Feature - An unexpected bounty of Near Earth Objects

Image of a near-earth object detected by the Sloan Digital Sky Survey. The blue, red and green streaks show the object as it moves through three of the five SDSS filters over a period of five minutes. The two white objects are distant stars. Image courtesy Stephen Kent.

While scanning through images from the Sloan Digital Sky Survey, Fermi National Accelerator Laboratory researcher Stephen Kent noticed something unusual — a few extended streaks scattered among the millions of point-like stars and galaxies.

Kent realized the streaks were produced by Near Earth Objects (NEOs), asteroids or extinct comets whose orbits bring them close to Earth — close enough that they could collide. They appear as streaks because the closer an object is to Earth, the more quickly it moves across our sky. That’s why the patterns of distant stars appear unchanged over the course of our lifetimes, whereas our closest neighboring planet, Venus, moves noticeably with respect to the stars from night to night.

Because the NEOs detected by the SDSS are so close to the Earth, they moved across the telescope’s field of view during the 52-second camera exposures, creating streaks against the far-away stationary background objects.

During its eight years of operation, the SDSS obtained images of more than a quarter of the night sky and identified almost 400 million objects. Although the survey was designed to detect stars and galaxies and determine their properties, it also helped identify more than 100 NEOs.

“It was an enormous job to whittle down the forest in order to pick out the interesting trees,” Kent said. “The project was extremely well suited for the grid because we were able to break the large volume of data into many small pieces and parcel them off to different computers on the grid.”

This graph depicts the near-earth objects found by four sky surveys, including Kent's grid-assisted search of the SDSS data. It shows that large NEOs such as the K-T Impactor that wiped out the dinosaurs 65 million years ago are quite rare. The SDSS NEO Survey, indicated by the thick red line, searched for the more common smaller objects. Although these are not small enough to cause mass extinction, they are still quite powerful. The Tunguska impactor, for instance, burst about five to 10 kilometres (3-6 miles) in the air above Northern Siberia in 1908, knocking over an estimated 80 million trees in a section of forest over 2150 square kilometres (830 square miles) in size. Image courtesy Stephen Kent

To sift through the data for NEOs, Kent divided the SDSS data into fields, each covering an area of sky about half the size of the full moon and containing about 1,000 candidate objects of all types. He then designed an algorithm that examined the properties of each object in a field to determine if it met the criteria of an NEO.

To run the application on the grid, Kent bundled several hundred fields together. Each bundle, about two gigabytes or so of data, was submitted as one job to a grid node and took about 12 hours to process. In total, more than 600,000 fields were searched.

Kent then examined the resulting 200 to 300 NEO candidates by eye to eliminate misclassifications and compile the final catalog of around 100.

The NEOs Kent found were all relatively small, ranging in size from about 20 to 200 meters in diameter. Based on his results, Kent was able to estimate the total population of NEOs in the same size range to be around one million. He was also able to estimate the Earth-NEO collision rate — about one every thousand years — but said that many uncertain factors go into the calculation.

“NEOs are very interesting and important to study because if any of the big ones with diameters several kilometers across collide with the Earth, they can cause all sorts of dramatic effects, such as the impact that led to the extinction of the dinosaurs,” Kent said. “Even the smaller ones like the ones discovered in the SDSS data could do many miles’ radius of damage around the impact point.”

—Amelia Williamson, for iSGTW