This time we’re going to look back at one of NASA’s most spectacular successes: the Chandra X-Ray Observatory. Launched by the space shuttle on July 23, 1999, Chandra has now far exceeded its originally projected lifetime, and still holds the distinction of being the most sophisticated X-ray observatory built to date. This single instrument has greatly added to our understanding of the universe, and when the history books of the future talk about these early days of space exploration, the Chandra Observatory is one of the names they will mention most fondly.
Before looking at the satellite, we should tip our hats to the man behind the name. Subramanyan Chandrasekhar was one of the foremost astrophysicists of the 20th century. While anything approaching a complete discussion of this man’s contributions to science would require several articles, we should acknowledge, in general, that he contributed some of the most fundamental ideas of modern astrophysics, and literally changed the entire field. He is widely regarded as the most influential thinker in this area since Albert Einstein. Unfortunately, Chandrasekhar died in 1995, and never got to see the success of the spacecraft that bears his name.
The Chandra Observatory, which is in a highly elliptical orbit around Earth, still holds several distinctions which remain unchallenged to this day. With a length of 45 feet, it is the largest satellite ever launched by the space shuttle. Chandra’s mirrors are the smoothest and most precisely aligned mirrors ever made. It has the highest resolution of any X-ray telescope, producing images 25 times sharper than those of its best predecessor.
Chandra’s telescope system consists of four pairs of mirrors with their support structure. X-ray particles have much more energy than photons of visible light, and would penetrate into a mirror’s surface if they hit it head-on. Because of this, it is necessary to turn the mirrors at an oblique angle to the direction the X-ray particles are traveling, which causes the particles to hit the surface and ricochet off. Thus the mirrors in Chandra are barrel-shaped rather than flat like conventional mirrors.
These mirrors focus the X-rays on a spot on the focal plane about half the width of a human hair. It is here that two instruments come into play, the High Resolution Camera (HRC) and the Advanced CCD Imaging Spectrometer (ACIS).
The HRC detects X-rays reflected from the mirror assembly, and is capable of taking images showing detail as small as one-half of an arc-second. This is the level of resolution that would be necessary to read a newspaper from a distance of half a mile. The HRC is especially useful for studying hot matter in the aftermath of stellar explosions, and in distant galaxy clusters, as well as identifying extremely faint X-ray sources.
The Advanced Charged Coupling Device Imaging Spectrometer (ACIS) is a series of charged coupled devices, more advanced versions of the ones used in camcorders. With this instrument, scientists can produce images using X-rays made only by one chemical element. For instance, multiple images of a single object might be taken in the light of oxygen ions, neon ions and iron ions. It is ideally suited for studying temperature and chemical variations across huge clouds of interstellar gas.
There are two instruments on-board Chandra that are used for X-ray spectroscopy, the High Energy Transmission Grating Spectrometer (HETGS) and the Low Energy Grating Spectrometer (LETGS). Each of these assemblies contains hundreds of gold gratings, which intercept the X-rays and diffract them as a prism diffracts visible light, separating them into individual X-ray lines. The resulting spectra allow analysis of the temperature, ionization and chemical composition of the source.
Using this unrivaled array of instruments, the Chandra Observatory has produced a staggering amount of science and images in its career. While a full discussion of this material would fill many books (and already has, no doubt) here we will present just a few of Chandra’s Greatest Hits:
1. Study of dark energy. A few years ago, scientists made a discovery that was completely unexpected: the universe’s expansion is speeding up. Before this discovery, it was assumed that the Big Bang had blown everything apart, and the expansion of the universe that we see today is that motion continuing. In that case, as the universe got bigger and bigger, it would lose energy and start to slow down. In other words, astronomical observations of very distant regions, which show things as they were in the early time of the universe, should show the universe expanding faster than it does today. Instead, it was found that the universe is expanding faster now than it was in earlier times, forcing scientists to conclude that some unknown force was making this expansion speed up. This force, which remains unknown today, is called dark energy. It seems to only come into play on a very large scale, since the attractive force of gravity obviously dominates on the local scale.
