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. I personally use Heavens Above and search for “CALIPSO” (or “ISS”) in the satellite database to try and find any near-zenith passes over my location.
LINKS TO MORE INFORMATION (and Definition of Acronyms):
Image of lidar pulse captured by Gregg Hendry of Ball Aerospacehttps://www.nasa.gov/larc/calipso-laser-flashhttps://www.facebook.com/nasalarc/photos/a.115596668486660.6213.115589531820707/823964850983168/
CNES: Centre National d'Études Spatiales (the French government space agency)
ESA: European Space Agency
Heavens Above website for finding satellite trajectories:http://www.heavens-above.com/Satellites.aspx
SATELLITES WITH LIDAR INSTRUMENTSThere are two satellite-borne lidars currently on-orbit: CATS and CALIOP, which are flying on the ISS and CALIPSO respectively. I’ve listed them here, along with other planned future missions (listed as satellite name / lidar instrument’s name, if different) and one past mission: ISS / CATS (Cloud-Aerosol Transport System) o 1064 nm, 532 nm, 355 nmo https://cats.gsfc.nasa.gov/o currently orbiting
CALIPSO / CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization)o 1064 nm, 532 nmo https://www-calipso.larc.nasa.gov/about/o downloadable ASCII files of the actual historical ground track: https://www-calipso.larc.nasa.gov/tools/overpass/coords/o currently orbiting
ADM-Aeolus / ALADIN (Atmospheric LAser Doppler INstrument)o 355 nmo https://en.wikipedia.org/wiki/ADM-Aeoluso planned launch in 2017
ICESAT-2 / ATLAS (Advanced Topographic Laser Altimeter System)o 532 nmo https://icesat-2.gsfc.nasa.gov/o scheduled for launch in 2018
EarthCARE (Clouds, Aerosols and Radiation Explorer)o 355 nm o https://earth.esa.int/web/guest/missions/esa-future-missions/earthcareo scheduled for launch in 2018
GEDI (Global Ecosystem Dynamics Investigation)o 1064 nm o https://science.nasa.gov/missions/gedio planned for 2019
MERLIN (Methane Remote Sensing Lidar Mission)o 1.6 micrometers o http://www.epj-conferences.org/articles/epjconf/pdf/2016/14/epjconf_ilrc2016_26001.pdfo scheduled for the 2021 timeframe
MESCAL (Monitoring the Evolving State of Aerosols and Cloud Layers)o future mission
ICESat (Ice, Cloud, and land Elevation Satellite) / GLAS (Geoscience Laser Altimeter System)o 1064 nm and 532 nmo https://attic.gsfc.nasa.gov/glas/ o launched in 2003o rentered Earth's atmosphere in 2010