By Robert Naeye
By the middle of the 20th century, astronomers had made enormous strides in understanding our universe. They had unlocked the secret of stars and discovered millions of galaxies – island realms unto themselves. They even had discovered that the universe is expanding.
Birthplace of stars - A Hubble Space Telescope image of a star-forming region of interstellar hydrogen gas and dust in the “Eagle Nebula” 7,000 light-years away in the constellation Serpens.
But astronomers were missing out on some of the most exciting action in outer space. They were blissfully unaware of exploding stars tapping unfathomable levels of energy. They had no idea that at the heart of every large galaxy lurks a giant vortex where matter plunges down the universe’s ultimate drain. And even their best telescopes were unable to peer deep into stellar nurseries to watch nascent stars incubating in clouds of gas and dust. They were missing out on an entire macrocosm where nature displays her most splendid creativity … and her fury.
The discovery of this universe would have to wait for the Space Age, when humans could launch their telescopes and detectors above Earth’s atmosphere. As astrophysicist Ed Weiler, director of NASA’s Goddard Space Flight Center, explained, “The amount of light that reaches the ground is an extremely tiny portion of what stars and galaxies actually put out.”
Like a radio receiver that can only pick up one station, our eyes are tuned to a narrow sliver of all the light that is emitted in the universe. We can’t see radio waves or infrared light because their wavelengths are too long to stimulate the chemical receptors in our retinas. On the other end, ultraviolet light, X-rays and gamma rays have wavelengths that are too short. Objects in space emit all these forms of light, so visible light paints an incomplete picture of what is out there.
But most of this light cannot penetrate Earth’s atmosphere. The gases we breathe absorb infrared light, ultraviolet light, X-rays and gamma rays. To see these invisible forms of light, astronomers must loft their telescopes into space. A series of successful space telescopes in the 1960s and '70s motivated the construction of NASA’s four Great Observatories: the Hubble Space Telescope, the Compton Gamma Ray Observatory, the Chandra X-ray Observatory and the Spitzer Space Telescope. A triumph of engineering and ingenuity, these orbiting platforms have ushered in a new era of scientific discovery by transforming human understanding of the universe.
The space-astronomy revolution began with little fanfare on June 18, 1962. A small group of scientists led by Italian-American physicist Riccardo Giacconi gathered at White Sands, N.M., to launch a small U.S. Air Force sounding rocket into space. The rocket would spend just a few minutes above the atmosphere before plummeting back to Earth. But those few precious minutes in space would fundamentally change astronomy.
Giacconi’s rocket carried a small instrument for detecting X-rays. It picked up a glow of X-rays coming from all directions, and an intense source of X-rays in the constellation Scorpius. No scientist had predicted the existence of an “X-ray star” giving off more energy than a million suns, and many didn’t think that X-rays could be detected from beyond our solar system. By proving the skeptics wrong, Giacconi’s experiment opened up a new window to the universe – a strange, unexpected cosmos that beckoned for exploration. The enduring legacy of this rocket experiment earned Giacconi a share of the 2002 Nobel Prize for Physics.
Center of the galaxy - The central high-energy regions of our Milky Way galaxy viewed in panorama by the Chandra X-ray Observatory.
“This initial discovery of X-rays from beyond the solar system encouraged others to continue and extend their efforts to open all of these bands with observations from space,” said astrophysicist Harvey Tananbaum of the Harvard-Smithsonian Center for Astrophysics. “Moreover, in the next decade or so, we came to appreciate the prevalence of explosions, collisions, and extreme physical conditions throughout the cosmos.”
During the remainder of the 1960s astronomers developed increasingly sophisticated, yet relatively low-cost, balloon- and rocket-borne experiments to see if the universe was worth exploring from outer space. These efforts culminated in 1970, when Giacconi and several colleagues launched for NASA Uhuru (the Swahili word for “freedom”), the first X-ray satellite. Among the several hundred X-ray objects detected by Uhuru, it found the first convincing black hole candidates. Further observations revealed that the X-ray star in Scorpius was a neutron star, the collapsed ultradense remnant of a massive star. By revealing new classes of objects hitherto unknown to science, space astronomy had come of age.
