Challenger, go at throttle up.
Roger, go at throttle up.
The disaster extended well beyond the manned space program. The Galileo mission to Jupiter was going to take Galileo into orbit where powerful rocket known as the Centaur-G would set it on a direct trajectory to Jupiter. New post-Challenger safety protocols and large schedule delays forced the Jupiter mission to use a less powerful booster. Getting to Jupiter now required an indirect path using gravity-assist flybys. First it aimed towards Venus and then made two close flybys of Earth gaining speed on each pass.
The delay and slower route frustrated many, but a bit of serendipity appeared. The idea came from Carl Sagan. As Galileo approached Earth its instruments were aimed Earthwards recording light reflected by the Sun. Would there be indications that life existed on Earth? Maybe it would be a clue to how we should be looking.
In fact four signals indicated there was life on Earth. The strongest indicator came as something of a surprise - a dramatic change in the reflectance of near infrared light. It turns out plants strongly absorb light in the visible part of the spectrum. We see a lot of green because photosynthesis doesn't use it so plants reflect some, but for the most part plants are roach moteles for visible light. At slightly longer wavelengths - moving from visible red to the near infrared - plants become more fussy. Having no use for these longer wavelength photons, they bounce them back. The result is a huge increase in reflectance at around 750nm (visible red light's wavelength is around 650 nanometers). Our planet has so much vegetation that the signal was easily spotted by Galileo as it sped by. Note that many photos taken in the infrared often color code near infrared as red - the "red" in this false color photo by Trev Lea is the type of near infrared light Galileo would have recorded.
The spectral feature became known as the red edge and sparked a lot of thinking about astrobiology. If a planet had life with an Earth-like photosynthesis, you could scan for a red edge as it turns out to be unlikely that other sources could generate such a feature. Of course extraterrestrial light harvesting life doesn't need to use light to make carbohydrates, but one can image photosynthetic processes with their own "red edges" at different wavelengths. It is possible that some of this life may not get energy from light and that creates other challenges. Astrobiology became a serious discipline.
If extraterrestrial life exists, it seems likely "dumb" lower life forms will be much more common that intelligent life trying to signal or visit us. Other than waiting for radio or physical contact, signatures in planetary atmospheres is one of the main tools we have until we develop interstellar travel. But you have to start somewhere ... like finding planets in other solar systems. But even the closest stars are a long way away and are much brighter than any orbiting planets. Fortunately people are clever and a few methods have been developed.
The first method is to recognize planets don't orbit stars. Rather stars and planets both orbit the barycenter - the center of mass - of their solar system. The Sun has a bit less than 99.9% of our solar system's mass, so the barycenter is close to it. Jupiter's mass is about one thousandth that of the Sun everything else is about a third of a Jupiter. Our solar system's barycenter wanders around a bit and is usually inside the Sun. If you were to look at the Sun from a distance over time you'd notice it wobbles a bit. You can detect the wobble using the doppler effect. When the wobble - the technical term is radial velocity - is towards you, its light is shifted a bit towards the blue end of the spectrum. Moving away it the light becomes a bit redder. You have to get lucky and stumble onto stars with wobbles along the direction between you and the star, but it doesn't need to be exactly lined up. The technique works best with huge planets in close orbits around small stars. Think of a Jupiter sized planet inside the orbit of Mercury. Usually they're really hot .. like thousands of degrees and are often called hot Jupiters.
Another technique is to watch a planet transit a star. Watching from Earth, some exoplanets will appear to pass in front of a star as they orbit. The star is so bright it washes out the planet completely, but it's apparent brightness dims a bit during the transit An alien observing the Sun from a great distance would see the Sun dim by about one part in a million the Earth transited the solar disk. Jupiter is so much larger that it would drop the Sun's apparent brightness by nearly one percent - an easy measurement for an alien with 1990 Earth technology. The real ticket is to observe from space. The Kepler Space Telescope bagged a few thousand exoplanets by staring at a small patch of sky for a few years watching for these minute dips in brightness.
While Kepler was great at finding new solar systems, the Hubble Space Telescope and the Spitzer Space Telescope would take a more detailed look at some of these candidates and wait for a transit to occur (many of these have planets that orbit their star quickly with years measured in days or a few weeks) During the transit some of the star's light goes through the atmosphere of the planet. It's a very delicate measurement, but it is possible to look at the chemical composition of some of these exoplanet atmospheres. Then, perhaps, you could look for signs of life. But the sensitivity isn't quite high enough to get too excited - yet.
Before talking about the signs of life that might be picked up and what might make a planet habitable, a bit on where we're headed. The James Webb Space Telescope and the Wide-Field Infrared Survey Telescope will extend sensitivity far beyond what we have now with Hubble and Spitzer. WFIST has a coronagraph - essentially it sticks its thumb in front of the star to create an artificial eclipse making it much easier to detect light from an exoplanet when it isn't in transit. Further down the road the ideas is to make a better coronagraph. The starshade is an umbrella-like objects tens of meters in diameter positioned about 50,000 kilometers in front of the telescope precisely blocking out light from the distant star cutting down even more on the glare from the star. Curiously the design is strongly influenced by the art of oragami.. a natural way to construct such an object.
A few other techniques to find exoplanets exit, but this should be enough for now. Now the task is to ask what do you look for and what are some of the characteristics life might require...
Out of time, so perhaps the next post.
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