Dr Duncan Steel is one of the world's foremost experts in detecting and defending the earth from asteroids and comets. He was one of six foreign members of NASA's Spaceguard committee in the early 1990s and has twice served as vice-president of the Spaceguard Foundation.
Today, Dr Steel - who studied physics at the University of Canterbury - works for the Centre for Space Science Technology, in Alexandra.
Photo: 123rf
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In 1980, a landmark scientific paper was released - the first to convincingly argue that the extinction of the dinosaurs was caused by a giant asteroid or comet striking the earth from outer space.
One of the key pieces of evidence for that paper came from New Zealand. Core samples taken from the K-Pg boundary at Woodside Creek, north of Kaikoura, showed high levels of iridium, an element which is rare on earth but relatively common in asteroids.
Samples taken from core samples at Woodside Creek helped prove that a massive asteroid or comet impact caused the extinction of the dinosaurs 65 million years ago. Photo: Chris Hollis / GNS Science
Comet 67P/Churyumov-Gerasimenko as seen by the OSIRIS wide-angle camera on ESA's Rosetta spacecraft on September 29 2016. Photo: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Duncan Steel was a graduate student at the University of Canterbury when that paper was released, building a radar to search for small to mid-sized meteors entering the earth's atmosphere. He says it led to a kind of epiphany.
"There were lots of objects we were detecting with this radar or you could see with your eye at night and every so often something really huge hits the earth," says Dr Steel. "But I realised there must be some in between which hit the earth - not frequently enough to be a daily or yearly problem, but often enough that we need to be worried about them."
This realisation sparked Dr Steel's career: discovering and tracking potentially dangerous objects in the solar system, and thinking about how to defend earth from them.
The mid-sized asteroids Dr Steel was thinking about in the 1980s are about 100-1000 metres across. That's much smaller than the dinosaur-killing asteroid, which is thought to have measured around ten kilometres.
The last mid-sized asteroid to hit earth was the Tunguska event in 1908. An asteroid estimated at 60-190 meters across exploded above Siberia, flattening two thousand square kilometres of forest.
"If it happened above Auckland it would kill nearly everybody," says Dr Steel. "It would set the whole city on fire and the blast wave would level the place."
Trees flattened by the 1908 Tunguska Event, caused by a large asteroid impact. Image taken during a 1931 Soviet scientific expedition. Photo: Vokrug Sveta / Public Domain
Fortunately, asteroids that size don't hit the earth often. Dr Duncan Steel says estimates range from one every hundred to a thousand years.
Scientists have plotted the orbits of more than 1400 potentially asteroids which are bigger than 140 meters across and pass within 7.5 million kilometres of earth. Photo: NASA/JPL-Caltech/Public Domain
Unfortunately it's virtually impossible to see these objects coming. They are simply too small to detect with telescopes until they get closer to the earth than the orbit of the moon.
Perhaps surprisingly, it's not these kind of asteroids which worry Duncan Steel the most. He's more concerned about smaller objects, less than 50 metres across - like the asteroid which exploded above Chelyabinsk in Russia in 2013, injuring more than two thousand people.
But it's not the direct damage which Dr Steel finds most concerning. It's the potential for a small asteroid to be mistaken for a thermonuclear attack.
"The thing which I worry about most is a half to one megatonne event occurring above a region with heightened international tension, and the people who are there don't understand what it is," says Dr Steel. "[They could] react to that by launching an attack on a neighbour that is unwarranted."
Given there's no lack of international tension in the world at the moment, we'd best hope no asteroids decide to strike anytime soon.
Listen to the full Our Changing World podcast to hear how we could potentially redirect a hazardous asteroid - and for the more positive side of asteroids and comets
