NASA's Hubble Space Telescope has been pushing the frontiers of astronomy since its launch in 1990. The orbiting observatory has watched a comet disintegrate as it passed by the Sun and pinpointed a massive star that exploded 10 billion years ago. It has provided a view of a bewildering zoo of young galaxies that existed when the cosmos was a youngster. It has measured the expansion rate of the universe and detected clumps of matter perhaps the seeds of planets swirling around nascent stars.
Now its time to expand Hubble's vision even further during Servicing Mission 3B, scheduled to begin Feb. 28 with the launch of the space shuttle Columbia. The mission will give the orbital observatory a series of midlife upgrades that includes the Advanced Camera for Surveys (ACS), a new instrument package that will increase Hubble's already formidable capacity for discoveries tenfold, according to the leader of the team that built it.
"If you had two fireflies six feet apart in Tokyo, Hubble's vision with ACS will be so fine that it will be able to tell from Washington that they were two different fireflies instead of one," says Holland Ford, professor of astronomy in the Krieger School of Arts and Sciences at The Johns Hopkins University and leader of the team that built the ACS over a five-year period.
Ford thinks there's an outside chance that the ACS might even be powerful enough to obtain "direct evidence" - i.e., an image of some type - of planets in other, nearby solar systems. Although planets have been detected around many stars, all of them have been inferred through the gravitational wobbles they impart to their stars, rather than detected through a direct image of the planets themselves.
"I think that there is a chance" we'll be able to directly image a planet, says Ford, clearly tantalized by the prospect. "It's going to be difficult, for sure, but we're going to try it."
The ACS will replace the Faint Object Camera, which is the last of Hubble's original instruments. After catching Hubble with the shuttle's robot arm and securing it in the shuttle's payload bay, spacewalking astronauts will open the servicing doors on Hubble, remove the Faint Object Camera, and install the ACS.
Scientists and engineers who contributed to the ACS came from across the country, but are primarily found at Hopkins, NASA's Goddard Space Flight Center, Ball Aerospace Corp., and the Space Telescope Science Institute. (A complete list of project staff is available at http://acs.pha.jhu.edu/general/personnel/sci-team/.)
The ACS weighs 870 pounds and is "about the size of an old-fashioned phone booth," according to Ford. Inside the ACS are three electronic cameras (the wide-field, high-resolution, and solar blind cameras), and a range of filters, polarizers, dispersers and other astronomical tools. ACS can detect radiation ranging from the ultraviolet portion of the spectrum, through visible light, to a portion of the spectrum known as the near infrared.
All the ACS instruments take advantage of new techniques and technology developed since Hubble's inception to deliver increased observing power at greatly reduced costs.
In comparison to the Wide Field Planetary Camera 2, another instrument already in use in Hubble, the ACS will provide two times the observational area, two times the resolution and four times the sensitivity.
"This means a single ACS image will capture more objects in more detail and at a faster rate than before," says Frank Summers, an astrophysicist at the Space Telescope Science Institute.
For example, astronomers like to use Hubble to probe the distant reaches of the universe in a project known as a deep-field survey. If they probe to the same distances as previous surveys, researchers should be able to finish their work approximately ten times faster, reducing their observation time on the telescope from twenty days to just a few days.
ACS also contains an instrument known as a coronagraph that will allow astronomers to block out small bright sources of light in order examine the details of structures around the light sources. Ford noted that this might allow astronomers to search for warps and gaps in the disks of gas and dust surrounding nearby stars that may be early signs of planet formation. The coronagraph will also be very useful to astronomers who study quasars, powerful distant objects in the farthest reaches of the universe that are thought to be highly active black holes in the center of galaxies.
"We're looking forward to taking images of quasars, and seeing the structures that surround the quasars much better with the ACS's higher resolution and higher sensitivity, but especially with the ACS's ability to block the extremely bright emissions coming from the quasar," explains Ford.
Ford and other astronomers have many other ideas for using the ACS, including taking a closer, more detailed look at the weather on planets in our solar system, and no less ambitious a project than verifying the celestial yardstick astronomers have used for several decades to gauge distances around the universe.
"ACS has a set of filters that lets us take pictures in polarized light, which in effect can allow us to see around corners," says Ford. "We plan to use the polarizers to make some geometric measurements of distances using light echoes from supernovae. This will give us very important checks on how we bootstrap distances across the universe."
Noting Hubble's history of astonishing images and breakthrough discoveries, Ford says he's positive that the ACS will help keep Hubble "on the astronomical forefront that the public has come to expect of the Space Telescope."
Space Telescope Science Institute, Baltimore, MD
Johns Hopkins University, Baltimore, MD