In order to clearly detect all radioactive waves traveling through space an observer would have to be above the shielding effects of Earth's atmosphere. The human eye can only detect one type of radioactive wave—visible light energy. These waves comprise only a small fraction of the radiation flying around the cosmos. This is why scientists invented instruments that can detect and measure waves that are invisible to humans. These instruments are put on satellites and launched into outer space to provide a clearer "picture" of the universe.
The largest and most powerful space telescopes in the world are NASA's Great Observatories. This is a family of observatories developed over many years. As of March 2004 NASA has put four Great Observatories into space:
- Hubble Space Telescope
- Chandra X-Ray Observatory
- Compton Gamma Ray Observatory
- Spitzer Space Telescope
These observatories were designed to observe the universe across a wide range of the light energies making up the electromagnetic spectrum.
The Electromagnetic Spectrum
Scientists use a scale to describe different types of light energies. This scale is called the electromagnetic spectrum. It categorizes energy by wavelength and frequency. A radio wave is the largest type, measuring between one millimeter and one hundred kilometers in length. This is a range of roughly 0.04 inches to 328,000 feet. At the other end of the scale is the gamma ray. Its length is the size of subatomic particles. Near the middle of the scale is visible light. This is the only kind of energy that humans can see.
For most celestial objects there is a relationship between their temperature and the main type of radiation that they emit. Astronomers use the Kelvin scale to measure temperature. A temperature of 0 degrees Kelvin is called absolute zero. This is equivalent to minus 460 degrees Fahrenheit.
Most stars have temperatures between 3,500 and 25,000 degrees Kelvin. Much of the energy they emit is visible. This explains why humans can see stars in the nighttime sky. However stars emit radiation at other wavelengths across the spectrum, particularly at infrared.
Everything that has a temperature above absolute zero emits infrared waves. This makes infrared observation an extremely important astronomical tool. There are huge murky clouds of dust and gas floating around in the universe. These clouds, called nebulae, can hide visible light from human sight. Scientists poetically refer to this phenomenon as the "cosmic veil." An infrared telescope is said to lift the cosmic veil by allowing humans to see through the clouds.
Many of the most mysterious objects and explosions in the cosmos are associated with very high temperatures and releases of gamma-ray, x-ray, and ultraviolet radiation. Only space-based telescopes equipped with special instruments can detect these waves.
This is important because X-rays and gamma rays are associated with phenomena such as black holes, supernova remnants, and neutron stars. Table 6.1 defines these terms and describes other cosmic objects. Astronomers learn about hot stars, quasars, and white dwarf stars using ultraviolet observations. Visible light is emitted by stars and emitted or reflected by cooler objects in the universe, including planets, nebulae, and brown dwarf stars. Radio waves are associated with cold clouds of molecules and radiation left over from celestial events that happened long, long ago.
Telescopes as Time Machines
It takes a long time for energy waves to travel through space. Although there are many different wavelengths of radiation, they all travel at a speed called the speed of light. The speed of light through a vacuum is 186,280 miles per second. That seems very fast. However, most celestial objects are so incredibly far away from Earth that it takes their energy a long time to reach us despite traveling at such a high speed.
Astronomers use a unit of measure called the light year to describe long distances in space. During one year
TABLE 6.1
Definitions
| Term | Meaning |
| Black hole | Invisible celestial object believed created when a massive star collapses. Its gravitational field is so strong that light cannot escape from it. |
| Brown dwarf | Celestial object much smaller than a typical star that radiates energy, but does not experience nuclear fusion. |
| Dark matter | Hypothetical matter that provides the gravity needed to hold hot gases within a galaxy cluster. |
| Event horizon | The boundary of a black hole. |
| Galaxy cluster | Huge collection of galaxies bound together by gravity. |
| Neutron star | Hypothetical dense celestial object consisting primarily of closely packed neutrons. Believed to result from the collapse of a much larger star. |
| Nova | Star that suddenly begins emitting much more light than before and continues to do so for days or months before returning to its former state of illumination. |
| Pulsar | Celestial body emitting pulses of electromagnetic radiation at short relatively constant intervals. |
| Quasar | Mysterious celestial object that resembles a star, but releases a tremendous amount of energy for its size. |
| Superflare | Massive explosion from a young star. A superflare is thousands of times stronger than a solar flare emitted by Earth's Sun. |
| Supernova | Catastrophic explosion of a star. |
| Supernova remnant | Bubble of multimillion degree gas released during a supernova. |
| White dwarf | Small dying star that has used up all its fuel and is fading. |
| SOURCE: Created by the author, 2004 | |
light travels 5.9 trillion miles. So a celestial object that is 5.9 trillion miles from Earth is said to be one light year away. If a spaceship could move as fast as the speed of light, it would reach that object in one year. Earth's Sun is eight light minutes away. This means that it takes eight minutes for an energy wave from the sun to reach the Earth. The sunlight visible on Earth at any given instant actually left the Sun eight minutes before.
