Where Is the Hubble Telescope and How Does It Work?

By: Craig Freudenrich, Ph.D. & Sarah Goddard  | 
The Hubble Space Telescope is deployed from the cargo bay of the space shuttle

Have you ever stared at the night sky and wondered what the universe looks like up close? Even if you're lucky enough to have access to a ground-based telescope, whose clarity depends on atmospheric factors like clouds, you won't get the lucidity these stunning celestial objects deserve. In 1946, an astrophysicist named Dr. Lyman Spitzer Jr. proposed putting a telescope in space to reveal clearer images.

Sounds logical, right? However, this was before anyone had even launched a rocket into outer space. Flash forward to 1990, the Hubble telescope launches. And where is the Hubble telescope? Space.


As the U.­S. space program matured in the 1960s and 1970s, Spitzer lobbied NASA and Congress to develop a space telescope. In 1975, the European Space Agency (ESA) and NASA began drafting the initial plans for it, and in 1977, Congress approved the necessary funds. NASA named Lockheed Missiles (now Lockheed Martin) as the contractor that would build the telescope and its supporting systems, as well as assemble and test it.

The famous telescope was named after U.S. astronomer Edwin Hubble, whose observations of variable stars in distant galaxies confirmed that the universe was expanding and gave support to the Big Bang theory.


Space Shuttle Discovery and the Hubble Space Telescope

After a long delay due to the Challenger disaster in 1986, the Hubble Space Telescope shot into orbit on April 24, 1990, piggybacking aboard the Discovery space shuttle. Since its launch, Hubble has reshaped our v­iew of space, with scientists writing thousands of papers based on the telescope's clear-eyed findings on important stuff like the age of the universe, gigantic ­black holes and what­ stars look like in the throes of death.

­In this article, we'll talk about how Hubble has documented outer space and the instruments that have allowed it to do so. We'll also talk about a few of the problems the venerable telescope/spacecraft has encountered along the way.


COSTAR Saves the Day

Preflight inspection of the Hubble Space Telescope's primary mirror
Photo courtesy NASA/STScI

Almost immediately after it was deployed in 1990, astronomers discovered a problem with their beloved $1.5 billion, 43.5-ft (13.3-m) telescope. Their new tractor-trailer-sized eye in the sky couldn't focus properly. They realized that the telescope's primary mirror had been ground to the wrong dimension. Although the defect in the mirror — roughly equal to one-fiftieth the thickness of a human hair — would seem ridiculously minute to most of us, it caused the Hubble Space Telescope to suffer spherical aberration and produce fuzzy images. Surely the astronomers didn't spend years working on the telescope only to be satisfied with unremarkable snapshots of outer space.­

­Scientists came up with a replacement "contact" lens called COSTAR (Corrective Optics Space Telescope Axial Replacement) to repair the defect in the HST. COSTAR consisted of several small mirrors that would intercept the beam from the flawed mirror, fix the defect and relay the corrected beam to the scientific instruments at the focus of the mirror.


NASA astronauts and staff spent 11 months preparing for what would be one of the most challenging space missions ever attempted. Finally, in December 1993, seven men aboard the space shuttle Endeavour rocketed into space for the HST's first servicing mission.

It took the crew one week to make all of the necessary repairs, and when the telescope was tested after the servicing mission, the images were vastly improved. Today, all of the instruments placed in the HST have built-in corrective optics for the mirror's defect, and COSTAR is no longer needed.

There's more to Hubble than COSTAR, though, and we'll talk about some of those critical parts next.


Anatomy of the HST

The Hubble Space Telescope in orbit
NASA/National Geographic/Getty Images

Like any telescope, the HST has a long tube that is open at one end to let in light. It has mirrors to gather and bring the light to a focus where its "eyes" are located. The HST has several types of "eyes" in the form of various instruments. Just as insects can see ultraviolet light or we humans can see visible light, Hubble must also be able to see the various types of light raining down from the heavens.­

Specifically, Hubble is a Cassegrain reflector telescope. That just means that light enters the device through the opening and bounces off the primary mirror to a secondary mirror. The secondary mirror in turn reflects the light through a hole in the center of the primary mirror to a focal point behind the primary mirror. If you drew the path of the incoming light, it would look like the letter "W," except with three downward humps instead of two.


At the focal point, smaller, half-reflective, half-transparent mirrors distribute the incoming light to the various scientific instruments. (We'll talk more about those instruments in the next section.) As you might have guessed, these aren't just ordinary mirrors that you might gaze in to admire your reflection.

HST's mirrors are made of glass and coated with layers of pure aluminum (three-millionths of an inch thick) and magnesium fluoride (one-millionth of an inch thick) to make them reflect visible, infrared and ultraviolet light. The primary mirror is 7.9 feet (2.4 meters) in diameter, and the secondary mirror is 1.0 feet (0.3 meters) in diameter.

