Is a deadly asteroid about to hit Earth? Meet the man who can tell you

Richard Binzel has been watching the skies for hazardous asteroids for more than 50 years. In 1995, he proposed the Near-Earth Object Hazard Index, later renamed the Torino scale, which rates asteroids from 0 to 10 based on how certain we are that they could hit Earth – and the potential devastation such an impact might cause.

Earlier this year, Binzel’s scale got a high-profile outing when asteroid 2024 YR4 briefly reached level 3 on the scale – the first space rock to get this high in two decades. While the risk has since faded, it won’t be the last time we need to fire up the Torino scale. But Binzel, who is at the Massachusetts Institute of Technology, says we can probably rest assured that we won’t see the very highest levels of the scale reached in our lifetimes, or even those of our grandchildren. He spoke to New Scientist about asteroid hunting, the chances of a devastating impact and the future of planetary defence.

Alex Wilkins: When you started your career, how did people view the threat of an asteroid impact?

Richard Binzel: I published my first paper in the 1970s, when I worked for [the geologist] Eugene Shoemaker, who understood that craters we see on the Earth are impact craters, so I grew up with the awareness of asteroid impacts as a natural process that still occurs in the solar system today.

In the public, it was a giggle factor. Shoemaker was just doing serious science, not paying too much attention to the public side of things, but people like [astronomers] Clark Chapman, David Morrison and Don Yeomans were beginning to see it was important to talk about this. There was a book called Cosmic Catastrophes that Chapman and Morrison wrote [in 1989], which was the first real treatment for the public. The Alvarez discovery of the K-T boundary layer [the geologic record of the Chicxulub asteroid thought to have wiped out the dinosaurs] was probably the wake-up call to greater scientific awareness that impacts can happen in modern geologic history.

Why did you come up with the Near-Earth Object Hazard Index?

There was an object named 1997 XF11, which had a non-zero impact probability based on its initial orbit. Email had just become a thing. I was in a small email list with people like Brian Marsden, Yeomans, Chapman, Morrison, and we were debating what to do with this information. We wanted to release it publicly, but we wanted to make sure [of the risk]. We thought maybe we should just get a little more data, because with longer measurements of that orbit, [the probability of collision] would probably go away. Why cry wolf if this object is going to go away in a few days?

Marsden decided to write a press release, and just as he was sending it out, we found some earlier observations that gave a sufficient orbit to say [the probability of impact was] zero. I remember an email from Yeomans, who did the analysis, and the email basically said, “That’s zero, folks.” Brian went ahead with his press release, because he thought it was important to get the issue out into the public. Most of us disagreed, that that was crying wolf.

This set into my mind the need for some means of communicating when you discover an asteroid that has a non-zero impact probability, however small. Just be a little patient, and we’ll get enough data to make it go away. That if we discovered another object like that, we don’t want to keep it secret. That’s the worst thing we could do, because then no one ever trusts you, because they never know what you’re not telling them. So, we collectively decided that we needed to tell people what we know as soon as we can, when we know it. Then later, when it goes away, it’s not that anyone made a mistake or made an error, it’s just that we now have better information to know it goes away. That was the genesis of what was first called a Near-Earth Object Hazard Index.

How was it received at the time?

There happened to be a United Nations conference on near-Earth asteroids, where I first presented the idea, and it was not well-received. There were people who said we don’t need that, because we can explain the longitude and latitude and the ascending node of the orbit, and we can explain this all perfectly well. We don’t need some simple thing like a small, 0-10 scale. So, the initial response was arrogance by some astronomers that they didn’t need this, because we are smart enough and capable enough to communicate all these three-dimensional orbital characteristics that most people wouldn’t know.

But I persisted. I brought it forward again to a conference in Torino, and I had the idea that we should call it the Torino scale, because it was presented at the conference. I didn’t want to put my name on it because it would look egotistic. But if we called it the Torino scale, everyone had ownership of it and everyone would feel it useful to use, anyway.

Has it worked as you thought it would?

I thought it would be called into play a bit more than it has, but I think it’s because discoverers have done a good job of following up objects right away, and so if they have a non-zero probability, they go away pretty quickly.

There have been a dozen or so objects that have reached 1 on the Torino scale without much news, which is perfect. That’s exactly the intent. It’s like the Richter scale, where if you tell someone in California there’s going to be a magnitude-1 or magnitude-2 earthquake tomorrow, they go on with their day and think nothing of it.

