In the little over one year that the Wide-field Infrared Survey Explorer (WISE) spacecraft surveyed the sky, it captured images of hundreds of millions of objects. Many of these were previously known stars, galaxies, and the like, but it also added a few newcomers to our catalogs, including a score of comets:
[Click to encomanate.]
Why did WISE find them, and not ground-based observers? Lots of reasons come to mind. Comets are not really the spectacular and brilliant objects commonly thought; at least, not all of them are. The solid part of a comet is usually a mix of rock and ice, the ice being made of water, ammonia, carbon dioxide, and other materials we tend to think of as gases here on Earth. But in the depths of space, where it's cold, they can remain frozen solid... until the comet nears the Sun. Then, the materials go from a solid directly to a gas, surround the solid nucleus, and reflect a whole lot more sunlight. The comet gets bright and can be spotted more easily.
Even then, it may not be easy. The comet may be small and faint in optical light. It may be too near the Sun to spot. It may be too far away to be seen easily. Or it may simply not be in a place anyone on Earth is looking.
WISE scanned the entire sky, and was prone to seeing such things. And the lack of optical light isn't so much an issue if the comet is warm enough to glow in the infrared, and that can be at temperatures a hundred degrees below 0 or more. And even then, WISE only found 20 such comets before anyone here on Earth did. I'll note it also did see quite a few comets discovered on Earth first, like the comet C/2007 Q3, also known as Siding Spring.
It also racked up a huge number of previously unknown asteroids, some of which are potentially dangerous to the Earth some time in the (far, hopefully) future. The point is -- and I've said this many times before -- the more eyes we have on and in the sky, the better. And by looking at different wavelengths we'll see even more.
Image Credit: NASA/JPL-Caltech/UCLA