Long exposure night sky photography can capture the beauty of the cosmos, whether it depict stars, solar system objects, the Milky Way or deep sky objects such as galaxies and nebulae. Sometimes though, such images include streaks of light that were unexpected. In some cases, we’ve luckily captured a speck of dust streaking through Earth’s upper atmosphere, glowing as it burns up, what we know as meteors. But often that streak across the photo is something else. For example, what about this one?
 |
| Photo by Nathan Eaton |
Anyway, below I’ll share examples of other types of objects that may show up in night sky images and ways to tell when you’re seeing them.
First, a distinction to make. My focus here is on things that appear in night sky images taken with exposures at least a few seconds long. Obviously, when viewing the night sky directly, if you see a streak of light across the sky it’s typically going to be a meteor while the other objects I’ll discuss would appear to the eye as a relatively slowly moving point (or points) of light. It’s when these things are photographed over the length of seconds or minutes that they appear as a streak of light.
Meteors
 |
| A bright meteor
passing below the Milky Way Photo by Martha Dean via EarthSky.org |
Although many streaks of light on long exposure images are taken for a meteor at first glance, there are a few key characteristics that can help confirm if that’s what you’re looking at. Generally, meteors in photographs (individually or from a meteor shower) usually:
- appear wider in the middle narrowing to a point on each end,
- may vary in color from end to end depending on the chemical elements released as the meteor burns up,
- in some cases may have a wispy, smokey appearance along its length,
- due to how fast they move, appear in just one frame of a sequence.
Of course there are exceptions. You might find that a streak that otherwise appears to meet the characteristics listed above for a meteor but has a blunt end. In this case, your exposure may have started or ended during a meteor mid-flight. Another possibility is when the meteor is a bolide or a fireball. In this case, the streak will grow much larger and brighter at one end. It may also last long enough to appear in more than one frame of a sequence.
Aircraft
 |
| Stack of 8 second exposures capturing DFW-area
aircraft Photo by Nathan Eaton |
Of all the things that might be mistaken for a meteor in a night sky photo, aircraft are usually the easiest to exclude. We have a number of things going for us in identifying when that streak of light is an aircraft:
- they most often have multiple lights so on close inspection the streak is actually multiple streaks,
- the different lights on aircraft are different colors, usually a combination of white, red and green,
- the navigation lights (red on left wing, green on right) blink which will appear in a long exposure image as series of dots and/or dashes,
- if flying at high altitude where you're unable to distinguish separate streaks, there will still be dots as the navigation lights blink on and off in flight,
- in image sequences, an aircraft will appear in multiple, consecutive images (though as in the image above, not always in a consistent direction).
Satellites
 |
| Starlink Satellites Photo by Jason Parnell-Brookes |
With the dramatic increase in satellites launched into Earth orbit in recent years (nearly 38% between early 2022 and mid-2023), capturing a satellite in a long-exposure photograph of the night sky is becoming more and more likely. Although they vary in speed, altitude and direction, they often appear as solid streaks of light having a consistent brightness and color front end to end and the ends appear blunt rather than tapered like a meteor. As with aircraft, in image sequences a satellite will typically appear in multiple, consecutive images (though in a consistent direction).
Satellites show up to us on the ground not because they have their own lights (like aircraft) but because they reflect sunlight. Sometimes, as a satellite passes over, the angle of its reflective surfaces may change, causing a brightening along part of the streak it leaves in your photograph.
Another characteristic of satellites: when they move into or out of the Earth's shadow, they no longer reflect sunlight. Instead of having a blunt end (caused by the abrupt start or end of your exposure), the end may taper. This can happen on either or both ends. Although this may look similar to the tapered ends of a meteor, a satellite trail will typically be longer, appear in multiple frames, and lack the distinctive color associated with meteors.
These days, that brightening is likely to be relatively minor. Prior to 2019, a constellation of 66 Iridium satellites were still in operation. Having highly reflective surfaces they commonly produced a streak of light in the sky called an Iridium flare, visible not just in photos but to viewers on the ground, much like a meteor. Often occurring shortly after sunset, they were bright enough to be visible even in the twilight sky. Since the last Iridium satellite was decommissioned in 2019 (burning up in the Earth’s atmosphere), there aren’t any other satellites that produce streaks of light across the sky quite so brightly.
That said, today there is a new type of satellite that may make a big appearance in your night sky images: Starlink. Although individual Starlink satellites would appear as any other satellite once on-station 342 miles up, two factors make Starlink more likely to show up:
- sheer numbers - as of last month, there are over 5000 of them in low Earth orbit (out of a grand total of about 12,000 total satellites in orbit, active or inactive),
- the number launched at once, usually about 20 or so at a time - within the days after a launch and before they achieve full orbit, all of a group of Starlink satellites launched together are flying in a line, appearing in the sky as a series of dots, often referred to as a “train”.
The good news is that there are tools that can help identify whether that streak in your photo is a satellite. My favorite is Stellarium (available as free open source software for Windows, Mac and Linux or as paid software for iOS, Android and Symbian). By setting the location where your shot was taken and the date/time of the image in Stellarium, you can zoom in on the area where the streak appears in your image and increase the time rate then watch for what crosses the same section of the sky. Sometimes it’s difficult to find what it was but I can usually confirm there was a satellite and which one it was. Another useful tool is Heavens Above.
While those tools are great for looking back in time at when/where a satellite was visible or for looking ahead at when they might be visible, another tool you can try if your goal is finding when you can see a Starlink satellite, give the SpaceX Starlink Satellites Tracker a try. Thanks to Stephen Buck for his comment on the Facebook group Astrophotography for Beginners for that link.
Space Stations
 |
| Stack of 8 second exposures of ISS fly-over Photo by Nathan Eaton |
For the most part, craft like the International Space Station (ISS) would appear in a long exposure night sky image as a long streak similar to any other satellite. In the case of the ISS and eventually the China space station Tiangong, given their large size relative to other satellites they can sometimes appear brighter and result in a wider streak across your photo. Similar to other satellites, as a space station passes overhead it may pass into the Earth’s shadow before reaching the horizon. When this is captured in an image such as the one above, it may appear to fade out, perhaps even with a tapered appearance, rather than the light streak having a blunt end representative of your camera’s shutter closing.
Another similarity space stations have to other satellites is that their orbits are well understood. This makes their appearance predictable. There are many tools available to forecast these appearances including NASA's Spot the Station. One of my favorites is Heavens Above. With it, you can see when various satellites and space stations are expected to be visible from your location.
Variation on a Theme: Solar and Lunar Transits
 |
| ISS Transiting the Moon Photo by Andrew McCarthy |
If having the ability to tell where satellites and space stations are in the sky piques your interest beyond just figuring out what that light streak across your long exposure night sky photo is, if you ever take solar or lunar photos, then another tool you should check out is Transit Finder. With it, you can identify when the ISS, the Hubble Space Telescope (HST) and other large craft in space will fly across the face of the Sun or Moon, giving you a chance to try your hand catching them in a photograph. Follow the link of the image above for information on what's involved.
The journey continues beyond 2009... check it out!
I've recently found a similar post on photographer Alyn Wallace's blog. It's worth a read.
ReplyDeleteLink