> ...why have a satellite waste time looking at the earth when it's dark? ... Exactly. Many NASA Earth-observing satellites use the same kind of sun-synchronou

> ...why have a satellite waste time looking at the earth when it's dark? ... Exactly. Many NASA Earth-observing satellites use the same kind of sun-synchronous orbit (https://en.wikipedia.org/wiki/Sun-synchronous_orbit https://en.wikipedia.org/wiki/Sun-synchronous_orbit). (One of several exceptions are the instruments opportunistically mounted on the ISS, which has its own orbit, as you note.) One of these sequences is called the A-train (https://atrain.nasa.gov https://atrain.nasa.gov) - that's what OCO-2, discussed in OP, is part of. Having it be near noon local time (1:30pm, say) when you pass over every patch of ground has great advantages for some instruments. The sunlight passes through less atmosphere, and does so in "the same way" for most passes. So you're not seeing some places at local dawn and others at local dusk. This makes the observation less sensitive to the difficulties of atmospheric modeling. (Because you need a good atmospheric model to retrieve the solar induced flourescence [SIF] signal.) OTOH, the sun-synchronous orbit does restrict your observations to specific times. The companion instrument, OCO-3 (which is mounted on ISS), plays a special role here. OCO-3 can provide information about the daily cycles of plants that would be undersampled by a consistent 1:30pm arrival. * Separately from orbits: The discovery that you could obtain insights on plant processes at global scales from SIF using the OCO-2 measurement has been a really big deal for Earth science.