Moment that mattered: Nasa conducts the world’s first planetary defence mission
In issue 48 we spoke to Lindley Johnson, planetary defence officer at Nasa, about how the DART probe successfully hit its target
26th September 2022 (Taken from: #48)
“I wasn’t nervous. I was excited to see it,” recalls Lindley Johnson, Nasa’s first planetary defence officer, of the moment he and his colleagues attempted to alter the orbit of a minor planet moon by crashing a purpose-built probe into it. “A lot of hard work by a lot of people over many years made it happen. But in a way, it was the fulfilment of an idea I had 30 years ago.”
For someone who has imagined what the apocalypse might look like for over three decades, Johnson is surprisingly laid-back. He speaks with calm authority and reassurance, which is a relief, given that it is his job to track asteroids that might one day collide with Earth, possibly creating an extinction-level event that eliminates all life. “We started out trying to find all of those [asteroids] that were a kilometre or larger in size, because we knew that it would be a very bad day to get hit by something that big,” he says with no little understatement. “It would be climate change in an afternoon.” So far his unit believes it has discovered up to 97 percent of the objects that will move into near-Earth space and could prove dangerous in coming years. There are around 900 one km-plus-wide planet killer-sized objects, and another 25,000 that wouldn’t end life on Earth but would, Johnson says, lead to “a regional disaster… Europe could be devastated by an impact that large.”
The good news, Johnson says, is that none of the objects he has tracked currently represent an impact hazard to the Earth, but he believes in preparing for the worst. While the plotlines of Armageddon, Deep Impact and Don’t Look Up, films in which Earth is threatened by an extinction-level asteroid, are clearly science fiction, Johnson and Nasa felt they needed a tool in their armoury that could, if needed, be called into action.
And on 26th September 2022 such a tool was put to the test for the first time. Johnson was one of the key scientists working on DART, the Double Asteroid Redirection Test. Ten months earlier Nasa, in collaboration with the European Space Agency, had launched a SpaceX Falcon 9 rocket from the Vandenberg Space Force Base in California, carrying the DART impactor. The plan was to crash the impactor, which weighed just over 600kg, into Dimorphos, a moon of the asteroid Didymos, nearly seven million miles from Earth. The mission was to change Dimorphos’ orbit sufficiently so that, if it had been on a collision course with Earth, it would shift it just enough to miss. “Asteroid deflection is based on the principle that you only need to change the orbital velocity of the asteroid by less than one per cent,” says Johnson. “If you change that velocity, just a hair, that changes its orbit. That’s all it takes.”
But first Johnson’s team had to work out how to hit Dimorphos head-on. Is this the equivalent of shooting a bullet with a bullet? “Yes, but where the distances are tens of millions of miles and the velocities are some 20 to 60 times the speed of a bullet,” he says. “But other than that, it’s a fairly accurate analogy.”
Johnson had always been interested in tracking what was going on in the stars. He studied astronomy at university before joining the US air force. His specialism was space surveillance. A huge technological leap forward had been made in the early 1990s with ‘charge-coupled devices’, imaging hardware which is now in all mobile phone cameras. For Johnson it revolutionised space surveillance and led to him writing a 1993 academic paper called ‘Preparing for Planetary Defence’. “That was the first use of the term ‘planetary defence’,” he says. “So when I retired from the air force in 2003, Nasa said, ‘Come over, we’ve got a job for you’. And that led to developing their Near-Earth Object observations programme, and later the creation of the Planetary Defence Coordination Office. ”
We can predict to the day when an object may be an impact threat to the Earth”
Although Johnson had been talking about setting up a test collision like DART for decades, the collaboration with the European Space Agency only began in 2015. Two years later Nasa approved the concept and it went into the design phase. Construction began in 2018. He was at Vandenberg Space Force Base in California when DART was launched in November 2021. And he was at mission control at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland on 26th September 2022 as DART approached its target. “I was very confident at that point,” he says, explaining how he watched the black and white pictures of the DART impactor’s front camera jerking towards Dimorphos until the screen went black on impact and the room erupted in cheers.
DART was a resounding success. Early results, according to Johnson, indicate that the orbital velocity of Dimorphos around Didymos was shifted by around four percent, almost four times the amount needed to deflect a near-Earth object if it threatened the planet. These are just initial results, however, and Johnson will be looking at new data from the collision for years to come. A European Space Agency mission, Hera, will launch in 2024 and visit Didymos and Dimorphos in 2026 to gather images of the impact crater and understand what the density and composition of the two astral objects are.
The next big close encounter with a near-Earth object will be in 2029. “We found Apophis back in 2004 and I’ve been tracking it ever since,” says Johnson. “It is a 340-metre asteroid and on 13th April 2029 it is going to come a little bit closer to Earth than our weather and communication satellites. That is a very, very close approach by an object large enough to do very significant damage.”
But DART won’t need to be scrambled. Johnson has done the maths. “We can calculate it accurately enough that we know it will not impact in 2029, or for the foreseeable future [when its orbit takes it past the Earth again],” he says confidently. Although it’s still a little close for comfort: Earth’s geosynchronous weather satellites are within 22,000 miles, a little less than the circumference of the planet.
“Projecting it out now, there will be multiple close encounters with Earth. These asteroids will approach Earth closely, decades, centuries, in advance of an actual potential impact,” says Johnson. “So we just need to be looking for them, cataloguing them and tracking them. We can predict to the day when an object may be an impact threat to the Earth.”
Which brings us to the one thing Johnson does worry about: any objects he and his team may have missed. With that in mind, he believes that just as important as DART is the launch of the Near-Earth Object Surveyor mission, or NEO Surveyor, an infrared space telescope which will drastically improve our monitoring of space objects. While DART has shown that it is possible for a probe to collide with any new dangerous near-Earth object that is discovered, the process takes time. “I would say a minimum of five years,” explains Johnson of the realistic period between finding an object deemed sufficiently dangerous and actually hitting it with a DART-style object. “And that would be a fairly chancy mission. I prefer a margin of at least ten years.”
The NEO Surveyor is due to launch in 2026 and will, it is hoped, give Johnson and his team of asteroid-watchers the heads-up they need to fire a missile capable of saving life on Earth. “We now have the technology, not only to find them, but also to divert one from its orbit,” says Johnson. “It’s a natural disaster, one of the few natural disasters we now have some way of preventing.”
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