Robotic Missions in Sun-Earth Space

The Universe is an explosive, energetic and continually changing place.

—National Aeronautics and Space Administration, “Chandra’s X-ray Vision” (February 19, 2004)

Sending humans into space is expensive and risky. It takes great resources to protect and sustain them every time they leave the planet. Losing a crewed spacecraft means the loss of life. This is a high price to pay to learn about the universe. This explains why robotic spacecraft are so vital to space science. Since the beginning of the space age, satellites and probes have been sent out to gather data about Earth’s surroundings. The earliest ones were rather crude in their technology. People referred to them simply as unmanned spacecraft. Times changed, and technology improved significantly. In the twenty-first century, these mechanized explorers are called robotic spacecraft.

Only a handful of robotic spacecraft are sent to other planets. The vast majority of them circle Earth or the Sun. Spacecraft in Earth orbit serve commercial, military, and scientific purposes. Scientists rely on satellites to collect data about Earth’s weather, climate, atmospheric conditions, sea levels, ocean circulation, and gravitational and electromagnetic fields. These satellites are not space explorers but Earth observers that reside in space.

Other satellites in Earth orbit look outward toward the cosmos. These space observatories carry high-powered telescopes that beam images back to Earth. They can detect the light and radiation of celestial objects hidden from human view. They peer into the deep, dark regions of space to explore what is out there. Scientists use the images sent back from such satellites to learn about the origins of stars and planets and unravel some of the mysteries of the universe.

Closer to Earth is the Sun. It emits radiation and heat and produces a flow of energetic particles called the solar wind that constantly blows against Earth. Gusts of solar wind can upset the planet’s electromagnetic field. Every so often the Sun spits out globs of plasma and intense bursts of radiation. These, too, can have a profound effect on Earth. Investigating the Sun-Earth connection is a major goal of modern space science. Robotic spacecraft are put into Earth orbit or sent out into space to collect data about this vital connection.

For decades, robotic spacecraft have been designed for one-way trips. Eventually, their batteries give out and their radios malfunction, and then they quit reporting back to Earth. They continue silently in their orbits or are incinerated by reentry into Earth’s atmosphere. During the 1990s scientists developed robotic spacecraft that can do more than report back—they can come back. These sampling ships are designed to grab samples of space particles and return safely to Earth. The first one returned in September 2005. It carried samples of solar wind collected a million miles from Earth. The spaceship’s name was Genesis. It marked the beginning of a whole new way for humans to explore space without even leaving the planet.

NASA’S SPACE SCIENCE MISSIONS

The vast majority of robotic spacecraft are operated by the National Aeronautics and Space Administration (NASA), under the agency’s Science Mission Directorate (SMD). This directorate focuses on areas of research including the origins and evolution of the universe, the nature of life, the solar system, and the relationship between Earth and the Sun. It also supports the Vision for Space Exploration by studying the formation of Mars and Earth’s Moon; searching for exoplanets (planets outside the solar system), particularly those that are potentially habitable; and investigating space phenomena that could harm robotic and human explorers.

The SMD categorizes space science missions by primary research target: Earth, the Sun, the solar system, and the universe. Table 6.1 provides a categorical break-down of the SMD’s operational and past missions as of October 2007. More than fifty science missions were ongoing at that time.

The SMD also categorizes space science missions administratively, that is, by mission class and by the NASA program under which they are developed.

Mission class is characterized primarily by complexity, leadership, and cost. These elements are defined in Summary of the Science Plan for NASA’s Science Mission Directorate 2007–2016 (January 2007, http://science.hq.nasa.gov/strategy/Science_Plan_07_summary.pdf). NASA has two complexity classes: strategic and competed. Strategic missions are major complex projects that address broad science goals and have long development times and high costs. These missions are managed and led directly by NASA. Competed missions are smaller missions with narrowly focused science goals, short schedules, and relatively low-cost caps. These missions are proposed and led by entities called principal investigators (PIs). A PI can be one or more universities, research organizations, companies, or even NASA centers. Strategic missions may include components that are competed (e.g., specific science instruments onboard a spacecraft).

NASA’s cost categories for space science missions are as follows:

• Small missions—life cycle costs less than approximately $300 million
• Midsize missions—life cycle costs between approximately $300 million to $700 million
• Large missions—life cycle costs more than approximately $750 million
• Flagship missions—life cycle costs more than approximately $1 billion

Major NASA programs engaged in space science include the Explorers and Discovery programs. The following describes these program in more detail.