What will future asteroid tracking look like?

The discovery rate of near-Earth asteroids is going to improve or accelerate dramatically as the Vera C. Rubin telescope and Near-Earth Object (NEO) survey telescope come online. We will discover near-Earth objects at an incredibly fast rate. Some of them will have very uncertain initial orbits that we will want to extrapolate forward for decades, so that means it’s a non-zero impact probability. It will simply take time to get enough orbital data for a long enough period that we can say more precisely where it’s going to be many decades from now, and entirely rule out an Earth impact.

We may see a few objects that get numbers like 4 or maybe 5 on the Torino scale, but never in the red zone (see diagram). I don’t expect we’ll see that in anyone’s current lifetime, or even in our great-grandchildren’s. It’s just incredibly, incredibly rare. But if we do, we have a method for people to immediately know, should I pay attention, or should I not?

The low end of the Torino scale will become so routine that we won’t need to pay attention, or the public won’t need to pay attention. They can rest assured that, for interesting objects like that, the astronomers are going to do their job and follow them up and make sure they go away. The Torino scale did its job.

When we saw asteroid 2024 YR4 reach Torino level 3, did the system work as intended?

My colleagues did an excellent job, clearly and consistently saying over and over again: “We expect, when we get more data, this object will go away.” That was always the message. If you read the description in each Torino scale category, especially on the lower end, it says this is of interest to astronomers, and we fully expect additional data will rule out any possibility of it intersecting the Earth.

I think what was confusing to most of the media and to the public was the impact probabilities. They were always low. (At its peak, 2024 YR4’s probability of impact reached 3.1 per cent.) The impact probabilities started going up, but that’s a natural consequence of what happens when you get more data. When you first discover an asteroid, you have watched it over a very short track, and now you want to extrapolate that track forward decades and decades into the future. Sometimes the probability number can go up, but it’s really just a function of the process of refining the orbit and shrinking the window to make sure that the Earth isn’t in it at all.

What about Apophis, a 340-metre asteroid that will pass very close to Earth in 2029, but ultimately miss. How can we be so confident?

When asked about Apophis, I give three answers. Apophis will safely pass Earth. Apophis will safely pass Earth. Apophis will safely pass Earth. How do we know that? This is an asteroid we’ve been tracking for more than 20 years, and that tracking includes pinging radar waves off this asteroid, which pins down its position to metres in space. The overall uncertainty for this asteroid passing safely by Earth is that it will be 38,000 kilometres away from Earth, plus or minus 3 kilometres.

Astronomers have taken this object very seriously for the past 20 years. In fact, when it was first discovered, it was a 4 on the Torino scale, the only object that ever reached 4. It didn’t stay there very long, maybe a week, but this object got astronomers’ attention back in 2004, right around Christmas time. I wanted to call it Grinch, because I was working late into Christmas Eve on different aspects of the asteroid’s orbit until my family yanked me downstairs from my office.

The DART mission, which saw NASA fly a spacecraft into an asteroid to try to change its orbit, felt like a new direction for planetary defence. How significant was this mission?

DART was a step forward in our maturity as a species, where we are no longer at the complete mercy of whatever space wants to throw at us. DART was simply a demonstration that we could target an object and have a consequential effect on its orbit. I think it’s a pivotal moment for humanity. It’s saying: “Wait a minute, folks, if we ever need to do something to mitigate an incoming asteroid, with enough time, we have the capability to do so.”

You often hear people still talk about the risk that a massive asteroid will be discovered that will wipe out humanity. How has this risk changed from when you started to now?

We are on the job. This isn’t a major problem. It isn’t a major threat, but it’s one that we now have the capability to understand. Speaking very personally, as a scientist who’s been in the field for 50 years, who has largely been supported by public funds, I feel a moral responsibility to push forward the idea that, because we now have the capability to find any serious asteroid threat, we have a moral obligation to do it. Otherwise, we are not doing our job as scientists.

Putting it another way, if we were to be taken by surprise tomorrow by an object that we could have discovered if only we had built that telescope 10 years ago, that would be an epic failure in the history of science. That’s the only thing that keeps me awake about asteroids: that somehow we haven’t done our job yet.

It’s tremendous progress to see Vera Rubin and the NEO surveyor coming online, and it’s finally about time that we get a thorough survey done and make sure that there is no imminent asteroid threat in the coming decades or centuries. Because we now have the capability to get the answer. It’s our responsibility to see that we go out and get the answer.