Introduction by David Morrison Radar astronomy has again demonstrated its incredible precision by measuring a deviation of only 15 km in the orbit of Near Earth Asteroid (NEA) Golevka. This small change in the orbit of the asteroid is the first direct evidence of an important force for long-term evolution of asteroid orbits, called the Yarkovsky effect. The Yarkovsky effect was first proposed early in the last century, and during the past decade it has repeatedly been invoked by theorists working on asteroid dynamics. This is the first positive evidence of the existence of this force, however. Following are a NASA press release, the abstract of the paper by Steve Chesley and co-workers published in Science in December 2003, and two news stories the discuss the discovery and its implications. ============================= NASA News Release NASA SCIENTISTS USE RADAR TO DETECT ASTEROID FORCE December 5, 2003 NASA scientists have for the first time detected a tiny but theoretically important force acting on asteroids by measuring an extremely subtle change in a near-Earth asteroids orbital path. This force, called the Yarkovsky Effect, is produced by the way an asteroid absorbs energy from the sun and re-radiates it into space as heat. The research will impact how scientists understand and track asteroids in the future. Asteroid 6489 "Golevka" is relatively inconspicuous by near- Earth asteroid standards. It is only one half-kilometer (.33 mile) across, although it weighs in at about 210 billion kilograms (460 billion pounds). But as unremarkable as Golevka is on a celestial scale it is also relatively well characterized, having been observed via radar in 1991, 1995, 1999 and this past May. An international team of astronomers, including researchers from NASA's Jet Propulsion Laboratory in Pasadena, Calif., have used this comprehensive data set to make a detailed analysis of the asteroids orbital path. The team's report appears in the December 5 issue of "Science." "For the first time we have proven that asteroids can literally propel themselves through space, albeit very slowly," said Dr. Steven Chesley, a scientist at NASAs Jet Propulsion Laboratory and leader of the study. The idea behind the Yarkovsky Effect is the simple notion that an asteroids surface is heated by the sun during the day and then cools off during the night. Because of this the asteroid tends to emit more heat from its afternoon side, just as the evening twilight on Earth is warmer than the morning twilight. This unbalanced thermal radiation produces a tiny acceleration that has until now gone unmeasured. "The amount of force exerted by the Yarkovsky Effect, about an ounce in the case of Golevka, is incredibly small, especially considering the asteroids overall mass," said Chesley. "But over the 12 years that Golevka has been observed, that small force has caused a shift of 15 kilometers (9.4 miles). Apply that same force over tens of millions of years and it can have a huge effect on an asteroids orbit. Asteroids that orbit the Sun between Mars and Jupiter can actually become near-Earth asteroids." The Yarkovsky Effect has become an essential tool for understanding several aspects of asteroid dynamics. Theoreticians have used it to explain such phenomena as the rate of asteroid transport from the main belt to the inner solar system, the ages of meteorite samples, and the characteristics of so-called "asteroid families" that are formed when a larger asteroid is disrupted by collision. And yet, despite its profound theoretical significance, the force has never been detected, much less measured, for any asteroid until now. "Once a near-Earth asteroid is discovered, radar is the most powerful astronomical technique for measuring its physical characteristics and determining its exact orbit," said Dr. Steven Ostro, a JPL scientist and a contributor to the paper. "To give you an idea of just how powerful our radar observation was like pinpointing to within a half inch the distance of a basketball in New York using a softball-sized radar dish in Los Angeles." To obtain their landmark findings, the scientists utilized an advanced model of the Yarkovsky Effect developed by Dr. David Vokrouhlicky of Charles University, Prague. Vokrouhlicky led a 2000 study that predicted the possibility of detecting the subtle force acting on Golevka during its 2003 approach to Earth. "We predicted that the acceleration should be detectable, but we were not at all certain how strong it would be," said Vokrouhlicky. "With the radar data we have been able to answer that question." Using the measurement of the Yarkovsky acceleration the team has for the first time determined the mass and density of a small solitary asteroid using ground-based observations. This opens up a whole new avenue of study for near-Earth asteroids, and it is only a matter of time before many more asteroids are "weighed" in this manner. =============================== Abstract of paper in Science DIRECT DETECTION OF THE YARKOVSKY EFFECT VIA RADAR RANGING TO ASTEROID 6489 GOLEVKA. S. R. Chesley, S. J. Ostro, D. Vokrouhlicky, D. Capek, J. D. Giorgini, M. C. Nolan, J. L. Margot, A. A. Hine, L. A. M. Benner, and A. B. Chamberlin. Science 302, 1739-1742 (2003). Radar ranging from Arecibo, Puerto Rico, to the 0.5-kilometer near-Earth asteroid 6489 Golevka unambiguously reveals a small nongravitational acceleration caused by the anisotropic thermal emission of absorbed sunlight. The magnitude of this perturbation, known as the Yarkovsky effect, is a function of the asteroid's mass and surface thermal characteristics. Direct detection of the Yarkovsky effect on asteroids will help constrain their physical properties, such as bulk density, and refine their orbital paths. Based on the strength of the detected perturbation, we estimate the bulk density of Golevka to be 2.7 (+0.4, -0.6) grams per cubic centimeter. ============================== THE ASTEROID AND THE PEA By David Tytell, Sky & Telescope December 17, 2003 Astronomers tracking asteroids in their orbits by radar have directly observed the Yarkovsky effect a long-proposed force imparted on asteroids by sunlight. The force is subtle in the case of minor planet 6489 Golevka, it's the equivalent of one ounce of thrust pushing against a 0.5 kilometer, 210 million-ton tumbling "mountain." A team led by Steven R. Chesley and Steven J. Ostro (NASA/Jet Propulsion Laboratory) focused on the spinning asteroid and measured the minute orbital change it incurred between 1991 and 2003. In that time