NASA’S EXPLORERS PROGRAM

The Explorers Program includes robotic missions that conduct relatively low-cost scientific investigations with specific objectives in the fields of astrophysics and heliophysics (physics of the Sun). The program dates back to the late 1950s. The first satellite launched into space by the United States was Explorer 1. It was the first of a series of scientific satellites named Explorer.

In 1958 Explorer 1 used temperature gauges and a Geiger counter to collect data in space. A Geiger counter is an instrument that detects the presence and intensity of radiation. Explorer 1 detected radiation levels as expected during most of its orbit. However, at the highest altitudes the Geiger counter recorded no radiation. This was most puzzling. Scientists expected radiation levels to increase, not decrease, farther from Earth. Data from later Explorers and the Soviet Sputnik satellites revealed that Explorer 1 had encountered radiation levels so high that its detector was overwhelmed.

These missions led to the discovery of the Van Allen radiation belts. These are two doughnut-shaped regions of high radioactivity that encircle Earth. The inner belt is centered approximately two thousand miles above Earth and is roughly four thousand miles thick. The outer belt lies between approximately eleven thousand and twenty-five thousand miles from Earth. All future spacecraft venturing far from Earth have to be designed to withstand the intense radiation of the Van Allen belts.

Over the next three decades the Explorers Program continued with mission series given different names, including the Atmosphere Explorer, the Electrostatic Particle Explorer, the Interplanetary Monitoring Platform, and the Solar Radiation Satellites. There were also Explorer spacecraft with colorful names such as Hawkeye, Injun, and Uhuru. (Uhuru means “freedom” in Swahili, an African language.)

NASA’s 2007 science plan describes the Explorers missions as competed missions led by the PIs. Cost-wise, they are considered “small” missions. NASA (June 11, 2007, http://explorers.gsfc.nasa.gov/missions.html) indicates that within the Explorers Program missions are further categorized by complexity and cost as follows:

• University-class Explorer/Student Explorer Demonstration Initiative Program—total cost less than $15 million
• Small Explorer—total cost less than $120 million
• Medium-class Explorer (MIDEX)—total cost less than $180 million

NASA stopped accepting proposals for MIDEX missions in 2004. The decision was due to budget constraints, the change in NASA’s focus to crewed Moon and Mars missions, and the lack of appropriate launch vehicles (mainly Delta II rockets) for spacecraft of the size typically used in MIDEX missions.

Another type of Explorer mission is the Mission of Opportunity (MO). This is a mission operated by another office within NASA or by the space agency of another country in which an SMD investigation “hitches a ride” with the mission of the other agency or country. The total cost to NASA of an MO mission cannot exceed $35 million.

Table 6.2 lists the Explorers Program’s operational missions as of December 2007. Most of the spacecraft are small observatories in low Earth orbit (LEO; 125 to 1,200 miles above Earth’s surface). The Advanced Composition Explorer and the Wilkinson Microwave Anisotropy Probe are positioned at Lagrange points around Earth (designated L1 and L2 in Table 6.2, respectively). These are points at which the gravitational pulls of the Sun and Earth are relatively even, meaning that a spacecraft can stay “parked” between the two heavenly bodies. Point L1 is approximately one million miles from Earth toward the Sun; point L2 is approximately one million miles from Earth in the other direction (away from the Sun). The locations of IMP-8, THEMIS, IMAGE, and Integral are given in terms of Earth radii. (Earth’s radius is approximately 3,960 miles.)

As of January 2008, NASA had two full-fledged Explorer missions and two MOs scheduled for launch before the end of the decade. The full-fledged missions are the Interstellar Boundary Explorer (an imager designed to detect specific atoms believed to have originated outside the solar system) and the Wide-Field Infrared Surveyor (a miniobservatory that will image nearby stars and galaxies). The scheduled MOs are the Two Wide-Angle Imaging Neutral-Atom Spectrometers and the Coupled Ion Neutral Dynamics Investigation. Both instruments will fly on U.S. Department of Defense spacecraft.

