Interesting Science

February 7 2007: A NASA podcast in which Drs. Steve Chesley and Don Yeomans of JPL and NASA's Near Earth Object office are interviewed by Jane Platt - listen to the podcast here.

Other contents:

1) Making Sure the Sky Is Not Falling - Transcript of above podcast
2) Quantifying the risk posed by potential Earth impacts - Abstract
3) NASA Scientists Use Radar to Detect Asteroid Force - Related PR
4) Asteroid 1950 DA - Video and Info

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1) Making Sure the Sky Is Not Falling - Transcript

Narrator: Making sure the sky is NOT falling. I'm Jane Platt with a podcast from JPL - NASA's Jet Propulsion Laboratory in Pasadena, California. Asteroids and comets are pretty cool, cosmic objects - as long as they keep their distance. Dr. Steve Chesley of JPL is with NASA's Near Earth Object office, which uses telescopes and radar to study objects that venture near Earth.

Chesley: Well, we have about 4,300 Near Earth objects in the catalog at the moment. That's all sizes. We're most interested in finding the large objects, what we consider to be those that could, say, threaten the climate of the Earth if they were to impact.

Narrator: Those are bigger than one kilometer, or four times the size of Pasadena's Rose Bowl. Scientists have found more than 700 of those. Most are too far away to pose any danger to Earth. On the other hand, they have identified about 120 objects - of varying sizes - that do have the potential to hit Earth. Still, no need to panic.

Yeomans: No one knows of a friend or a loved one who has been hurt by a Near Earth object, that's true. So these are very low probability events but very high consequence events. It's very unlikely that one of these large objects will hit us.

Narrator: Dr. Don Yeomans heads NASA's Near Earth Object office. His colleague, Dr. Steve Chesley, says an ounce of prevention is worth a pound of cure.

Chesley: NASA's approach to the Earth hazard problem so far, and I think rightly, has been to focus on discovery. You can not deflect an object that you haven't discovered. And so finding the asteroids, finding them early is the most important thing in this process.

Narrator: So the Near Earth Object office, with input from astronomers around the world, detects objects, tracks their location, size, speed and movement over a period of time. They rate them on the 10-point Torino scale. Sort of like the Richter scale for earthquakes.

Chesley: Torino scale zero is where 99 percent of our cases fall, which means it's just not worth any public attention, although we continue to monitor those routinely. Torino scale one means it's more than ordinary, but still not particularly alarming. We get on average a few, maybe several Torino scale one cases per year. We have had a couple of Torino scale twos. We even had one Torino scale four, which was quite extraordinary.

Narrator: Now a four is enough to worry about. But with more detailed observations, the four and the twos were downgraded.

Chesley: After an object is discovered and observations continue to arrive at our office, we continue to update and refine the orbital predictions and the impact assessments. That allows us to refine the Torino scale ratings for the object and so on.

Narrator: And so far, every single worrisome object they've tracked, with further observations, has been ruled out as a hazard to Earth. Interested in keeping tabs on these objects? Check out the Near Earth object website at http://neo.jpl.nasa.gov .

Chesley: We have the impact probabilities, and you can click on any one of the objects and get details. But at the top level, there's a summary for each object. The speed at which it passes the Earth is present there, and that of course is important for the impact energy. The size of the object, and the Torino scale.

Narrator: Again, that's http://neo.jpl.nasa.gov , and it includes a lot of links. Oh, and in case you're wondering what would happen if scientists did find an asteroid that really could threaten Earth? Researchers are scoping out different ways to handle a scenario like that.

Chesley: Probably the best and most obvious way of deflecting an asteroid is to simply slam another, a spacecraft, into it, and to slow it down, or speed it up if you overtake it from behind, and that gives it enough change in velocity to steer it off the Earth impact trajectory.

Narrator: But again, none of the known near-Earth objects have scientists staying up nights worrying. So for now, they say - the sky is not falling. Thanks for joining us for this podcast from NASA's Jet Propulsion Laboratory.

Source: NASA February 7 2007

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2) Quantifying the risk posed by potential Earth impacts - Abstract:

Steven R. Chesley (JPL), Paul W. Chodas (JPL), Andrea Milani (Univ. Pisa), Giovanni B. Valsecchi (IASF-CNR) and Donald K. Yeomans (JPL)
Icarus 159, 423-432 (2002)

Predictions of future potential Earth impacts by Near-Earth Objects (NEOs) have become commonplace in recent years, and the rate of these detections is likely to accelerate as asteroid survey efforts continue to mature. In order to conveniently compare and categorize the numerous potential impact solutions being discovered we propose a new hazard scale that will describe the risk posed by a particular potential impact in both absolute and relative terms. To this end we measure each event in two ways, first without any consideration of the event's time proximity or its significance relative to the so-called background threat, and then in the context of the expected risk from other objects over the intervening years until the impact. This approach is designed principally to facilitate communication among astronomers, and it is not intended for public communication of impact risks. The scale characterizes impacts across all impact energies, probabilities and dates, and it is useful, in particular, when dealing with those cases which fall below the threshold of public interest. The scale also reflects the urgency of the situation in a natural way, and thus can guide specialists in assessing the computational and observational effort appropriate for a given situation. In this paper we describe the metrics introduced, and we give numerous examples of their application. This enables us to establish in rough terms the levels at which events become interesting to various parties.

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3) NASA Scientists Use Radar to Detect Asteroid Force - Related PR

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 asteroid's 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 asteroid's orbital path. The team's report appears in the December 5 issue of Science [1].

