Hubble Space Telescope still pushing the frontiers of astronomy
Nearly a quarter of a century after its 1990 launch, the Hubble Space Telescope is still pushing the frontiers of observational astronomy, thanks to the sensitivity of its instruments, the ultra precise way the observatory can be controlled and ingenious new techniques that are allowing astronomers to peer deeper into the cosmos than ever before.
"That's why the Hubble is still so exciting," said Matt Mountain, director of the Space Telescope Science Institute at Johns Hopkins University in Baltimore. "We're learning more and more about how to use it even better and better, whether it's looking for exoplanet atmospheres, measuring dark energy to a precision we never thought possible or using gravitational lenses to push Hubble to look even further back in time."
In recent observations, Hubble has been used to search for dim, difficult-to-detect minor planets beyond the orbit of Pluto, possible candidates for a flyby after the New Horizons probe streaks past Pluto in 2015. Hubble has monitored Jupiter's Great Red Spot, which appears to be shrinking, and a comet -- Siding Spring -- that will make a close flyby of Mars in October.
But it's Hubble's ability to capture light from galaxies shining when the universe was a fraction of its present age that continues to intrigue scientists and the public alike, providing a glimpse into the depths of cosmic history.
To many astronomers, one of Hubble's most mind-boggling observations was a 1995 time exposure of an apparently empty region of space. The resulting "Hubble Deep Field" image, built up over 10 days, revealed thousands of previously unseen galaxies sprinkled like colored jewels on black velvet.
Similar images using newer, more sensitive instruments have revealed a universe populated by uncounted galaxies and fragments of galaxies that somehow began assembling shortly after the big bang birth of the cosmos 13.7 billion years ago.
Now, 20 years after the original Deep Field, Hubble is making another series of long-exposure photographs known as "Frontier Fields." But this time around, Hubble is using the titanic gravity of galaxies and dark matter in nearby clusters to magnify images of even more remote -- and thus younger -- galaxies in the far background.
The result, astronomers hope, will be a glimpse of the universe when it was only 400 million years old, the age when stars and galaxies first began shining as the infant universe expanded and cooled.
NASA's $8 billion James Webb Space Telescope, scheduled for launch in 2018, is optimized to directly image that early epoch in the infrared region of the spectrum, but Hubble's gravity-assist Frontier Fields may provide a tantalizing preview of what's to come.
"Gravity bends light, that was Einstein's discovery, general relativity, and that cluster of galaxies and dark matter can actually behave like a lens and actually magnify objects behind it in the very distant universe," Mountain said in an interview with CBS News. "That allows Hubble to see things even farther away than it could normally."
The resulting gravitationally magnified images are distorted and smeared into arcs "but if you understand the lens, you can recreate the actual shape back where you're going," Mountain said. "Because of our experience over the last few years, we've worked out how to calculate the prescription of the lens so when we see one of these objects we know now how far away it is and how bright it is, which we wouldn't have known before."
How far away is far? And how old is old?
"It's increasing Hubble's ability to go back in time, in very specific areas, back to about 400 million years after the big bang," Mountain said. "That's the incredible thing, that we've managed to calibrate the prescription of these gravitational lenses and now we can use them as tools. Four or five years ago, that wasn't possible."
Closer to home, both in time and space, the hunt for planets orbiting other stars is one of the hottest fields in astronomy, thanks in large part to NASA's Kepler space telescope, a 50-megapixel camera that has discovered thousands of exoplanet candidates.
Mountain said Hubble is using a new technique to study starlight passing through the atmosphere of a confirmed exoplanet as it moves in front of its parent star to measure at least some of its chemical constituents. The trick is being able to separate out the light passing through an atmosphere from the total output of the vastly brighter star.
Simply pointing Hubble at a nearby target star will not work because the starlight will saturate the camera's CCD detector, resulting in a blob-like image that cannot be studied with the required precision.
"The problem here is we have to look at very bright stars, and Hubble is very sensitive," Mountain said. "It basically smears the light over the whole camera. It's a bit like when you've got a digital camera and you look at a street lamp by accident at night and you get a streak across your camera. That's the problem Hubble has when it looks at bright stars.
"So the guys here came up with this really cunning idea. Because Hubble can point so accurately, we actually (move the telescope and) drift the star down the camera all the time so you're producing a very straight, linear streak, but it smears the light over the whole CDD and it doesn't saturate."
The resulting streaks can be precisely measured and subtle changes teased out of the data.
"They found a way to very accurately move the telescope while we took the exposure so the light got spread out in these columns and it didn't saturate the camera," Mountain said. "But because we collect all the light over the exposure, we sum up those streaks and we can see those very, very small differences and actually see for the very first time even fainter planets than we could see before."
So far, Hubble has been able to use the technique with a handful of Jupiter-class planets, but Mountain said he is confident researchers eventually will be able to look for signs of water vapor in Neptune-size worlds as observations improve.
Hunting for bigger game, Nobel Laureate Adam Riess, who earlier used Hubble to help confirm the existence of dark energy and its role in speeding up the expansion of the universe, figured out a way to use the streak-exposure technique to improve cosmic distance measurements by a factor of 10.
He came up with the idea while swimming laps in a Baltimore pool, Mountain said. "He thought, oh my God, I could use this technique to help me with my dark energy research."
To directly measure the distance to a star, it must be close enough to Earth that it shifts position when viewed from one side of Earth's orbit and the other. Hold a finger up at arm's length and look at it with one eye and then the other. The finger will change position slightly due to this parallax effect.
Because the 186-million-mile diameter of Earth's orbit is known, astronomers only need to measure the angular shift of a distant star to calculate how far away it must be. But given the scale of the galaxy, even a 186-million-mile baseline means exceedingly small angles. To directly measure the distance to the nearest star, for example, astronomers had to discern angles equivalent to the width of a dime two miles away.
That star, Alpha Centauri, is just 4 light years from Earth. The disk of the Milky Way spans 100,000 light years and millions to billions of light years separate galaxies.
To extend the distance ladder across the gulfs separating galaxies, astronomers use Cepheid variables, stars that pulsate in a predictable manner and have a known intrinsic brightness. By measuring the apparent brightness of a Cepheid in a distant galaxy, and comparing it to the brightness of a Cepheid a known distance from Earth, astronomers can indirectly calculate the distance to that galaxy.
The key is first directly measuring the parallax of a Cepheid in the Milky Way to calibrate the cosmic distance ladder.
Up to this point, direct measurements of stellar distances using parallax extended a few hundred light years. Using the streak-exposure technique, Riess and co-worker Stefano Casertano were able to directly measure the distance to a Cepheid variable star some 7,500 light years out.
"Inside our own galaxy, instead of just looking at very local objects, we can look very far out," Mountain said. "He has managed to change the measurement precision of the universe from 10 percent, he thinks, down to 2 to 3 percent. Why is that important? Well, it's all about dark energy."
A more accurate distance scale allows a more precise characterization of dark energy's effects on the universe at different times in its evolution, shedding light on how the cosmic expansion is changing and how that plays into the ultimate fate of the universe.
Other spacecraft now in development will probe that new frontier in great detail, but Hubble is helping fill in the blanks today by "using its stability and being very smart with new math and new techniques," Mountain said. "So suddenly we've given Hubble a new ability to measure things 10 times more accurately than it could do before."