NASA jumped on the bandwagon by deepening its involvement and funding. The 1970s would witness the launches of successful missions such as the International Ultraviolet Explorer in 1978 and the Einstein Observatory in 1979. Einstein was the first X-ray observatory that could actually take X-ray images of objects outside the solar system, revealing new details about giant clusters of galaxies and supernova remnants – the tattered remains of exploded stars.
NASA continued its successes in the 1980s, with a series of modest but successful missions. Perhaps most notable, the Cosmic Background Explorer (COBE) satellite, launched in 1989, confirmed that our universe originated from an expanding fireball known as the big bang. Two of COBE’s leading scientists, John Mather of Goddard and George Smoot of the University of California, Berkeley, shared the 2006 Nobel Prize for Physics.
With space astronomy firmly established as a mature field by the late 1970s, it was time to take it to the next level. Space telescopes had racked up a dazzling array of accomplishments, but they still lagged far behind their terrestrial counterparts in size and sophistication. This was about to change.
Construction of the space telescope that would become Hubble commenced in the late 1970s. The concept was the brainchild of Princeton professor Lyman Spitzer. As early as 1946, Spitzer called for an “extra-terrestrial observatory” to study the universe in visible light. Even though visible light reaches Earth’s surface, Spitzer was keenly aware that light from distant stars and galaxies travels trillions upon trillions of miles across space, only to be blurred during the final 100-mile passage through our planet’s atmosphere. A telescope above Earth, Spitzer correctly reasoned, could probe the universe with unprecedented clarity.
Astrophysicist Charlie Pellerin took Spitzer’s idea a step further. He conceived a “Great Observatories” program, which would include Hubble. Pellerin, who became NASA’s director of astrophysics in 1983, envisioned the construction of four large space telescopes that would operate simultaneously to cover a large swath of the spectrum. “We were trying to sell them individually, but, all being billion-dollar missions, we weren’t having much success,” recalled Weiler. “But Pellerin came up with a way to bundle them into a single program.”
NASA and Congress bought in to Pellerin’s dream, ultimately committing billions of dollars to turn it into reality. NASA would go on to launch four spectacularly successful astronomical satellites under the Great Observatories banner, although there was never a moment when all four operated simultaneously.
On April 25, 1990, astronauts aboard NASA’s space shuttle Discovery deployed Hubble into an orbit about 360 miles above Earth. The telescope was named for American astronomer Edwin P. Hubble (1889-1953), who in 1923 proved that distant “spiral nebulae” are galaxies, and that our Milky Way is just one of billions of galaxies in a universe so vast it defies human comprehension. Hubble’s later observations led to the realization that our universe is expanding.
NASA’s Mount Rushmore of space observatories - Clockwise from top, left: The Hubble Space Telescope, the Compton Gamma Ray Observatory, the Spitzer Space Telescope and the Chandra X-ray Observatory.
At first, things seemed to be going smoothly after the space telescope’s deployment. Ground controllers established contact and powered up its electrical systems. But over the next few weeks, astronomers realized something was terribly wrong: Every image appeared blurry. Hubble’s 8-foot-wide primary mirror had been figured to the wrong curvature, like eyeglasses with the wrong prescription. The figure was off by only one-fiftieth the width of a human hair, but that was enough to derail most of Hubble’s primary science objectives. The prospect of a $2-billion orbiting turkey was a looming disaster for the future of space astronomy.
Fortunately, Hubble was designed to be serviced by shuttle astronauts. This capability would save the telescope and redeem NASA’s reputation. Astronomers identified the problem and developed a solution. In a daring December 1993 mission, four spacewalking astronauts (Story Musgrave, Jeffrey Hoffman, Thomas Akers and Kathryn Thornton) installed corrective optics. Within a month, the space telescope was beaming back razor-sharp images of stars and galaxies.
Today, Hubble’s images have seeped into popular culture, gracing posters, T-shirts and stamps. By capturing the interest of people worldwide, Hubble has become an icon of astronomy. As astrophysicist Mario Livio of the Space Telescope Science Institute noted, “Hubble has done something unique: It has not only given scientists incredible discoveries, it has brought the excitement of those discoveries into the homes of people worldwide. Ask any person on the street to name a playwright and they’ll say ‘Shakespeare.’ For the name of a scientist, they’ll say ‘Einstein.’ For the name of a telescope, they’ll say ‘Hubble.’”
