We're conducting this survey as part of a probabilistic risk assessment of the potential hazards presented to the general public by spaceborne lidar systems. NASA and its partners (including CNES and ESA) use lidar on satellites to make measurements of Earth's surface and atmosphere. Examples include CALIPSO, CATS, ALADIN, and GLAS. Lidar works on the same time-of-flight principle as radar, but using a laser instead of a beam of radio radiation.
The lidar lasers on satellites greatly exceed eye safety standards for observers on the ground for direct, unaided viewing of the laser beam. But, with sufficiently powerful light-collecting optics, it would, in theory, be possible to exceed the retinal damage threshold. So please, NEVER LOOK DIRECTLY INTO A LASER WITH YOUR EYE. We really don’t want to injure anyone! To that end, we're attempting to calculate the odds of one of these lasers causing injury to make sure the risk has been mitigated to an acceptably low level (e.g. lower than the odds of being injured by a piece of orbital debris from the satellite itself when it reenters Earth’s atmosphere someday, which is another risk we must calculate and mitigate to internationally-agreed-upon acceptably low levels).
We’ve so far done our best to model the problem, but many of our input parameters are only educated guesses. For instance, while we can calculate the energy per area that would result from standing in the laser footprint and viewing the beam through a given size and type of telescope, we don’t know how many such telescopes are in use at any given time. Larger telescopes carry a higher risk of injury, but we suspect larger telescopes are less common since they cost more. And if someone is doing all-night observations, we suspect it’s more likely that they’re using a sensor/camera rather than their eye, e.g. for recording star trails or doing long exposures of faint objects.
The particular orbital parameters of our various satellites also affect our modeling. A laser on the ISS has a non-repeating ground track that paints the mid-latitudes but never the polar latitudes and can pass overhead at different times of night. The A-Train constellation of satellites, on the other hand, has a repeating ground track, flies in a solar-synchronous orbit, so it paints all latitudes and always passes overhead at approximately the same time of night. So, the odds of injuring someone depend on things like the percentage of people still out observing at 2 am and whether active astronomers are distributed by latitude in proportion to the overall population.
If you decide this is a phenomenon you want to see for yourself, be sure to make your attempt using a camera rather than your eye; if you are successful, not only will you avoid the risk of retinal damage by using a camera, you'll also have an image to prove you saw the laser flash. A picture is much more convincing than, "Hey guys! I saw a flash of light from space!" And if you're feeling generous, we'd love it if you'd share your images with us. See contact information below.
Definition of Acronyms and Links to More Information:
Image of lidar pulse captured by Gregg Hendry of Ball Aerospace
https://www.nasa.gov/larc/calipso-laser-flash
https://www.facebook.com/nasalarc/ph...3964850983168/
CNES: Centre National d'Études Spatiales (the French government space agency)
ESA: European Space Agency
CALIPSO: Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation.
CALIPSO website:
https://www-calipso.larc.nasa.gov/
CATS: Cloud-Aerosol Transport System
CATS website:
https://cats.gsfc.nasa.gov/
ALADIN: Atmospheric LAser Doppler INstrument
ALADIN website:
http://www.esa.int/Our_Activities/Ob...Aeolus/Payload
GLAS: Geoscience Laser Altimeter System
GLAS website:
https://attic.gsfc.nasa.gov/glas/