When a telescope captures an image of an object in the universe, the image shows what that object looked like in the past. The farther away a telescope can see, the farther back into time it looks. NASA's Great Observatories capture images of celestial objects that are very far from Earth. This means that the radiation reaching the observatories left its source a long time ago. This is why NASA refers to its observatories as time machines. They allow humans to look back into time and learn about the origins of the universe.
Most astronomers believe that the universe started ten to twenty billion years ago with a massive explosion called the Big Bang. The energy released during this explosion would have been tremendous. Since then the universe has been cooling and expanding. Scientists believe that most radiation left over from the Big Bang now travels throughout space as radio waves and microwaves. This is called cosmic background radiation.
Ground-Based Telescopes
Telescopes first appeared in Europe around the beginning of the 1600s. Glass lenses had been used to make eyeglasses for several centuries. The first telescopes included a series of lenses within a long slender tube that magnified distant objects. In 1610 the Italian scientist Galileo Galilei (1564–1642) popularized the telescope when he published Sidereus Nuncius (Starry Messenger). The book described Galileo's astronomical observations, including the discovery of four moons around Jupiter.
Like the human eye, optical telescopes can discern only visible light. During the 1800s scientists first developed detectors for infrared radiation coming from outer space. Because the water in Earth's atmosphere blocks out most infrared radiation, these detectors were placed atop mountains where the air was thinner. Later they were sent up in high-altitude balloons and even airplanes.
During the 1930s an American engineer named Karl Jansky built an instrument capable of detecting radio waves. His radio telescope picked up waves generated by thunderstorms and from another unknown source in outer space. Over time scientists constructed larger and more powerful radio telescopes to pick up the radio waves beaming toward Earth.
RADIO ARRAYS.
In the 1980s dozens of radio telescopes in New Mexico were linked together to enhance their capability. This was called the Very Large Array. In 1993 the National Science Foundation created a more powerful system called the Very Long Baseline Array. It includes ten eighty-two-foot radio antennas located across the United States, from Hawaii in the west to the Virgin Islands in the east. The telescopes are connected by a computer network and provide the best radio wave images in the world.
ARECIBO OBSERVATORY.
The largest single-dish radio telescope on Earth is the Arecibo Observatory in Puerto Rico. The observatory was built in the 1960s and is operated by Cornell University for the National Science Foundation. NASA also provides funding support. On November 16, 1974, scientists used the observatory to send a radio message out into the galaxy. The message was coded in binary, meaning that a series of zeroes and ones were transmitted by shifting frequencies. The total broadcast took less than three minutes. It was a pictorial message.
The message shows the numbers one through ten, the atomic numbers of five chemical elements, the chemical formula of DNA, information about the human form and Earth's population, a stick figure person, the location of Earth in relation to the Sun and the other planets, a representation of the Arecibo telescope, and information about its size.
KECK OBSERVATORY.
In 1996 the twin optical/infrared telescopes of the Keck Observatory were completed.
FIGURE 6.3
The configuration of the Hubble space telescope
This facility sits atop an extinct volcano named Mauna Kea in Hawaii. The very thin atmosphere at this location allows the Observatory to detect a type of infrared radiation called near-infrared. This is a narrow band of infrared radiation that lies closest to visible light in the electromagnetic spectrum.
The Keck Observatory is operated by NASA in conjunction with the California Institute of Technology and the University of California. Other universities and institutions around the world also operate telescopes atop Mauna Kea.