But the telescope’s long list of scientific accomplishments will be its ultimate legacy. Hubble is arguably the most important astronomical instrument since the great Italian scientist Galileo Galilei first aimed his telescope at the heavens in 1609, discovering the moons of Jupiter, mountains on the moon, sunspots, and the phases of Venus. Hubble has achieved all of its scientific objectives, and far surpassed the expectations of its builders. Hubble’s images and data have touched on virtually every astronomical field of investigation, from studies of the planets in our own backyard to galaxies at the edge of the visible universe. To date, Hubble has taken 500,000 images, and scientists have published 7,000 research papers on its findings.
When studying objects in our solar system, Hubble has given us sharp views of planets even if no spacecraft is paying them a visit. When 21 fragments of comet Shoemaker-Levy 9 rained down on Jupiter in 1994, Hubble returned the sharpest pictures – revealing towering plumes and dark splotches the size of Earth. When flight controllers needed to know weather conditions on the Red Planet prior to the landing of Mars Pathfinder in 1997, Hubble pictures gave them the information they needed. Hubble has observed aurorae on other planets, and discovered two new moons of Pluto.
When Hubble turns its gaze into our Milky Way galaxy, it gives astronomers amazing views of star clusters and the gaseous nurseries in which they form. Images of the most famous stellar nursery of all, the Orion Nebula, show that half of all newborn stars are surrounded by disks of gas and dust – nascent planetary systems in the process of formation. Hubble’s image of the Eagle Nebula is perhaps the most famous astronomical image of all time, revealing “Pillars of Creation” where gas is condensing into new stars.
With its ability to peer deep into the cores of other galaxies, Hubble has found gas and stars whirling at nearly light speed around seemingly nothing at all. But something – an invisible concentration of mass – must be accelerating this material to breakneck speeds. The most likely explanation involves one of nature’s most bizarre creations: black holes. Black holes are objects where nature crams so much mass into such a small volume of space that gravity triumphs over all other forces of nature. Like an escape-proof prison, anything that falls into a black hole, including light, can never get out.
Only monster black holes with the gravity of millions or billions of suns can accelerate matter to the incredible speeds measured by Hubble. Even more amazing, astronomers using Hubble and other telescopes have discovered that the larger the host galaxy, the larger its central black hole. For reasons that astronomers are only now beginning to fathom, the black hole somehow “knows” about the size of its host galaxy.
Hubble photo album - A “light echo” illuminates dust around supergiant star V838 Monocerotis.
But some of Hubble’s most astonishing images and discoveries involve its studies of the distant universe. Like all telescopes, Hubble is a time machine. Since light travels at a finite (albeit very fast) speed, the farther away an object exists in space, the further back we see it in time. Hubble’s deepest images, some involving images taken by staring at a single spot in space for more than 10 days, transport us so far back in time that we’re nearly back to the beginning of the universe itself. These so-called “Deep Fields” give humanity glimpses of the universe in its infancy, when galaxies were first “turning on,” setting the universe ablaze with the light of billions of stars.
The Hubble Deep Fields show that galaxies assembled with amazing rapidity – much faster than anyone expected. Some galaxies had already reached a mature state less than a billion years after the big bang – 13.7 billion years ago. It is like a mother giving birth to a son who grows into 230-pound NFL linebacker by the age of 5. And by catching galaxies at many different distances from Earth, astronomers using Hubble can literally see the universe evolve before their very eyes. Studies of galaxies from different epochs of cosmic history have revealed the crucial role of galaxy collisions and mergers in shaping the universe we see today.
Working in conjunction with ground-based telescopes, Hubble observations of exploding stars in distant galaxies have confirmed what many astronomers consider to be their most important and unexpected discovery of the past decade: The expansion of our universe is not slowing down under the pull of gravity, as most scientists predicted. Rather, the expansion is revving up. Some unknown form of “dark energy” must be overpowering gravity on large scales and causing galaxies to rush away from one another at increasing speeds. The nature of this dark energy is one of the most vexing mysteries in science.
But Hubble is just one of NASA’s four Great Observatories. In April 1991, only a year after Hubble launched, shuttle astronauts deployed the Compton Gamma Ray Observatory. The satellite, named after the great American physicist Arthur Holly Compton (1892-1962), winner of the 1927 Nobel Prize for Physics, studied the exotic, extreme universe: where nature harnesses extraordinary amounts of energy to produce gamma rays, the most energetic form of light.
