The MAXIM Mission
In one of its most spectacular attempts yet to capture the black hole phenomenon, the Hubble Space Telescope peered into the core of galaxy M87 and found a jet of particles racing out at nearly the speed of light. This jet of electrons and sub-atomic particles stretches for 5,000 light years -- just out to beyond the galaxy's borders -- and is powered by a supermassive black hole.
|Hubble Space Telescope image of the core of galaxy M87.|
But don't let that term "supermassive" fool you. A black hole is tiny compared its host galaxy. That bright core that you see is about 200 light years across. This is roughly 1/50th the size of the entire galaxy. The supermassive black hole is buried in this core, and it is about 1/3000th of a light year across or about the size of our solar system. So that's nearly a half a million times smaller than the core of the galaxy.
It took Hubble's fantastic resolution to capture such a clear image of the M87 galaxy core. Can we go a million times deeper to capture a picture of the black hole?
Yes, we can. This is the primary goal of the MAXIM mission, a proposed NASA X-ray observatory that will have the resolution to image a black hole. Such a feat is comparable to seeing the details of a dinner plate on the surface of the sun.
To attain this caliber of resolution, MAXIM will employ an emerging technology called X-ray interferometry. Think of interferometry as telescope teamwork. Essentially, MAXIM will utilize a multitude of separate X-ray reflecting mirrors orbiting Earth in unison and teaming up to create the effect of one massive X-ray telescope. A description of interferometry is below.
One might ask how we plan to go about imaging a black hole, an object that is by definition invisible. What we hope to image, actually, is the region bordering the black hole, called the event horizon. This region forms a halo of radiation around the invisible black hole.
Once light or matter crosses the event horizon into the black hole, it is lost forever. The force of gravity is so great that even a photon moving at light speed cannot escape the pull. Think of the event horizon as the point of no return. Light can escape near the event horizon. Yes, gravity is fantastically powerful. This gravity may distort the light, but nonetheless, light can escape from very close to a black hole and travel across space to our telescopes to tell the tale.
Imaging this very region, where matter and energy are put to the ultimate tests of gravity, will be a windfall for the fields of astronomy and physics. For example, at the event horizon, we can test the predictions put forward in Albert Einstein's Theory of General Relativity: Does time crawl to a standstill, and is the fabric of space distorted by gravity? Gravity is a force that is all around us, yet scientists know surprisingly little about what produces this force. A black hole is the ultimate gravity laboratory.
What will the black hole look like? We envision it to be a black void surrounded by gas glowing brightly in X rays, distorted by gravity in bizarre ways to look like an image in a funhouse mirror. To image a black hole, therefore, we must turn to the X-ray band of the electromagnetic spectrum.
Matter falling into a black hole emits energy in a variety of forms, including radio waves, optical light and ultraviolet radiation. X-ray photons, however, detail the action closest to the black hole. Further away from the black hole, lower-energy ultraviolet radiation or radio waves may dominate. But at the event horizon, where gravity is strongest, temperatures are the highest and X rays dominate.
X-rays also have the unique ability to pierce through a galaxy's worth of dust and travel great distances through space en route to our orbiting X-ray detectors. Even if a next-generation Hubble-type optical telescope had a million-fold increase in resolution, it still could not peer into the heart of a galaxy to see a black hole. Galaxy cores are simply too dusty, and optical light cannot penetrate this dust -- just as a flashlight cannot shine through a wall. X rays, as you may know, can pass through walls.
As telescopes become more and more powerful, we essentially travel closer and closer to the stars. Galileo first turned an optical telescope toward the sky in 1610. Before this, humans relied solely on their eyes to observe the heavens. The unaided eye can see detail as fine as approximately 100 arcseconds. Galileo was able to resolve 10 arcseconds features. This mere 10-fold increase in optical resolution allowed Galileo to see moons around Jupiter and craters on our own Moon. The Hubble Space Telescope sees at about 0.1 arcseconds, a hundred time improvement over Galileo's scope.
Mind you, MAXIM's goal of imaging a black hole requires 0.000001 arcsecond resolution! Building an X-ray detector with such superior resolution is a great undertaking. Currently, optical telescopes achieve greater resolution than X-ray telescopes. There are some physical reasons for this, based on the nature of optical light waves compared to X-ray photons. There is also the fact that optical telescopes have enjoyed a 400-year head start.
Cosmic X-ray detectors didn't come about until 1960. We had the technology to detect X rays for about 60 years before this, but we had no cosmic X rays to detect. Our atmosphere shields us, fortunately, from these X-rays. We needed to go to space to "see" the X-ray realm of the universe. The first X-ray detectors were carried on sounding rockets that flew an arch-shaped path in a near-space environment.