Early Space Telescopes
In 1923 German scientist Hermann Oberth (1894–1989) proposed a space telescope in Die Rakete zu den Planetenraumen (The Rocket into Planetary Space. Two decades later an American physicist named Lyman Spitzer (1914–1997) wrote a paper in which he outlined the scientific benefits of putting telescopes in space. At the time space travel was not even possible. Telescopes were placed on top of mountains or sent up in high-altitude balloons and airplanes to avoid as much of Earth's atmosphere as possible.
In April 1968 NASA launched its first space telescope, the Orbital Astronomical Observatory (OAO-1). It was small and sensitive to ultraviolet light. This launch was first in a series of many satellites put into Earth orbit to detect energy sources in space. Most of these projects included NASA and international partners. In 1975 the European Space Agency (ESA) mission COS-B provided the first complete gamma-ray map of the galaxy.
Spitzer and other astronomers continued to urge the National Academy of Sciences and NASA to develop more powerful space telescopes for the scientific community. During the 1970s development began on a project called the Large Space Telescope (LST). Following the U.S. Moon landings NASA's budget was severely cut. This forced scientists to downsize the LST project several times. In 1975 the ESA joined the project and agreed to fund a percentage of LST costs in exchange for a guaranteed amount of telescope time for its scientists. Two years later the U.S. Congress authorized funding for construction and assembly of the LST.
At the same time that the LST was under construction, the space shuttle was also undergoing development. LST planners decided to utilize shuttle crews to deploy and service the telescope and ultimately return it to Earth. This decision would turn out to be a fateful one. By 1985 the observatory was finished and given a new name, Hubble Space Telescope.
The Hubble Space Telescope
The Hubble Space Telescope (HST) was the first of NASA's Great Observatories. It is named after the American astronomer Dr. Edwin Hubble (1889–1953). HST detects three types of light radiation: ultraviolet, visible, and near-infrared. It is the only one of the Great Observatories that captures images in visible light.
Figure 6.3 shows the general layout of the Hubble spacecraft. It can hold eight science instruments in addition to the primary mirror and secondary mirrors. These instruments are powered by sunlight captured by the satellite's two solar arrays. Gyroscopes and flywheels are used to point the telescope and keep it stable.
Table 6.2 provides design, cost, and operational data about the HST. NASA's Goddard Space Flight Center (GSFC) in Greenbelt, Maryland, is responsible for oversight of all HST operations and for servicing the satellite. The Space Telescope Science Institute (STSI) in Baltimore, Maryland, selects targets for the observatory based on proposals submitted by astronomers. The STSI also analyzes the astronomical data generated by the observatory.
Hubble transmits raw data to one of NASA's Tracking and Data Relay Satellite s (TDRS s). There are six of these satellites in low-Earth orbit. The TDRS relays the data via radio frequency communication links to a complex in White Sands, New Mexico. From there the data are relayed to GSFC and then to STSI.
The HST was originally supposed to go into space in 1986. The explosion of the space shuttle Challenger that year delayed Hubble's launch for four years. On April 24, 1990, HST was carried into orbit by the space shuttle Discovery on mission STS-31. One day later the astronauts deployed the satellite approximately 380 miles from Earth. It assumed an orbit that is nearly a perfect circle.