Like most NASA astronomical satellites, Compton would go on to far exceed its expected mission lifetime. Designed to last for two years, Compton served as an orbiting sentinel for more than nine years.
Compton made a series of remarkable discoveries. It found dozens of galaxies that are shooting black-hole-powered jets directly toward Earth at virtually the speed of light. Fortunately, these galaxies are so far away that they cause no harm, but their power and sheer numbers flabbergasted astronomers. Closer to home, Compton mapped out vast clouds near the center of our galaxy where particles of ordinary matter are smashing into and annihilating their antimatter counterparts.
Compton’s most celebrated findings involved gamma-ray bursts, or GRBs. These powerful cosmic explosions were discovered in the 1960s by U.S. surveillance satellites, which were patrolling the heavens looking for clandestine Soviet nuclear explosions in violation of the Test-Ban Treaty. GRBs are bursts of gamma rays that suddenly appear out of nowhere, and then disappear in a matter of seconds to minutes – before astronomers can pinpoint their locations for follow-up observations. As Compton project scientist Neil Gehrels of NASA Goddard pointed out, “How do you research a phenomenon when it doesn’t last long enough for detailed studies?”
Compton provided a vital clue. It detected more than 2,700 GRBs, and found that they come from random directions in space. This was the breakthrough that astronomers had been eagerly anticipating. If GRBs originated in the Milky Way, most of them should line up with the plane of our galaxy. The fact that GRBs showed no preference for our galactic plane meant that they have to originate in the distant universe. This breakthrough, along with subsequent observations from the European BeppoSAX satellite and NASA’s Swift satellite (launched in 2004 and still in operation), has convinced astronomers that most GRBs are triggered by the explosive deaths of massive stars.
Next on the Great Observatory docket was the Chandra X-ray Observatory, named for Indian-American astrophysicist Subrahmanyan Chandrasekhar (1910-1995), winner of the 1983 Nobel Prize for Physics. Chandra studies X-rays, which are one notch down in energy from gamma rays. Launched aboard the space shuttle Columbia in July 1999, Chandra continues its groundbreaking science and shows few signs of slowing down. With its large and ultrasmooth mirrors, Chandra is providing sharp X-ray images that reveal a wealth of new detail about cosmic objects. As Tananbaum noted, “Chandra has opened new chapter after new chapter in our exploration of the universe.”
Star hatchery - NASA’s Spitzer Space Telescope shows infant stars “hatching” in the head of the hunter constellation Orion. Astronomers suspect that shockwaves from a supernova explosion in Orion’s head, nearly 3 million years ago, may have initiated this newfound birth.
Just months after launch, Chandra beamed down dramatic images of the inner sanctum of a supernova remnant known as the Crab Nebula. This cloud is the gaseous remains of a massive star that was seen to explode in 1054 A.D. Subsequent images taken about a month apart were turned into a motion picture of the Crab’s beating heart, the site of a city-sized neutron star produced by the explosion. Chandra showed that dramatic changes are taking place in real time, a rare thing in a science dominated by large distances and slow processes. Through careful study of these images, astronomers now understand how the rapidly spinning neutron star acts like a flywheel gone berserk, providing the staggering amount of energy needed to illuminate the entire nebula.
Chandra’s exquisite resolution has also enabled astronomers to map the abundance and distribution of various chemical elements inside supernova remnants, helping theorists reconstruct how massive stars explode. Supernovae produce and disperse elements such as carbon, oxygen and nitrogen — the elements that make up life as we know it. These studies are thus providing new insights into how supernovae – extraordinarily violent events – actually play a creative role by seeding our galaxy with materials necessary for life.
Chandra hasn’t just looked at supernovae in our galaxy. It has detected X-rays from comets and planets in our solar system, and has turned its eye to the distant universe. It has resolved the X-ray glow first seen in Giacconi’s 1962 rocket experiment into individual X-ray emitting objects. The glow is, in fact, the combined output of millions upon millions of distant galaxies, each powered by a monster black hole in its core. Chandra observations of clusters of galaxies show that these black holes repeatedly blow out vast amounts of gas in explosions so powerful they boggle the imagination.
