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Giant space rocks are definitely going to hit the Earth again. We actually do know how to deflect
them, but only if we find them and correctly assess their risk first. But the solar system is a
chaotic place. How is it even possible to tell if a space rock will one day collide with the Earth?
Hey everyone! Before we dive in, a quick update on our experiment. A few episodes ago,
we asked for your help in an experiment to see if more likes, comments, and subscribers could
give Space Time a boost in the algorithm—and yeah, did you deliver some impressive datapoints! In
the first 24 hours of our latest episodes, we saw over 8,000 comments (a 1043% increase) and
45,000 likes (up 275%). Overall this helped boost impressions by 143%, spreading Space Time to more
of the community. Even better? Many of you told us Space Time is popping up on your homepages again!
Honestly, this has been the best 10-year anniversary gift we could ask for. So,
what’s next? Phase 2. This time, let’s push even further. Our goal:
50,000+ likes in 24 hours and a thriving comment section. That means asking questions,
replying to others, and keeping the conversation going. We will track the results and report back
soon! And if you haven’t seen yet, we’ve got special 10-year anniversary merch to
celebrate a decade of breaking brains—check it out at the merch store! Now, let’s get into it.
To you this may look like a moving smudge. But it’s actually the asteroid 2024 YR4,
discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS). Based on early observations,
NASA calculated a 3.1% chance of this striking the Earth on the 22nd of December, 2032.
Since then, further observations have lowered that impact probability to close
to zero. essentially eliminated the threat Low enough that we don’t need to scramble our
spaceship full of oil rig workers, and also low enough that this isn’t an episode about
whether 2024 YR4 will hit us. Rather, this is an episode about how it’s even possible to go from a
fuzzy dot hopping across a screen to a precise probability of that dot manifesting as a giant
space rock burning through our atmosphere and flattening a city or a continent.
In our recent episode we talked about the OSIRIS-REX mission, which successfully
intercepted the asteroid Bennu and returned a sample to Earth. Bennu is what we call an
Earth-crossing asteroid, which means orbital path crosses right through Earth’s path. That
means it has a potential risk if hitting us if that crossing occurs when Earth happens to
also be at the same spot in its orbit. Bennu won’t do that any time soon, although to land
on the thing we still needed incredibly precise knowledge of its trajectory through 3-D space,
which had to be determined from those fuzzy moving dots. Now that OSIRIS-REX has finished with Bennu,
it’s been renamed OSIRIS-APEX, and it’ll intercept the asteroid Apophis
which has a series of uncomfortably close approaches over the next several decades.
But space is very very big. How is it possible to know the location and orbit
of a faint space rock with the precision to land on it, or know whether it’ll land on us?
Imagine that you’re an astronomer doing some astronomy of some sort on a big telescope. You
notice that an unknown blob moves across the patch of sky that you’re observing over the course of
a few nights. Looks like an asteroid because of the speed of motion, and the lack of tail
means probably not a comet. You check against databases of all known solar system bodies and
it’s not one of those. So it's new. How do you know whether it’s coming our way?
From these grainy images, you don’t know much. It could be big, far away,
and fast, or small, closer, and slower—or anything in between. We would say that
these properties are degenerate—they look the same from this 2-D image.
Now you as an astronomer have seen Don’t Look Up and so you know that time is of the essence
when evaluating a potentially threatening object, so you get to work figuring out its
orbit. The first step is to determine the true location of the object. You know its
exact angular position—the direction relative to Earth, because you know where your telescope was
pointing in each image and, more importantly, you can calibrate very precisely relative to
the well-known stars in the background. But you have no idea how far away it is,
and so we have this degeneracy I mentioned between close, slow orbits and distant,
fast orbits. Fortunately the problem of determining an orbit has been solved for
a couple of hundred years by some of the greats of old—Laplace and Gauss in particular. They in
turn used the even older work of Johanne Kepler, who provided the first mathematically accurate
description of elliptical orbits. So with only three observations, reasonably separated in time,
it’s possible to determine which Keplerian orbit your moving smudge is on.
These days there are improved versions of the Laplace and Gauss methods, but the principle is
the same. From your few observations, you manage to solve for the six classical Keplerian orbital
parameters, which tell you the path the asteroid would follow around the Sun assuming that the only
thing affecting that motion was the Sun’s gravity. Which isn’t the case, but I’ll come back to that.
