Hello, and welcome to the Physics World weekly
podcast. I'm Hamish Johnston.
Our guest this week is Rahil
Macadiah
who has just completed a PhD
in aerospace
engineering
at the University of Illinois
Urbana Champaign.
There, he explored how we could deflect
asteroids away from Earth.
We chat about the potential threats
posed by near Earth asteroids
and how they could be sent on their
way.
Macadea also stresses the importance
of getting a deflection right the first time.
Here's that conversation.
Hi, Rahil. Welcome to the podcast.
Hey. Hey, Mish. Thanks for having me. So,
Rahil, before we talk about deflecting
asteroids,
can you give our listeners an idea of
the threat?
What sort of threat is posed
to Earth from asteroids?
Yeah. Sure. I wanna start off by saying
that none of the asteroids that we know
about are currently a significant threat of impacting
the Earth. So for the next hundred years
or so,
none of the objects that we know about
are gonna potentially hit the Earth, come close,
enough to to pose a danger.
So we should all be resting easy.
The the threat really comes from the fact
that we don't know about all of the
asteroids that are out there, so not all
the asteroids in the solar system have been
discovered yet. And if one were to hit
the Earth,
there'd be potentially dire consequences. So
even a smaller object,
say, I don't know, tens of meters in
size
can potentially cause massive infrastructure damage and cause
injuries to to people. Like, there was an
event in
2013
in Chelyabinsk,
Russia, where a meteor exploded over the ground,
and, the the shock wave from that explosion,
you know, shattered windows. And people naturally, when
they see something like a meteor flying through
the sky, they'll flock to the windows. And
as a result of this shock wave, you
know, exploding windows, there are a lot of
cuts and bruises to to a lot of
people.
But if you go, larger up the the
asteroid size scale, so if you have, say,
an asteroid that is hundreds of meters in
size, that one can cause,
potentially massive damage. You know? It can wipe
out cities, potentially do damage on continental scales.
If you go even bigger, which there are
asteroids that are kilometer sized and plus, then
we start getting into the realm of, you
know, the the ones that that, made the
dinosaurs extinct, and there'd be global consequences,
if one that one that size were to
hit the Earth. I see. And the the
Russian asteroid, how how big was that?
I think it was around 25 to 30
meters in diameter. Okay.
So so fairly small, but,
good given that the other asteroids, some of
them are more than kilometers in size. But,
you know, even a a small one can
pack a bunch. And
smaller asteroids routinely go through the atmosphere,
they usually end up not making it to
the ground. So, you know, around five to
10 meters, there were, I think, around four
small impactors just last year in 2024.
But,
those didn't cause any damage. Some of them
make it to the ground in the form
of small meteorites that teams can go out
and recover,
but it's the larger ones that we need
to worry about potentially hitting the earth. I
see. And and so let's say at some
time in the future,
unfortunately,
we detect
a relatively large object
that's bearing down on Earth.
Could we deflect,
such an object? Do we have the the
technology at the moment?
And and and how how would that work?
Yeah. So the answer to that question depends
on a a couple of factors. One of
them is how big the asteroid is.
For for asteroids that are extremely large, some
of the deflection techniques we have, would take
too long or so you need a large
notice period before,
the the potential impact date. But if we're
talking about the realm of, you know, these
hundreds of meter sized objects,
we have what's called the kinetic impact method
where the basic idea is you just take
a spacecraft and smash it into the asteroid,
and
that causes a small deflection because the size
of the spacecraft relative to the size of
the asteroid is pretty small. But if you
give it enough time to accumulate, then the
trajectory of the asteroid is altered enough that
it would miss the Earth. And we successfully
demonstrated this technique with, a mission called DART,
the double asteroid redirection test,
which was an asset mission that launched in
2021.
And in 2022,
it impacted,
Dimorphos,
which was the the smaller moonlet asteroid in
the Didymos binary asteroid system.
And it successfully altered the trajectory of the
secondary around the primary,
and it,
you know, proved that we can do this
kinetic impact method if we ever have the
need to. And Dimorphos was around,
a 150 meters, in diameter, so we've successfully
proven that we can successfully hit asteroids that
are this small. Because the smaller the asteroid
gets, the harder it becomes to target it
because especially if you're coming in very fast
because you have
a small amount of time to to actually
do the targeting when the spacecraft's on its
way.
There are other techniques, that have been studied.
There are there are options that have studied
the use of nuclear devices,
what's called a standoff detonation,
where they would detonate a device at a
certain distance above the the surface of the
asteroid and let the radiation melt away a
layer,
a a top layer of the of the
surface of the asteroid,
and that would impart some momentum as the
the material is getting vaporized and kicked off
the asteroid.
Those are the the sort of two shorter
term deflection methods. There are a couple others
that need a longer time scale between,
discovery and potential impact.
There's what's called the ion beam deflection method
where you can redirect,
an ion beam up up of a spacecraft
towards the asteroid, and that slowly erodes, the
surface of the asteroid to to part momentum
onto it.
