* Astronomy

Asteroid 29075 (1950 DA) makes its closest approach to the Earth (0.238 AU) on the 21st May, 2012.

(1950 DA = 2000 YK66 )

Armageddon

An asteroid nearly a mile wide is on a collision course with Earth. But thankfully we have time - plenty of time.

There is a one in 300 chance that the asteroid 1950 DA will collide with Earth on March 16, 2880. The collision would smash millions of tons of TNT into the Earth and could wipe out a large city, trigger widespread fires and tidal waves.

Jon Giorgini of NASA's Jet Propulsion Laboratory explains he isn't trying to sound a warning bell.

"It's so far in the future that it's nothing anyone should worry about now," he said. "But it illustrates the value of understanding things sooner rather than later."

Lost and found

1950 DA was discovered on 23 February 1950 by C. Wirtanen at Lick Observatory. Mount Hamilton, Calif It was observed for 17 days and then It faded from view for five decades . An object, designated 2000 YK66, was discovered by the Near Earth Object Search

(LONEOS) program in Arizona, was later recognized as being 1950 DA.

 

"Once an asteroid is discovered, radar is the most powerful way to find its exact orbit and, apart from sending a spacecraft, the only way to see what it looks like.

Radar observations are helping us push predictions 5 to 10 times further into the future."

said JPL's Dr. Steve Ostro, who led the radar observations of 1950 DA.

After being under surveillance using optical facilities for the following two months after the LONEOS recovery, delay-Doppler radar observations were carried out at Goldstone and Arecibo on 3-7 March 2001 while 1950 DA approached the Earth at 7.79 million km. After measuring a mean diameter of 1.1 km, the team led by Dr. Giorgini at NASA JPL facility in Pasadena, California, provided an accurate analysis of future close encounters with the Earth for the next one thousand years.

A bit later, in mid-2001, the Spaceguard Central Node was inquired by Dr. Jon Giorgini both for the existence of precovery observations and for the possibility of re-measuring the old discovery images from the plate collection at Lick Observatory in order to remove systematic effects on the old data thanks to the use of much better astrometric catalogues. Dr. Klemola, who works at Lick Observatory, kindly did all the remeasurements from the old data and supplied the SCN with interesting information regarding the Lick photographic archive for future searches. As for precovery data, after a general inquiry among the teams involved in this task, only a faint image from the UK Schmidt archive was found.

"This is not something to worry about," said Giorgini, "We're showing that searches with optical telescopes and follow-up observations with radar telescopes can provide us centuries of advance notice about potential close encounters of asteroids with Earth. That's plenty of time to consider the options -- 35 generations, in fact."

Quick rock

The data on 1950 DA suggest it is travelling at a velocity of about 15 km (9 miles) per second relative to the Earth.

This means that if 1950 DA were to collide with the planet, it would do so with an explosive force of approximately 44,800 megatonnes of TNT.

If it struck land, it would produce a crater about 22 km (14 miles) across, with a blast radius of intense damage of around 300 km (190 miles).

If it struck water, it would also produce a huge tsumami (3.39 Mb download)

False Alarms

Astronomers have warned of possible asteroid collisions in the past. Perhaps most notorious was the 1998 announcement from a scientist at the International Astronomical Union that a massive asteroid would smash into Earth in 2028. After much public hand-wringing, NASA dismissed the prospect as impossible a day later.

The uncertainty of Armageddon announcements generally comes from a lack of data. Although NASA has set up a program to identify and monitor all of the large asteroids that pass near Earth, so far astronomers have only located about half of these estimated 1,200 potentially hazardous asteroids. Additional observations often quickly reveal asteroids as non-threatening.

But asteroid 1950 DA is different. Thanks to a quirk in the asteroid's orbit, observation records dating back to 1950 and precise radar readings from NASA's Arecibo station in Puerto Rico when the asteroid hurtled by more than 70 million miles from Earth in 2000 and 2001, researchers understanding of this asteroid and its path of orbit is unusually complete.

