Thank you to DeleteMe for Supporting PBS.

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 firs.

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 wow,  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’ll 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 do 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 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 a couple of hundred kilometers 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 one in millions.

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.

Thank you to DeleteMe for Supporting PBS.

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