Dec 14, 2001

Bright Asteroid

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


Marshall Space Flight Center

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Dec. 14, 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.

Right: High school students in Amtsgymnasiet Sønderborg, Denmark, captured this 8-minute exposure of 1998 WT24 streaking through the constellation Gemini on Dec. 11, 2001. [movies]

"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.


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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.

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Left: 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.

"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.

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Right: Just passing by... Earth as seen from asteroid Toutatis in 2004. [more]

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.

Below: 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.

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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.

Web Links & More...

1998 WT24 Ephemeris -- (JPL) this utility will help you pinpoint the asteroid for telescopic observations.

1998 WT24 Video Gallery -- ( movies and images of 1998 WT24

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Radar Asteroid Research -- (JPL) Radar is a uniquely powerful source of information about asteroid physical properties and orbits.

Right: Astronomers will use the 70-meter diameter Goldstone radar to study 1998 WT24 as it passes by Earth. [more]

Trick or Treat: It's Toutatis! -- (Science@NASA) learn more about another bright asteroid that will visit Earth in 2004.

The orbit of 1998 WT24 -- (JPL) explore the orbit of this asteroid in 3 dimensions.

What is a Potentially Hazardous Asteroid? -- (JPL)

1998 WT24's close approaches to Earth, Venus, and Mercury -- (NeoDys) orbital elements and planetary encounters from 1950 to 2100.

1999 RD32 -- This big asteroid flew close to Earth in 1969, but no one noticed.

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