Earthorbiting Telescopic Observations

The Hubble Space Telescope (Figure 12.1, [4]) continues to operate as of this writing and will continue to do so for several years. Its primary mirror is 2.4 m in diameter. It can resolve details as small as 0.05 arcseconds across. That corresponds to sizes near

Figure 12.1. An image of the Hubble Space Telescope (HST) shortly after its release from the bay of the Space Shuttle. HST has undergone several servicing missions since its launch, including one to correct a flaw in the primary mirror shape. HST's contribution to ring studies at all four giant planets is considerable.

Figure 12.2. The Spitzer Space Telescope was placed into solar orbit by an unmanned spacecraft. This is an artist's concept of the Spitzer Telescope in orbit. Although few planetary ring studies have yet been carried out using the Spitzer Telescope, it has the capability to collect infrared images and spectra on planetary rings.

Figure 12.2. The Spitzer Space Telescope was placed into solar orbit by an unmanned spacecraft. This is an artist's concept of the Spitzer Telescope in orbit. Although few planetary ring studies have yet been carried out using the Spitzer Telescope, it has the capability to collect infrared images and spectra on planetary rings.

150, 310, 660, and 1,050 km at Jupiter, Saturn, Uranus, and Neptune, respectively. While much poorer than achieved by spacecraft encountering or orbiting one of the giant planets, these resolutions are sufficient to reveal changes in ring appearance over the decades since the Voyager encounters. Imagery of the Neptune ring arcs at comparable resolutions recently revealed changes in the relative brightness and positions of several of the arcs [5].

The Hubble Space Telescope orbits Earth at an altitude of 600 km. A newer infrared telescope, the Spitzer Space Telescope (Figure 12.2 [6]), is actually in orbit around the Sun rather than around the Earth. It follows the Earth in its orbit, at a distance that increases about 15,000,000 km each year. While its main goals are associated with astronomy beyond the solar system, it also studies the giant planets circling our own star, the Sun, and could provide temperatures, images, and other data on planetary ring systems.

The Hubble Space Telescope will eventually be replaced by the infrared-optimized James Webb Space Telescope (Figure 12.3, [7]). With a primary mirror 6.5 m in diameter, its light-gathering power and resolution will improve substantially on that

Figure 12.3. A mock-up of the James Webb Space Telescope (JWST), scheduled for launch no earlier than 2013, is on display at NASA's Goddard Space Flight Center in Greenbelt, Maryland. JWST is a more capable version of the Hubble Space Telescope and will orbit the Sun in Earth's L2 Lagrangian point.

of the Hubble Space Telescope. However, it is not scheduled for launch until at least 2013. The James Webb Space Telescope will be inserted into the Earth's L2 Lagrangian point [8], nearly 1,500,000 km from Earth (about four times the distance of the Moon), where it will orbit the Sun with the same orbital period as Earth, thanks to the combined gravitational forces of the Sun and the Earth.

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