Posted on 12/05/2003 8:40:15 AM PST by blam
Sunlight's gentle nudge on asteroids detected
19:00 04 December 03
NewScientist.com news service
Astronomers have detected the delicate force of sunlight on an asteroid's orbit for the first time.
The long-predicted effect has been blamed for propelling some asteroid fragments from the Solar System's main asteroid belt into the region near Earth, where they can threaten the planet. But its subtle action on asteroid orbits has been difficult to measure with optical telescopes.
Instead, Steve Chesley, Steve Ostro and colleagues at NASA's Jet Propulsion Lab in Pasadena, California, used the radar range finder at Arecibo in Puerto Rico. They used this to track a near-Earth asteroid called Golevka.
The team took precise measurements of Golevka's position regularly between its discovery in 1991 and its most recent Earth pass in May. This showed it was not quite in the position that would be predicted based on gravitational forces alone.
But the orbit can be explained if Golevka also experiences a "Yarkovsky effect", a recoil effect caused when absorbed solar photons are re-emitted from the asteroid's surface as heat.
"Golveka is a very good radar target and this group are the world experts in radar," says Alan Harris of the German Aerospace Centre in Berlin, who works on thermal emissions from asteroids. "They've now got 12 years' worth of data, a long enough timebase to detect the slight changes in its orbit that you can only measure with radar. It's certainly very significant."
Missing miles
In May, the JPL group measured Golevka to be about 16 million kilometres from Earth. But its precise position was 15 kilometres further away from the point predicted by gravitational calculations.
"That was so large compared to the radar's uncertainty of a few ten or hundred metres," says Ostro. The position fits with a prediction of the Yarkovsky force on Golevka, based on the asteroid's rate of spin and its surface properties, including its reflectivity. This means astronomers can have more confidence in predictions of other asteroid orbits.
The distance Golevka was deflected by the Yarkovsky force also reveals the mass of the 350-metre-wide asteroid. This remote calculation of mass has not been done before for a solitary asteroid.
The researchers will now move on to calculating the masses of other asteroids. "This is going to allow us to explore asteroids remotely by getting their mass, density and their surface properties," says Chesley, avoiding the difficult and expensive requirement of sending out spacecraft.
Journal reference: Science (vol 302, p1739)
Eugenie Samuel Reich
Maybe it's the radar beams that are pushing it off course...
Interesting comment.
It would have been preposterous for them to assume that radiation pressure was not a real perturbation for asteroids. The only interesting thing here is that they were finally able to measure the effects on great big bodies.
That one achievement, however, suggests something else that's helpful: assuming they have a good handle on the absorptivity/emissivity of the asteroid, this approach can be used to estimate the mass of the body -- something they couldn't do before -- and thereby gain an estimate of its density.
And from that, they can begin guessing at the composition of the asteroids. If there were ever a viable asteroid mining industry, that kind of knowledge would be incredibly valuable.
The reason I pung you is to point out that knowledge of asteroid density would be incredibly useful for selecting among the huge number asteroids. It would save years -- for metal, you go to a dense asteroid. For water and propellants, you go to a "fluffy" one.
Yeah, you can buy those "radiometers" for as little as $15.
I'm trying to recall a "Quirks & Quarks" broadcast last summer (Radio Canada, 9.515Mhz, 11AM Eastern, Saturdays).
The guest [physicist] said his favorite method was to "paint" the asteroid white to reflect the energy. My memory is hazy
as to what effect absorbtion/radiation v. reflection has.
IIRC the effect of pure specular reflection is precisely twice that of pure absorption. The question of re-radiation of heat at low IR frequencies complicates this some, but the pressure models I'm used to seeing have specular reflection being the most "energetic."
Looking at the radiometers, it is the dark side that "pushes", causing the spin.
What you describe is counter intuitive to that. Probably the reason I can't remember the discussion.
In the asteroid case, I couldn't remember if they wanted to speed it up or slow it down to alter the course with the light color.
There was also the discussion on the effect of the color of solar sails...
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.