Enlarge / Artist impression of a large asteroid impacting on Earth, such as the Chicxulub event that caused the end-Cretaceous mass extinction 66 million years ago. (credit: Mark Garlick)

Some 66 million years ago, an errant asteroid wiped out three-quarters of all plant and animal species on Earth, most notably taking down the dinosaurs. That has long been the scientific consensus. However, three years ago, Harvard astronomers offered an alternative hypothesis: The culprit may have been a fragment of a comet thrown off-course by Jupiter's gravity and ripped apart by the Sun.

Now an international team of scientists have reaffirmed the original hypothesis, according to a new paper published in the journal Science. They analyzed ruthenium isotopes from the Chicxulub impact crater and concluded the impact was due to a carbonaceous-type asteroid, likely hailing from beyond Jupiter.

As previously reported, the most widely accepted explanation for what triggered that catastrophic mass extinction is known as the "Alvarez hypothesis," after the late physicist Luis Alvarez and his geologist son, Walter. In 1980, they proposed that the extinction event may have been caused by a massive asteroid or comet hitting the Earth. They based this conclusion on their analysis of sedimentary layers at the Cretaceous-Paleogene boundary (the K-Pg boundary, formerly known as the K-T boundary) found all over the world, which included unusually high concentrations of iridium—a metal more commonly found in asteroids than on Earth. (That same year, Dutch geophysicist Jan Smit independently arrived at a similar conclusion.)

Enlarge / Artist's illustration of NASA's Wide-field Infrared Survey Explorer spacecraft. (credit: NASA/JPL-Caltech)

Last week, NASA decommissioned a nearly 15-year-old spacecraft that discovered 400 near-Earth asteroids and comets, closing an important chapter in the agency's planetary defense program.

From its position in low-Earth orbit, the spacecraft's infrared telescope scanned the entire sky 23 times and captured millions of images, initially searching for infrared emissions from galaxies, stars, and asteroids before focusing solely on objects within the Solar System.

Wising up to NEOs

The Wide-field Infrared Survey Explorer (WISE) spacecraft launched in December 2009 on a mission originally designed to last seven months. After WISE completed checkouts and ended its primary all-sky astronomical survey, NASA put the spacecraft into hibernation in 2011 after its supply of frozen hydrogen coolant ran out, reducing the sensitivity of its infrared detectors. But astronomers saw that the telescope could still detect objects closer to Earth, and NASA reactivated the mission in 2013 for another decade of observations.