A big and bright near-Earth asteroid will glide by our planet on Dec. 16th
within easy range of powerful radars and backyard telescopes.

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 km-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.


The approximate location of 1998 WT24 on Dec. 16, 2001,
at 0600 UT, when the space rock will be closest to Earth. This
region of the sky is easy to find: The constellation Perseus appears
directly overhead from mid-Northern latitudes at 10 p.m. local time. A
detailed ephemeris is available from JPL:
A HREF=”http://neo.jpl.nasa.gov/cgi-bin/db?name=1998+WT24#ephemeris”>http://neo.jpl.nasa.gov/cgi-bin/db?name=1998+WT24#ephemeris

“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

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.


Mercury, Venus, Earth and 1998 WT24 on Dec. 14, 2001. Click on the
image to manipulate a 3D model of the asteroid’s orbit. Credit: JPL’s
Near-Earth Object program. (http://neo.jpl.nasa.gov/cgi-bin/db?name=1998+WT24)

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.

Editor’s Note: 1998 WT24 will be closest to Earth around 0600 UT on Dec.
16th. (10 p.m. PST on Saturday, Dec. 15th). Amateur astronomers who wish to
observe the space rock shouldn’t worry too much about catching it at the
precise moment of closest approach. The asteroid will be brighter than 10th
magnitude — and an easy target for telescopes — all weekend.