Powerful New Technique to Measure Asteroids' Sizes and Shapes

About this image: Artist’s impression of the asteroid (234) Barbara. Thanks to a unique method that uses ESO’s Very Large Telescope Interferometer, astronomers have been able to measure sizes of small asteroids in the main belt for the first time. Their observations also suggest that Barbara has a complex concave shape, best modelled as two bodies that may possibly be in contact.
Credit: ESO/L. Calçada

Wednesday, February 04, 2009

A team of French and Italian astronomers have devised a new method for measuring the size and shape of asteroids that are too small or too far away for traditional techniques, increasing the number of asteroids that can be measured by a factor of several hundred. This method takes advantage of the unique capabilities of ESO's Very Large Telescope Interferometer (VLTI).

"Knowledge of the sizes and shapes of asteroids is crucial to understanding how, in the early days of our Solar System, dust and pebbles collected together to form larger bodies and how collisions and re-accumulation have since modified them," says Marco Delbo from the Observatoire de la Côte d'Azur, France, who led the study.

Direct imaging with adaptive optics on the largest ground-based telescopes such as the Very Large Telescope (VLT) in Chile, and space telescopes, or radar measurements are the currently favoured methods of asteroid measurement. However, direct imaging, even with adaptive optics, is generally limited to the one hundred largest asteroids of the main belt, while radar measurements are mostly constrained to observations of near-Earth asteroids that experience close encounters with our planet.

Delbo and his colleagues have devised a new method that uses interferometry to resolve asteroids as small as about 15 km in diameter located in the main asteroid belt, 200 million kilometres away. This is equivalent to being able to measure the size of a tennis ball a distance of a thousand kilometres. This technique will not only increase the number of objects that can be measured dramatically, but, more importantly, bring small asteroids that are physically very different from the well studied larger ones into reach.

The interferometric technique combines the light from two or more telescopes. Astronomers proved their method using ESO's VLTI, combining the light of two of the VLT's 8.2-metre Unit Telescopes. "This is equivalent to having vision as sharp as that of a telescope with a diameter equal to the separation between the two VLT Unit Telescopes used, in this case, 47 metres," says co-author Sebastiano Ligori, from INAF-Torino, Italy. The researchers applied their technique to the main belt asteroid (234) Barbara, which was earlier found, by co-author Alberto Cellino, to have rather unusual properties. Although it is so far away, the VLTI observations also revealed that this object has a peculiar shape. The best fit model is composed of two bodies each the size of a major city – with diameters of 37 and 21 km – separated by at least 24 km. "The two parts appear to overlap," says Delbo, "so the object could be shaped like a gigantic peanut or, it could be two separate bodies orbiting each other."

If Barbara proves to be a double asteroid, this is even more significant: by combining the diameter measurements with the parameters of the orbits, astronomers can then compute the density of these objects. "Barbara is clearly a high priority target for further observations," concludes Ligori.

Having proven the validity of their new and powerful technique, the team can now start a large observing campaign to study small asteroids.

2004 MN4: swing and a miss

December 28, 2004

Despite the holidays, a herculean effort went into acquiring additional observations of asteroid 2004 MN4 and in searching for the object in old images. A couple of days ago, Jeff Larsen and Anne Descour of Spacewatch Observatory managed to locate the asteroid on images taken March 15 of this year, greatly extending the time span of the observations. Measurements of 2004 MN4's positions on these images, combined with additional observations, enabled members of the Spaceguard Survey to reduce the uncertainty in the orbit and rule out an impact in 2029. Furthermore, it looks like this asteroid will not pose a significant threat for the next 100 years.

It's still going to pass close — current estimates put it missing us by about 40,000 miles. At this distance, observers located in the right place — say near London, England — around 10 P.M. on Friday, April 13, 2029, should be able to see the asteroid with the unaided eye. It'll be a dim 5th magnitude star near the constellation Gemini moving about 2.5° per minute — about half as fast as the International Space Station appears to move across the sky. At this speed, the asteroid will be very hard to track with a telescope.

December 27, 2004

For researchers involved with the Spaceguard Survey, the international effort to locate asteroids that may pose a hazard to Earth, 2004 has been an interesting year. It began and ended with an apparent asteroid threat.On January 14, initial observations of an asteroid named AL00667 seemed to indicate it had about a 25 percent chance of hitting Earth within 36 hours. The situation prompted asteroid hunters to discuss who should be warned of the impending impact. Further observations — made by an amateur that same evening — ruled out a hit. Although AL00667 missed us by over a million miles, the incident forced astronomers to think about the practical consequences of an unlikely event.

Now, we have 2004 MN4, an asteroid that, as of this writing, astronomers give a 2.7 percent chance of striking Earth — odds of 1 in 37 — on Friday, April 13, 2029. The asteroid was discovered June 19, 2004, by Roy Tucker, David Tholen, and Fabrizio Bernardi from Kitt Peak, Arizona, but it was observed for only 2 nights. On December 18, Gordon Garradd of Australia's Siding Spring Survey rediscovered the object. Further observations from around the globe over the next several days confirmed its connection to the June discovery. At this point, automatic systems recognized the impact possibility and provided predictions.

In the case of AL00667, observations almost immediately reduced the chance of impact to zero. But additional observations of 2004 MN4 (169 of them from June to the present) actually have increased its chances of collision from 1 in 300 (December 23rd) to 1 in 60 (December 24th) to the current 1 in 37. The impact probability has gone up, rather than down.

MN4 is the first object to break a rating 2 on the Torino impact hazard scale, a classification, similar to the Ritcher scale for earthquakes, designed to quantify the impact risk of near-Earth asteroids. The asteroid is currently standing at 4, which signifies an "event meriting concern" and is defined as "a close encounter with 1 percent or greater chance of causing regional devastation."

The current guess of 2004 MN4's size is about 1,300 feet (400 meters) across, which is too big to be broken up by the atmosphere. If it were to hit, astronomers estimate its striking power at 1,400 megatons, nearly 28 times greater than the largest nuclear weapon ever detonated. This asteroid would leave a hole big enough to swallow a small city — and this ignores the effects of the blast wave, which would damage a much larger area. (For a closer look at the potential effects of MN4's impact, see the University of Arizona Lunar and Planetary Lab's interactive web page, Effects of an impact on Earth.)

If the rock is as small as 300 feet (100m) in diameter, the atmosphere will probably break it apart before it hits the ground. The estimated size depends on how much light the surface of the asteroid reflects, and astronomers do not have a good idea of its composition at this time.

But before we get on the phone to Bruce Willis and tell him to break out the mining gear, bear in mind that the impact probability likely will decrease as more observations become available — especially radar observations. Because it permits precise determinations of speed and distance, radar is used for refining asteroid orbits. NASA's Near Earth Object Program impact risk web page will reflect changes to our understanding of 2004 MN4's orbit, but things may not change very much until an opportunity for radar observation occurs.

If the chance of collision continues to climb, we'll have more than 24 years to prepare for a possible impact, and there are many folks already working on the problem of deflecting an asteroid.

It's likely the chances of a collision with 2004 MN4 will soon be reduced to zero, but until then, members of the Spaceguard Survey are closely watching this asteroid.

Sooner or later, the improbable does happen. Just ask the dinosaurs.