Wednesday, April 24, 2013

Happy Birthday, Hubble Space Telescope!

23 years ago today, NASA launched the space shuttle Discovery which carried the Hubble Space Telescope into low-Earth orbit. Since then, despite the infamous initial problems with Hubble's primary mirror, Hubble has been a workhorse of the astronomical community and is probably the most well-known telescope in human history. It's hard to encounter someone who hasn't seen one of the classic images from Hubble at some point in their lives, whether they know it or not. Even with its many scientific successes, I think that Hubble's greatest success has been its unmatched ability to bring the universe to the public through the beautiful, iconic images it has taken over the years. Without any doubt in my mind, Hubble lives up to being one of NASA's Great Observatories and may well be the greatest observatory we've ever had (for now).

In 2001 NASA conducted an online poll to determine what object Hubble should image to celebrate its eleventh year of operation, with the overwhelming winner being the Horsehead Nebula, in the Orion Molecular Cloud Complex. The result was the following image, taken in visible light (with some very near infrared).
Image Credit: Space Telescope Science Institute.
The Horsehead Nebula is a cloud of cool, mostly molecular, gas and dust in which star formation is currently occurring. The gas and dust of the Horsehead Nebula are dark because, unlike stars or the brilliant planetary nebulae that form when stars like the Sun die (see: Ring Nebula), are cold. The average temperature for the cool molecular gas and dust found in star formation regions is between 10 and 50 Kelvin or so. That doesn't mean that all of the gas is this temperature. A large portion of the gas in the Horsehead Nebula is ionized because of young, massive nearby stars whose intense radiation are essentially blowing the nebula apart. One of these stars can just be seen through the clouds of the nebula toward the upper left side of the nebula.

The almost uniformly bright background that can be seen in the image against which the nebula is silhouetted is actually a bright background emission nebula known as IC-434. The Wikipedia page on this object isn't particularly enlightening, except to point out that it was first observed by William Herschel. Unlike the Horsehead Nebula, IC-434 is an emission nebula, which means that it is giving off light because it is being heated by some source. This source could be a particularly hot star or a white dwarf (as in a planetary nebula).

On Friday, April 19, the Space Telescope Science Institute released another anniversary image of the Horsehead Nebula that was taken in purely near-infrared light. Because it was taken at completely different wavelengths of light than the image taken in 2001, you'll see that the image will look very different.
Image Credit: Space Telescope Science Institute.
But why does it look so different? Astronomers use infrared light to see through clouds of gas and dust because its longer wavelength means it doesn't get scattered as easily as visible light. So where you previously saw a dark cloud of gas and dust obscuring everything inside and behind it, now you see way more stars than you did before purely because their light can make it through the gas. (The wider field of vision may also aid the number of stars you can see compared to the previous image).

You've probably also noticed that the background cluster IC-434 no longer provides a luminous background  for the Horsehead Nebula. Instead, you can see through the optically bright gas to see the stars (and even some galaxies!) that were otherwise not visible in or behind IC-434 for the same reason as is stated above.

All of this talk of pretty astronomical pictures is not to diminish Hubble's importance as a scientific instrument, of course! Hubble has been involved in some of the most important scientific work of the past decade. Hubble is responsible for the first precision measurement of the Hubble Constant, the deepest visible light image of the universe (shown below), obtaining spectra of exoplanet atmospheres, detecting exoplanets through both direct and indirect techniques, and some of the best images of our neighbors in the solar system (like the picture of Mars also shown below). Hubble discovered the existence of dark energy, the ubiquity of central supermassive black holes in galaxies, protoplanetary disks, optical counterparts of gamma-ray bursts,  and the many moons of Pluto. And that's probably just a very small taste of what Hubble has accomplished during its time.

The Hubble Ultra Deep Field. To really get an idea of how awesome this picture is, you absolutely must view it in its full resolution, completely zoomed in. Image Credit: Space Telescope Science Institute
Hubble image of Mars. Image Credit: Space Telescope Science Institute
The Hubble Space Telescope is due to be replaced in the not-too-distant future by the James Webb Space Telescope, which is still on schedule to launch in 2018. JWST will have increased capabilities in the infrared part of the spectrum, along with a much larger collecting area than Hubble. Of course, this is very exciting for astronomers everywhere, because the advances we expect to make with JWST are on par with those the astronomical community was able to achieve with Hubble.

The future of the Hubble Space Telescope itself is a bit more sad. Hubble was placed in low Earth orbit so it could easily be serviced and upgraded with shuttle missions (which has been a major part of its longevity). However, because NASA's shuttle program is now defunct, Hubble will not be retrieved at the end of its life, and will most likely be de-orbited to safely burn up in Earth's atmosphere. After all it's accomplished for humanity, I think Hubble deserves better. I, for one, would love to see a one-off mission designed to retrieve Hubble and bring it safely back to Earth. Hubble is part of our history that deserves its place in a museum for future generations. It was the first truly and universally great space telescope, which paved the way for greater still missions like JWST.

But let's save the sadness for another time. For now, as long as it's still chugging away, it deserves to be celebrated for what it has accomplished over the years, and what it will continue to accomplish well into the foreseeable future.

Happy birthday, Hubble Space Telescope!
Image Credit: NASA (except for the birthday hat; that was all me.)

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