NEO Visible This Weekend

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http://www.space.com/scienceastronomy/solarsystem/neo_bright_011214.ht
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Near-Earth Asteroid Visible in Telescopes This Weekend

By NASA
posted: 04:25 pm ET, 14 December 2001

There's no danger of a collision, but astronomers are nevertheless
keeping a watchful eye on "1998 WT24" -- a large near-Earth asteroid
that will glide by our planet this weekend.

"It's a great opportunity to study an Earth-approaching asteroid,"
says Donald Yeomans, the manager of NASA's Near-Earth Object Program
at JPL. At closest approach on Sunday, Dec 16th, the kilometer-wide
space rock will be about five times farther from Earth than the Moon.
That's near enough for Earth-based radars to ping, and it will be
bright enough for amateur astronomers to see through backyard
telescopes.

"The last time a kilometer-sized object came so close to Earth was
August 27, 1969, when 1999 RD32 passed within 3.7 lunar distances of
our planet," says Yeomans. Thirty-two years ago, no one noticed 1999
RD32 because it hadn't been discovered yet. But thanks to modern
asteroid search programs that do a better job spotting near-Earth
objects, astronomers are well prepared for the coming close encounter
with 1998 WT24.

In the days ahead, JPL's Steve Ostro and colleagues will monitor 1998
WT24 using NASA's Goldstone planetary radar in the Mojave desert and
the powerful Arecibo radar in Puerto Rico. Their observations will
substantially reduce uncertainties in the object's orbit, which will
allow scientists to better predict its future path. The radar data
will also delineate the 3D shape of the asteroid. Radar maps of other
asteroids have revealed a surprising variety of shapes and sizes.
Some are binary systems (one space rock orbiting another) and one
even looks like a dog bone.

"We need to know the structure of these Earth-approaching objects in
case we ever have to deflect or destroy one," says Yeomans.

It's not unusual for small asteroids to fly by Earth as close as 1998
WT24 will do this weekend, but 1998 WT24 is big and thus remarkably
bright. On Dec. 15th and 16th the asteroid will glow as bright as a
9th magnitude star as it races through the northern constellations
Auriga and Perseus. That's not bright enough to see with the unaided
eye, but 1998 WT24 should be easy to spot by peering through the
eyepiece of a 6" or larger telescope.

"This is a rare chance for amateur astronomers to visually observe a
near-Earth asteroid," says veteran asteroid watcher John Rogers. "I
encourage astronomy clubs to try." Because 1998 WT24 will be moving
as fast as 1 degree per hour, observers using a telescope with a 60
or so power eyepiece might be able to perceive the asteroid's motion
as it glides past background stars.

"A fine opportunity for observers in the Americas to identify 1998
WT24 comes on Saturday morning, December 15th, when it glides through
the northern fringe of the bright open star cluster M38 in Auriga,"
says Roger Sinnott, a senior editor of Sky & Telescope
magazine. "This takes place near 8:20 Universal Time (that is, 3:20
a.m. EST or 12:20 a.m. PST). The asteroid will then be at its
brightest, about magnitude 9.5."

"This is only one of two known near-Earth asteroids that will be
brighter than 10th magnitude before the year 2027," adds Rogers. The
other one is 4179 Toutatis, which will glow at 8.9th magnitude during
a close encounter with Earth in 2004. Although Toutatis will be a
little brighter than 1998 WT24 is now, Toutatis will be harder to see
because of a glaring full Moon.

Both 1998 WT24 and Toutatis are considered potentially hazardous
asteroids -- or "PHAs" for short. "An asteroid is a PHA if it can get
within about 0.05 astronomical units (AU) of Earth's orbit and if
it's larger than a few hundred meters," explains Yeomans. "We know of
354 PHAs and the list is growing as new ones are discovered."

Fortunately, none of the known PHAs pose an immediate threat to our
planet. But, says Yeomans, that could change. Gravitational tugs felt
by such asteroids when they fly by planets could set one on a
collision course with Earth.

Indeed, the orbit of 1998 WT24 has already been substantially altered
by such encounters.

Yeomans explains: "This object probably began its life long ago in
the main asteroid belt between Mars and Jupiter." Gravitational
interactions with Jupiter and Mars, and then later with Earth,
transformed 1998 WT24's wide orbit into the much smaller one it
follows today. The asteroid now approaches the Sun even closer than
Mercury does. And its maximum distance from the Sun is only 1.02 AU,
scarcely beyond the 1.0 AU orbit of Earth.

As 1998 WT24 travels back and forth through the inner solar system on
its 222-day orbit, it can have close encounters with three planets:
Mercury, Venus and Earth. Indeed, it often comes close to all of them
in a single Earth-year. Those orbit-altering flybys are one reason
that astronomers are keeping tabs on this large asteroid.

1998 WT24 is intriguing for another reason, too: Its greatest
distance from the Sun, at 1.02 AU, almost exactly coincides with
Earth's orbit. That makes the asteroid an attractive target for space
missions. "As a rule of thumb," says Yeomans, "the more an object's
orbit is like Earth's orbit, the easier it is to reach with minimal
fuel expenditure. It would be relatively simple and cheap to fly a
spacecraft by an asteroid like 1998 WT24."

There are no plans to do so at the moment, but who knows what the
future holds? "Now we know almost nothing about 1998 WT24 -- that is,
except for its approximate size and orbit. This weekend's close
approach to Earth should vastly improve our knowledge," says Yeomans.