The Hubble Space Telescope’s self-propelled guns celebrate 20 years of groundbreaking discovery

The Hubble Advanced Camera for Surveys celebrates 20 years of discovery. credit: NASA

ACS continues to deliver groundbreaking science.

When astronauts installed the Advanced Camera for Surveys (ACS) on March 7, 2002,[{” attribute=””>Hubble Space Telescope was already famous for taking deep images of the distant universe. ACS went even deeper, pushing humanity’s view of the universe back to within 435 million years of the Big Bang and capturing images of the earliest objects in the cosmos. It also helps map the distribution of dark matter, searches for massive planets and studies the evolution of clusters of galaxies. The longevity and consistency of ACS is critical for monitoring cosmic phenomena over time. In its 20 years aboard Hubble, ACS has taken over 125,000 pictures and spawned numerous discoveries.

Over the course of 20 years, the Advanced Camera for Surveys (ACS) has revealed intriguing new secrets of the universe, looking into the depths of space with unprecedented clarity from aboard.[{” attribute=””>NASA’s Hubble Space Telescope. Astronauts installed ACS during Hubble Servicing Mission 3B, also known as STS-109, on March 7, 2002. With its wide field of view, sharp image quality, and high sensitivity, ACS has delivered many of Hubble’s most impressive images of deep space.

Former astronaut Mike Massimino, one of the two spacewalking astronauts who installed ACS, remembers, “We knew ACS would add so much discovery potential to the telescope, but I don’t think anybody really understood everything it could do. It was going to unlock the secrets of the universe.”

ACS has lived up to that promise. Following its installation, ACS became Hubble’s most frequently used instrument. Among its many accomplishments, the camera has helped map the distribution of dark matter, detected the most distant objects in the universe, searched for massive planets and studied the evolution of clusters of galaxies.

“When ACS was installed on Hubble, the telescope was already famous for taking deep images of the distant universe, like the Hubble Deep Field,” explained Tom Brown, Head of the Hubble Space Telescope Mission Office at the Space Telescope Science Institute (STScI) in Baltimore, Maryland. “However, because ACS was so powerful relative to the earlier cameras, it became routine to see very distant galaxies in the background of Hubble images, even when we were looking at nearby objects.”

Hubble’s Advanced Camera for Surveys (ACS) forever changed our view of the universe. Two decades into its epic mission, ACS continues to deliver ground-breaking science and stunning images. ACS has taken over 125,000 pictures and spawned numerous discoveries. Here is a portfolio of some of the ACS’s most striking images. In this six-panel collage, the photos are (left to right): the Spire in the Eagle Nebula, V838 Monocerotis, the Hubble Ultra Deep Field (HUDF), the Whirlpool Galaxy (M51), Saturn, and the Orion Nebula (M42). Credit: NASA, ESA, STScI

One example of this is a spectacular disrupted galaxy called the Tadpole (UGC 10214). Astronomers photographed the Tadpole shortly after ACS’s installation to demonstrate the camera’s capabilities. With its long tail of stars, the Tadpole looked like a runaway pinwheel firework. But what was really stunning was the backdrop — a rich tapestry of 6,000 galaxies captured by ACS.

“The Advanced Camera for Surveys represented a new paradigm for Hubble Space Telescope instruments when it was designed. It has lived up to expectations, proving to be one of Hubble’s most scientifically productive instruments,” said Mark Clampin, Director of the Sciences and Exploration Directorate at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Prior to joining Goddard, Clampin was the ACS Group Lead at STScI, where he worked on three Hubble Servicing Missions. 

In January 2007, an electronics malfunction rendered the two most-used science channels on ACS inoperable. Thanks to engineering ingenuity, spacewalking astronauts on Hubble Servicing Mission 4 (STS-125) repaired the Wide Field Channel, the workhorse responsible for 70 percent of the pre-2007 ACS science. The High Resolution Channel, however, could not be repaired. Still, two decades into its mission, ACS continues to deliver ground-breaking science.

