We’re gonna do it. We are going to launch a rocket that will whack an asteroid and, it is hoped, change its path through space. No, really, we’re going to do it.
As early as this July but maybe later, NASA will, it says, launch a rocket aimed at a double asteroid. Well, actually, it will launch a rocket aimed at the place the double asteroid will be when the rocket gets there, but you know what I mean. (Figuring out the location of that intersection is in and of itself a tough piece of arithmetic.)
The rocket will meet up with the asteroid Didymos in September 2022, we are told. It will crash into Dimorphos, a small moon of the already small Didymos. It is hoped that this will alter the orbital velocity of the dinky Dimorphos. (Double asteroids are surprisingly common.
Anyway, Didymos is about a half mile in diameter, and Dimorphos is a little more than 500 feet, which is still pretty big. At astronomical speeds, either would be big enough to do quite a job on our planet – think of those films you may have seen of a bullet hitting a jug of water and you’ll get the idea. Or ask a dinosaur.
Like many minor objects in the solar system, Didymos and Dimorphos have an egg-shaped orbit around the sun. At their closest point they come near Earth’s orbit; at its most distant they’re a bit over twice Earth’s distance from the sun. They are considered of nontrivial danger to Earth – in 2023 they will pass about 3 2/3 million miles from us. They have also been an object that scientists have wanted to play with for awhile. A plan to bang something into one or the other in hope of altering their path was canceled in 2016. Now it’s on again.
A Didymosian month, the amount of time it takes for Dimorphos to orbit it, lasts 11.9 hours. That’s important to what we’re discussing here, because the idea is not to crash something into Didymos but instead into littler Dimorphos. I haven’t been able to find out the mass of the probe that will hit the tiny moon, but I do know that it will be traveling about 4 miles per second when it does.
The idea, the researchers say, is to alter the length of the Didymosian month by about 10 minutes. That doesn’t seem like much, but over time it is likely to have a considerable effect. If Dimorphos speeds up, it will move farther from Didymos; if it slows down, it will come in closer. To complicate things further, its orbit around its small host is likely to become elliptical, or more elliptical. (Orbital mechanics is a tricky discipline. Astronauts have written about how changing the speed of their spacecraft actually alters its altitude.) Conceivably, the change could be enough to cause the two to collide, or to separate permanently.
What is far above my small knowledge is whether the change in the orbit of Dimorphos is likely to have much effect on the path of Didymos, but altering the larger body’s orbit doesn’t appear to be a big part of the experiment. Still, it could well have some effect, just as our moon has an effect on Earth.
Even very small changes over time have a big effect. We all know that the tides are in large measure the result of the moon’s gravitational pull. This is especially true during full and new moons, when the moon and sun are aligned or in opposition. At those points we have what are called “astronomical” tides, and high tide is higher and low tide lower. There are other effects, as well. Research published three years ago told us that “[p]eriodic variations in Earth’s orbit and rotation axis occur over tens of thousands of years, producing rhythmic climate changes known as Milankovitch cycles.” That’s interesting, but to our point today, the scientists found that a day on Earth was about 18 hours long 1.4 billion years ago, but our planet’s rotation has slowed since then – and the moon is to blame.
That’s because the moon is getting farther away. You needn’t worry that it will spin off into the cosmos any time soon, because its orbit is increasing by only an inch and a half per year. As it moves away – it was a lot closer 1.4 billion years ago – its gravity has slowed the Earth’s rotation.
(While we’re here, it’s worth noting that Earth returns the favor, and would do so even more if, for instance, the use of tides to provide power were to become widespread: Just as the moon produces tides, alteration of the tides would have an effect on the location and velocity of the moon. If just a few people use tidal power to generate electricity, the effect would be small, but if a lot were to do it the result would be correspondingly larger. Strange, isn’t it.)
Back to Didymos and its little orbiting buddy. The collision of Dimorphos and NASA’s lump of stuff will certainly change the relationship of the two bodies, and the goal is to see how and to what extent. I am going to presume, without evidence, that the one guy with the slide rule has confirmed what the computers say, which is that there’s no chance that it will cause either of the two to careen into the Earth year after next.
Added to the uncertainty is our lack of knowledge of the composition of smallish objects in the solar system. Some, the ones we all think of, seem to be great big rocks. But some aren’t. They’re more like sand held together by gravity.
Its not as exciting as some movie about a last minute effort to save the Earth from an approaching asteroid, but it’s useful and interesting nonetheless. And what we learn may well contribute to research toward knocking asteroids off course in the future, so that they don’t do to us what one is believed to have done to most of the life on our planet 66 million years ago. And remember, dinosaurs were around for 230 million years, while we’ve been here for barely a million.
It’s a statistical certainty that without our intervention at some point one big space rock or another will slam into the Earth, possibly killing us all, so it’s important that we try to find a way to prevent it.
Unless, I guess, it’s expected to hit Georgia.