"This one is distant in the future but it's most interesting because it's not likely to disappear after a few observations,"

said Steven Chesley, a co-author of the Science paper and a researcher at the Jet Propulsion Laboratory.

Astronomers have learned the asteroid swings around the sun every 2.2 years. It passes within 77 million milesof the sun and then streaks back into space, passing Mars' orbit and reaching a point about 241 million miles from the sun. The rocky body is 1.1 km (0.68 miles) in diameter, it has at least a couple large craters on its surface and it spins very fast - about once every 2.1 hours.

"If you were to stand on the surface you might get dizzy watching the stars zip by," said Giorgini.

"It's like predicting a 15-bank shot in a pool game," Giorgini said. "We know the cue stroke extremely well because it is right now and we can measure it. But at each future bank, small variations accumulate and change the next bounce, which changes the following one and so on. What we've done is find the range of changes possible due to tilt, imperfections and fuzz on the table, the bounce of the cushions, and wind blowing across the room. We need to know more about the 'cue ball' to really be sure of how the last three banks in 2809, 2840 and 2860 will line things up for 2880."

Devising Plan Deflection

It would not be good, for example, to blow up an asteroid with a nuclear warhead only to have the pieces slam into Earth.

Spitale, meanwhile, has been thinking of ways of using the Yarkovsky effect to mankind's advantage. He suggests if we know far in advance that an asteroid is heading our way, we might be able to deflect the rock by altering the asteroid's surface and changing the Yarkovsky effect on the body.

Spitale suggests this could be done by blanketing the surface with a layer of dirt to decrease how readily it absorbs heat. Or, he says, we could "paint" the asteroid white - perhaps by dropping tons of white powder across its surface - to increase its reflectivity. Another alternative is to break up the asteroid's surface using a series of planted explosives.

"All you have to do is change properties like its thermal conductivity or its colour and that will affect its trajectory," Spitale said.

Altering the asteroid's surface may sound like an elaborate, expensive task, but it's simple compared to other suggestions. Some have proposed dropping a shrink-wrap material on an asteroid, attaching a giant solar sail to it or blasting it with a nuclear bomb - as characters did in the 1998 movie Armageddon.

No matter the method, Spitale believes it's vital that people develop some method of deflecting big incoming rocks, whether we use it in the next few decades or 878 years in the future.

"It definitely is a credible threat," he said. "We know an asteroid caused mass extinction among the dinosaurs 65 million years ago. These things happen rarely, but they do happen."

 

Integration of the orbit of asteroid (29075) 1950 DA, based on radar and optical measurements spanning 51 years, reveals a 20-minute interval in March 2880 when there could be a non-negligible probability of the 1-kilometer object colliding with Earth. The impact scenario is three orders of magnitude more probable than any previous case, or about 1.5 times greater than the entire background risk through the year 2880, but depends on the physical properties of the asteroid. Trajectory knowledge remains accurate until 2880 because of extensive astrometric data, an inclined orbit geometry that reduces in-plane perturbations, and an orbit uncertainty space modulated by gravitational resonance. Analysis of sources of uncertainty in this long-term prediction include the effects of numerical integration error, galactic tides, perturbations due to encounters with other asteroids, solar mass loss, solar particle-wind, solar oblateness and radiation pressure, the uncertainties in planetary masses, and accelerations due to the time-delayed anisotropic thermal re-radiation of incident solar radiation, also known as the "Yarkovsky" effect. This latter effect depends on the shape, mass, spin axis, composition, and surface properties of the asteroid, all of which are either unknown or weakly determined, such that refinement to a specific collision probability may require direct inspection by a spacecraft

Impact in 2880?

That said, there's still a lot that scientists don't know about 1950 DA, including small factors that, over 878 years, could significantly skew predictions. They don't know, for example, the exact mass of the asteroid or its rotation or its thermal conductivity. Even the computing tools astronomers use and the way programs round off to the nearest number after so many digits muddy the prediction of an encounter.