Right away you can figure out some useful stuff. The asteroid has an eccentric orbit that takes
it from midway between the orbits of Earth and Mars when furthest from the Sun—its aphelion,
to midway between Earth and Venus when closest, at perihelion. This means you’ve found a new
Apollo group asteroid—which means it crosses Earth’s orbit and has a “semi-major axis”,
or average orbital radius, is larger than Earth’s. In contrast the other
group of Earth-crossers—the Atens, have smaller semi-major axes than Earth.
Either way, these Earth-crossing asteroids are dangerous because they pass through
Earth’s path twice during each of its orbits, and if that happens while Earth is in that
same spot then kablooee. You calm yourself by remembering that 2024 YR4 and Apophis,
which did not hit the Earth, were both Apollos, but then you recall that the
Chelyabinsk and Tunguska objects were also Apollos and they did hit.
But how big a hit would this be? Depends on the object’s size, which you can also now
estimate. You know that the light you receive from this asteroid is reflected sunlight,
and if you assume a typical rocky composition you can guess its albedo—the fraction of light
reflected. Now that you know its current distance from the Sun and from the Earth,
you can compute what surface area it would need in order to have the observed brightness. That
in turn gives you the approximate size, with some assumptions about its shape. You estimate
around 40m in diameter assuming that it’s roughly spherical. There’s a very large uncertainty in
this, so it could actually be as little as half or as much as twice that size. If the former,
it’s the same size as the one that exploded over Chelyabinsk in 2014 with the explosive
equivalent of a smallish thermonuclear bomb. But the upper end of that size range is in the
realm of the Tunguska impactor that flattened 2000 square km of Siberian forest in 1908 with
an explosive power an order of magnitude larger than the Chelyabinsk explosion.
Given a bit of time this size can be determined by taking the spectrum of the object would determine
its composition, and so that would give you a much better constraint on its albedo and
hence size. And observations with a more powerful telescope or radar could resolve the object and
literally measure the size—but that’ll have to wait for a close approach to the Earth.
For now, you have enough information about this new, potentially hazardous object to keep digging.
Now that you know the orbital elements you could just run a simulation of both Earth
and the asteroid’s orbits far, far into the virtual future to see if they ever collide.
And you know that would be useless. The real solar system is a messy place. Your
asteroid is going to be tugged off its perfect Keplerian orbit by the gravitational fields of
the planets. It’s also going to be pushed ever so slightly by sunlight—by radiation
pressure. And there can even be a significant nudge caused by a sun-heated rock radiating
away its own heat in different directions as it rotates—that’s the Yarkovsky effect.
The only way to understand the possible trajectory of this object
is to simulate that future motion step by step by step, and at each step figure out the tiny
deflections caused by all these influences. The gravitational pulls of Venus, Mars, Jupiter,
Saturn and even sometimes the Moon have small but noticeable effects, but the real wild card
is Earth’s own influence. When the asteroid makes a close approach to the Earth, it’s deflected
by the Earth’s gravitational field, sometimes getting slung into a very different orbit. That
deflection depends strongly on exactly where the asteroid passes the Earth on that close approach,
and that’s very hard to predict due to the various uncertainties. You find that the
main uncertainties are the effects of solar radiation and the Yarkovsky effect due to the
unknown geometry and composition of the object. These have only a tiny influence in the moment,
but that influence and so the uncertainty compounds over time. But worse than that
is the fact that your starting position and velocity for the asteroid are already uncertain,
in particular due to the difficulty in knowing the exact distance from the Earth.
Due to these uncertainties, there’s no way to find the exact path—the best you can do
is to simulate many, many possible paths that explore the range of the uncertain
parameters—in other words, you need run a Monte Carlo simulation. Doing that,
you discover a huge range of possible futures for this asteroid. On each close encounter with Earth,
it passes either in front or behind the Earth, or, sometimes, straight into it.
But you notice something interesting. Hypothetical trajectories that do end up hitting the Earth tend
to be deflected onto those deadly paths by the Earth itself during a previous close encounter. And
during those close encounters, all of the subsequently cataclysmic trajectories pass
through small region in space or one of a very few regions. These regions are called
gravitational keyholes, and they can be as small as hundreds of meters in diameter.
If a trajectory misses a keyhole, Earth is safe for the time being, but if it threads one of the
keyholes for a given close encounter then impact at the next close encounter is extremely likely.
