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Teacher Page: Science Background



Science Background
Words from the Scientist

Science Background:

The following information is provided to give the teacher some additional knowledge about the Hubble Space Telescope's Servicing Mission 3B. You can choose to share this information with the students so that they can do research on the questions that you see mentioned here or use them as a review for class discussion.

1. What is the Hubble Space Telescope?

The Hubble Space Telescope (HST) is a space-based telescope that was launched in 1990 from the space shuttle. From its position 380 miles above the Earth's surface, the HST has expanded our understanding of the universe, in particular star birth, star death, galaxy evolution, and black holes.

The telescope's instruments are the astronomer's eyes to the universe. Its instruments include the Wide Field Planetary Camera 2, Space Telescope Imaging Spectrograph, Near Infrared Camera and Multi-Object Spectrometer, Faint Object Camera, and the newly installed Advanced Camera for Surveys.

When first launched, the HST's lens was out of shape on the edges by 1/50 of the diameter of a human hair. This very small defect made it difficult to focus faint objects being viewed by Hubble. Because the HST is in low Earth orbit, it can be serviced by a shuttle. The defect was corrected in the first servicing mission. The Hubble Space Telescope is scheduled for one more servicing mission in May 2009.

2. Why service the Hubble Space Telescope?

NASA decided early in the telescope's development to design the observatory for on-orbit servicing. Instruments were designed as modular units, comparable to dresser drawers that could be easily removed and replaced. In addition, designers equipped the telescope with handholds and other special features to make servicing tasks less difficult for astronauts wearing bulky spacesuits.

By periodically upgrading the science instruments, NASA also reasoned that it could provide the science community worldwide with state-of-the-art technology that takes advantage of Hubble's unique position high above Earth's obscuring atmosphere. "Over the past 11 years, the public has seen the power of the Hubble Space Telescope," says GSFC Project Scientist David Leckrone. "They haven't seen anything yet. We're opening the doors for more discoveries."

3. What instruments and equipment did the astronauts add to the Hubble Space Telescope during servicing mission 3B?

The 2002 mission was aimed at improving Hubble's science capabilities and ensuring the continued effective operation of the telescope. Adding the Advanced Camera for Surveys (ACS) and restoring the use of the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) by adding a cryocooler achieved the objective of improving the science capabilities of the telescope.

4. What is the Advanced Camera for Surveys and what will it do?

The Advanced Camera for Surveys (ACS) is a third-generation instrument to be installed on the Hubble Space Telescope during Servicing Mission 3B. By design, the three cameras of the ACS will advance HST's capability for surveys and discovery by about a factor of ten. The Wide Field Camera will make deep imaging surveys to search for galaxies and clusters of galaxies in the early universe, the High Resolution Camera will study the light in the centers of galaxies with massive black holes, as well as ordinary galaxies, star clusters, and gaseous nebulae, and the Solar-Blind Camera will be used to find hot stars and quasars to study aurora on Jupiter and investigate the formation of planetary systems.

5. What is a cryocooler?

A cryocooler is a refrigerator for an infrared detector on a telescope. Infrared light falls between visible light and radio waves on the electromagnetic spectrum and, although our eyes are not sensitive to this light, our skin can sometimes detect it as heat. When you put your hand near a hot iron, you can feel the heat on your hand due to the infrared light being emitted by the hot iron. People and animals emit infrared radiation - that's the light night-vision goggles detect. Since most objects emit infrared light, detectors are most sensitive when they are cooler than the light they are detecting. Our skin feels the infrared light from a hot iron but can't detect it from another person since we are at the same temperature as the other person. Infrared detectors need to be cooled, so they can efficiently record the infrared light emitted by celestial objects.

6. Why do astronomers want to detect infrared light?

Infrared light isn't absorbed or scattered like visual light by the clouds of gas and dust found abundantly in the universe. Therefore, astronomers are able to see through the dust around newly forming stars and measure the properties of the disks of dust particles believed to give birth to planetary systems around these stars. They are able to peer into the dusty centers of galaxies, including our Milky Way, to study quasars and other exotic objects. Astronomers are able to observe some of the universe's oldest and most distant objects, observations that could someday answer questions about the size, structure, and future of the universe.