"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 NASA's Jet Propulsion Laboratory and leader of the study.

The idea behind the Yarkovsky Effect is the simple notion that an asteroid's 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 asteroid's 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 asteroid's 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.

In addition to Chesley, Ostro and Vokrouhlicky, authors of the report include Jon Giorgini, Dr. Alan Chamberlin and Dr. Lance Benner of JPL; David Eapek, Charles University, Prague, Dr. Michael Nolan, Arecibo Observatory, Puerto Rico, Dr. Jean-Luc Margot, University of California, Los Angeles, and Alice Hine, Arecibo Observatory, Puerto Rico.

Arecibo Observatory is operated by Cornell University under a cooperative agreement with the National Science Foundation and with support from NASA. NASA's Office of Space Science, Washington, DC supported the radar observations. JPL is managed for NASA by the California Institute of Technology in Pasdena.

More information about NASA's planetary missions, astronomical observations, and laboratory measurements are available on the Internet at: http://neo.jpl.nasa.gov/

Information about NASA programs is available on the Internet at: www.nasa.gov

JPL is managed for NASA by the California Institute of Technology in Pasadena.

Source: NASA Press Release December 5, 2003-163

[1] Direct Detection of the Yarkovsky Effect by Radar Ranging to Asteroid 6489 Golevka
Steven R. Chesley, Steven J. Ostro, David Vokrouhlicky, David Capek, Jon D. Giorgini, Michael C. Nolan, Jean-Luc Margot, Alice A. Hine, Lance A. M. Benner, and Alan B. Chamberlin

Science 5 December 2003 302: 1739-1742 [DOI: 10.1126/science.1091452] (in Reports)

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 +0.4/-0.6 grams per cubic centimeter.

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4) Asteroid 1950 DA - Video and Info

History of Observation

Asteroid 1950 DA (29075) was discovered on 23 February 1950. It was observed for 17 days and then faded from view for half a century. Then, an object discovered on 31 December 2000 was recognized as being the long-lost 1950 DA. (As an aside, this was New Century's Eve and exactly 200 years to the night after the discovery of the first asteroid, Ceres.)

Radar observations were made at Goldstone and Arecibo on 3-7 March 2001, during the asteroid's 7.8 million km approach to the Earth (a distance 21 times larger than that separating the Earth and Moon). Radar echoes revealed a slightly asymmetrical spheroid with a mean diameter of 1.1 km. Optical observations showed the asteroid rotated once every 2.1 hours, the second fastest spin rate ever observed for an asteroid its size.

Detection of A Potential Hazard

When high-precision radar meaurements were included in a new orbit solution, a potentially very close approach to the Earth on March 16, 2880 was discovered to exist. Analysis performed by Giorgini et al and reported in the April 5, 2002 edition of the journal Science [2] determined the impact probability as being at most 1 in 300 and probably even more remote, based on what is known about the asteroid so far. At its greatest, this could represent a risk 50% greater than that of the average background hazard due to all other asteroids from the present era through 2880, as defined by the Palermo Technical Scale (PTS value = +0.17). 1950 DA is the only known asteroid whose hazard could be above the background level.

Understanding the Risk

However, these are maximum values. The study indicates the collision probability for 1950 DA is best described as being in the range 0 to 0.33%. The upper limit could increase or decrease as we learn more about the asteroid in the years ahead.

Expressing the risk as an interval is necessary because not enough is known about the physical properties of the asteroid. For example, radar data suggests two possible directions for the asteroid's spin pole. If one pole is correct, solar radiation acceleration could mostly cancel thermal emission acceleration. Collision probability would then be close to the maximum 0.33%. If the spin pole is instead near the other possible solution, there would be little chance of collision. There are other factors also.

The situation is similar to knowing you have a coin that is biased so one side will land up 80% of the time - but you don't know which side. You can only say that when you flip the coin, the chance of heads is 80% or 20%.

Source: NASA - Info obtained via the Near Earth Object Program's Impact Risk page ("Where is 1950 DA?")

[2] Asteroid 1950 DA's Encounter With Earth in 2880: Physical Limits of Collision Probability Prediction
J. D. Giorgini et al.

Science 5 April 2002:
Vol. 296. no. 5565, pp. 132 - 136
DOI: 10.1126/science.1068191

Abstract

Integration of the orbit of asteroid (29075) 1950 DA, which is based on radar and optical measurements spanning 51 years, reveals a 20-minute interval in March 2880 when there could be a nonnegligible probability of the 1-kilometer object colliding with Earth. Trajectory knowledge remains accurate until then because of extensive astrometric data, an inclined orbit geometry that reduces in-plane perturbations, and an orbit uncertainty space modulated by gravitational resonance. The approach distance uncertainty in 2880 is determined primarily by uncertainty in the accelerations arising from thermal re-radiation of solar energy absorbed by the asteroid. Those accelerations depend on the spin axis, composition, and surface properties of the asteroid, so that refining the collision probability may require direct inspection by a spacecraft.

Excerpt

The next radar opportunity for this asteroid is in 2032. The cumulative effect of Yarkovsky acceleration since 2001 might be detected with radar measurements obtained then, but this would be more likely during radar opportunities in 2074 or 2105. Earlier Yarkovsky detection or orbital uncertainty reduction might be possible with space-based optical astrometric systems. Ground-based photometric observations might better determine the pole direction of 1950 DA much sooner. Depending on the results of such experiments, a satisfactory assessment of the collision probability of 1950 DA may require direct physical analysis with a spacecraft mission.

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