“The Advanced Camera for Surveys has opened our eyes to a deep and active universe for two decades,” said Jennifer Wiseman, NASA’s Hubble Senior Project Scientist. “We are anticipating still more discoveries with this camera, in conjunction with Hubble’s other science instruments, for many years to come.”

To date, ACS has taken over 125,000 pictures. These observations have spawned numerous discoveries, some of which are highlighted below.

Abell 370 is a cluster with several hundred galaxies at its core. It was one of the first clusters where astronomers observed gravitational lensing and part of the Frontier Fields project.
Credit: NASA, ESA, R. Bouwens and G. Illingworth (University of California, Santa Cruz)

The Hubble Ultra Deep Field

In undoubtedly its most important observations, ACS revealed a series of the deepest portraits of the universe ever achieved by humankind. In the original Hubble Ultra Deep Field (HUDF), unveiled in 2004, ACS teamed up with Hubble’s Near Infrared Camera and Multi-object Spectrometer (NICMOS) to capture light from galaxies that existed about 13 billion years ago, some 400 to 800 million years after the Big Bang. This million-second-long exposure revealed new insights into some of the first galaxies to emerge from the so-called “dark ages,” the time shortly after the Big Bang when the first stars reheated the cold, dark universe. 

In later versions, ACS teamed with other Hubble instruments to refine the depth and reach of the original Hubble Ultra Deep Field. These portraits pushed humanity’s view of the universe back to within 435 million years of the Big Bang, capturing images of the earliest objects in the cosmos. They forever changed our view of the universe and spawned innumerable collaborations.

The Frontier Fields

Following in the spirit of the Hubble Ultra Deep Field, the Frontier Fields extended Hubble’s reach even farther with the help of giant cosmic lenses in space. The immense gravity of massive clusters of galaxies warps the light from even-more-distant galaxies beyond, distorting and magnifying the light until those galaxies — too faint to be seen by Hubble directly — become visible. Frontier Fields combined the power of Hubble with the power of these “natural telescopes” to reveal galaxies 10 to 100 times fainter than could be seen by Hubble alone. Astronomers simultaneously used ACS for visible-light imaging and Hubble’s Wide Field Camera 3 for its infrared vision.

Over the course of three years, Hubble devoted 840 orbits around the Earth — that’s more than 1,330 hours — to six clusters of galaxies and six “parallel fields” — regions near the galaxy clusters. While these parallel fields could not be used for gravitational lensing, Hubble performed “deep field” observations on them — long looks far into the depths of space. Through the power of gravitational lensing, Hubble peered more deeply into space than ever before, while the parallel field observations expanded our knowledge of the early universe that began with the Hubble Deep Fields and Hubble Ultra Deep Field.

Helping the New Horizons Mission by Photographing Pluto

ACS captured the most detailed images ever taken of the dwarf planet Pluto years before the New Horizons flyby. The images reveal an icy, mottled, dark molasses-colored world undergoing seasonal surface and brightness changes. The ACS images were invaluable to planning the details of the New Horizons flyby in 2015 by showing which hemisphere looked more interesting for the spacecraft to take close-up snapshots during its brief encounter.

The Mysterious Fomalhaut b 

In 2008, ACS made the first visible-light snapshot of what was initially thought to be a planet, dubbed Fomalhaut b, orbiting the nearby, bright southern star Fomalhaut. The diminutive-looking object appeared as a dot next to a vast ring of icy debris that ACS observed to be encircling Fomalhaut. In following years, researchers tracked the object along its trajectory. But over time the dot expanded and became fainter as it moved out of sight. Instead of a planet, it is now thought to be an expanding cloud of very fine dust particles from two icy bodies that smashed into each other, according to some researchers. The nature of the object is still being debated, and follow-up studies may unravel this mystery.

V838 Monoceroti became the brightest star in the entire Milky Way Galaxy in January 2002. Then, just as suddenly, it faded.
Credit: NASA, ESA

The Light Echo of V838 Monocerotis  

The ACS captured an unusual phenomenon in space called a light echo, where light from an erupting star reflects or “echoes” off the dust and then travels to Earth. The echo came from the variable star V838 Monocerotis (V838 Mon). In early 2002, V838 Mon increased in brightness temporarily to become 600,000 times brighter than our Sun. The reason for the eruption is still unclear.

