Why Do This From Space?
Let's get this out of the way from the start: doing astronomy from
space is much harder than from the ground. You can't go up there to
fix your telescope if a part fails, for example, and the size of the rocket
limits the size of the telescope.
Ground-based image (left) and Hubble image (right) of
the crowded star field in the cluster 30 Doradus. Note the number
of stars visible in the Hubble image.
But the advantages far outweigh the problems. For one thing, in space
there is no air. This is a huge advantage, in fact. If you've ever
looked down a street on a hot summer day, you can see distant objects
appear to shimmer and dance, even though you know they're motionless.
This shimmering is due to the hot air from the pavement rising and
distorting the light from more distant objects. The Earth's atmosphere
does the same thing to the light from astronomical objects. Through a
ground-based telescope, the targets appear to constantly move, blurring
them. In space, with no air, that's not a problem. Objects appear rock-solid.
This means two objects spaced closely together can be distinguished
from each other instead of being blurred together. This is important in
separating a distant supernova from its host galaxy's light, or
gravitational lensing of narrowly separated galaxies.
In this Shuttle image of the Earth at night, the "airglow" is
visible as the luminous arc across the frame.
Another reason is that the Earth's atmosphere absorbs some kinds of infrared
light. When a star explodes, observations of it
using visible light can tell astronomers quite a bit about it, such as the
chemical elements in it. This can, in turn, yield
critical information needed
to use the supernova to measure dark energy. However, very distant
supernovae will be
the visible portion of the spectrum to the infrared-- which will
then get absorbed by the Earth's air. A telescope on the ground
cannot clearly see the supernovae at this critical distance.
A third reason is that although it looks transparent to the eye,
the Earth's atmosphere
actually does glow faintly, and at infrared wavelengths the
glow is very bright. This background glow can swamp the light from
very dim objects-- and dim objects are what we need to observe to
categorize dark energy! Again, from space, fainter objects are far
easier to observe.
Diagram showing the absorption of light by the Earth's atmosphere. Note
that infrared and ultraviolet are absorbed well above the ground; spacecraft
are needed to see these wavelengths. Credit: NASA
So the advantages of getting above the Earth's interfering atmosphere
are not only important, they are vital. The kind of work SNAP will
do cannot be done from the ground.
Another key issue is that a space telescope
can observe nearly 24 hours a day, which a ground-based telescope cannot do.
Coupled with the other factors, SNAP will be able to observe
far fainter objects than ground-based telescopes many times its size.
In fact, it will do far better even than Hubble: it will observe
a patch of
sky 9000 times the size of the
Hubble Deep Field (one of the deepest images
ever taken of the Universe), observe it in more colors,
and still be able to see much fainter objects
than Hubble could -- as little as one-quarter as bright as the faintest
objects in the Deep Field.
To tease out the faint, tiny details of dark energy from the Universe,
SNAP is the right tool for the job.