In a mere 40 years, X-ray astronomy has progressed rapidly. The Chandra X-ray Observatory, launched in 1999, is a marvel. In many regards, Chandra is a sister to the Hubble mission, imaging star explosions and extremely distant galaxies with unprecedented resolution. At about 0.5 arcsecond resolution, Chandra cannot quite achieve the clarity of Hubble. But Chandra is seeing a huge slice of the universe that Hubble cannot see due to the limitations of optical light. In fact, after just six months in space, Chandra was detecting new types of galaxies and black hole activity so distant and faint that Hubble and Keck, a powerful ground-based telescope, couldn't even follow-up with optical observations.
Chandra is a massive telescope, one of the heaviest ever to be lifted into orbit by the space shuttle. Building a bigger X-ray telescope -- that is, with greater collecting area like the huge telescopes we see on earth -- would simply be too expensive and too impractical to launch into space. But X-ray astronomy is not licked. The future lies in the MAXIM mission and X-ray interferometry.
Interferometry is the process of coupling two or more telescopes together to synthetically build an aperture equal to the separation of the telescopes. Sounds too good to be true, but it is a real phenomenon and we use this technique today.
Radio astronomers were the first to capitalize on interferometry. In the 1970s, the National Science Foundation funded the construction of the Very Long Array (VLA), a 40-kilometer chain of 29 radio dishes that essentially serves as one single 40-kilometer wide radio telescope. With such integrated collecting power capable of 0.04 arcsecond resolution, the VLA opened up a world of quasars and particle jets never before resolved with even the largest individual radio telescopes. In the late 1990s, the Very Long Baseline Array (VLBA) opened for business. This string of ten 25-meter radio telescopes scattered from Hawaii to St. Croix and as far north as Washington and New Hampshire achieves a resolution 400 times better than Hubble -- over 0.001 arcsecond resolution.
Optical interferometry is now coming of age as well. The Very Large Telescope (VLT), over 2.5 kilometers atop Chile's Cerro Paranal, comprises four 8.2-meter telescopes that can act independently or together. The Keck Observatory atop Hawaii's Mauna Kea comprises two 10-meter telescopes that also employ interferometry.
Dr. Webster Cash at University of Colorado and his colleagues have achieved 0.1 arcsecond resolution in the laboratory with their design of an X-ray interferometer. This resolution is comparable to Hubble. NASA is now considering this design for the MAXIM mission. MAXIM, of course, must fly in space. An X-ray interferometer large enough to image a black hole would entail a fleet of up to 33 optics spacecraft flying in formation, a challenging task but possible with today's technology.
In general, X-ray telescopes are difficult to build because, to obtain a true focus, X-ray photons must reflect twice from very precisely figured hyperbolic and parabolic surfaces. These surfaces are nested cylinders that are very expensive to shape to the required precision. In this regard, the Chandra X-ray Observatory is a technological marvel and is at the forefront X-ray technology. Chandra achieve 0.5 arcsecond resolution.
This disadvantage for X-ray astronomy -- the need for smooth mirrors -- becomes an advantage when it comes to interferometry. Instead of precisely focusing X-rays with expensive mirrors onto a detector, the University of Colorado team used readily-made flat mirrors to mix the X-ray wavefronts. This technique can produce an even sharper image, similar to the way sound waves can be combined to either cancel each other out (resulting in silence) or amplify the sound. The very nature of X rays being difficult to focus means that they can be mixed more easily than visible light and radio waves.
Flying the various parts of an X-ray interferometer in unison is tricky business. NASA is considering to first test the X-ray interferometer technique in space with a "pathfinder" mission. The MAXIM Pathfinder would have a one-meter separation between the mirrors so that all the X-ray optics are on one spacecraft. The MAXIM Pathfinder would provide 100 microarcsecond resolution.
A larger separation between mirrors provides greater resolution. The MAXIM mission -- an X-ray interferometer large enough to image a black hole -- would entail a fleet of up to 33 optics spacecraft flying in formation with a precision of 20 nano-meter, plus a detector spacecraft 500 kilometers behind the mirrors. MAXIM would achieve 100 nanoarcsecond resolution.
With 100 microarcsecond resolution, astronomers could image the coronae of nearby stars, seeing the actual disks of other stars which appear now only as points of light. With 300 nanoarcsecond resolution, astronomers could attain one of astronomy's ultimate goals -- imaging a black hole.
(In comparison, Hubble and Chandra attain around 100 and 500 milliarcsecond resolution, respectively. The lower the number, the higher the resolution. It is a thousand-fold increase in each jump from "milli" to "micro" to "nano".)
Through the power of ultra-high resolution, we could journey to distant places without need for a warp drive. We could see stars as clearly as we now see our own Sun. This is the promise of MAXIM and the MAXIM Pathfinder.