NASA’S DISCOVERY PROGRAM

The Discovery Program was initiated in 1989 to develop missions that could be accomplished using small spacecraft at a low cost in a relatively short time to address focused scientific goals. According to NASA’s 2007 science plan, Discovery missions are competed PIled missions with a cost classification of “medium.” The focus of the program is celestial bodies (mainly planets, moons, comets, and asteroids) within and outside the solar system.

The program allows scientists to conduct small space investigations that complement NASA’s larger and more expensive interplanetary missions. Each Discovery mission must have a fast development time and relatively low cost. NASA calls it the “faster, better, cheaper”approach to space science. MO projects are also allowed under the program.

As of December 2007, there were ten current and past missions listed under the Discovery Program. (See Table 6.3.) Three of the missions were operational: ASPERA-3, Dawn, and MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging). (ASPERA-3 and Dawn are described in Chapters 7 and 8, respectively.) MESSENGER is the only operational mission focused on Sun-Earth space. It launched on August 3, 2004, on a voyage to Mercury. The spacecraft will conduct several flybys of the planet before achieving orbit in 2011. It will be the first spacecraft to orbit that planet. NASA’s Mariner 10 flew by Mercury in the 1970s, but it captured limited images of the planet’s surface. As such, much of Mercury has remained unseen territory. MESSENGER includes a complex suite of instruments that will capture detailed images of the planet and investigate its geology and atmosphere. (See Figure 6.1.) The payload was specially designed to endure the high-temperature environment near the Sun.

Past Discovery Missions

Seven Discovery missions (NEAR Shoemaker, Sojourner, Lunar Prospector, Stardust, Genesis, CONTOUR, and Deep Impact ) ended their primary missions during the late 1990s or

early 2000s. The CONTOUR spacecraft, which was intended to encounter two comets, was lost six weeks after launch. The following describes the remaining missions. (Sojourner is described in Chapter 7.)

NEAR SHOEMAKER.

Near Earth Asteroid Rendezvous Shoemaker (NEAR Shoemaker) was the first mission launched under the Discovery Program. The spacecraft investigated the asteroid 433 Eros. Asteroids are small celestial bodies that orbit larger ones. Most of the asteroids in the solar system are found in a massive asteroid belt that circles around the Sun between the orbits of Mars and Jupiter. 433 Eros is twenty-one miles long and has an elliptical orbit that carries it outside the Martian orbit and then in close to Earth orbit. On February 12, 2001, NEAR softly touched down on the asteroid. It was the first time in history that a spacecraft had lande on an asteroid. When it happened, 433 Eros was 196 million miles from Earth.

NEAR orbited 433 Eros for nearly a year before landing and returned dozens of high-resolution photographs of the asteroid. Before it landed, the spacecraft was renamed NEAR Shoemaker in honor of the geologist Eugene M. Shoemaker (1928-1997). The first Discovery mission was considered an overwhelming success.

LUNAR PROSPECTOR.

Lunar Prospector launched on January 7, 1998, and assumed an orbit around Earth’s Moon to collect scientific data. On July 31, 1999, the spacecraft was purposely crashed into the lunar surface.

STARDUST.

Stardust collected particle samples from comet Wild 2 and returned them to Earth. Wild 2 (pronounced “Vilt” 2) is a relatively small comet with a

nucleus measuring about three miles in diameter. Stardust carried a particle collector about the size of a tennis racket that could capture and hold tiny comet dust particles. (See Figure 6.2.) It collected and analyzed interstellar dust and successfully sampled particles from the coma of Wild 2. These particles are believed to be more than 4.5 billion years old.

As the spaceship approached Earth, it released the sample-return capsule. On January 15, 2006, the capsule entered Earth’s atmosphere. Parachutes were deployed, and the capsule landed safely in the Utah desert. The main spacecraft was put into orbit around the Sun and will likely be used on a future mission. As of March 2008, scientists continued to analyze the particles collected by Stardust.

GENESIS.

Genesis was a revolutionary spacecraft designed to collect charged particles emitted from the Sun and return them safely to Earth. It would assume a tight orbit around LI for 2.5 years, collect samples, and then head back to Earth. After reentering Earth’s atmosphere, Genesis would deploy its parachutes for a slow descent toward the surface. A specially equipped helicopter was to snag the spaceship midair and carry it to land.

