SIRTF will make infrared observations, but in the manner of a stronger, more focused son of Hubble. Scientists expect SIRTF to provide images that are visually and scientifically as grand as those Hubble has been beaming back to Earth for a decade.
"The difference is that Hubble just dips its toe into the infrared wavelength band, and SIRTF is right smack in the middle of it," says SIRTF project scientist Michael Werner.
The last Great Observatory's 33.5-inch (85-centimeter) mirror will allow it to peer back to the early universe, at objects that barely emit any heat, and through dusty clouds of interstellar material. As Werner puts it, SIRTF will explore "the old, the cold, and the dirty."
NASA ran a naming contest for SIRTF, and the name is expected to be announced sometime after launch.
About infrared
Infrared light has a longer wavelength and less energy than visible light, and it is invisible to the human eye [diagram of wavelengths]. It is most commonly associated with heat, so most objects with a temperature emit it, including a cup of coffee, you, and the entire planet Earth. This makes observing at room temperature difficult because any signal from an astronomical object would be overwhelmed by the infrared energy coming from the telescope itself.
Therefore, the equipment must be cooled to extremely low temperatures.
Infrared astronomy is also challenging because the atmosphere absorbs most of the light coming from space except in a few narrow bands. The whole range of infrared light is best seen from space, above the atmosphere.
The European Space Agency operated a highly successful mission, called the Infrared Space Observatory, from 1995 to 1998. But with recent advances in detector technology, making them much larger and more sensitive, SIRTF will have up to 1,000 times the power of the ISO.
"The big gain comes from the detector arrays, and they're really at the heart of the instrument," Werner said.
The detector arrays will be paired with two imaging instruments and a spectroscope, which is used to analyze light waves, and astronomers around the world will be able to apply for time to use them to study the cosmos.
"There are some areas where SIRTF will excel and maybe be out there by itself," Werner said. "And there will be many others where the work will complement that from Hubble, Chandra and ground-based observatories."
The old
SIRTF will especially complement Hubble's Deep Field data, as the new telescope is slated to recreate these observations of narrow but distant fields of galaxies. SIRTF, like Hubble, will act like a "time machine," Werner says, and look back in space and time to the early universe. The farther it looks, the more the light coming from objects is red-shifted, or shifted toward the red end of the spectrum.
Most of the Hubble Deep Field data are in the ultraviolet range of the spectrum, Werner says. And the difference is that "what SIRTF tends to see is red-shifted near infrared radiation, which is more indicative of the underlying mass and age of the stellar constituents of the galaxy."
These data will expand the understanding of the early universe and how galaxies formed and evolved over time.
The cold
SIRTF will also investigate sub-stellar objects known as brown dwarfs. These are, Werner explains, "objects that are too low in mass to be stars but may still be very important astrophysically."
There is an undetermined, fuzzy line between what constitutes a small brown dwarf and a big planet. Astronomers are arguing the point right now.
When SIRTF was being designed, scientists imagined using the observatory to discover a brown dwarf that was theorized to exist. Since the, several have already been discovered. Now astronomers believe SIRTF will be useful to learn more about the failed stars. "This is an interesting example of where time caught up with us, in a sense," Werner says. "We've gone from a search or survey or discovery mission into more of an analytical mode."
SIRTF data will help astronomers understand what brown dwarfs are made of, how they formed, and maybe even find some that have so far gone unseen. Understanding these things may help scientists understand more about Earth and its neighbors.
"Because they're sort of a halfway house between stars and planets, understanding how brown dwarfs form could help us understand how planets and solar systems form," Werner says.
The dirty
Astronomers believe that our solar system formed out a dense disk of dust swirling around the young Sun. SIRTF will more directly investigate planet formation by studying one leftover portion of this disk, the outer region called the Kuiper Belt. It is a sea of icy objects beyond Neptune and Pluto.
Other stars ought to have similar disks. Around younger stars, the dust is still thick.
Looking at faraway protoplanetary disks, as they are called, in the infrared is easier than in the optical because the light from the star at the center of the disk is less overwhelming. Also, infrared data of a star and disk will help astronomers understand the circumstances surrounding planet formation -- the structure and composition of the disks, the type of star involved. The investigations should also show how frequently these disks occur.
Another "dirty" theme of SIRTF's scientific goals is to investigate ultraluminous infrared galaxies. These objects can have an energy output of more than a trillion Suns but emit mostly in the infrared.
These bright infrared galaxies may be powered by bursts of star formation at their centers or may be similar to active galactic nuclei, which astronomers believe are powered by black holes. Scientists do not know for sure, however, because they cannot see through the surrounding dust to peer at the center.
"In the infrared you can peer into the heart of these very luminous objects," Werner said.
Falling behind
SIRTF has suffered delays and changes in funding and design. In the end, financial pressure led to a unique orbital solution.
Many of the objects Werner expects SIRTF to observe have temperatures only tens of degrees above absolute zero. The Kuiper Belt objects are about 50 to 70 Kelvin (below -333 degrees Fahrenheit). The coldest grains of dust and clouds of gas have temperatures of about 15 to 25 degrees Kelvin, according to Werner. For best results, SIRTF will be cooled to even lower temperatures, or about 1.5 degrees above absolute zero for the coldest parts of the telescope.
To accomplish this, SIRTF will carry liquid helium and be set into an Earth-trailing orbit. In other words, SIRTF will be in the same path around the Sun as Earth, but will be slower, falling 15 million kilometers behind every year, or one-tenth the distance from the Sun to the Earth. This way, the telescope will cool to ambient space temperatures, which is about 30 Kelvin, without any added energy or coolant.
SIRTF designers had to resort to this novel orbit after budget cuts to the mission. As recently as 1990, SIRTF was supposed to be a massive 6-ton (5,700-kilogram) telescope with 3,800 liters of coolant. After two redesigns, and being canceled completely, the resurrected SIRTF now weighs in at under a ton (750 kilograms) and carries only 250 liters of liquid helium. It will be launched by a less powerful rocket than originally planned, saving money.
Despite the change in funding and shifting attitudes towards NASA's Great Observatories program since its inception with Hubble, expectations for the new infrared telescope are high.
"We fully hope and expect to get images that will have a comparable visual and scientific impact as the Hubble images," Werner said.
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