Spaced Out ::

I was asked in-class this week about imaging the Earth’s surface at night…… does it happen? The short answer is yes, it can, but generally doesn’t for much of the Earth-resource imaging that takes place. Here are some instances where it does (and I’m using examples from the fantastic NASA Earth Observatory Image of the Day):

Astronaut photo: plenty of examples using a standard digital camera with a short focal length, wide f-stop and high ISO. I like this one of northern Europe (Nikon D3S digital camera using a 28 mm).

Thermal Imagery: if there isn’t much visible light around, use a different part of the EM spectrum. Thermal wavelengths are commonly monitored, such as this image of a lava flow in Chile.

Microwave Imagery: thermal imagery uses naturally occurring raditation at longer wavelengths than visible light and the strong contrast (at night) to the surrounding terrain allows “hot” objects to be visible. Microwave, in contrast, is at much longer wavelengths (mm to cm) and uses energy generated by the satellite to illuminate the Earth’s surface. This allows it to operate at night (and through cloud). I couldn’t find any nighttime examples, but this one shows an oil spill off South Korea.

NPP Day/Night Mode: NPP VIIRS (see earlier blog) has a day-night band (wide bandwidth) and can therefore operate at night to image clouds (particularly when there is moonlight) and auroras (EOID explanation). A good image of Hurricane Sandy.

These are them most common nighttime imaging techniques you’ll come across, however most Earth surface remote sensing is achieved using a sun-synchronous orbit and so not at night.