On September 8, 2004, Genesis began its descent to a Utah landing site. However, its parachutes did not open,

and the spacecraft plummeted at high speed into the ground. The capsule containing the samples split open during the crash, exposing the sample medium to the outside atmosphere. However, approximately 4 milligrams of samples were saved. As of March 2008, they were being analyzed by scientists. Genesis was the first spacecraft to return extraterrestrial materials to Earth since the Apollo missions.

DEEP IMPACT.

On July 4, 2005, Deep Impact encountered the comet Tempel-1 and sent a probe into the comet’s surface. The comet was discovered by the French astronomer Ernst Tempel (1821-1889) in 1867. It orbits the Sun between Mars and Jupiter with a 5.5-year orbital period. The probe relayed data to the flyby portion of the spaceship. NASA scientists are using the data to determine the physical and chemical makeup of the comet. As of March 2008, the Deep Impact flyby spacecraft remained in space and was expected to be used for a future Discovery MO.

Future Discovery Missions

As of March 2008, future Discovery missions include one full-fledged mission called Kepler and an MO called Moon Mineralogy Mapper (M3). The Kepler space telescope was scheduled to launch in 2008, and will assume an Earth-trailing heliocentric orbit (an orbit behind Earth around the Sun) and survey a region of the Milky Way looking for planets similar to Earth. Kepler will be pointed toward the constellations Cygnus and Lyra. The primary stars in this region include Vega, Deneb, and Albireo.

The M3 is an instrument suite aboard the spacecraft Chandrayaan-1. Chandrayaan-1 is a lunar orbit spacecraft scheduled to be launched in 2008 by the Indian Space Research Organization.

SPACE OBSERVATORIES

Celestial objects emit all kinds of radioactive waves that give clues about their shape, size, age, and location. Earth’s atmosphere filters and blocks out most of this radiation, keeping it from reaching the planet’s surface. This is one reason biological life is possible on planet Earth. The atmospheric shield is good for human health but bad for observing the universe.

To clearly detect all radioactive waves traveling through space, an observer has to be located above the shielding effects of Earth’s atmosphere. The human eye can only detect one type of radioactive wave: visible light. These waves comprise only a small fraction of the radiation located throughout 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. These observatories were developed over many years. As of March 2008, NASA had four Great Observatories orbiting in 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 the different types of light energies. This scale is called the electromagnetic spectrum. (See Figure 6.3.) 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.

For most celestial objects there is a relationship between their temperature and the main type of radiation that they emit. Astronomers use the kelvin temperature scale to measure temperature. A temperature of zero kelvin is called absolute zero (the lowest possible temperature in the universe, at which all atomic activity ceases). This is equivalent to –459.7 degrees Fahrenheit.

Most stars have temperatures between thirty-five hundred and twenty-five thousand 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 with a temperature above absolute zero emits infrared waves. This makes infrared observation an extremely important astronomical tool. There are huge clouds of dust and gas located throughout 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 extremely high temperatures and releases of gamma-ray, x-ray, and ultraviolet radiation. 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 6.4 defines these terms and describes other cosmic objects. According to NASA, ultraviolet observations are useful for studying

the youngest, hottest, and most massive stars, besides quasars and white dwarf stars. 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 billions of years ago.

Telescopes as Time Machines

It takes a long time for energy waves to travel through space. Even though 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 186,282 miles per second. This seems extremely fast. However, most celestial objects are so incredibly far 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 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. The Sun is eight light-minutes from Earth. This means that it takes eight minutes for an energy wave from the Sun to reach Earth.

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 extremely distant 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 of the 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 were first developed 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 is thinner. Later, they were sent up in high-altitude balloons and even airplanes.

During the 1930s the American engineer Karl Jansky (1905-1950) 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 that arrived from outer space.

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. 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 deoxyribonucleic acid, 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.

MAUNA KEA OBSERVATORIES.

Mauna Kea is a fourteen-thousand-foot-high extinct volcano in Hawaii. A number of observatories have been erected atop Mauna Kea, because the thin atmosphere allows detection of near-infrared radiation. This is a narrow band of infrared radiation that lies closest to visible light in the electromagnetic spectrum.

The Keck Observatory is operated at Mauna Kea by NASA in conjunction with the California Institute of Technology and the University of California. The Subaru Telescope is a project of the National Institutes of Natural Science of Japan. The Gemini Observatory is an international collaboration between the United States, the United Kingdom, Canada, Chile, Australia, Argentina, and Brazil. Other universities and institutions from around the world also operate telescopes atop Mauna Kea.

