Large Constellations of Satellites and Optical Astronomy

There are currently over 20,000 artificial objects in Earth orbit larger than 10 cm. These frequently leave trails in astronomical images. This population has started to change dramatically with the launch of the initial Starlink (1584 satellites) and OneWeb (648 satellites) constellations into low Earth orbit (LEO). These new satellites will be brighter than 99 percent of all objects in Earth orbit and may be visible to the unaided eye. Future constellations include the Amazon/Kuiper constellation of 3236 satellites. There are three phases of such a constellation: launch and checkout, operational phase where the satellite is in its final circular orbit, and the deorbit phase. All of the reports of very bright Starlinks (the ‘strings of pearls’) have occurred in the first phase, but the effect on astronomy of the other two phases need to be considered as well. I’ll show examples of the effects of bright satellites in Dark Energy Camera images obtained on the Blanco 4-m telescope. For a satellite to be visible by reflected sunlight the observer must have a dark sky and the satellite must be in sunlight. When and for how long this happens depends on the observatory latitude, time of year, orbital altitude, and orbital inclination. I have modelled when and where these satellites are visible for a specific observatory location and time of year by propagating the actual orbits of the satellites in the above constellations. Several general conclusions for the steady state configuration of a constellation are: 1. Only a small fraction of a constellation is visible above 30 degrees elevation at any one time. But this number could be very large if 10s of thousands of new satellites are launched. 2. Since the Earth’s shadow is a cone, satellites at a higher altitude are visible longer into the night than ones at a lower orbit. During summertime, for example, satellites at 1200 km can be visible during the entire night at a mid-latitude observatory such as Cerro Tololo or Kitt Peak. 3. Even though the satellites are streaked in astronomical images, the streaks can be saturated in astronomical images obtained by large facilities such as the Vera Rubin Observatory (formerly the LSST). I’ll show how you can compute whether a satellite of a given brightness and orbit will saturate your particular project.