7. What is NICMOS and why does it need a new cryocooler?

NICMOS stands for: Near Infrared Camera and Multi-Object Spectrometer. NICMOS can see the universe at near infrared wavelengths more sensitively and in sharper detail than any other existing telescope. The instrument is a system of three cryogenically cooled cameras that operate independently and simultaneously but with different fields of view. The cameras were originally cooled by solid nitrogen, which evaporated at a faster rate than expected, resulting in a reduced lifetime. By installing the new cryocooler, NICMOS can be brought back to life and fully restore its science capabilities.

8. Why replace the solar panels and power control unit (PCU) if the old ones still work?

Over time, space debris and radiation take their toll on the sensitive equipment. The old solar panels have been on the telescope since the first servicing mission in 1993. So, scientists question the solar panels' reliability. The solar panels, therefore, are being replaced to ensure continued, uninterrupted service. Another benefit is that the new solar panels are smaller, which will result in less drag so the telescope will maintain its orbit longer. The new panels also produce slightly more power, which will be used by the new equipment.

The power control unit has been on the job since the telescope was deployed in space in 1990. This unit controls and distributes electricity from the solar arrays and batteries to other parts of the telescope. Replacing it means that the telescope will be completely shut down for the first time since it was deployed. Although this shutdown could mark the end of the telescope's career if it fails to start up again, the potential benefits outweigh the risks. The old unit is no longer reliable and could damage the telescope if it is not replaced. In addition, the new PCU enables the telescope to utilize the slight extra power supplied by the new solar arrays.

9. How often are servicing missions scheduled?

Your car needs a tune-up; so you take it to your mechanic for servicing. Engineers and scientists designed the Hubble Space Telescope with the same principle in mind. When a component breaks or a more technologically advanced instrument becomes available, astronauts install the new equipment using wrenches, screwdrivers and power tools during periodic servicing missions. By the time Hubble's mission is supposed to end, astronauts will have serviced it five times - 1993, 1997, 1999, 2002, and 2009 - and will have installed many new instruments featuring more advanced technology.

10. When were the prior servicing missions and what instruments were replaced?

Servicing Mission 1:

Servicing Mission 1, launched in December 1993, was the first opportunity to conduct planned maintenance on the telescope. In addition, new instruments were installed and the flaw of the optics in Hubble's primary mirror was corrected. The new instruments and the most important equipment installations are listed below:

New Science Instruments:

  • Astronauts removed the original Wide Field Planetary Camera and installed an updated model equipped with built-in corrective optics that compensated for the incorrect figure on the telescope's primary mirror. The instrument, also built at the Jet Propulsion Laboratory, is called the Wide Field Planetary Camera 2 (WFPC2).
  • Astronauts removed the High-Speed Photometer and replaced it with the Corrective Optics Space Telescope Axial Replacement (COSTAR), which includes an assembly of coin-sized mirrors that are polished to a special prescription to compensate for the incorrect figure on the telescope's flawed primary mirror.

Other important equipment replacements:

  • Solar Array Panels: wing-like structures that produce electricity to power the observatory
  • Solar Array Drive Electronics: modular hardware that controls the position of the solar arrays
  • Magnetometer: a device that measures the telescope's orientation within Earth's magnetic field to help in attitude control
  • DF-224 Flight Computer Co-processor: computer memory that improves backup and computational power

Servicing Mission 2:

The Second Servicing Mission, launched February 11, 1997, greatly improved Hubble's productivity. The installation of new instruments extended Hubble's wavelength range into the near infrared for imaging and spectroscopy, allowing Hubble to probe the most distant reaches of the universe. The replacement of failed or degraded spacecraft components increased the efficiency and performance of the telescope.

New Science Instruments

  • The Space Telescope Imaging Spectrograph (STIS) provides Hubble with unique and powerful spectroscopic capabilities. A spectrograph separates the light gathered by the telescope into its spectral components so that the composition, temperature, motion, and other chemical and physical properties can be analyzed.
  • The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) has provided valuable new information on the dusty centers of galaxies and the formation of stars and planets. NICMOS consists of three cameras. It is capable of both infrared imaging and spectroscopic observations of astronomical targets.