GRAN TELESCOPIO CANARIAS.

In August 2007 Spanish astronomers unveiled the largest terrestrial telescope to date: Gran Telescopio Canarias. The observatory is located on the Canary Islands off the coast of Morocco in northern Africa. NASA notes in “World’s New Largest Telescope Gets Ready for Planet-Hunting” (August 2, 2007, http://planetquest.jpl.nasa.gov/news/telescopePlanetHunting.cfm) that the new telescope is only 4% larger than the Keck I and II telescopes, the previous record holders. Gran Telescopio Canarias was expected to begin scientific operations in 2008. It will be used to search for exoplanets, brown dwarfs, nearby stars, and other celestial phenomena.

Early Space Telescopes

In 1923 the scientist Hermann Oberth (1894-1989) proposed a space telescope in “Die Rakete zu den Planet- enräumen” (“The Rocket into Interplanetary Space”). Two decades later the physicist Lyman Spitzer Jr. (1914-1997) more fully outlined the scientific benefits of putting telescopes in space. At the time, space travel was not even possible.

In April 1966 NASA launched its first space telescope, the Orbital Astronomical Observatory. This launch was the 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 August 1975 the European Space Agency (ESA) launched COS-B, which provided the first complete gamma-ray map of the galaxy.

Spitzer and many 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 the Large Space Telescope (LST). Following the Moon landings, NASA’s budget was severely cut, which forced scientists to downsize the LST project several times. In 1975 the ESA joined the project and agreed to fund a percentage of the LST’s costs in exchange for a guaranteed amount of telescope time for its scientists. Two years later Congress authorized funding for the 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 use 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 Edwin P. Hubble (1889-1953). The 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 6.4 shows the general layout of the HST. It can hold eight science instruments besides 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 6.5 provides design, cost, and operational data about the HST. NASA’s Goddard Space Flight Center (GSFC) in Greenbelt, Maryland, is responsible for over-sight 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. The STSI is operated by the Association of Universities for Research in Astronomy, an international consortium of dozens of universities.

The HST transmits raw data to one of NASA’s Tracking and Data Relay Satellites (TDRSs). There are six of these satellites in LEO. 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 the GSFC and then to the STSI.

The HST was originally supposed to go into space in 1986. The explosion of the space shuttle Challenger that year delayed the HST’ s launch for four years. On April 24, 1990, the HST was carried into orbit by the space shuttle Discovery. One day later the astronauts deployed the satellite approximately 380 miles from Earth. It assumed an orbit that is nearly a perfect circle.

When scientists first started using the HST, 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 Endeavour 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 the HST’ s performance, and reboosted the satellite to a higher altitude. Like all objects in LEO, the HST experiences some drag and gradually loses altitude. The satellite does not have its own propulsion system.

THE HST NEEDS SERVICING.

The HST 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 February 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 the NASA administrator Sean O’Keefe (1956–) announced his decision to cancel the fifth servicing mission to the HST. A shuttle flight to the observatory was considered too risky because the HST orbits far from the International Space Station. In the event of an emergency, the space station is the only safe haven available to shuttle astronauts.

The decision caused an uproar in the scientific community. Without a reboost, the HST would gradually fall out of orbit and be destroyed by reentry to Earth’s atmosphere. In October 2006 NASA announced that the agency was reinstating the HST servicing mission. As of March 2008, the mission had not taken place, but it was tentatively scheduled for August 2008.

In the fact sheet “Hubble Space Telescope Servicing Mission 4” (October 2006, http://www.nasa.gov/pdf/161640main_SM4-Mission_Fact%20Sheet%203.pdf), NASA describes the dire conditions aboard the HST in 2006. At that time, scientists had been operating the HST for two years with only two gyroscopes, the minimum necessary to accurately point the telescope at its target. Only four of the HST ’s six original gyroscopes were still operational. Two of the four were shut down to hold in reserve as a backup. NASA estimated that the existing

gyroscopes on the telescope would allow it to operate only through the end of 2008. In addition, NASA was concerned about the satellite’s deteriorating batteries and outside heat shielding. In 2004 the HST’s Space Telescope Imaging Spectrograph (STIS) quit working when its power supply failed. In January 2007 the Advanced Camera for Surveys (ACS) suffered a similar failure. The loss of the ACS was particularly troublesome, because it has supplied many of the most sensitive and dramatic images captured by the HST over the years. NASA’s plans for the servicing mission include replacing all of the HST’s batteries and gyroscopes and installing new external thermal protection sheets. These modifications are expected to extend the HST’s useful life through 2013. Repairs of the STIS and the ACS and the installation of two new instruments were also planned for the servicing mission.