And there is the the gravity tractor method
where you basically station keep,
or hover around an asteroid,
with a big enough spacecraft and just the
gravitational interaction between the spacecraft and the asteroid
over decades, maybe even, can cause,
a big enough change to potentially deflect it
away from the Earth. And you you mentioned
that we we we think we're safe for
for about a hundred years or so. Does
that does that mean that, for example, tomorrow,
we could spot,
a large asteroid
that will reach us, let's say, within
a hundred years, or I should say, in
over a hundred years? And so we would
have time
to
really think about how we would deflect it.
And
Yeah. I don't know. Maybe maybe get cracking
in ten years from now when we've when
we've developed some technology
that's suitable for that particular object. Is that
I mean, is that how it would play
out, I suppose?
Yeah. I should say that for the larger
objects, so let's say kilometer size plus, most
of our population models indicate that we've discovered
most, if not all of them,
especially, you know, the 10 kilometer plus size.
Those are the the sort of planet killers
you might think of as.
We we're pretty sure that we've we've discovered
pretty much all of them, because the the
bigger they are, the easier they are to
detect, and telescopes keep getting better and better.
So it would be pretty, difficult to detect
a massive,
asteroid at this point. But, yeah, like you
said, if we're, if we detect one that's
massive enough, it it will likely be that
it's far, far away, the the potential impact
date. So I'll give you a specific example.
There's this asteroid called nineteen fifty d a,
which is larger than a kilometer, we think,
and its potential impact date is the year
2880.
So there are, you know, special cases like
this one where we can predict that, you
know, eight hundred years in advance.
Obviously, we don't need to do anything about
that one anytime soon. But,
a lot of these large asteroids, you know,
we're able predict their trajectories for a pretty
long term and,
conclude whether or not they're a significant impact
hazard.
And I'd just like to talk about some
recent work that you've done.
You've you've looked at the possibility
of an impact,
putting an asteroid into a trajectory
in which it could return to Earth,
which is probably something that we'd like to
avoid
if possible. Yeah.
Can you explain how how this would happen?
Is it is it simply a matter of
of hitting the the asteroid,
you know, with the wrong sort of impulse
or in the wrong direction
and setting it into a alternative orbit that
eventually brings it back to Earth.
Yeah. So it's, you know, the the the
it's the opposite of being in the right
place at the right time. So you're sort
of deflecting the asteroid at the wrong place
at the wrong time,
and what you can do is potentially so
let's say in this hypothetical scenario, there's an
asteroid headed towards the Earth, and we need
to deflect it away. Right? And we send
a deflection mission, let's say, a kinetic impact
there because that's the one we know how
to do.
And we do this deflection, and we, you
know, dismiss the immediate impact threat from the
asteroid.
But lo and behold, because we weren't that
careful, we've put it now in a trajectory
that will return to an to the Earth
on an impacting trajectory forty years later.
So in this scenario, what we've done is
we've triggered what's called a gravitational keyhole.
And these keyholes are basically, you know, predictors
of whether or not an asteroid might hit
the earth in the future. So,
if we blindly deflect an asteroid and push
it through this gravitational
keyhole, it's guaranteed to return to the earth
on an impacting trajectory at a future date.
But the the the nice thing about these
keyholes is that we can compute them beforehand
and map them,
given an asteroid's orbit.
And so if we're careful about it, we
can directly map these keyholes when we're designing
the initial deflection mission in the first place,
such as to avoid any keyholes while deflecting
it clear of the the immediate impact threat.
I see. And and are these these keyholes,
are they are the I mean, do they
tend to be large, or are there lots
of them in the sense that, you know,
if if we didn't think of this,
we you know, what are the odds, I
suppose, if we didn't think of this and
just hit the the, the asteroid as hard
as we could at a certain point in
time?
Would it be very likely that we could
go into one of these keyholes by mistake,
or would we have to be really unlucky?
So that sort of delves into specifics,
because the sizes of the keyholes, they're usually
pretty small. I think the largest ones I've
seen so far are around a kilometer or
a few kilometers in size. And given how
vast space is a kilometer or a few
kilometers is is minuscule,
but,
once again, we start delving into the problem
of, you know, likelihood and consequence. So the
likelihood might be low. And I think
the,
I should mention, yeah, that the largest number
of keyholes I've seen for an asteroid is
is around a couple 100 or so, in
a in a massive
uncertainty region or the the region which you
could find the asteroid at any given day.
But once again, if we were to send
an asteroid through one of these keyholes,
because it's, you know, a
a future predictor of an Earth impact, the
same consequences apply as if that size asteroid
were to hit the Earth. So it's a
it's a trade off between do we wanna
just blindly deflect it, and, obviously, if there
is no way of deflecting an asteroid without
avoiding,
a a keyhole, we need to dismiss the
immediate impact threat. So that's the primary goal.