Other important equipment replacements:

  • Refurbished Fine Guidance Sensor: needed to point the telescope at celestial objects
  • The addition of an Optical Control Electronics Enhancement Kit: provides the electronic pathway for commanding the alignment mechanisms in the refurbished Fine Guidance Sensor
  • The Solid State Recorder: stores ten times more data and replaced one of Hubble's three reel-to-reel tape recorders, called the Engineering Science Tape Recorders

Servicing Mission 3A:

Servicing Mission 3A (SM3A) took place in December 1999. NASA decided to split the Third Servicing Mission (SM3) into two parts, SM3A and SM3B, after the third of Hubble's six gyroscopes failed. In accordance with NASA's flight rules, a "call-up" mission was quickly approved, and developed and executed in a record seven months! What was originally conceived as a mission of preventive maintenance turned more urgent on Nov. 13, 1999 when the fourth of six gyros failed, and Hubble temporarily closed its eyes on the universe. The telescope needs a minimum of three gyros to function. The new, improved, and upgraded equipment included six gyroscopes; six battery voltage/temperature improvement kits; a faster, more powerful, main computer; a next-generation solid state data recorder; a new transmitter; another enhanced Fine Guidance Sensor; and new insulation.

11. How do astronauts train for a servicing mission?

The payload team trains in NASA's deep water Neutral Buoyancy Lab at Johnson Space Center in Houston, which simulates the weightlessness of space. The astronauts practice procedures using tools specifically designed for the mission. In addition, the payload team travels to the NASA Goddard Space Flight Center's (GSFC) clean room, where they train on the actual equipment or mock-ups that will be used in space. For example, one of the training activities for Servicing Mission 3B included a demonstration of a specific alignment tool that will be used on the Advanced Camera for Surveys, and information concerning how to roll up the flexible solar array panels that will be removed and returned to Earth. The flight crew also uses simulators to practice the capture and deployment of the telescope. The robotic arm operator must practice transporting the astronauts and the new equipment from the cargo bay to the telescope. The robotic arm also will become a temporary work platform for one or both of the astronauts performing an EVA.

12. Who are the flight crew and payload specialists for Servicing Mission 3B?

Two-time shuttle veteran Scott Altman will command the mission. With him on the flight deck are pilot Duane Carey, who is making his first space flight, and flight engineer and robotic arm operator Nancy Currie, who has three previous missions to her credit.

Payload Commander John Grunsfeld is no stranger to Hubble. This high-energy astrophysicist is a veteran of three flights, including the 1999 Hubble servicing mission, when he performed two spacewalks. Now he will lead the spacewalking team, which includes veteran astronauts James (Jim) Newman and Richard (Rick) Linnehan, and first-time flyer Michael (Mike) Massimino. The four will work in alternating pairs to perform the five planned spacewalks.

13. What impact has Hubble had on society?

Not since Galileo turned his telescope towards the Heavens in 1610 has any event so changed our understanding of the universe as the deployment of the Hubble Space Telescope. Hubble orbits 600 kilometers (375 miles) above Earth, working around the clock to unlock the secrets of the universe. It uses excellent pointing precision, powerful optics, and state-of-the-art instruments to provide stunning views of the universe that cannot be made using ground-based telescopes or other satellites. Hubble was originally designed in the 1970s and launched in 1990. Thanks to on-orbit service calls by the space shuttle astronauts, Hubble continues to be a state-of-the-art, model year 2001 space telescope.

Hubble's accomplishments are extraordinary. Before Hubble, distances to far-off galaxies were not well known. Questions such as how rapidly the universe is expanding, and for how long, created great controversy. Hubble data has changed all of that.

14. What are some of the science highlights from Hubble?

Hubble has turned its eye to various parts of the universe and provided scientists with many images that have had an impact on numerous areas of astronomy. Looking at objects in our own solar system, Hubble provided spectacular views of Comet Shoemaker-Levy 9's collision with Jupiter, delivered the first detailed images of Pluto and its satellite Charon, afforded new understanding of the atmospheres of Uranus and Neptune, and revealed stunning views of the northern and southern lights on Jupiter, Saturn, and Ganymede, as well as imaging the dynamic electrical interactions between Jupiter and its satellite Io.

Moving from planets to stars, the telescope documented in colorful detail the births and deaths of these bright celestial objects. It provided visual proof that pancake-shaped dust disks around young stars are common, and showed for the first time that jets of material rising from embryonic stars emanate from the centers of disks of dust and gas, thus turning what was previously merely theory into an observed reality. Hubble delivered many stunning pictures of stellar deaths, such as the glowing shrouds surrounding Sun-like stars (called planetary nebulae) and the mysterious rings of material around the exploding, massive star called Supernova 1987A.

Hubble also managed to probe the crowded central regions of galaxies, where stars, dust, and gas compete for space, and provided decisive evidence that supermassive black holes - compact "monsters" that gobble up any material that ventures near them - reside in the centers of many galaxies.