they found that the object's path had shifted 15 kilometers from what would be expected based on gravitational interactions with other solar-system bodies. The Yarkovsky effect arises when sunlight warms a rotating asteroid. Absorbed thermal energy is radiated away when the heated face of the rock rotates to one side. The departing infrared photons create a feeble "rocket effect" that imparts a sideways force. Over time, the exiting photons gradually change the asteroid's orbit. Rocks that rotate in the same direction as they revolve will slowly achieve wider orbits. The converse is true opposite spinners slowly spiral into the Sun. In addition to spin, Yarkovsky involves more than a dozen other factors. One important variable is the material property of the object. The amount of sunlight absorbed and the time lag for re-radiation depend respectively on the reflectivity and heat capacity of the asteroid's surface. Extremely precise radar observations spanning many years were critical for measuring the minuscule orbital deflection. "We need three widely spaced [in time], high-quality radar apparitions," says Chesley. In a 2000 Icarus paper, Chesley and others predicted that the May 2003 apparition of well-studied Golevka would reveal that the asteroid had shifted its orbit due to sunlight. Three springtime observations with the Arecibo Observatory in Puerto Rico later, the 2000 prognostication proved positive. "It's always exciting when you can make an prediction and observations bear it out," says Chesley. Several factors needed to be addressed for the researchers to be sure that what they were seeing was really Yarkovsky in action. The largest potential for error was a perturbation from an unknown passing asteroid. But after careful analysis, the team became convinced that interloping asteroids weren't the culprits. "There is no room for confusion as to what the measured effect was." Nailing down Yarkovsky allowed the team to make another measurement the first-ever mass determination of a lone asteroid. (Rocks with satellites, such as 243 Ida with its moonlet Dactyl, are "weighed" by timing the orbit of one around the other.) Astronomers are watching about two dozen other objects whose orbits are refined enough to look for the effect. Determining how and if those rocky princesses feel the Yarkovsky peas will speak volumes about their differing thermal conductivities and other characteristics, as well as providing masses for previously immeasurable bodies. The results appeared in the December 5, 2003, issue of Science. ================================== WILL DISCOVERY HELP REPEL ASTEROIDS HEADED FOR EARTH? Sarah Ives for National Geographic News December 11, 2003 NASA scientists have detected a small but significant force that has the power to change an asteroid's orbital path. The discovery may help scientists predict asteroids' travel routes with greater accuracy and even provide crucial data to help deflect asteroids headed for Earth, researchers say. Scientists detected the force, called the Yarkovsky Effect, on a near-Earth asteroid (NEA) known as asteroid 6489, or Golevka, that has been tracked for a dozen years. An ounce of force, caused by slight variations in thermal radiation, caused the Asteroid Golevka to shift its orbital path by almost 10 miles in 12 years. While the distance is small compared to the billions of miles the asteroid traveled during that time, Steven Chesley, leader of the study, believes it is significant. "Over tens of millions of years that [change in orbit] can have a big effect," he said, noting that understanding the effect of the force can help scientists predict how asteroids move. The researchers described their findings in the December 5 issue of Science. The Yarkovsky Effect is produced by an asteroid's absorption of solar energy. During the day, the sun heats the asteroid. At night, the asteroid cools off. Because of this heating and cooling, an asteroid tends to emit more heat on its "afternoon" sidethe same phenomenon that makes the Earth's evening twilight warmer than its morning twilight. The result is unbalanced thermal radiation that produces a small acceleration. For years, scientists have applied the theory of the Yarkovsky Effect to explain aspects of asteroids, from rates of travel to the age of meteorite samples. But until now, the force had never been directly observed. "We measured a force of about one ounce [28 grams] acting on an asteroid that weighs 460 billion pounds [208 billion kilograms]," Chesley said. By his calculations, that figure amounts to the weight of a strawberry moving an asteroid that is longer than five football fields. Using the Yarkovsky Effect, scientists also determined, for the first time, an asteroid's mass using ground-based observation. "This is a unique method to determine a mass of a single&asteroid without sending a spacecraft there," said Vokrouhlicky. Determining the mass of an asteroid could have enormous implications, said Steven Ostro, a study co-author also based at NASA's Jet Propulsion Laboratory. Knowing an asteroid's mass and volume from radar imaging enables scientists to determine the celestial body's density, a crucial piece of information. Ostro said: "If we ever have to deflect an asteroid, in order to design a procedure to do the deflecting, we have to know a lot about the interior." Most asteroids in the solar system travel along an asteroid highway that lies between the orbits of Mars and Jupiter known as the main belt. Occasionally, an asteroid will break free of the belt and move into the inner solar system. If the asteroid comes within 121 million miles (195 million kilometers) of the sun, the asteroid is known as a near-Earth asteroid. Compared to other near-Earth asteroids, Golevka is relatively unremarkable, measuring only half a kilometer (0.3 mile) in diameter. Scientists believe that asteroids that collide with Earth must be larger than a kilometer (0.6 mile) in diameter to create massive climate change and greater than 10 kilometers (6.2 miles) wide to cause mass extinction, Chesley said. None of the large near-Earth asteroids astronomers are currently observing appear on track to hit Earth any time soon. Data from Golevka, however, will help make predictions more accurate. David Vokrouhlicky, a study co-author based at Charles University in Prague, Czech Republic, said he can imagine cases where the discovery of the Yarkovsky Effect will help scientists "when we would try to decide whether [an] asteroid will hit the Earth or not." |