During its mission the HST has captured more than seven hundred thousand images of celestial objects throughout the universe, including nebulae, galaxies, stars, and even some bodies believed to be planets. In late 2005 NASA began using the telescope to search Earth’s Moon for oxygen-bearing minerals. Scientists hope to find a potential source of oxygen for use by future astronauts visiting the lunar surface. The HST provided the first-ever high-resolution ultraviolet images of the Moon.

The 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 American physicist Arthur Holly Compton (1892-1962).

On April 5, 1991, the CGRO was launched into space aboard the space shuttle Atlantis. The satellite was deployed at an orbit altitude of 280 miles above Earth. It was equipped with four highly sensitive detecting instruments and a pair of solar arrays. (See Figure 6.5.) 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 the CGRO to higher altitudes.

The original plan was for a five-year mission. However, 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 the CGRO’s gyroscopes had failed. Even though the observatory’s instruments were still in working order, NASA decided to purposely deorbit the satellite. Scientists feared that if left to reenter on its own, the CGRO could rain pieces down on populated areas. This was of particular concern because the satellite was unusually massive at seventeen tons and included a fair amount of titanium in its components. Titanium is not easily disintegrated during atmospheric reentry.

On June 4, 2000, the 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 United Kingdom, and the ESA. 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. The CGRO recorded more than twenty-five hundred gamma-ray bursts, far more than had ever been detected before. It also mapped hundreds of gamma-ray sources.

The 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 Indian-American scientist Subrahmanyan Chandrasekhar (1910– 1995). The CXRO can detect x-ray sources that are billions of light-years from Earth.

On July 23, 1999, the CXRO was launched into space by the shuttle Columbia. The CXRO 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 Earth.

At that point, the CXRO’s twin solar wings were unfolded and began converting sunlight into electrical power. Figure 6.6 shows a diagram of the spacecraft. Important design information is included in Table 6.6. Solar power is used to run the CXRO’s equipment and charge its batteries.

The CXRO circles Earth once every sixty-four hours. The satellite’s orbit is highly elliptical. At its closest point the observatory is 6,214 miles from Earth, and at its farthest point it is 86,992 miles. (See Table 6.6.) This is nearly one-third of the distance to the Moon. The CXRO’ s flight path carries it through the Van Allen radiation belts that encircle Earth. When the satellite is in or near this highly radioactive area of space, its sensitive electronic instruments are turned off.

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 May 2007 the CXRO imaged the brightest stellar explosion ever recorded. The supernova known as SN 2006gy is in the galaxy NGC 1260, which is approximately 240 million light-years from Earth.

The CXRO was designed for a five-year mission. As of March 2008, the telescope had been operating for more than eight years and was expected to remain in service for up to seven more years. The satellite will likely continue to orbit for decades after that.

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 collects and maintains all astronomy data obtained from the CXRO and NASA’s smaller gamma-ray, x-ray, and extreme ultraviolet observatories.

The 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 many space shuttle missions. In 1983 NASA decided the telescope needed to be in continuous orbit and able to fly alone. 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 many 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 6.7 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 6.8 shows details of the observatory’s structure.

The 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 seven thousand people from around the world entered the contest. Some of the most frequently proposed names were Red Eye, Sagan (after the scientist Carl Sagan [1934-1996]), Herschel (after the scientist John Herschel [1792-1871], who in 1800 discovered infrared radiation), and Roddenberry (after the science-fiction writer Gene Roddenberry [1921-1991], the creator of the Star Trek television show). In the end NASA chose the name Spitzer to honor the late American physicist. It was Spitzer’ s groundbreaking work in 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 Earth in an Earth-trailing heliocentric orbit. 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 an extremely cold temperature to operate properly. Everything with 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.

Spitzer 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 179,000 light-years from Earth.