Giorgini says the biggest wildcard is a factor called Yarkovsky's effect. This effect, believed to be first penned by a Polish engineer named IO Yarkovsky in the early 1900s, describes how unbalanced heating of a small body can propel it in a particular direction.

Like on Earth, the surface of an asteroid experience seasons, with a "summer" in which the Northern Hemisphere is hotter. It also experiences "afternoons" (on 1950 DA an "afternoon" would amount to minutes rather than hours) that are warmer than "mornings."

The warmer regions of an asteroid absorb more thermal energy and so give off more thermal energy. The radiation propels the asteroid slightly, like a rocket.

If you add the Yarkovskey effect, it could be this asteroid doesn't have a chance of hitting us" - Joseph Spitale of the Lunar and Planetary Laboratory at the University of Tucson, Ariz.

So far scientists still aren't sure how or even if the Yarkovsky effect will change the trajectory of 1950 DA. Unknown factors like the roughness of the asteroid's surface (dirt absorbs heat less readily than flat rock) and the asteroid's colouring (a darker surface will absorb more heat) all influence the effect on the asteroid.

It may be the only way to fill in these blanks will be by sending a robotic or manned mission to the asteroid.

"We now know what we need to know," said Giorgini. "That's important in science - you've got to know your limits."

"Yarkovsky's dark side

Ironically, the Yarkovsky effect has natural consequences that seem to herd space rocks toward Earth.

Smaller asteroids, especially, are more vulnerable to being propelled by the uneven solar heating. A study found that bits and pieces created when two asteroids collide undergo changes in their orbits do to their new surface properties.

Computer simulations led by the Southwest Research Institute's William F. Bottke, Jr. found that some of these smaller asteroids can be propelled to the edges of gaps in the main asteroid belt -- regions of the belt that have been swept clean by the gravitational effects of Jupiter. Unfortunately, when asteroids enter these gaps, Jupiter's gravity tends to fling them into near-Earth orbits.

Small force

If the impact occurred in the ocean, a tsunami 25 metres (80 feet) high would be generated 1,000 km (620 miles) from the impact site.

Currently, 1950 DA has been rated as a level two event on the Torino warning scale, which means it is an event "meriting concern". Experts say the size of the asteroid is close to that which could affect the Earth's global climate. However, it is not thought a collision with 1950 DA would threaten the continued existence of our species.  By comparison, the impact 67 million years ago, which has been implicated in the extinction of the dinosaurs, is thought to have been caused by an object 10 times the diameter of 1950 DA.

The next 877 years

All the while, the precisely calculated chance of doom on the eve of Saint Patrick's Day in 2880 has the potential to hang over the heads of 35 generations of human beings.

So, should our descendents or we worry about a gargantuan space rock named 1950 AD?

"The most likely thing is that Saint Patrick's Day parades in the year 2880 may be a little more festive than usual as this object recedes into the distance, having passed Earth by," he said.

 

(1950 DA = 2000 YK66 )

 

Discovery Circumstances:	C. A. Wirtanen - Lick Observatory, Mount Hamilton   22 Feb 1950	(1)
Orbital Type:	AP	(*)( 2)
Taxonomic Type:	-	-
Albedo:	-	-
Diameter:	1.2 - 2.7 km	($)
H:	17.00	(#)
G:	0.15	(#)
U-B:	-	-
B-V:	-	-
Rotation Period:	2.1216 hr	( 3)
Quality:	p	( 3)
Lightcurve Amplitude:	0.2 mag	( 3)
Spin Vector:	-	-
Radar Observations:	Y	( 4)
Spectral Observations:	-	-
IR Observations:	-	-

Remarks

* - orbital type derived from osculating orbital elements

$ - diameter range derived from H and assumed albedo for C and S type' - 0.04 and 0.20 respectively

# - absolute magnitude H derived from astrometric observation via orbit determination - G value assumed to be 0.15

p - as rotation quality code means period derived, not yet published