TABLE 6.2
The Hubble space telescope at a glance
| Hubble's name: | NASA named the world's first space-based optical telescope after American astronomer Edwin P. Hubble (1889–1953). Dr. Hubble confirmed an "expanding" universe, which provided the foundation for the Big Bang theory. |
| Launch: | April 24, 1990, from space shuttle Discovery (STS-31) |
| Deployment: | April 25, 1990 |
| Mission duration: | Up to 20 years |
| Servicing Mission 1: | December 1993 |
| Servicing Mission 2: | February 1997 |
| Servicing Mission 3A: | December 1999 |
| Servicing Mission 3B: | February 2002 |
| Length: | 43.5 ft (13.2 m) |
| Weight: | 24,500 lb (11,110 kg) |
| Maximum diameter: | 14 ft (4.2 m) |
| Cost at launch: | $1.5 billion |
| Orbit: | At an altitude of 380 statute miles (612 km), inclined 28.5 degrees to the equator (low-Earth orbit) |
| Time to complete one orbit: | 97 minutes |
| Speed: | 17,500 mph (28,000 kph) |
| Hubble can't observe: | The Sun or Mercury, which is too close to the Sun |
| Sensitivity to light: | Ultraviolet through near infrared (110-2,500 nanometers) |
| First image: | May 20, 1990: Star Cluster NGC 3532 |
| Data stats: | Each day the telescope generates enough data—3 to 4 gigabytes—to fill six CD-ROMs. The orbiting observatory's observations have amounted to more than 7 terabytes of data. Hubble's digital archive delivers 10 to 20 gigabytes of data a day to astronomers all over the world. |
| Power mechanism: | Two 25-foot solar panels |
| Power usage: | 3,000 watts. In an average orbit, Hubble uses about the same amount of energy as 30 household light bulbs. |
| Pointing accuracy: | In order to take images of distant, faint objects, Hubble must be extremely steady and accurate. The telescope is able to lock onto a target without deviating more than 7/1000th of an arcsecond, or about the width of a human hair seen at a distance of 1 mile. Pointing the Hubble Space Telescope and locking onto distant celestial targets is like holding a laser light steady on a dime that is 200 miles away. |
| Primary mirror | |
| Diameter: | 94.5 in (2.4 m) |
| Weight: | 1,825 lb (828 kg) |
| Secondary mirror | |
| Diameter: | 12 in (0.3 m) |
| Weight: | 27.4 lb (12.3 kg) |
| Power storage | |
| Batteries: | 6 nickel-hydrogen (NiH) |
| Storage capacity: | Equal to 20 car batteries |
| SOURCE: "Hubble at a Glance," in About Hubble, Space Telescope Science Institute, Baltimore, MD, undated [Online] http://hubblesite.org/newscenter/news_media_resources/reference_center/about_hubble/glance.php [accessed January 12, 2004] | |
When scientists first started using Hubble they discovered that its images were fuzzy due to a defect in the telescope's optical mirrors. The defect had existed before the satellite was launched into space. Media publicity about the problem caused an uproar and brought harsh criticism of NASA. In December 1993 astronauts aboard the space shuttle Endeavor (STS-61) were sent to intercept the HST and install corrective devices. The mission was a success.
This was the first of four servicing missions conducted by shuttle crews between 1993 and 2002. During these missions astronauts performed repairs and maintenance, installed new components to enhance HST's performance, and reboosted the satellite to a higher altitude. Like all objects in low-Earth orbit the HST experiences some drag and gradually loses altitude. The satellite does not have its own propulsion system.
Hubble was originally designed for a twenty-year lifetime. Planners assumed that five shuttle servicing missions would be required over this time period to keep the observatory in orbit and functioning properly. At the end of 2002 four of these missions had been performed. A fifth servicing mission was anticipated in 2004 followed by satellite retrieval in 2010.
The loss of the space shuttle Columbia in January 2003 altered the fate of the HST. The shuttle fleet was grounded while NASA examined the cause of the accident and developed ways to make shuttle flight safer. In January 2004 NASA announced its decision to cancel the fifth servicing mission to HST. A shuttle flight to the observatory is considered too risky, because HST orbits far from the International Space Station (ISS). The ISS is the only safe haven available to shuttle astronauts in the event of an emergency. Future shuttle missions are expected to be devoted entirely to projects at or near the ISS.
Without a reboost the HST will gradually fall out of orbit and be destroyed by reentry to Earth's atmosphere. This is anticipated sometime between 2007 and 2010. Reentry does not always completely incinerate space objects. HST reentry over a populated area could pose a danger to people on the ground. When the time comes NASA plans to send a robotic "tow truck" to guide the satellite to reentry over a desolate area of the Pacific Ocean.
The observatory could cease to be useful even before it falls out of orbit. Any failures of the satellite's scientific instruments, computer, gyroscopes, data relay system, or solar arrays would require repair by shuttle astronauts. It is highly unlikely that NASA would approve such an effort.
Abandoning the observatory is a controversial decision. Many scientists are disappointed that such a valuable resource may be lost before the end of its useful life. During its mission Hubble has captured thousands of dramatic images of celestial objects throughout the universe, including nebulae, galaxies, stars, and even some bodies believed to be planets.