In early 2005 the telescope captured the first-ever infrared images of two exoplanets. The two large gas planets, known as HD 209458b and TrES-1, orbit close to their respective suns. This makes them impossible to image directly with visible light telescopes. HD 209458b circles a yellow sun 150 light-years from Earth in the constellation Pegasus. TrES-1 is located five hundred light-years from Earth in the constellation Lyra.

In October 2005 NASA released infrared images of the spiral galaxy Messier 31, which is commonly called Andromeda. The galaxy is about 2.5 million light-years from Earth. Spitzer recorded nearly eleven thousand separate snapshots of the galaxy that were pieced together to form a dramatic mosaic. The images reveal old giant stars at Andromeda’ s center and a system of arms spiraling out to a prominent ring of stars.

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 high-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.

Spitzer is managed by NASA’s Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, California. The university also includes the Spitzer Science Center, which conducts science operations for the telescope. According to John R. Stauffer et al., in The Spitzer Warm Mission Science Prospects (2007, http://ssc.spitzer.caltech.edu/mtgs/warm/wp/003stauffer.pdf), Spitzer’s cryogenic helium supply will be depleted by mid-2009, and its primary mission will end. Because some of the instruments will continue to operate, astronomers hope to conduct limited astronomical observations during a so-called warm mission that could last up to five years.

The Next Great Observatory

As of 2008, NASA’s next Great Observatory was under development and scheduled for launch in 2013. It is called the James Webb Space Telescope (JWST) in honor of James Edwin Webb (1906-1992), the NASA director of flight operations during the Apollo program. The JWST will detect infrared radiation from its orbit location at L2 approximately one million miles from Earth in the direction away from 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 will also make it easier to shield the telescope from the massive infrared radiation coming from the Sun, Earth, and Moon. The JWST will look outward into space.

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 Explorers Program. (See Table 6.2.) The projects are conducted with the help of scientists from academic institutions and/or foreign space agencies.

The ESA operates two observatories of its own: the X-ray Multimirror Mission (XMM-Newton) and the International Gamma Ray Astrophysics Laboratory (INTEGRAL). XMM-Newton was named after Isaac Newton (1642-1727). This powerful telescope is similar 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. It was launched into space on December 10, 1999, by an Ariane rocket. XMM-Newton was expected to have a ten-year lifetime. Its images have provided crucial information to scientists researching black holes and the formation of galaxies.

INTEGRAL was launched in October 17, 2002, and images celestial gamma-ray sources. It orbits Earth once every three days. In “Five Years of INTEGRAL” (October 17, 2007, http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=41414), the ESA summarizes the major findings of the observatory. Scientists are using INTEGRAL data to study black holes, explosive stars, and other objects in the Milky Way and in other galaxies.

THE SUN-EARTH CONNECTION

One of NASA’s primary research goals is to learn about the Sun-Earth connection. The Sun bathes Earth with life-sustaining warmth. It also emits radiation, charged particles, and other substances that can disrupt

electrical systems and electronic equipment vital to modern societies. Since the beginning of the space age scientists have put satellites in Earth orbit to monitor the Sun and its effects on Earth. During the 1990s NASA begin sending spacecraft even farther out into space to investigate solar phenomena that directly affect planet Earth.

Solar Wind

The solar wind is a constant flow of charged particles from the Sun into space. It moves away from the Sun at a speed of two hundred to four hundred miles per second and flows across the solar system reaching just past Pluto. The region of outer space exposed to the solar wind is called the heliosphere.

The solar wind constantly pushes and shapes Earth’s magnetosphere. This is the region of space around Earth dominated by the planet’s magnetic field. (See Figure 6.9.) The magnetosphere helps protect Earth from dangerous electromagnetic radiation moving through space. The outer boundary of Earth’s magnetosphere is called the bow shock.

The speed, composition, density, and magnetic field strength of the solar wind are not constant but vary depending on conditions on the Sun. A gust of solar wind can energize Earth’s magnetosphere, producing beautiful auroras, but wreaking havoc on sensitive electronics.

Sunspots

Sunspots look like dark blemishes on the surface of the Sun. They are actually areas of plasma that are slightly cooler than their surroundings due to magnetic activity. Sunspots can be enormous in size, even bigger than planet Earth. Sunspots can appear, change size, and disappear. Each typically lasts from a few hours to a

few months. Most sunspots are visible only through telescopes.

