars and planets are at negative magnitudes.
The visual magnitude of the space station is often listed at a brilliant minus 0.9, though at its very brightest it can exceed this level and sometimes appear to rival even Jupiter in brilliance.
Windows of opportunity
During the next couple of weeks, North Americans will have many opportunities to see the international space station, due chiefly to a seasonal circumstance. From now through the beginning of July, nights are shortest and the time that a satellite in a low Earth orbit (like the space station) 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 the station circles Earth about every 90 minutes on average, it’s possible to see it on several consecutive passes in a single night.
Because the space station revolves around Earth in an orbit that is inclined 51.6 degrees to the equator, there are two types of passes that are visible. In the first case (we’ll call it a "Type I" pass), the station initially appears toward the southwestern part of the sky and then sweeps over toward the northeast. About seven or eight hours later, it becomes possible to see a second type of pass (we’ll call it "Type II"), but this time initially showing up toward the northwestern and sweeping toward the southeast.
During mid-May, because of the shortness of the nights, North Americans will get a chance to see the station in a series of Type I passes after sunset in the evening sky, then see it again the following morning before sunrise, undergoing a series of Type II passes.
This weekend, only the Type II morning passes will be available for most North American locations. In some cases, the station might make as many as three such pre-sunrise trips. A good example is New York City, where, on Monday morning, the giant shining batch of metal will make three overflights beginning, respectively, at around 2:01 a.m., 3:35 a.m., and 5:10 a.m. ET.
Then, beginning Tuesday and lasting (depending on your location) until about May 14, it will be possible to see both Type I passes in the evening and Type II passes the following morning. For some locations, like Chicago on the night of May 13-14, there may be as many as five chances to see the station during a single night!
For much of North America, the prime viewing period for both evening and morning passes will be from May 12 through 14. After May 14 or 15, the window of opportunity for the Type II morning passes will close and only Type I evening passes will be possible, and then only for several more days thereafter.
Below are links to web sites that provide specific times to look for the space station. But first, some tips to help you find it:
- Some passes are superior to others. If the space station is not predicted to get much higher than 20 degrees above your local horizon, odds are that it will not get much brighter than second or third magnitude (10 degrees is roughly equal to the width of your fist held at arm’s length). In addition, with such low passes, the event will likely be visible for only a minute or two. Conversely, those passes that are higher in the sky — especially those above 45 degrees — will last longer and will be noticeably brighter.
- The very best viewing circumstances are those that take the station on a high arc across the sky about 45 to 60 minutes after sunset, or 45 to 60 minutes before sunrise. In such cases, you’ll have it in your sky upwards to four or five minutes; it will likely get very bright and there will be little or no chance of it encountering Earth’s shadow.
- While the space station looks like a moving star to the unaided eye, those who have been able to train a telescope on it have actually been able to detect its T-shape as its whizzed across their field of view. Some have actually been able to track the object with their scope by moving it along the projected path. Those who have gotten a good glimpse describe the body of the space station as a brilliant white, while the solar panels appear a coppery red.
- With evening passes, the station will usually start out rather dim, then tend to grow in brightness as it moves across the sky.
- With morning passes, the reflection of the sun will already be quite significant when it first appears and will tend to fade somewhat toward the end of its predicted pass. This is due to the change in the angle of sunlight hitting the vehicle.
- Remember that in certain cases, the space station will either quickly disappear when it slips into Earth’s shadow (during evening passes) or quite suddenly appear when it slips out of Earth’s shadow (during morning passes). This becomes increasingly more likely for passes that take place more than 90 minutes after sunset or more than 90 minutes before sunrise.
When and where to look
You can easily find best viewing times for your location by visiting one of these web sites:
Each will ask for your ZIP code or city, and respond with a list of suggested spotting times. 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 reboosts to higher altitudes. Check the sites frequently for updates.
The station is visible at other times during the year. And other satellites are worth noting, too. See Space.com's general guide to satellite spotting.