Calculating an impact probability is now simplified to calculating the probability
of threading a gravitational keyhole. It’s also true that deflecting a potential impactor is
simplified to diverting it from passing through a keyhole if you can catch it before that happens.
That means you only have to offset its position by kilometers at the keyhole, rather than the several
thousand km needed to avoid an Earth collision after it already passes a keyhole. In 2022,
NASA’s DART mission proved we can indeed deflect a large asteroid by the requisite amount by slamming
a mass into it. As long as we do it early enough. But, assuming that keyhole passage happens,
now you can at least calculate with some accuracy when and where the asteroid will impact the Earth.
Each different keyhole focuses the asteroid towards an impact corridor—a trail across
the planet that’s defined by the fairly precise location that the asteroid crosses our orbit, with
the uncertainty in the crossing time stretching that impact point out into an impact line.
So now you have a probability of impact for the next few close approaches and a
rough idea of possible impact locations for each. Further observations over time
will narrow all of that down as the orbit of the asteroid becomes
better known. Ultimately you’ll have a very reliable probability of an impact
in the next few close encounters. But beyond that the uncertainties blow up.
So, what did you discover? Do we need to launch the nukes? Organize
an apocalypse party? I have no idea. This whole scenario is just made up.
Feel free to choose your own ending. But the process I just described is more or
less what we do to turn a newly noticed moving smudge into an impact probability.
And it’s more or less what was done with the most recent scale, 2024 YR4—although
in this case by the ATLAS robotic survey, not by some astronomer who happened to look at the
right spot. Anyway, as I said, simulations based on our first set of observations revealed a 3.1%
chance of a December 2032 impact by 2024 YR4. The impact corridor was equatorial,
and included parts of Africa, India and South and Central America But as further observations were
made, the uncertainty in its orbital parameters dropped, and so too did the number of simulated
trajectories impact the Earth. By March this year NASA revised the impact probability to 1 in around
5500, while further analysis by ESA set the chance to nearly only one in a million. It’s actually
more likely that the rock will hit the Moon in 2032 than it will the Earth. For the time being,
2024 YR4 isn’t a threat. Which is good, because this thing is between 40 and 90
meters diameter, which is around the size of the Tunguska impactor. It could make a mess.
A more perennial threat is the asteroid Apophis, which is nearly half a kilometer long. Big enough
that it gets to have a cool name - in this case after the Egyptian god of darkness and chaos,
which would certainly ensue if Apophis decided to descend upon the Earth. If it ever hits,
it would be with an energy just shy of all existing nuclear
weapons, or several Krakatoa eruptions. That means regional devastation and tsunamis for
an ocean hit, as well as some years of global cooling from dust and ash
in the atmosphere. Apophis is too small to be an extinction level impactor—for example,
the object that ended the dinosaurs 66 million years ago was around 20 times larger than apophis.
Still, we should be happy that Apophis won’t hit us any time soon. Although when it was
first discovered in 2004, early observations and orbital simulations indicated a 2029 impact chance
of up to 2.9%. Further observations, including radar by Arecibo, slowly downgraded the risk for
a 2029 impact, instead indicating a possibility of passing through an 800m wide gravitational keyhole
for a 2036 impact. Happily, yet more observations show that’s no impact is likely for the next
century. Still, Apophis is a great test case for planetary defense. As I mentioned in the recent
episode, that’s one of the reasons the newly renamed OSIRIS-APEX will intercept Apophis on its
now-benign 2029 encounter—in particular to assess how its interaction with Earth’s gravitational
field changes the structure and rotation of the asteroid—all of which are important for
future simulations of NEO trajectories following a close encounter with Earth.
As far as we know, there are no giant space rocks about to drop into our atmosphere. The fact that
we’re still finding new Earth-crossing objects is a bit worrying. However the
Rubin observatory’s LSST survey that starts this year will find many of the missing ones,
NASA’s Near Earth Object Surveyor to launch in 2027 or so is supposed to find at least
⅔ of objects larger than 140m. In 2005 the US Congress mandated NASA to find at least 90% of
the estimated 15,000 NEOs larger than 140 meters. So, you know, we’re getting there. At least we
know that when we do spot a new space rock hopping across the grainy starfield of our survey images,
we know exactly how to figure out its potentially Earth-bound trajectory through space time.
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