Next, we'll talk about what Hubble does with all that light after it hits the telescope's mirrors.


Hubble's Scientific Instruments: WFC3 and More

A picture of the Eagle Nebula, captured by Hubble's old camera, the WFPC2
Photo courtesy STScI and NASA

By looking at the different wavelengths, or the spectrum of light, of a celestial object, you can discern many of its properties. To do this, HST is equipped with several scientific instruments. Each instrument uses charge-coupled devices (CCDs) rather than photographic film to capture the light. The light detected by the CCDs is turned into digital signals, which are stored in onboard computers and relayed to Earth. The digital data are then transformed into amazing photos. Let's look at how each instrument contributes to those images.

Wide Field Camera 3 (WFC3)

The Wide Field Camera 3 (WFC3) is one of Hubble's primary imaging instruments. Featuring two channels, WFC3 captures both ultraviolet and infrared light, extending Hubble's observational reach. It uses two distinct rectangular chips for its ultraviolet/visible and infrared channels. Coupled with an extensive array of filters, WFC3 allows astronomers to glean intricate details about celestial objects, making it a pivotal upgrade from the Wide Field and Planetary Camera 2 (WFPC2) in Hubble's long-standing mission.


Near Infrared Camera and Multi-Object Spectrometer (NICMOS)

Often, interstellar gas and dust can block our vision of the visible light from various celestial objects. No problem: Hubble can see the infrared light, or heat, from the objects hidden in the dust and gas. To see this infrared light, HST has three sensitive cameras that make up the Near Infrared Camera and Multi-Object Spectrometer (NICMOS).

Space Telescope Imaging Spectrograph (STIS)

Besides illuminating a celestial object, the light emanating from that object can also reveal what it's made of. The specific colors tell us what elements are present, and the intensity of each color tells us how much of that element is present. The Space Telescope Imaging Spectrograph (STIS) separates the incoming colors of light much as a prism makes a rainbow.

In addition to describing the chemical composition, the spectrum can convey the temperature, density and motion of a celestial object. If the object is moving, the chemical fingerprint may shift toward the blue end (moving toward us) or the red end (moving away from us) of the spectrum. Unfortunately, the STIS lost power in 2004. It was repaired in 2009.

Advanced Camera for Surveys (ACS)

During a servicing mission in February 2002, astronauts added the Advanced Camera for Surveys (ACS), doubling the Hubble's field of view and replacing the Faint Object Camera, which served as the HST's telephoto lens.

The ACS, which sees visible light, was installed to help map the distribution of dark matter, detect the universe's most distant objects, search for massive planets and examine the evolution of clusters of galaxies. Scientists estimated it would last five years, and right on cue, an electrical shortage disabled two of its three cameras in January 2007.

Fine Guidance Sensors (FGSs)

The final instrument on board the HST is its Fine Guidance Sensors (FGSs), which point the telescope and precisely measure the positions and diameters of stars, as well as the separation of binary stars. The Hubble has three of these sensors overall; two to point the telescope and keep it fixed on its target, looking for "guide" stars in the HST field near the target. When each FGS finds a guide star, it locks on to it and feeds information back to the HST steering system to keep that guide star in its field. While two sensors are steering the telescope, one is free to make astrometric measurements (star positions). Astrometric measurements are important for detecting planets because orbiting planets cause the parent stars to wobble as they move across the sky.

Now you know how Hubble takes all those pictures. We'll learn about Hubble's other life as a spacecraft next.


Hubble's Spacecraft Systems: Generating Power and Talking to Ground Control

Hubble's communication system
Photo courtesy STScI and NASA

Hubble isn't only a telescope with highly specialized scientific instruments. It's also a spacecraft. As such, it must have power, communicate with the ground and be able to change its attitude (orientation).

­All of the instruments and computers on board the HST require electrical power. Two large solar panels fulfill this responsibility. Each winglike panel can convert the sun's energy into 2,800 watts of electricity. When the HST is in the Earth's shadow, energy that has been stored in onboard batteries can sustain the telescope for 7.5 hours.­


In addition to generating power, the HST must be able to talk with controllers on the ground to relay data and receive commands for its next targets. To communicate, the HST uses a series of relay satellites called the Tracking and Data Relay Satellite (TDRS) system. Currently, there are five TDRS satellites in various locations in the sky.

Hubble's communication process is also helped by the two main computers that fit around the telescope's tube above the scientific instrument bays. One computer talks to the ground to transmit data and receive commands. The other computer is responsible for steering the HST and various housekeeping functions. Hubble also has backup computers in the event of an emergency.