Compton Gamma-Ray Observatory
The Compton Gamma-Ray Observatory (CGRO) was the second of NASA's Great Observatories. It was designed to detect high-energy gamma rays, the most powerful type of energy in the electromagnetic spectrum. The observatory was named after the late American physicist Dr. Arthur Compton.
On April 5, 1991, CGRO was launched into space aboard the space shuttle Atlantis as part of mission STS-37. The satellite was deployed at an orbit altitude of 280 miles
FIGURE 6.4
Compton Observatory instruments
above Earth. It was equipped with four highly sensitive detecting instruments and a pair of solar arrays. (See Figure 6.4). The satellite also had a small propulsion system and three gyroscopes that were used to point the telescope and maintain flight stability. The propulsion system was not powerful enough to boost CGRO to higher altitudes.
The original plan was for a five-year mission. The observatory proved to be much more durable than expected and remained in space for nine years.
In early 2000 NASA learned that one of CGRO's gyroscopes had failed. Even though the observatory instruments were still in working order, NASA decided to purposely deorbit the satellite. Scientists feared that left to reenter on its own, CGRO could rain pieces down on populated areas. This was of particular concern because the satellite was unusually massive at seventeen tons and also included a fair amount of titanium in its components. Titanium is not easily disintegrated during atmospheric reentry.
On June 4, 2000, CGRO's propulsion system was used to guide the satellite to a safe reentry over a deserted area of the Pacific Ocean.
The CGRO program included participation by scientists from Germany, the Netherlands, the European Space Agency, and the United Kingdom. During its mission the observatory investigated high-energy phenomena associated with black holes, novas, supernovas, quasars, pulsars, solar flares, cosmic rays, and gamma-ray bursts. Gamma-ray bursts are sudden short flashes of gamma rays that do not seem to occur at predictable locations. CGRO recorded
FIGURE 6.5
The Chandra X-Ray Observatory's telescope system
more than 2,500 gamma-ray bursts, far more than had ever been detected before. It also mapped hundreds of gamma ray sources.
Chandra X-Ray Observatory
The Chandra X-Ray Observatory (CXRO) is the third of NASA's Great Observatories and the world's most powerful x-ray telescope. It was named after the late Indian-American scientist Dr. Subrahmanyan Chandrasekhar. The CXRO can detect x-ray sources that are billions of light years away from Earth.
On July 22, 1999, the space shuttle Columbia blasted off from Cape Canaveral, Florida. Mission STS-93 carried the CXRO as its primary payload. The CXRO spacecraft was equipped with a rocket called an Inertial Upper Stage (IUS). The shuttle crew released the satellite and then moved a safe distance away. The IUS was fired for four minutes to move the satellite into a temporary orbit high above the Earth.
At that point, the CXRO's twin solar wings were unfolded and began converting sunlight into electrical power. Figure 6.5 shows a diagram of the spacecraft. Solar power is used to run CXRO's equipment and charge its batteries. The spacecraft's own propulsion system was used to boost it to its final orbit.
Chandra circles the Earth once every sixty-four hours. The satellite's orbit is highly elliptical. At its closest point the observatory is about 6,200 miles away from Earth. At its farthest point it is about 87,000 miles away. This is nearly one-third of the distance to the Moon. Chandra's flight path carries it through the Van Allen radiation belts that encircle Earth. The satellite's sensitive electronic instruments are turned off while the spacecraft is in or near this highly radioactive area of space.
Scientists around the world are using CXRO images to create an x-ray map of the universe. They hope to use this map to learn more about black holes, supernovas, superflares, quasars, extremely hot gases at the center of galaxy clusters, and the mysterious substance known as dark matter.
In February 2004 researchers at the Max Planck Institute for Extraterrestrial Physics in Germany announced that CXRO had recorded an event called a stellar tidal disruption (STD). This is when a star wanders too close to a massive black hole and gets ripped apart. STDs are predicted by astronomical theory, but had never been observed before.
CXRO detected a tremendous burst of x-ray energy coming from the center of a galaxy called RXJ1242-11. This galaxy is about 700 million light years from Earth. Scientists believe that the energy burst was due to super-heating of gases from the star as it was being devoured by the black hole.