Last but certainly not least, NASA launched the Spitzer Space Telescope in August 2003. Named after the visionary who first conceived of space telescopes, Spitzer observes the universe with an infrared eye. Infrared light is familiar to anyone who has worn night-vision goggles. Objects as diverse as dust particles in deep space to the human body radiate infrared light, more commonly known as “heat.”
To prevent the spacecraft’s own heat from swamping the faint infrared signals from distant objects, a refrigeration system keeps Spitzer’s detectors chilled to minus 457 degrees Fahrenheit, just two degrees above absolute zero — the coldest possible temperature in nature. And unlike the other three Great Observatories, which orbit Earth, Spitzer directly orbits the sun. “In its unique solar orbit, Spitzer follows the Earth around the sun, lagging farther behind with time like a playful puppy out for a walk,” said Spitzer project scientist Michael Werner of NASA’s Jet Propulsion Laboratory.
Spitzer has made its mark in a variety of fields, including key insights into galaxy, star, and planet formation. However, its biggest headlines have come from detailed studies of planets orbiting other stars, known as exoplanets. At the time of this writing in February 2008, astronomers have discovered more than 260 exoplanets. But the vast majority of these worlds have been discovered by the motion they induce on their host stars through the tug of their gravity. This method has been spectacularly successful at discovering planets, but it yields only limited information about the planets themselves: just their orbits and minimum masses.
Fortunately, a small fraction of planets have orbits that are aligned perfectly with Earth’s line of sight to the host star. Once every orbit, these planets pass in front of the star. During one of these so-called transits, the planet blocks a tiny portion of the star’s disk, leading to a small but measurable drop in its brightness. When the planet is on the other side of its orbit, it passes behind its host star, where it is completely blocked from view — an event known as a secondary eclipse.
Next up - A NASA Marshall Space Flight Center employee inspects a mirror segment of the James Webb Space Telescope, scheduled for launch in 2013.
With its heat-seeking detectors, Spitzer is perfectly poised to take advantage of this orbital geometry. All of the transiting exoplanets discovered to date hug their stars tightly, with orbits of just a few days. The stars are roasting the outer atmospheres of these planets to temperatures hotter than a blast furnace. By observing the system as the planet goes around the star, Spitzer catches the planet at different places in its orbit. When the planet is positioned to one side of the star, Spitzer captures the heat from both the star and planet. But during a secondary eclipse, Spitzer sees just the star. The difference between the two observations reveals the heat of the planet alone.
These studies have shown a planet known as HD 189733b has substantial water vapor in its atmosphere. (HD stands for Henry Draper, the 19th century American astronomer who cataloged stars that had not yet been named. Newly discovered planets that orbit these stars are given a letter such as b.) “Finding water on this planet implies that other planets in the universe, possibly even rocky ones, could also have water,” said Sean Carey of NASA’s Spitzer Science Center. Studies of other planets have found hints of high-altitude clouds made of silicate dust.
Astronomers have made temperature maps of HD 189733b and another planet, HD 149026b. This information reveals how 1,000-plus-mile-per-hour winds transport heat from the planets’ day sides to their night sides. For the first time, these planets have been transformed from data points to actual worlds, with astronomers possessing detailed information about their compositions and physical characteristics. “Spitzer has provided us with a powerful new tool for learning about the temperatures, atmospheres, and orbits of planets hundreds of light-years from Earth,” said Drake Deming of Goddard, one of the astronomers who conduct these studies.
Three of NASA’s four Great Observatories remain in operation, and a fleet of smaller, low-cost missions such as Swift, Galaxy Evolution Explorer (GALEX) and the Wilkinson Microwave Anisotropy Probe (WMAP) are still on patrol. NASA’s space astronomy program remains remarkably healthy and vibrant. But NASA has never been known to rest on its laurels. In 2008, NASA will launch the Gamma-ray Large Area Space Telescope (GLAST), which will follow up many of Compton’s discoveries, and undoubtedly break new ground. In 2009, NASA will launch Kepler, which could discover hundreds of Earth-sized planets by watching them transit their parent stars. And around 2013, NASA will launch the James Webb Space Telescope (JWST), a gigantic infrared observatory that will peer deep into the universe to catch the light of the first stars and galaxies. Space observatories past and present have revolutionized astronomy, but with next-generation telescopes on the near horizon, the best is yet to come.