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.

image of a ground based and Hubble image of a star

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 observing gravitational lensing of narrowly separated galaxies.

image of the Earth at night showing the airglow

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 redshifted, shifting 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.

Quick Links

Why Do This From Space?