This story was updated at 5 p.m. EDT.
SpaceX made history with its first launch of NASA astronauts into orbit Saturday (May 30) on a private Crew Dragon spacecraft, weather permitting, you may be able to spot them in the night sky tonight.
A SpaceX Falcon 9 rocket launched astronauts Bob Behnken and Doug Hurley on a test flight, called Demo-2, that will take 19 hours to reach the International Space Station (ISS). Their Crew Dragon spacecraft will dock with the station on Sunday morning (May 31).
If the skies are reasonably clear Saturday evening, residents living across the central and northern United States, as well as southern Canada, will be in for a real treat as they will get an opportunity to see Crew Dragon move across their local sky.
And as a bonus, roughly 6 to 10 minutes later, skywatchers will have a chance at sighting Crew Dragon's target: the ISS.
You can watch the Demo-2 mission live here and on Space.com's homepage, courtesy of NASA TV, through docking.
Full coverage: SpaceX's historic Demo-2 astronaut launch explained
This is a sight that should easily be visible to anyone, even from brightly lit cities. The appearance of either the International Space Station or the Dragon capsule moving across the sky is not in itself unusual.
Truth be told, on any clear evening and with no optical aid, you can usually spot at least one or two Earth-orbiting satellites, and sometimes as many as a half dozen, creeping across the sky like moving stars. Satellites become visible only when they are in sunlight and the observer is in deep twilight or darkness. This usually means shortly after dusk or before dawn.
What makes this Saturday's passage so interesting is that you'll be able to see the very first crewed commercial spaceflight in the evening sky, followed shortly by the largest orbiting space vehicle. The two will appear widely separated when they move across the sky, but in the hours that follow, the large gap between them will close.
If all goes according to plan, the SpaceX capsule will dock with the ISS on Sunday at 10:29 a.m. EDT (1429 GMT) to the Harmony module’s International Docking Adapter.
Related: How SpaceX's Crew Dragon Demo-2 mission will work in 13 steps
There will be three overflights of Crew Dragon and the ISS over the central and northern United States and southern Canada on Saturday evening. Each overflight will be separated by roughly 90 minutes, with both vehicles moving at 17,150 mph (26,200 km/h) at an altitude of about 260 miles (418 kilometers), and all three passes will take northwest-to-southeast trajectories. Many localities should be able to catch a glimpse of at least one, and possibly even two, of the ISS passes.
You can see the ISS' location and speed here, while this link provides live high-definition views of Earth from the space station.
Unfortunately, for much of the southern third of the United States, no sightings of Crew Dragon will be possible, either because the space vehicle will be in Earth’s shadow when passing overhead and therefore will not be illuminated by sunlight, or it will be too close to the horizon to be easily seen.
Related: How to spot the International Space Station, Hubble & more
Differences in brightness
Because of its size and configuration of highly reflective solar panels, the ISS is, by far, the brightest human-made object currently in orbit around Earth. On its most favorable passes, it can become as brilliant as magnitude -4.0, which approaches the planet Venus in brightness and is nearly 10 times brighter than Sirius, the brightest star in the night sky. Some observers have even caught a glimpse of the ISS just prior to sunset or shortly after sunrise. And as a bonus, sunlight glinting directly off the solar panels can sometimes make the ISS appear to briefly flare to super-brilliance.
The Crew Dragon capsule, on the other hand, is much smaller than the space station, measuring about 26.7 feet (8.1 meters) high and 13 feet (4.4 m) in diameter. The capsule's solar arrays are mounted on the body of the spaceship’s unpressurized trunk. The trunk is attached to the aft end of Crew Dragon’s pressurized crew cabin, and a thermal radiator sits on the opposite side of the cylindrical trunk from the solar arrays.
Consequently, in terms of overall brightness, Dragon will appear much dimmer compared to the ISS, perhaps appearing roughly as bright as a second-magnitude star. But that still can be categorized as "fairly bright." For instance, Polaris, the North Star, is a second-magnitude star.
But, for perspective: the ISS is about 250 times brighter than Polaris!
When and where to look
So what is the viewing schedule for your particular hometown? For Crew Dragon as well as the ISS, you can visit Heavens-Above.com to find out.
That website is dedicated to helping people observe and track satellites orbiting Earth without the need for optical equipment such as binoculars or telescopes. It provides detailed star charts showing the trajectory of the satellites against the background of the stars as seen during a pass. Special attention is paid to the ISS, SpaceX's Starlink internet satellites and others. Users click on a map of the world to set their viewing location. Lists of objects, their brightness and the time and direction to look to see those objects are given.
Preliminary orbital elements for Crew Dragon during Demo-2 have been provided by Netherlands-based satellite tracker Marco Langbroek, enabling Heavens-Above to provide an advance look at what locations will have a chance at making a sighting.
"I have seen no sub-minute launch times directly from SpaceX or NASA anywhere yet, so treat this cautiously, as without an authoritative source this remains rather apocryphal," Langbroek noted.
A more accurate set of orbital elements will hopefully will become available soon after launch, so check frequently late Saturday afternoon and early evening for updates.
For a viewing schedule for the ISS, go to NASA's Spot the Station site.
Spot the Station will ask for a 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.
Finally, another great site is N2YO, which provides real-time satellite tracking and shows you at any given moment during the day or night over what part of the Earth the ISS or Crew Dragon happens to be.
If the weather forces a postponement of the launch of the Falcon 9 rocket on Saturday, it will be rescheduled for Sunday (May 31) at 3 p.m. EDT (1900 GMT).
Visit Space.com for complete coverage of SpaceX's Crew Dragon Demo-2 flight.
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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Follow us on Twitter @Spacedotcom and on Facebook.
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Wouldn't a skywatcher see the ISS first followed by the crew dragon?