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. During the mission, spacewalking astronauts will install the Advanced Camera for Surveys (ACS), a powerful instrument that will allow Hubble to that will significantly increase Hubble's abilities and enable a broad array of new astronomical discoveries. Although Hubble was deployed in 1990, it has remained in the forefront of astronomy because frequent visits by astronauts have kept it humming with cutting-edge science instruments.
The ACS covers twice the area, has twice the sharpness, and is up to five times more sensitive to light than Hubble's workhorse camera, the Wide Field and Planetary Camera 2 (WFPC2). Hubble's deepest visible-light view of the universe, called the Hubble Deep Field, required 10 straight days of observing time with the WFPC2. The ACS will be capable of completing the observation in just a few days.
The ACS is really three cameras in one: a wide-field camera, a high-resolution camera (with a smaller field of view), and a solar-blind camera, which is sensitive to the ultraviolet light. The camera also is equipped with a coronagraph, which blocks out the glare of quasars and stars so that astronomers can examine fainter objects that lie near them. The new camera's vision ranges from ultraviolet to near-infrared light. The wide wavelength range and its three modes of operation make the ACS a very versatile camera. Astronomers will take advantage of this versatility by using the camera for a variety of science observations, from probing far-flung galaxies to hunting for planet-forming disks around nearby stars.
Using the coronagraph, astronomers will hunt for low-mass companions around our nearby stars. A challenging but tantalizing project is to look for small objects orbiting Alpha Centauri A and B. Alpha Centauri A is the nearest Sun-like star to Earth.
The ACS coronagraph is four to six times better than that of Hubble's Space Telescope Imaging Spectrograph. Looking far across the universe, the coronagraph will be used to study the host galaxies of distant quasars, brilliant light beacons which astronomers believe are powered by massive black holes. Astronomers hope to pinpoint the types of galaxies that produce them.
In addition to distant quasars, the ACS will search for very young galaxies that existed less than a billion years after the Big Bang, the explosive birth of the universe. Hubble has provided intriguing clues to how galaxies form and evolve. The ACS's greater sensitivity and higher efficiency will allow astronomers to see objects and details that could never have been seen before. The camera, for example, will be able to spot galaxies that are several times fainter than those detected by the WFPC2, providing more details about how matter combines to form galaxies. The ACS has better vision because it can see deeper into the infrared part of the spectrum. Due to the expansion of space, light from distant galaxies is stretched into the spectrum's infrared region.
One way astronomers will hunt for faint, distant galaxies is by using massive clusters of galaxies as large magnifying lenses. Light from faint, remote galaxies that passes through these clusters is bent and magnified, an effect called gravitational lensing. Using this boost from nature, undetectable galaxies become detectable. Astronomers have assembled a large survey program to hunt for hefty clusters. The survey, spanning 300 orbits, represents about 10 percent of the ACS's yearly observational program. For WFPC2, the same program would eat up a year's worth of observation time. The addition of the more efficient ACS enables astronomers to conduct large surveys for the first time with Hubble.
Astronomers have planned several more sky surveys of more than 100 orbits each. Here is a sampling.
Astronomers will use a suite of space- and ground-based observatories, including the ACS, to probe the universe's earliest galaxies. The ambitious program, called Great Observatories Origins Deep Survey (GOODS), will try to reconstruct the history of galaxy and star formation. They also will study the relationship between supermassive black holes and their host galaxies. These hefty black holes are millions to billions times more massive than the Sun. Astronomers hope to answer several questions, such as, which came first: galaxies or black holes? Or, did they grow together? And, is there a relationship between a galaxy's mass and the mass of its black hole?
This survey will encompass the northern and southern skies. The ACS program alone will require 398 orbits to complete. By comparison, the WFPC2 would need more than 6,000 orbits to complete the program. That is more than two years of observational time.
In scrutinizing distant galaxies in the GOODS survey, astronomers hope to nab eight to 20 supernovae that reside about 8 to 10 billion light-years from Earth. Astronomers will analyze those faraway supernovae to obtain more clues about the mysterious "dark energy," which may be shoving galaxies apart at ever-increasing speeds. Last year, Hubble pinpointed an exploding star, called a supernova, about 10 billion light-years from Earth. The supernova appeared brighter than scientists expected, lending support that at an early epoch the universe slowed down but later began to accelerate. Astronomers must analyze more supernovae to confirm their suspicions that the early cosmos was decelerating and to determine whether dark energy actually exists.
The ACS will scrutinize large clusters of galaxies that lie about 1.3 to 8 billion light-years away. They hope that these clusters will surrender clues why star formation has declined dramatically since about 7.5 billion years ago.
Moving to Andromeda, the nearest large galaxy to Earth, the ACS for the first time will resolve the oldest normal stars in a galaxy other than our Milky Way. The stars, which reside in the galaxy's halo, are the best laboratory specimens for testing theories of galaxy formation and evolution. Astronomers hope to use this information to pinpoint the age of the universe.
Astronomers will use the ACS to analyze the mechanism that triggers gamma-ray bursts, the most powerful explosions in sky, to test the standard model that collapsing, massive stars produce the outbursts.
Turning to our solar system, the ACS will take a census of objects in the Kuiper Belt, a vast region of leftover material from the creation of our solar system. Ground-based observations have found a handful of large objects. Theories suggest, however, that the region is populated mostly with smaller bodies. Astronomers will use the ACS to obtain an accurate sampling of the sizes of Kuiper Belt inhabitants.