Detailed observations by NASA’s Wide-field Infrared Survey Explorer (WISE) have cleared the asteroid family that has long been on trial for its suspected role in killing the dinosaurs 65 million years ago.

A 2007 study using visible light data from ground-based telescopes pointed the finger at a particular group of asteroids, suggesting that an asteroid named Baptistina collided with another main belt asteroid some 160 million years ago, creating the Baptistina family from the resulting debris. One of these mountain-sized chunks, an estimated ten-kilometre wide rock, collided with Earth 65 million years ago, wiping out the dinosaurs and clearing the path for new life forms to develop. The presence of a large crater off the coast of Mexico dating to this epoch, and the layer of extraterrestrial minerals common to meteorites found in the geological record at the same time, above which no fossils of dinosaurs are seen, supports the widely-accepted impact theory. Yet the latest infrared observations by NEOWISE – the asteroid hunting element of WISE – rules out Baptistina as the culprit.

"The original calculations with visible light estimated the size and reflectivity of the Baptistina family members, leading to estimates of their age, but we now know those estimates were off," reveals Lindley Johnson, program executive for NASA's Near Earth Object (NEO) Observation Program. "With infrared light, WISE was able to get a more accurate estimate, which throws the timing of the Baptistina theory into question."

NEOWISE measured the reflectivity and size of 120,000 asteroids in the main belt, including 1,056 members of Baptistina. The new results reveal that the original parent Baptistina asteroid broke up closer to 80 million years ago, half as long as originally proposed. The data also indicates how much time would have been required for the asteroid fragments to reach their current locations, since larger asteroids would not disperse in their orbits as fast as smaller ones.

"Asteroids the size of this impactor take on average 40 million years to move from Baptistina's region of the main belt to Earth-impacting orbits," Joseph Masiero, lead author of the study, tells Astronomy Now. "This means that with the break-up happening about 80 million years ago, it becomes much more difficult to deliver a large impactor to Earth 65 million years ago." In other words, if the Baptistina asteroid was guilty of the alleged crime, it needed to hit Earth within just 15 million years of being flung onto its planet-colliding orbit.

Masiero adds that there are a few possible source regions for objects in Earth-threatening orbits. "The history of the main belt appears to be one of turmoil," he says. "Though many NEOs are believed to come from the main belt, the search is still on for the precise origin of the dinosaur-killing impactor."