February 2004
Deep Space Network 2-for-1
Sale!
By Patrick L. Barry
Call it a "buy one, get one free" sale for astronomers: Build a network of radio
dishes for communicating with solar-system probes, get a world-class radio
telescope with a resolution nearly as good as a telescope the size of Earth!
That's the incidental bonus that NASA's Deep Space Network (DSN) offers the
astronomy community. Designed to maintain contact with distant spacecraft in
spite of the Earth's rotation, the large, widely spaced dishes of the DSN are
ideal for performing a form of radio astronomy called "very long baseline
interferometry" (VLBI).
VLBI produces very high resolution images of the cosmos by combining the output
from two or more telescopes. The result is like having a giant "virtual"
telescope as large as the distance between the real dishes! Since bigger
telescopes can produce higher resolution images than smaller ones, astronomers
need to use dishes that are as far apart as possible.
That need dovetails nicely with the DSN's design. To maintain continuous contact
with deep space missions, the DSN has tracking stations placed in California,
Spain, and Australia. These locations are roughly equally spaced around the
Earth, each about 120 degrees of longitude from the others-that way at least one
dish can always communicate with a probe regardless of Earth's rotation. That
also means, though, that the straight-line distance between any two of the
stations is roughly 85 percent of Earth's diameter-or about 6,700 miles. That's
almost as far apart as land-based telescopes can be.
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"We often collaborate with other VLBI groups
around the world, combining our dishes with theirs to produce even better
images," says Michael J. Klein, manager of the DSN Science Office at NASA's Jet
Propulsion Laboratory. "Since our 70-meter dish in Canberra, Australia, is the
largest dish in the southern hemisphere, adding that dish in particular makes a
huge difference in the quality of a VLBI observation."
Even though only about 1 percent of the DSN's schedule is typically spared from
probe-tracking duty and scheduled for radio astronomy, it manages to make some
important contributions to radio astronomy. For example, the DSN is currently
helping image the expanding remnant of supernova 1987A, and Dr. Lincoln
Greenhill of the Smithsonian Astrophysical Observatory is using the DSN dishes
to explore a new way to measure the distances and velocities of galaxies.
And all this comes as a "bonus" from the dishes of the DSN.
To introduce kids to multi-wavelength astronomy, NASA's website for kids, The
Space Place, has just added the interactive demo, "Cosmic Colors," at
http://spaceplace.nasa.gov/cosmic.
This article was provided by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.