While we may not know what dark energy is, we now know something about what it does, thanks largely to Chandra. Observations from the observatory have shown us the action of dark energy in detail. For instance, observations of the galaxy cluster Abell 85, about 740 million light years from Earth, reveal that the predictable collapse of dust and gas that forms galaxies has been slowed down by the repulsive force of dark energy, stifling the growth of galaxies. By comparing the expected rate of collapse with the real, observed rate, we can quantify the repulsive force of dark energy. Chandra is ideally suited for observations of this nature, and when scientists finally figure out what this stuff is, their discovery will probably owe something to the data gathered by Chandra.
2. The “Hand of God” image. This one is an interesting scientific observation, and a humdinger of a picture, too. A tiny, dense pulsar only 12 miles wide is spinning madly and spraying energy in a huge pattern spanning 150 light years. Called PSR B1509-58, it spins about seven times a second. Its enormous release of energy is thought to stem from an extremely powerful magnetic field, estimated to be 15 trillion times as strong as Earth’s. As the pulsar’s wind of electrons and ions travels through the magnetized gas, it creates the elaborate nebula seen by Chandra. In the false-color image released by NASA, the X-rays with the lowest energy are red, those in the mid-range are green, and the ones with the highest energy are blue- a color scheme that makes for a dramatic image. The resulting picture bears an uncanny resemblance to a spread hand- hence, the name.
3. Clearest-ever image of the Crab Nebula. This one is similar to the last one, in that it is produced by a combination of an intense magnetic field and a rapidly-rotating pulsar. In this case, the resulting energy is spraying out jets of matter and anti-matter from the pulsar’s north and south poles, as well as an intense wind flowing from the middle region. In the image at the NASA website, the motivating pulsar is clearly visible, looking like the bull’s eye of a target surrounded by a shock wave generated when the pulsar’s emissions hit the surrounding nebular gas. The cloud surrounding the pulsar is twisted in elaborate swirls, which are actually tracing the lines of the nebula’s magnetic field. While the Crab Nebula has been observed many times before, this is the first time these lines could be seen so clearly.
4. Brightest supernova ever observed. In collaboration with ground-based telescopes, Chandra has taken pictures of the supernova SN 2006gy, which is the brightest and most energetic explosion ever recorded. In its spectacular death-throes, the original star expelled two lobes of gas before it exploded, and these show up as two lights of pale lavender in the Chandra image. The explosion has slammed into the surrounding gas, causing a shock wave that is producing some of the visible light. The rest of the visible light is made by debris that has been heated by radioactivity. This observation allowed astronomers to determine that the feature was definitely caused by the collapse of an extremely dense star, and not the collapse of a white dwarf star, an alternative theory that had been discussed.
The list goes on, and it’s not finished yet. Chandra, living up to its reputation, is still going strong, and will certainly give us a lot more great science. Having survived for more than twice its planned lifetime, it is a great example of a space project that has exceeded even the wildest dreams of its designers, and is still doing so. There is no reason to think that Chandra won’t keep going for years to come- and if the past is any indication, its achievements will be amazing and beautiful.
When they happen, you can read about them here, of course.
(Of course, you want to see this stuff, right? Please, hang out here for a while and enjoy our articles- but when you’re through doing that, go to the “10 Years of Chandra” address below and check out “Cool Stories From the Hot Universe.”)
Sources:
“Space Topics: Chandra X-Ray Observatory” at the website of the Planetary Society: planetary.org/explore/topics/space_missions/chandra/
“Chandra Feature 5-11-10: X-Ray Discovery Points to Location of Missing Matter” at the NASA website: nasa.gov/mission_pages/chandra/news/10-048.html
“Chandra X-Ray Observatory: CXC Operated for NASA by the Smithsonian Astrophysical Observatory” at the Harvard website: chandra.harvard.edu/
“Chandra X-Ray Observatory: the Chandra Mission” at the Harvard website: chandra.harvard.edu/about/axaf_mission.html
“10 Years of Chandra” at the Harvard website: chandra.harvard.edu/ten/