Thursday, May 14, 2009

Herschel and Planck launch

With all eyes on the shuttle's final servicing mission to the Hubble Space Telescope, many of you probably have not heard of today's other giant space news: the launch of the European space telescopes Herschel and Planck. The single Ariane 5 rocket containing both satellites launched successfully at 9:12am EDT this morning, and the latest news is that both satellites successfully separated from the rocket and are on their way.

Planck is the latest satellite to study the Cosmic Microwave Background, the "echo" of the Big Bang discovered by Bell Labs engineers Penzias and Wilson back in the early 1960s. While NASA's COBE and WMAP missions have learned a lot about the early Universe from studying the Cosmic Microwave Background in detail, there is still a lot more information encoded in the light from this echo. Planck has two big advantages over the previous missions. It has sharper vision (high angular resolution), allowing Planck to study the smallest structures in the early Universe. This will allow for some of the first strong tests of hypotheses like inflation, the idea that in the first instants of the Big Bang, the Universe expanded at a rate much faster than the speed of light (space can do this). Many of the ideas behind inflation are similar to many of the ideas behind Dark Energy, so it is possible that Planck will be able to shed light on Dark Energy, as well.

Planck also can measure the polarization of light from the Cosmic Microwave Background. If you think of light as a wave travelling through space, polarization is the direction in which the wave is "waving." Imagine standing on the beach and watching the waves come in. The waves are moving toward the beach, but if you are out in the ocean, you will bob up and down as the waves pass. So, the polarization of ocean waves is up and down (more or less).

Light waves can have polarization, too. They can be polarized up and down, left and right, and even as a circle (meaning the wave sort of corkscrews through space).

Light from a star comes out as unpolarized, on average. But if the light reflects off of something, it tends to become polarized. This helps explain why polarized sunglasses work -- the sunglasses only let through light polarized in directions different from the polarization of reflected light, so sunlight glare from reflections gets filtered out by the sunglasses.

For hundreds of thousands of years after the cosmic microwave background light was created, it travelled freely through the Universe. The Universe was filled with neutral hydrogen and helium atoms, and light can pass right by neutral hydrogen or helium without interacting. (Don't believe me? Let the helium out of a helium-filled balloon. You won't see the helium, because light passes right through it without interacting.)

When the first stars formed in the Universe, they were very hot. The light from these stars was sufficient to ionize the hydrogen and helium, in other words, the starlight was able to strip electrons away from their parent atoms. Light and electrons interact very strongly; light tends to bounce off of electrons. (This is kinda why metal is used in mirrors; the electrons in many metals are more or less free to roam around, and light bounces off of them.) Bouncing light = reflecting light = polarized light!

So, since Planck can detect polarized light, it can determine when the light in the Cosmic Microwave Background started to become polarized. This tells us when the first stars formed in our Universe, which is still unknown.

Herschel, the other telescope launched today, is an infrared telescope. It can look at light that has wavelengths of 55 to 672 micrometers (or about 0.05 to 0.67 millimeters). Optical light has wavelengths of a fraction of a micrometer, and radio waves have wavelengths of millimeters to many meters long.

This regime of light is hard to study. Everything around us emits infrared light, including us and the air around us. The hotter something is, the more light it emits. Therefore, to study the Universe in this infrared light, our telescope needs to be outside of Earth's atmosphere and cooled to temperatures near absolute zero. The Herschel Telescope uses liquid helium to cool its instruments to 0.3 Kelvin, or a third of a degree above absolute zero, the coldest possible temperature.

There's another trick to looking at infrared light in this wavelength range. As you may remember, light can be thought of as either a particle (a photon) or a wave. Our current technology makes it easy to detect photons at short wavelengths (your digital camera detects individual photons), and easy to detect waves at long wavelengths (radio antennas work by detecting waves).

At wavelengths of around a few hundred microns, it is hard to detect photons and hard to analyze waves. So, infrared telescopes working in this range have a technological challenge, no matter whether they decide to detect waves or photons. Herschel is going for the wave detection, and the technology required to analyze these waves has proven quite challenging to develop. But the engineers did it!

Herschel's science is designed around looking for cold things. This includes dust and gas clouds in deep space, and the cold outer regions of forming stars, where planets might be forming. Herschel can also study some of the very first galaxies in our Universe, galaxies so far away that normal infrared light has been stretched by the expansion of the Universe into this far infrared. We have learned a lot about nearby galaxies in infrared light from telescopes like the Spitzer Space Telescope (Hubble's infrared cousin); Herschel will allow us to compare galaxies in the early universe to the nearby galaxies that we think we understand.

Congratulations to the Planck and Herschel teams on a successful launch!

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