Most scientists believe that black holes are the "engines" that power quasars, powerful light beacons located more than halfway across the universe. Hubble has surveyed quasars, confirming that nature's brightest "light bulbs" reside in distant galaxies.

Hubble looked back in time by probing the distant cosmos to reveal much smaller and more irregularly shaped galaxies than those astronomers seen in our nearby universe. These smaller structures, composed of gas and young stars, may be the building blocks from which the more familiar spiral and elliptical galaxies formed.

In the 1920s, astronomer Edwin Hubble observed that the universe doesn't remain still: it's expanding. Since then, astronomers have debated how fast it is expanding, a value called the Hubble constant, an essential ingredient needed to determine the age, size, and fate of the universe. In May 1999, a team of astronomers announced they had obtained a value for the Hubble constant and then determined that the universe is 12 to 14 billion years old.

One of the most dramatic astronomical discoveries of this century came in 1998, when two independent teams, using Hubble and other telescopes, found strong evidence that the cosmic expansion is accelerating. Hubble teamed up with a fleet of X-ray, gamma-ray, and visible-light observatories in a quest to analyze the sources of gamma-ray bursts. Gamma-ray bursts may represent the most powerful explosions in the universe since the Big Bang. Before 1997, astronomers were stumped: although they had observed more than 2,000 "bursts," they couldn't determine whether these fireballs occurred in our galaxy or at remote distances. Hubble images showed unambiguously that the bursts actually reside in far-flung galaxies rife with star formation.

15. Fun Facts about the Hubble Space Telescope and astronomy:


Words from the Scientist about Servicing Mission 2:

Imagine what the world would look like if it were always covered by clouds; by day, the sky would be a bright grayish-white and by night, pitch black. We would be blind to the heavens. We would not know what made night into day and have no knowledge of moons, planets, or stars. Then, one day when we are able to fly. We break through the clouds and behold an entire new world! We see the tiny disk that makes our days bright. We marvel at the beauty of the stars, the moon, and the mysterious worlds that lie beyond.

In reality, we are not perpetually blanketed by clouds. We do "see" heavenly bodies, but our atmosphere blurs our vision. Telescopes extend that vision to see distant stars and galaxies. There are even instruments that probe beyond visible light into the ultraviolet and infrared spectra - too red or too blue for human eyes. Still, the atmosphere "fogs" their sight. But we are never satisfied. Our atmosphere allows us glimpses of what lies beyond our capabilities and teases us. We want more. The answer has a simple solution: for clearer views we must rise above the atmospheric filters that blur our vision.

In 1990, NASA took a leap in this direction. The Hubble Space Telescope was launched, taking our "eyesight" to new heights. It increased our visible-light detection ten-fold. It gave us ultraviolet capabilities and took images of celestial bodies that were the province of imaginations. In February 1997, the Hubble Space Telescope will be fitted with state-of-the-art instruments that will probe deeper into the universe than any other telescope in history. Technology improves so quickly that electronics are not the same as they were a decade ago. TV cameras in 1997 are far different from cameras built in 1987. Hubble scientists knew this and planned decades ago for advancements. The Space Shuttle Discovery will deliver a new generation of instruments to the Hubble Space Telescope. The first refurbishment mission in 1993 proved that planned servicing missions to upgrade technology can take our vision beyond expectations, open new vistas and provide the spark that ignites our imagination.

-Abhijit Saha



NASA (1994) Spaced Based Astronomy: Teacher's Guide with Activities EG-102, August 1994, NASA Headquarters, Washington, DC. This guide has a wealth of useful activities that will give students opportunities to build simple telescopes, spectroscopes, and participate in other related hands-on experiences.

Pasachoff, Jay M., (1991), Astronomy: From the Earth to the Universe, Saunders College Publishing, Philadelphia, PA.

NASA, Hubble Space Telescope Servicing Mission, Education Briefs, EB:115, NASA Headquarters, Washington, DC.

Maton, Anthea (1994), Exploring the Universe, Prentice Hall, Englewood Cliffs, NJ.

NASA (1996), The Hubble Space Telescope Second Servicing Mission (SM-2), Hubble Facts FS-96 (03)-005-GSFC, NASA, Goddard Space Flight Center, Greenbelt, MD, 20771. This handout is very useful and specific to the mission.

NASA, Exploring the Universe with the Hubble Space Telescope, NP-126, GPO, Washington, DC.

Bickel, Charles L. et al, (1979), Physical Science Investigations, Houghton Mifflin Company, Boston, MA.