But what's used to retrieve data? And what happens to that information after it has been collected? Four antennae positioned on the telescope transmit and receive information between Hubble and the Flight Operations Team at the Goddard Space Flight Center in Greenbelt, MD. After receiving the information, Goddard sends it to the Space Telescope Science Institute (STScI) in Maryland, where it's translated into scientific units such as wavelength or brightness.

Learn how Hubble navigates next.


Hubble's Spacecraft Systems: Steering and Focusing the Eye in the Sky

If Hubble couldn't focus, it wouldn't have been able to take this image of a dying star
NASA/Getty Images

Hubble zooms around the Earth every 97 minu­tes, so focusing on a target can be difficult. Three onboard systems allow the telescope to remain fixed on an object: gyroscopes, the Fine Guidance Sensors that we talked about in the previous section, and reaction wheels.

The gyroscopes keep track of Hubble's gross movements. Like compasses, they sense its motion, telling the flight computer that Hubble has moved away from the target. The flight computer then calculates how much and in what direction Hubble must move to remain on target. The flight computer then directs the reaction wheels to move the telescope.


Hubble's Fine Guidance Sensors help keep the telescope fixed on its target by sighting on guide stars. Two of the three sensors find guide stars around the target within their respective fields of view. Once found, they lock onto the guide stars and send information to the flight computer to keep the guide stars within their field of view. The sensors are more sensitive than the gyroscopes, but the combination of gyroscopes and the sensors can keep the HST fixed on a target for hours, despite the telescope's orbital motion.

The HST can't use rocket engines or gas thrusters to steer like most satellites do, because the exhaust gases would hover near the telescope and cloud the surrounding field of view. Instead, the HST has reaction wheels oriented in the three directions of motion (x/y/z or pitch/roll/yaw). The reaction wheels are flywheels, like those found in a clutch. When the HST needs to move, the flight computer tells one or more flywheels which direction to spin in and how fast, which provides the action force. In accordance with Newton's third law of motion (for every action there is an equal and opposite reaction), the HST spins in the opposite direction of the flywheels until it reaches its target.

Is there anything Hubble can't do?


Hubble's Limitations

Although Hubble can't point at the sun, it can still take some pretty awesome images, like this one, which shows a halo of hot gas surrounding spiral galaxy NGC 4631 that's similar to the Milky Way
NASA/Getty Images

Although the HST is responsible for countless incredible images and discoveries, it does have a few limitations.

One of these limitations is that the HST can't observe the sun because the intense light and heat would fry its sensitive instruments. For this reason, the HST is always pointed away from the sun. That also means that Hubble can't observe Mercury, Venus and certain stars that are close to the sun either.


In addition to the brightness of objects, Hubble's orbit also restricts what can be seen. Sometimes, targets that astronomers would like Hubble to observe are obstructed by the Earth itself as Hubble orbits. This can limit the time spent observing a given object.

Lastly, the HST passes through a section of the Van Allen radiation belts, where charged particles from the solar winds are trapped by the Earth's magnetic field. These encounters cause high background radiation, which interferes with the instruments' detectors. It's impossible for the telescope to make observations during these periods.

Next, learn what the future holds for the great observatory in the sky.


Plans for Hubble

A model of the James Webb Space Telescope on the lawn at Goddard Space Center
Photo courtesy ­NASA

As with any technology, questions about Hubble's future viability and role in space research persist. Originally intended for a 15-year mission, it has outlived expectations, thanks in part to several servicing missions by NASA astronauts. These missions have not only repaired and replaced aging equipment but also upgraded its instruments, allowing Hubble to remain at the forefront of astronomical research.

NASA has not set a definitive retirement date for the Hubble. Instead, it is expected to continue operating as long as its instruments remain functional and provide valuable data. Its continued contributions, even amidst uncertainties, stand as a testament to the enduring impact of well-designed space missions and the resilience of the human spirit to explore and understand our universe.


Hubble's Successor: James Webb Space Telescope (JWST)

The James Webb Space Telescope (JWST), named after former NASA administrator James Webb, studies every phase in the history of the universe. From its orbit approximately 1 million miles (1.6 million km) from Earth, the telescope uncovers information about the birth of stars, other solar systems and galaxies, and the evolution of our own solar system.

To make these fascinating discoveries, the JWST relies primarily on four scientific instruments: a near-infrared (IR) camera, a near-IR multi-object spectrograph, a mid-IR instrument and a tunable filter imager.


But before we move on to the JWST and forget about Hubble, perhaps the hardworking telescope deserves a moment. Thanks to Hubble's unparalleled discoveries, captivating images of what lies beyond the Earth's atmosphere have been made accessible for everyone to enjoy. From a rare alignment between two spiral galaxies to a powerful collision between galaxy clusters, Hubble has brought a little piece of the heavens closer to home.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.


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