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2019-2020: CLI
THE DAILY ASTRONOMER
Wednesday, May 20, 2020
Remote Planetarium 38: Telescopes II - Orbiting Telescopes
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Helpful term:
Air
beneficial to the lungs, detrimental to astronomy.
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Astronomers have learned what they've learned about the cosmos by studying the light that celestial objects either emit or absorb. Each light beam is akin to a long tapestry on which information pertaining to the associated celestial body is contained. For instance, we can know a star's chemical composition through its spectrum and its luminosity by its apparent brightness and distance. The particular challenge confronting astronomers is the thin gaseous layer enshrouding this planet. When light leaves its origin, the beam travels at light speed through the outer space vacuum for most of its journey, be it less than 4.5 years (Alpha Centauri) or 2.2 million years (Andromeda Galaxy). Yet, during the last millionth of a second of its trip, the beam penetrates our atmosphere where it can be scattered, absorbed and distorted. Just think of the elite ultra marathoner who maintains a steady 4 minute mile over a 72 mile stretch only to falter clumsily and disastrously a quarter stride shy of the finish line. Air is the astronomer's bane. Yes, it sustains their lives, but often hampers sky watching. Unfortunately (ok, most fortunately) the astronomical community can do nothing to eradicate the atmosphere. However, the advent of rocketry has given them a means by which they can circumvent most of it.
The year 1923 marks the true beginning of orbiting telescope history. in that year physicist Hermann Oberth (1894-1989) published "Die Rakete zu den Planetenraumen," or "The Rocket in Planetary Space." Contained within that seminal paper was the outline for the deployment of a telescope above most of Earth's atmosphere. Recall that while Earth's atmosphere extends hundreds of miles above the planet's surface, 99.999% of it is located below 100 kilometers.
American physicist Lyman Spitzer (1914-1997) published a paper in 1946 entitled "Astronomical Advantages of an Extraterrestrial Observatory." In that paper he cited two distinct advantages to an orbiting observatory:
- angular resolution Angular resolution is the smallest separation by which two separate objects can be distinguished. For instance, the star Albireo (in Cygnus the Swan) is a binary star, meaning it consists of two stars gravitationally bound together. To the unaided eye, Albireo looks like a single star. Through most telescopes, one can see both of Albireo's stars. On the ground atmospheric disturbance and diffraction are two the two main causes that reduce resolution. Diffraction is the process by which a light beam is spread out when passing through a narrow aperture. In space, only diffraction would limit angular resolution.
- Infrared and UV Earth's atmosphere absorbs radiation along many parts of the electromagnetic spectrum. The image below reveals how far into the atmosphere the various radiation types can penetrate:
The small band comprising the "visible spectrum" penetrates all the way down to Earth's surface, which is why we can see each other. Some UV (ultraviolet) rays and Infrared radiation also reach the ground. (The former is responsible for sunburns, the latter for the Sun's heat.) We also see a radio "window," a large section of the radio segment of the EM spectrum. Higher energy EM waves, such as middle to far UV, X-rays and Gamma rays do not reach the planet's surface, which is why we're so happily alive right now. Notice that the orbiting satellites featured on the graphic above are positioned at a high enough altitude to receive all the incoming radiation.
Between 1962 - 1975, NASA launched eight Orbiting Solar Observatories (OSO)'s intended, naturally, to study the Sun. The OSO's studied solar flares, gamma ray bursts and even observed a flare emitted from Scorpius X-1, a strong x-ray source approximately 9000 light years from Earth.
In 1965 NASA appointed Spitzer to be the head of a committee whose aim was to develop a list of primary scientific objectives for any such orbiting telescope. Over the next decade, NASA, encouraged by the successes of the OSOs continued to pursue other orbiting telescope ventures. Although they faced widespread spending cuts in 1974 due to economic pressures, NASA finally obtained the initial funding of $38 million in 1978. (A much more substantial amount in the 1970s than it is today.) With the assistance of the ESA space agency, NASA started construction of an orbiting telescope that would be named the Hubble Space Telescope in honor of American astronomer Edwin Hubble (1883-1959) whose observations led to the discovery of the cosmic expansion.
American astronomer Edwin Hubble
The grinding of the Hubble Space Telescope's primary mirror.
The Hubble Space Telescope was initially scheduled to launch in October 1986. Unfortunately, the Space Shuttle Challenger exploded in January 1986. This tragedy not only cost the lives of seven astronauts, including New Hampshire school teacher Christa McAuliffe, but it also suspended all space shuttle flights for more than two years. The first Space Shuttle to be launched following the Challenger Disaster (STS 26 Discovery) lifted off on September 29, 1988. On April 24, 1990, STS-31 was launched and deployed the Hubble Space Telescope into orbit. After years of political wrangling, funding cuts, and the other logistical difficulties that attend all space science missions, the HST was finally going into orbit. The HST's launch was hailed as a triumph for NASA, ESA and the entire astronomical community that would benefit from the first major observatory to have ever be stationed well above Earth's surface.
The triumph was short lived. Within weeks of the launch, researchers received the first Hubble images. Though slightly superior in image quality than most ground based telescopes, the images lacked the resolution the astronomers expected. The outer perimeter was ground 2200 nano meters too fine: too large an error in the unforgiving world of optics. The HST images were deemed far inferior to what they could have been had the optics been working perfectly. If the 1986 Challenger disaster was NASA's worst tragedy, the 1990 HST deployment proved to be its most embarrassing debacle.
On December 2, 1993 STS-61 was launched. The crew's mission was to install corrective equipment designed to fix Hubble's faulty optics. After three years, NASA was hoping to finally have a well working space observatory and, in the process, to redeem itself. This redemption effort was an astounding success.
The three images below of the galaxy M100 show us the extent of the improvement. The left image was captured before the installation of the corrective equipment. The central image was captured soon after the installation. The right image is was taken in 2018.
Ever since that vital serving mission, the Hubble Space Telescope has become the most scientifically productive space craft ever launched. As we proceed through this course, we will see many of the images the HST has captured over the last twenty-seven years: star clusters, nebulae, galaxies and much more!
In March 2021, NASA plans to launch the James Webb Space Telescope, Hubble's worthy successor. Unlike the Hubble Space Telescope that orbits Earth at an approximate altitude of 250 miles, the Webb Telescope will be stationed around the Second LaGrange point between the Earth-Sun orbit
Who knows what the Webb telescope will reveal? If the HST is any indication, the Webb telescope's images and discoveries will be beyond all imagination.
Tomorrow, we begin our journey through the Universe beyond solar system.
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