Did an asteroid kill the dinosaurs?

Space rocks come in sizes. The dinosaur killer was 10 km wide.

Most space rocks are tiny. They burn up high in the sky. Sometimes a falling stone punches a hole in someone's roof.

But sometimes the rock is not small. Keep this ruler in mind. Every story below sits somewhere on it.

The ruler is logarithmic: each step is ten times bigger. Sizes are estimates. The 1991 rock is 5 to 10 m by the astronomers who found it.

In 1908 a rock blew up over Siberia and flattened 80 million trees.

It did not need to hit the ground. It exploded about 5 miles up. The blast knocked down trees across 830 square miles.

The trees all fell pointing away from one spot. But no crater, and no piece of rock, was ever found.

Most scientists say a small asteroid, or an icy chunk of comet, burst in the air. The 150-foot size and the "skipping stone" picture are from the old text. The energy is argued: 3 to 50 megatons, with 10 to 15 the usual guess. The old text said 1,200 square miles. Modern surveys say 830 (2,150 km²).

Small rocks pass us all the time. Sometimes they hit.

In January 1991 astronomers in Arizona spotted a rock called 1991 BA. It came within about 106,000 miles. That is less than half the way to the Moon.

It was only about 26 feet across. But it was found while passing, not before.

In 2013 a similar surprise did not miss. A 19 m rock blew up over Chelyabinsk in Russia, with about 30 Hiroshimas of energy. Nobody saw it coming.

Chelyabinsk hurt about 1,500 people, mostly from broken window glass. About 1,100 sought care and 112 went to hospital. No one died. The old text says only a dog has ever been killed, in Egypt in 1911. That tale is old and I could not confirm it.

The proof was hiding in a half-inch layer of clay.

In 1980 Walter Alvarez and his father Luis found a thin clay layer in Italy. It sat exactly where the dinosaurs vanish from the rocks.

The clay was full of iridium. Iridium is rare on Earth but common in space rocks. Other places around the world showed the same spike.

The old text says the idea came in the early 1980s. The key paper was published in June 1980. Their first number was about 30 times more iridium than the rocks around it.

Not everyone agrees. India's volcanoes were erupting at the same time.

Of course, this is theory. Many paleontologists accept that an impact happened but doubt it caused the extinction. One reason: many animals lived through it.

The rival suspect is the Deccan Traps, a huge lava field in India. It was already erupting before the rock hit.

Two 2019 studies disagreed on the timing. A 2020 study in Science argued the big gas release ended before the impact, and only the impact lines up with the die-off. A middle view says both mattered, and the impact may have sped up the eruptions. About 75% of species were lost.

This time we are looking, and we have already moved an asteroid.

In September 2022 NASA's DART spacecraft crashed into a small asteroid called Dimorphos. It was a test. Dimorphos was never a threat.

NASA hoped to shorten its orbit by 73 seconds. It shortened it by 32 minutes.

NASA's latest count I found: 874 asteroids over 1 km found, with about 50 left to find. For 140 m rocks: 11,373 found, about 14,000 still unknown. A new space telescope, NEO Surveyor, is due in 2027, per recent reports.