Asteroid Near-Miss or Impact?: The Yarkovsky effect

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In a few hours, the asteroid 2012 DA14 will have a close, but safe, pass by Earth on Feb. 15, 2013 at 2:25 p.m. EST (19:25 UTC). While there is no chance for an impact tomorrow, the fact that this object will come so close to the Earth (about 17,200 miles or 27.700 km) makes it a great opportunity for the scientists to get an up close and personal view of the near-Earth asteroid, which is half the size of a football field. This distance is well outside Earth’s atmosphere, but it is inside the belt of satellites in geostationary orbit, which is located 22,200 miles (35,800 kilometers) above Earth’s surface. But what makes the study of such “near earth objects (NEOs)” so important? How is the “Yarkovsky” effect related with the possibility of an asteroid’s impact?

map_tunguska_eventThe asteroid 2012 DA14 is a small near-Earth object approximately 150 feet (45 meters) in diameter. According to nasa this “is the closest ever predicted Earth approach for an object this large” [1]. However when we call “small” a 150 feet asteroid doesn’t mean that an impact to Earth will go unnoticed. Scientists believe that the famous “Tunguska Event” over Siberia in 1908 was caused by a 100-foot (30 m) asteroid.It is believed to have flattened about 750 square miles (1,200 square kilometers) of forest in and around the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Russia.

Since no impact or another “Tunguska” event will occur tomorrow, it will be a very good chance for scientists to observe this fascinated moment. “NASA’s Goldstone Solar System Radar, located in California’s Mojave Desert, will observe the asteroid on Feb. 16, 18, 19 and 20. Due to the asteroid’s small size, the radar images generated are expected to be no more than a few pixels across. It will also be observed by numerous optical observatories worldwide to attempt to determine its spin rate and composition.” [1]

Yarkovsky effect

The Yarkovsky effect is a force acting on a rotating body in space and is caused by the anisotropic emission of thermal photons, which carry momentum. It is usually considered in relation to meteoroids or small asteroids (about 10 cm to 10 km in diameter), as its influence is most significant for these bodies. [2]

The effect was discovered by the Russian civil engineer Ivan Osipovich Yarkovsky (1844–1902), who worked on scientific problems in his spare time. Writing in a pamphlet around the year 1900, Yarkovsky noted that the diurnal heating of a rotating object in space would cause it to experience a force that, while tiny, could lead to large long-term effects in the orbits of small bodies, especially meteoroids and small asteroids. Yarkovsky’s insight would have been forgotten had it not been for the Estonian astronomer Ernst J. Öpik (1893–1985), who read Yarkovsky’s pamphlet sometime around 1909. Decades later, Öpik, recalling the pamphlet from memory, discussed the possible importance of the Yarkovsky effect on movement of meteoroids about the Solar System.

The Yarkovsky effect is a consequence of the fact that change in the temperature of an object warmed by radiation (and therefore the intensity of thermal radiation from the object) lags behind changes in the incoming radiation. That is, the surface of the object takes time to become warm when first illuminated; and takes time to cool down when illumination stops.

In order to have an insight of how the Yarkovsky effect (among others) impacts on an asteroid’s orbit, NASA is planning a mission called OSIRIS-REx in 2016. “The OSIRIS-REx spacecraft will travel to a near-Earth carbonaceous asteroid (101955) 1999 RQ36, study it in detail, and bring back a sample (at least 60 grams or 2.1 ounces) to Earth”. “This sample will help us investigate planet formation and the origin of life, and the data collected at the asteroid will also aid our understanding of asteroids that can impact Earth.” [3]

“Every six years, 1999 RQ36’s orbit takes it near the Earth – to within 448,794 km (278,867 mi, 0.003 astronomical units). Calculation of its future orbits indicate that 1999 RQ36 has on of the highest impact probabilities in the next few centuries of any known asteroid. Understanding the orbit, effects on orbit and composition will help scientists better determine the orbit of the asteroid to provide more conclusive data on how and when it might impact Earth. These data will be available to all. With time, policymakers will be able to decide what (if any) steps should be taken to mitigate the risk of impact.” [4]

Goldstone Delay-Doppler images of 1999 RQ36 (cr: nasa)

Goldstone Delay-Doppler images of 1999 RQ36 (cr: nasa)

Observations that Michael Nolan at Arecibo Observatory in Puerto Rico made in September 2011, along with Arecibo and Goldstone radar observations made in 1999 and 2005, when 1999 RQ36 passed much closer to Earth, show that the asteroid has deviated from its gravity-ruled orbit by roughly 100 miles, or 160 kilometers, in the last 12 years, a deviation caused by the Yarkovsky effect. [5]

“The Yarkovsky force on 1999 RQ36 at its peak, when the asteroid is nearest the sun, is only about a half ounce — about the weight of three grapes on Earth. Meanwhile, the mass of the asteroid is estimated to be about 68 million tons. You need extremely precise measurements over a fairly long time span to see something so slight acting on something so huge.”

Nolan and his team measured the distance between the Arecibo Observatory and 1999 RQ36 to an accuracy of 300 meters, or about a fifth of a mile, when the asteroid was 30 million kilometers, or 20 million miles, from Earth.

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