Watching Earth from space
For close to three years now, the Kepler spacecraft has been staring into one region of space, a part of our galaxy that very much resembles our own neighborhood. Within that region, Kepler is searching for planets like Earth.
Kepler is the first in a series of spacecraft that will stare into this region of space. Future spacecraft will focus on Kepler’s list of Earth-like planets, looking for those that possess atmospheres and oceans. Further in the future, spacecraft will search these moist, airy planets for signs of life.
Hundreds, thousands of light-years away, these planets are too distant to visit and study close-up. What we can study is the light from their stars that reflects off their surfaces, off their clouds, that shines through their atmospheres. As it reflects off an object, as it travels through the air, light is altered (think, for example, of light coming through stained glass). Sometimes the alteration is so distinctive, it becomes a signature for the material it’s bounced off, or passed through.
Plants eat sunlight … red and blue light are especially tasty, absorbed by chlorophyll to energize the chemistry that joins carbon dioxide and water to make sugar.
Leaves eat green light, too, but not so completely. Some green-hued leftovers – leftover light – escapes, which is why leaves look green.
At a glance, we might interpret anything green as vegetation. But other green things exist: The aptly-named mineral, olivine; the fungus penicillium, turning bread and oranges moldy; artificial turf on a football field.
Sound waves – music – are vibrations in the air. High notes vibrate fast, low notes, slow.
Light is a form of electromagnetic radiation: Vibrating electromagnetic waves. Violet waves vibrate fast, red waves more slowly. Much as we can move down a musical scale, from high notes to low, we can move down the electromagnetic spectrum, from high frequency to low: Violet to blue, blue to green, green to yellow, to orange, to red.
Don’t stop there. Dropping down below red, we come to infrared: IR (“infra” means “below”). If we drop only a hair below red, we’re in the near-infrared (“near” as in close to red): NIR. Near-infrared is invisible to our eyes, but visible to the detector on the TV that “sees” the NIR signal from the remote.
Near-infrared is invisible to plant leaves, too – chlorophyll doesn’t absorb it. But the sun emits a lot of it, and, were it to soak in – just as sunlight soaks into a car seat – the leaf would be cooked.
The outer “skin” of a leaf reflects away some NIR, but most NIR goes right through, into the body of the leaf.
A leaf is a complex structure. On its upper surface, cells grow in closely-packed rows, catching as much sunlight as possible. But in the heart of the leaf, the cells grow in clumps, with lots of air between clumps. The arrangement acts like a funhouse hall of mirrors – NIR bounces hither and yon, until it bounces right back out of the leaf.
Leaves are intensely green, but they glow even more intensely in the near-infrared. We can’t see that glow, but NIR sensors – cousins to the sensor on a TV – can see it. That NIR glow distinguishes green vegetation from other things green.
For four decades, Landsat spacecraft – most recently, Landsat 7 – have been observing sunlight reflected from Earth, watching the evolution of forests and deserts, farms and cities.
Space is a harsh environment. If Landsat 7 were to die before its replacement was launched, it would be hard to know if an apparent change on Earth’s surface was real … or just the result of a new spacecraft seeing things differently.
I’m heading down to Vandenberg Air Force Base, this weekend, to observe the launch of the spacecraft that, upon reaching orbit, will become Landsat 8. I’ll be talking about the launch, and the spacecraft, at noon Feb. 19, on KVMR.
Al Stahler’s science programs can be heard on KVMR (89.5 FM). He teaches classes to students of all ages, and may be reached at firstname.lastname@example.org
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