FIGURE 6.6
Delta II Rocket launches Spitzer space telescope
The CXRO is managed by the Marshall Space Flight Center for NASA's Office of Space Science. NASA's Jet Propulsion Laboratory in Pasadena, California, provides communications and data links. The Smithsonian Astrophysical Observatory in Cambridge, Maryland, controls science and flight operations. NASA's High-Energy Astrophysics Science Archive (HEASARC) collects and maintains all astronomy data obtained from the CXRO and NASA's smaller gamma-ray, x-ray, and extreme ultra-violet observatories. The CXRO was designed for a five-year mission. Important design information about it is included in Table 6.3.
Spitzer Space Telescope
The Spitzer Space Telescope is the fourth of NASA's Great Observatories and detects infrared radiation. When it was originally conceived during the 1970s the observatory was called the Shuttle Infrared Telescope Facility (SIRTF). Its planners hoped to carry the telescope into space as a science payload on numerous space shuttle missions. In 1983 NASA decided the telescope needed to be in continuous orbit and able to fly by itself. The SIRTF was renamed the Space Infrared Telescope Facility.
During the 1990s NASA's science programs suffered harsh budget cuts. The SIRTF was redesigned several times and downsized from a large $2 billion telescope with numerous capabilities to a modest telescope costing less than half a billion dollars.
On August 25, 2003, the telescope was launched into orbit atop a Delta rocket. It was the only Great Observatory not carried into space by a space shuttle. Figure 6.6 shows the components of the two-stage launch vehicle. The telescope was housed inside a protective case called a fairing that fell away when the satellite reached orbit. Figure 6.7 shows details of the observatory's structure. The
TABLE 6.3
The Chandra Observatory at a glance
| Mission duration | |
| Chandra science mission | Approx. 5 yrs |
| Orbital activation & checkout period | Approx. 2 mos |
| Orbital data | |
| Inclination | 28.5 degrees |
| Altitude at apogee | 86,992 sm |
| Altitude at perigee | 6,214 sm |
| Orbital period | 64 hrs |
| Observing time per orbital period | Up to 55 hrs |
| Dimensions | |
| Length - (Sun shade open) | 45.3′ |
| Length - (Sun shade closed) | 38.7′ |
| Width - (solar arrays deployed) | 64.0′ |
| Width - (solar arrays stowed) | 14.0′ |
| High resolution mirror assembly | |
| Configuration | 4 sets of nested, grazing incidence araboloid/hyperboloid mirror pairs |
| Mirror weight | 2,093 lbs |
| Focal length | 33 ft |
| Outer diameter | 4 ft |
| Length | 33.5 in |
| Material | Zerodur |
| Coating | 600 angstroms of iridium |
| Science instruments | |
| Charged coupled imaging spectrometer (ACIS) | |
| High resolution camera (HRC) | |
| High energy transmission grating (HETG) | |
| Low energy transmission grating (LETG) | |
| SOURCE: Adapted from "Chandra at a Glance," in STS-93 Press Kit: Chandra X-Ray Observatory, National Aeronautics and Space Administration, Washington, DC, July 7, 1999 [Online] http://www.shuttlepresskit.com/STS-93/payload45.htm [accessed February 9, 2004] | |
dark circle at the top of the observatory is actually a door that opens to expose the telescope to space.
In December 2003 NASA chose a new name for the SIRTF from thousands of names suggested during a naming contest. More than 7,000 people from around the world entered the contest. Some of the most frequently proposed names were Red Eye, Sagan (after scientist Carl Sagan), Herschel (after scientist William Herschel, who in 1800 discovered infrared radiation), and Roddenberry (after science-fiction writer Gene Roddenberry, creator of the Star Trek television show). In the end NASA chose the name Spitzer to honor the late American physicist Dr. Lyman Spitzer. It was Spitzer's groundbreaking paper of the 1940s that inspired astronomers to build space telescopes.
The Spitzer Space Telescope is unique among the Great Observatories because it orbits the Sun instead of the Earth. It actually follows along behind Earth as the planet travels around the Sun. This is called an Earth-trailing heliocentric orbit. Heliocentric means sun-centered.