In 1826 the amateur German astronomer Samuel H. Schwabe (1789-1875) began keeping detailed records of the number of sunspots he observed every day. After nearly two decades of doing this, he noticed a cyclical pattern to the data. By the early 1900s scientists knew that the number of sunspots peaks approximately every eleven years and then drops off dramatically. (See Figure 6.10.) This is called the solar cycle. The period of peak sunspot activity is called the solar maximum, and the period of lowest activity is called the solar minimum.

In 1952 the International Council of Scientific Unions proposed that scientists around the world cooperate in conducting extensive earth science investigations between July 1957 and December 1958. This was the next expected solar maximum. Scientists hoped to collect large amounts of data on the relationship between sunspots and solar activity and the resulting effects on Earth’s magnetic fields. This research remained a priority throughout the space age. The most recent solar maximum occurred in 2000-01. The next peak is expected in 2011-12.

Solar Flares

Solar flares are sudden energy releases that burst out from the Sun near sunspots. They are most common during the solar maximums. Solar flares emit electromagnetic radiation, and can include energy particles and bulk plasma. They can last from minutes to hours.

Solar flares that erupt in Earth’s direction can shower the planet with energetic particles within thirty minutes. Solar flares are associated with auroras and magnetic storms on Earth and can cause radio interference if large amounts of x-ray radiation are released.

Solar Prominences

Solar prominences are giant clouds of dense plasma suspended in the Sun’s corona (outermost atmosphere). They are usually calm, but occasionally erupt and snake out from the Sun along magnetic field lines. Prominences can break away from the Sun and hurtle through space carrying large amounts of solar material.

Coronal Mass Ejections

Coronal mass ejections (CMEs) occur when billions of tons of particles are slung away from the Sun by broken magnetic field lines. CMEs move away from the Sun into space at millions of miles per hour. These ejections happen frequently and are most common during the solar maximum, when they can occur several times a day. CMEs are known informally as solar storms.

When a CME blasts toward Earth, the planet’s magnetosphere is hit within two to four days by a cloud of electrically charged particles. This cloud distorts the entire magnetosphere. (See Figure 6.11.) The resulting disturbance is called a geomagnetic storm. These storms can harm satellites and disrupt telecommunications and electric power generation around the world.

Space Weather

The term space weather was created by scientists to refer to the overall effects of solar activities on the space around Earth (or geospace). Space weather encompasses all the solar phenomena (solar wind, sunspots, flares, prominences, and CMEs) and the resulting conditions in Earth’s magnetosphere and upper atmosphere.

The National Oceanic and Atmospheric Administration (NOAA) monitors space weather and issues warnings about geomagnetic and solar radiation storms (increases in the number of energy particles in geospace). NOAA’s Space Environment Center is located in Boulder, Colorado. It serves as the national and worldwide warning center for space weather disturbances. The Space Weather Operations branch is jointly operated by NOAA and the U.S. Air Force.

The monitoring of space weather is important. A major solar event such as a CME can cause a tremendous inflow of electrical energy into Earth’s atmosphere. This is disruptive to electrical power grids and telecommunication systems on the ground and dangerous for Earth-orbiting satellites. NASA also worries about possible harmful exposure of astronauts to excessive radiation levels during solar storms.

Ongoing Solar Exploration Missions

NASA and other space agencies operate a number of missions dedicated to studying the Sun-Earth connection. Table 6.7 describes the major missions as of December 2007. Most of these spacecraft are stationed in Earth orbit and study the effects of solar activity on the magnetosphere or upper atmosphere. Some of the spacecraft are in orbits far from Earth or on other paths designed for optimal study of the Sun-Earth connection.

INTERNATIONAL SOLAR-TERRESTRIAL PHYSICS PROGRAM.

During the 1990s NASA teamed with the ESA and the Japan Aerospace Exploration Agency in the project known as the International Solar-Terrestrial Physics Science Initiative. The purpose of this project is to combine international resources to conduct long-term investiga-

tions of the Sun-Earth space environment. The program includes both ground-based studies and space missions. The space activities fall into two categories: terrestrial (Earth directed) and solar (Sun directed).

Terrestrial space missions rely on satellites in Earth orbit that gather data about the Sun’s effects on the planet. This effort is supported by two other U.S. federal agencies: NOAA and the Los Alamos National Laboratory.