(CNN) -- The meteor that blew up over southwestern Russia in February had survived long-ago collisions that blackened its surface, making it harder to spot, a scientist who studied its pieces said Wednesday.
The roughly 60-foot space rock plunged into Earth's atmosphere and exploded over the Russian city of Chelyabinsk with the force of about 30 early nuclear bombs. Of the roughly 100 fragments found so far, the biggest weighs about 11 pounds (5 kilograms), said Maria Gritsevich, a researcher at the University of Helsinki.
Gritsevich and others who have studied those fragments have found the small asteroid was mostly made of ordinary rock.
But it was marbled with metallic elements such as iron -- and when the asteroid collided with other bodies floating around the solar system, the heat and force of that impact melted the metal, which seeped into fissures in the surface of the stone and blackened its surface.
"Dark asteroids are harder to detect because they reflect less light," Gritsevich, who presented her findings at an American Astronomical Society conference this week, told CNN.
While most ordinary rocky asteroids reflect up to 20% of the sunlight that strikes them, and Earth reflects about 40%, dark asteroids reflect 5% of the light or less, she said.
The meteorites collected from the Chelyabinsk incident show varying signs of darkening, with some completely blackened, others made up entirely of light-colored stone and some with a mixture of coloration, Gritsevich said.
"All Chelyabinsk meteorites look different," she said. "You can clearly distinguish three different groups."
She said scientists are still working to determine the age of the Chelyabinsk meteor, the largest to hit Earth since the 1908 Tunguska incident in Siberia.
The blast left more than 1,500 injured, mostly by glass from shattered windows, and raised concerns about humanity's vulnerability to stray asteroids.
Two months later, NASA announced a goal of sending a spacecraft out to seize and asteroid and tow it into orbit around the moon, where it could be studied by astronauts -- a project billed in part as a planetary defense mission. But it ran into opposition in Congress, where a House committee voted to block any funding for the mission in July.