Mini-Throughput: We’re All Doomed Edition

Comet Hale-Bopp, as photographed by the author on March 29, 1997. This was a 10 minutes exposure on a tracking mount with a Pentax.

We’re all going to dieeeeeeeee:

AN ASTEROID with the power of destroying an entire continent if it hits is rapidly approaching Earth, the US space agency NASA has found.

Well, no, actually. The asteroid in question, 2002 PZ39, has not been “found”. We’ve known about it for 20 years and have been tracking its orbit for a long time. And “approaching Earth” is a rather alarmist description of “will pass relatively close to Earth in two days along its well-known and measured orbit”.

NASA’s asteroid trackers believe the space rock is trapped on an orbit that will bring it incredibly close to our planet this weekend. The asteroid has been dubbed 2002 PZ39 and it is estimated to measure up to 3,280ft (one kilometre) across.

It’s not “trapped in an orbit”. It’s in an orbit. It’s no more trapped in an orbit than the Moon is. And while it will get relatively close to Earth, it’s closest approach on February 15 will be…3.6 million miles. We’ve had asteroids pass way closer than that.

Now it is true that PZ39 is massive as asteroids go and could threaten destruction on a continental scale. The reason is velocity. Objects orbiting the Sun move fast. The Earth itself moves at a speed of over 100,000 kilometer per hour. When two orbits intersect, the relative speed can be tremendous. The energy of an impact grows linearly with the mass (i.e, double the mass, double the impact energy). But it grows as the square of the velocity (e.g., double the speed, quadruple the energy). So an asteroid that size striking the Earth at that speed would unleash an energy equivalent to thousands of atomic bombs. The result would be a massive heat wave that would incinerate anything nearby and the ejection of millions of tons of Earth into the atmosphere, causing the equivalent of a nuclear winter. Alternatively, if it hit the ocean, it would cause gigantic tsunami which could wipe coastal cities off the map.

(As a historical comparison, the Chelyabinsk meteor exploded in our atmosphere exactly seven years ago. It was probably only 20 meters in size but detonated with the power of 30 Hiroshima bombs damaging over 7000 buildings. And that was in a relatively isolated area.)

So the potential danger of an asteroid like PZ39 is very real. But the danger of this particular asteroid is zero. It’s not going to get particularly close to us this time. In fact, the overall danger — at least from asteroids — is reasonably well understood. We have identified a little over 2000 asteroids that pose a danger to Earth but only about 40 pose any real danger over the next century. And by “pose a danger”, I mean that the uncertainties in their orbits are enough that they have a non-zero chance of hitting us in the next hundred years. Not a single one has even a large chance of impact.

Furthermore, these asteroids “approach” us all the time. Many of them have orbits that are roughly similar to Earth’s, so they pass us or we pass them from time to time. 1999 JM8 is the largest of the Near-Earth asteroids. It orbits the Sun every 4.5 years so it “approaches” us about once a year, albeit usually at a distance of a couple of hundred million miles. It’s gotten “close” — 8.5 million miles in 1998. But its orbit shows no danger for at least the next century or three. And even that isn’t particularly close. The only time we really notice is within something passes within the lunar orbit of 250,000 miles.

But…that doesn’t mean we can rest on our laurels. Because there’s another kind of object out there: comets. Comets originate from the most distant reaches of the Solar System and can have orbits thousands of tens of thousands of years long. They can come in upon us quite suddenly and unexpectedly. For example, the spectacular 1996 Comet Hyakutake was spotted only two months before it made its closest passage to Earth. Hyakutake was about a mile in size — large enough to a danger had it come close to the Earth. And we had almost no warning that it was coming.

What’s more, comets can be…capricious. As they approach the Sun, their icy cores sublimate and they begin to send out massive eruptions of water and gas. But comets are so small that these eruptions can alter their rotation or their orbit. So while we’re reasonably confident that we won’t be killed by an asteroid anytime soon, a comet could suddenly and nastily surprise us.

The good news on this front is that we are taking the danger of impactors somewhat seriously. NASA has been investing in asteroid tracking for the last twenty years and we are now beginning to study the Oort Cloud and Kuiper Belt — the homes of comets. Moreover, this is one of the few initiatives that the Trump White House has gotten behind. He’s asked for $100 million to star working on so-called “space defense” — developing the technology to rendezvous with and nudge a dangerous asteroid off course¹. This is one of the few Trump initiatives I absolutely support. Think of it as Civilization Insurance. The chance that we will have to use it is low; the danger it could avert is … well, everything.

So, no, PZ39 is not a danger. This is not a sudden discovery that we are worried about. It’s not passing particularly close to us nor is it likely to within our lifetimes. But the overall danger of objects like PZ39 — while low — is real. And we can do something about it.

1. In Hollywood movies, they usually blow the menacing asteroid up. This is actually the worst thing you could do and most scientists think you’d be better off just letting it hit. Because the energy of the impact has to go somewhere. Having a thousand asteroids burn up in our atmosphere and cause a massive global heatwave might be worse than a nuclear winter or tsunami