If someone sneaks a bite of your chocolate chip cookie, they
leave behind evidence of their pilferage in the form of a crescent of
missing cookie. The same is true in our solar system, where an impact can
take a bite out of a planet or moon, leaving behind evidence in the form of
a crater. By combining modern technology with a historical telescope,
astronomers have discovered that the asteroid Juno has a bite out of it. The
first direct images of the surface of Juno show that it is scarred by a
fresh impact crater.

Juno, the third asteroid ever discovered, was first spotted by astronomers
early in the 19th century. It orbits the Sun with thousands of other bits of
space rock in the main asteroid belt between Mars and Jupiter. One of the
largest asteroids, at a size of 150 miles across, Juno essentially is a
leftover building block of the solar system.

Astronomer Sallie Baliunas (Harvard-Smithsonian Center for Astrophysics) and
colleagues photographed Juno when it was located relatively nearby in
astronomical terms, about 10 percent further from the Earth than the Earth
is from the Sun. Even at that distance, Juno appeared very tiny in the sky,
subtending only 330 milli-arcseconds – the equivalent of a dime seen at a
distance of 7 miles. Imaging Juno at the high resolution needed to resolve
surface details thus presented a challenge.

To solve the problem, the scientists used an adaptive optics system
connected to the 100-inch Hooker telescope at Mount Wilson Observatory.
Adaptive optics enables astronomers to compensate for the distortion created
by air currents in our planet’s atmosphere, yielding images as sharp and
clear as those taken in space.

Their surface maps showed that Juno, like other asteroids, is misshapen
rather than round, and that it has “sharp” edges. Even better, as Juno
tumbled through space during the night of observing, a “bite” came into view
– an area that appeared dark as seen at near-infrared wavelengths. The
astronomers concluded that the asteroid had recently (in astronomical terms)
collided with another object, resulting in a 60-mile-wide crater, or
possibly a smaller crater that is surrounded by a 60-mile blanket of ejecta

“I look at an asteroid as a garden – a garden not of flowers and leaves, but
one of rubble and dust churned up by constant impacts. This process of
gardening pulverizes the asteroid’s surface into a fine-grained regolith,”
said Baliunas. “The recent, large impact on Juno gives us an opportunity to
see through the regolith and study excavated material from beneath the
surface – a rare look into the material out of which the early Earth was

The blast that knocked a bite out of Juno may also have provided researchers
with a convenient way of studying that asteroid up close without ever
leaving our planet. Some meteorites found on the Earth are actually pieces
of large asteroids like Juno. Those pieces were broken off and launched into
space by an impact, and then fell on our planet. The newly-found impact
crater on Juno may have sent samples of that asteroid to the Earth.

This remarkable result demonstrates how technology can be used to renew
historical observatories, giving them a new lease on life. The Hooker
telescope, now nearing the end of its first century of observing, can use
adaptive optics systems to obtain views of the cosmos as clear as though the
telescope were in space. Hence, the telescope that Edwin Hubble and his
assistant used to discover evidence of the expanding universe continues to
make groundbreaking discoveries today.

These results were published in the May 2003 issue of the astronomy journal

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian Center
for Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists organized into six research divisions study the origin,
evolution, and ultimate fate of the universe.

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