Light from V838 Mon propagated outward through a cloud of dust surrounding the star. Because of the extra distance the scattered light traveled, it reached the Earth years after the light from the stellar outburst itself. ACS monitored the light from the stellar outburst for several years as it continued to reflect off shells of dust surrounding the star. The phenomenon is an analog of a sound produced when an Alpine yodeler’s voice echoes off the surrounding mountainsides. The spectacular light echo allowed astronomers to view continuously changing cross-sections of dust surrounding the star. This is a dramatic illustration of the power of ACS and Hubble to monitor phenomena over time. The longevity and consistency of ACS is critical for this type of research.  

Collision of the Milky Way and Andromeda Galaxies

By measuring the tiny, sideways motion of a group of stars in our neighboring Andromeda galaxy, ACS allowed astronomers to calculate that Andromeda and our Milky Way will collide head-on in about 4 billion years from now. Andromeda, also known as M31, is now 2.5 million light-years away, but it is falling toward the Milky Way under the mutual pull of gravity between the two galaxies. The prediction is that they will merge into a single elliptical galaxy similar to the kind commonly seen throughout the universe.

Hubble peered straight through the center of Abell 1689, one of the most massive galaxy clusters known.
Credit: NASA, N. Benitez, T. Broadhurst, H. Ford, M. Clampin, G. Hartig, G. Illingworth, the ACS Science Team, and ESA

Galaxy Cluster Abell 1689’s Gravitational Lens

In 2002, ACS delivered an unprecedented and dramatic new view of the cosmos when it demonstrated the power of gravitational lensing. The ACS peered straight through the center of one of the most massive galaxy clusters known, called Abell 1689. The gravity of the cluster’s trillion stars – plus dark matter – acts as a 2-million-light-year-wide “lens” in space. This gravitational lens bends and magnifies the light of galaxies located far behind it, distorting their shapes and creating multiple images of individual galaxies.  

ACS’s sharpness, combined with this behemoth natural lens, revealed remote galaxies previously beyond even Hubble’s reach. The results shed light on galaxy evolution and dark matter in space.

Mature and “Toddler” Galaxies Far Back in Time 

Using ACS to look back in time nearly 9 billion years, an international team of astronomers found mature galaxies in a young universe. The galaxies are members of a cluster of galaxies that existed when the universe was only 5 billion years old. This compelling evidence that galaxies must have started forming just after the Big Bang was bolstered by observations made by the same team of astronomers when they peered even farther back in time. The team found galaxies a mere 1.5 billion years after the birth of the cosmos. The early galaxies reside in a still-developing cluster, the most distant proto-cluster ever found. 

The ACS was built especially for studies of such distant objects. These findings further support observations and theories that galaxies formed relatively early in the history of the cosmos. The existence of such massive clusters in the early universe agrees with a cosmological model wherein clusters form from the merger of many sub-clusters in a universe dominated by cold dark matter. The precise nature of cold dark matter, however, is still not known.

Clues about the Accelerating Universe and Dark Energy

Astronomers using ACS found supernovas that exploded so long ago they provide new clues about the accelerating universe and its mysterious “dark energy.” ACS can pick out the faint glow of these very distant supernovas. The ACS can then dissect their light to measure their distances, study how they fade, and confirm that they are a special type of exploding star, called a Type Ia supernova, that are reliable distance indicators. Type Ia supernovas glow at a predictable peak brightness, which makes them reliable objects for calibrating vast intergalactic distances.

In 1998, Hubble astronomers found such a far-off supernova that provided the unexpected revelation that galaxies appeared to be moving away from each other at an ever-increasing speed. They’ve attributed this accelerating expansion to a mysterious factor known as dark energy that is believed to permeate the universe. Since its installation, ACS has been hunting Type Ia supernovas in the early universe to provide supporting evidence.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.