A new study using data from NASA’s NEOWISE space telescope has traced some dark Near-Earth Objects (NEOs) back to their likely source – a family of large, low albedo asteroids known as the Euphrosynes.
The asteroid 31 Euphrosyne glides across a field of background stars in this time-lapse view from NASA’s WISE spacecraft. WISE obtained the images used to create this view over a period of about a day around May 17, 2010, during which it observed the asteroid four times. The moving asteroid appears as a string of orange dots because it is much cooler than the distant background stars. Stars have temperatures in the thousands of degrees, but the asteroid is cooler than room temperature. Thus the stars are represented by shorter wavelength blue colors in this view, while the asteroid is shown in longer wavelength reddish colors. Image credit: NASA / JPL-Caltech.
The Euphrosyne asteroid family occupies a unique place in orbital element space among families, located in the outer main asteroid belt at very high inclination.
The asteroid after which they are named, 31 Euphrosyne is about 156 miles (260 km) across and is one of the ten largest asteroids in the belt.
This asteroid is thought to be a remnant of a massive collision about 700 million years ago that formed the family of smaller asteroids bearing its name.
The new study used NASA’s NEOWISE telescope to look at the Euphrosynes to learn more about NEOs and their potential threat to Earth.
According to the authors of the study, this asteroid family may be the source of some of the dark NEOs found to be on long, highly inclined orbits.
They found that, through gravitational interactions with Saturn, the Euphrosyne asteroids can evolve into NEOs over timescales of millions of years.
“The Euphrosynes have a gentle resonance with the orbit of Saturn that slowly moves these objects, eventually turning some of them into NEOs. This particular gravitational resonance tends to push some of the larger fragments of the Euphrosyne family into near-Earth space,” said Dr Joseph Masiero of NASA’s Jet Propulsion Laboratory in Pasadena, lead author of a paper accepted for publication in the Astrophysical Journal (arXiv.org preprint).
By studying the Euphrosynes with NEOWISE, Dr Masiero and co-authors have been able to measure their sizes and the amount of solar energy they reflect.
“Since NEOWISE operates in the infrared portion of the spectrum, it detects heat. Therefore, it can see dark objects far better than telescopes operating at visible wavelengths, which sense reflected sunlight. Its heat-sensing capability also allows it to measure sizes more accurately.”
The 1,400 Euphrosyne asteroids studied bythe team turned out to be large and dark, with highly inclined and elliptical orbits. These traits make them good candidates for the source of some of the dark NEOs the NEOWISE telescope detects and discovers, particularly those that also have highly inclined orbits.
There are over 700,000 asteroidal bodies currently known in the main belt that range in size from large boulders to about 60 percent of the diameter of Earth’s moon, with many yet to be discovered. This makes finding the specific point of origin of most NEOs extremely difficult.
“With the Euphrosynes it’s different. Most near-Earth objects come from a number of sources in the inner region of the main belt, and they are quickly mixed around,” Dr Masiero said.
“But with objects coming from this family, in such a unique region, we are able to draw a likely path for some of the unusual, dark NEOs we find back to the collision in which they were born.”
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Joseph R. Masiero et al. 2015. The Euphrosyne family’s contribution to the low albedo near-Earth asteroids. ApJ, accepted for publication; arXiv: 1507.07887