This orbit was chosen for several reasons. It allows the spacecraft to avoid interference from Earth's infrared-absorbing atmosphere. The orbit also provides a colder environment for the telescope than would an Earth orbit. Space infrared telescopes must be maintained at a very
FIGURE 6.7
Exterior of the Chandra X-Ray Observatory
cold temperature to operate properly. Everything that has a temperature emits infrared radiation. The spacecraft must be kept as cold as possible to prevent heat within it from interfering with its own detection equipment. The relatively cold Earth-trailing heliocentric orbit was selected to minimize the amount of liquid helium required onboard to keep the telescope cool. Scientists believe that Spitzer's liquid helium supply will last for a minimum of 2.5 years and possibly as long as five years. NASA estimates the total mission cost at around $1.2 billion.
The Spitzer Space Telescope can detect infrared energy with wavelengths of 3 to 180 microns. A micron is one-millionth of a meter. Detection of infrared waves at these lengths allows the telescope to image celestial objects hidden by dense clouds of dust and gas. During its first few months of operation Spitzer provided images of galaxies and nebulae up to three billion light years away. In January 2004 NASA released colorful Spitzer images of massive newborn stars at the core of the Tarantula Nebula. This nebula is a known star-forming region 170,000 light years from Earth.
Humans cannot see infrared radiation. The energy data that Spitzer collects are transformed into visible pictures by assigning different colors to different energy levels. The resulting pictures are called false-color images. Scientists either choose colors to highlight a specific area of scientific interest or to make a celestial object appear realistic to the human eye. For example, coloring the highest energy areas of an image with white or red indicates brightness and heat to a human looking at the picture. Likewise low-energy regions are shaded with darker colors to indicate relative coolness.
The Next Great Observatory
As of March 2004 NASA's fifth Great Observatory is under development and scheduled for launch in 2011. It is called the James Webb Space Telescope (JWST) in honor of NASA's director of flight operations during the Apollo program. The expected cost of the JWST mission is $825 million.
JWST will detect infrared radiation from a vantage point approximately one million miles from Earth in the direction away from the Sun. This point is called the second Lagrange point or L2. Lagrange points are places found in the space around a large body (like the Earth) orbiting another large body (like the Sun). At a Lagrange point, the gravity of the Earth and the gravity of the Sun are balanced, so that a satellite can remain at one of these points and not be pulled away by the gravity of either of the larger bodies. The concept was discovered by Joseph-Louis Lagrange (1736–1813), a mathematician born in Italy who lived in France.
There are five Lagrange points in the Sun-Earth system. The L1 point is roughly one million miles from the Earth, between the Earth and the Sun. The L2 point is approximately the same distance in the opposite direction, outside the Earth's orbit. Point L3 is located on the opposite side of the Sun from the Earth at a distance equal to that of the distance from the center of the Sun to the center of the Earth. The L1, L2, and L3 points lie on a center-line that bisects (cuts through the middle of) the Earth and the Sun. The L4 and L5 points are located at the end points of two equilateral (equal-sided) triangles whose shared base is the segment of centerline that falls between the center of the Earth and the center of the Sun.
Because the Earth constantly orbits the Sun, the Lagrange points constantly move also. However, they remain a fixed distance from the Earth and the Sun. When it is stationed at the L2 point, the JWST will make a small circle called a halo orbit around the L2 point. This point will not only provide the JWST with a relatively cold location in space, but make it easier to shield the telescope from the massive infrared radiation coming from the Sun, Earth, and Moon. JWST will look outward into space.
Any object at the L3 point is always hidden from Earth view by the Sun. Point L3 has always been an intriguing location to science fiction writers who fantasize that an unseen world called Planet X could be there. The idea was popularized in the 1951 movie The Man from Planet X.
Other Space Observatories
Besides the Great Observatories NASA operates a number of smaller space observatories in Sun-Earth orbits. These spacecraft were funded and built under NASA's Explorer program. The projects are conducted with the help of scientists from academic institutions and/or foreign space agencies.
The European Space Agency operates an observatory of its own called the XMM-Newton. This powerful telescope is similar in scope to one of NASA's Great Observatories. The XMM-Newton is the largest science satellite ever built in Europe and includes three x-ray telescopes.
Table 6.4 gives more details about observatory missions ongoing as of March 2004.
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