But if we can be smart about designing
the deflection mission and, you know, kill two
birds with one stone, so to speak,
and dismiss the impact threat, maybe permanently even
from one asteroid. That gives us less to
worry about in the future.
And, I think that the you've found that,
that this
the technique or or or, I suppose, avoiding
gravitational keyholes
involves,
having an understanding of the topography
of the asteroid. So I'm I'm guessing there
that its shape,
etcetera. Why
why is that important?
Yeah. So, yeah, like you mentioned, we need
an understanding of both how the asteroid's local
topography looks like, so what its shape is,
as well as how it's spinning because most
asteroids, are rotating.
So we need to know what orientation of
the asteroid we're gonna be looking at when
we're deflecting it and what the sort of
local topography
it at the is at the point,
on which we're gonna deflect the asteroid because
the local topography
dictates where
would where the ejecta will travel. So if
you deflect an asteroid using a kinetic impact,
what happens is that surface material around the
impact site actually gets kicked up off of
the surface,
and gets released in the opposite direction that
the impactor came from, and that acts as
an additional boost,
on top of the momentum that's imparted by
the spacecraft.
So this additional boost can be significant. It
can be, you know, similar to or larger
than the the the contribution of the spacecraft
itself.
So we wanna be able to accurately model
this deflection,
and that involves accurately modeling the momentum that's
imparted onto the asteroid,
which involves knowing where this ejecta went,
which dictates that we need to know what
the local topography looks like because that's the
local orientation is what dictates the direction of
this ejecta.
But if we have a rough idea, you
know, there are techniques even from the ground
to,
get the sit the shape of an asteroid.
There's, what's called radar,
astrometry. Radar imaging of an asteroid can tell
us what an asteroid looks like.
There's also light curve methods. So you you
sort of measure the amount of light you're
receiving from a given asteroid, and you can
invert that to to get a preliminary shape
of the asteroid.
But the the best way to get an
asteroid shape is obviously go visit it with
a spacecraft beforehand. That'll give you as much
information as as my heart desires and and,
you know, allow us to do this deflection
the best way possible.
And, I mean, I'm just thinking about, you
know, this this idea of hitting
an asteroid. And, you know, you mentioned that
these asteroids, well, they're it's probably going to
be rotating.
It do you I mean, the the problem
I I sort of I suppose a problem
I can think of is that you really
want,
I assume that you want sort of maximum
momentum transfer
from your spacecraft
to the,
to the object. And do you want to
avoid having some of that energy converted into
rotational energy in the sense that you you
whack the
the the the object, and instead of knocking
it off course, you just spin it up
or you reduce its,
spin, and it sort of continues on in
the same direct in its same trajectory. Is
is that a problem as well?
Yeah. That is definitely something to think about
is, yeah, if you hit it off center,
you're imparting a moment onto the asteroid, and
that will change its its spin.
But,
sometimes, you know, the the spin state change
can be overruled by the amount of deflection
an off center,
deflection would give you. So even if you
were hitting it off center, you might get
enough deflection to to put it clear of
the earth. And if, you know, the safe
zone that avoids these keyholes,
is sitting, let's say, 25 meters off of
the center of the asteroid, that's not that
much, that you're losing.
And with this technique, we actually put a
band around the asteroid
such as to avoid missing it. So if
you're too far away from the center, you
know, the the spacecraft can't perfectly target,
the asteroid at the point you've told it
to because there's, you know, algorithmic uncertainties and
and just uncertainties in general in the world
we live in.
And
that would mean that you might miss it,
miss the the aim point, let's say.
And we wanna make sure that in the
first place, we hit the asteroid because that's
the the goal of the deflection. So we
actually put a safety band around the the
perimeter of the asteroid, the visible perimeter of
the asteroid to make sure that we're in
pursuit of doing a better deflection. We're not
missing the deflection altogether because that would, you
know, defeat the entire point of the existence.
And and what's next for you in terms
of your research? You you you've looked at
these gravitational keyholes.
Are you are you gonna do more studies,
I suppose, on different types of asteroids, different
shapes,
different distances from Earth, different orbits, or
are are you go going to be looking
at another aspect of, of asteroid impact?
Yeah. Yeah. I think,
so I do wanna continue looking into this
this better deflection,
you know, a more optimal deflection strategy,
in terms of avoiding keyholes while deflecting asteroids.
One thing I do want to look at
is what happens if we if we use
one of the other techniques I was talking
about earlier to deflect an asteroid because kinetic
impact is the one that we've demonstrated so
far. It's the one we already have in
our arsenal. But, obviously, there are are other
message, the longer term message that I was
talking about, like, I NBM deflection and gravity
tracker have been got getting a lot of
attention in the in the research world after,
you know, the kinetic impact,
success.
So I would like to see how it
how avoiding keyholes while doing these other deflection
techniques, what that looks like.
Another thing that I wanna look into is
potentially better ways of computing these keyholes in
the first place. So it's a very nitpicky
thing. Let's see. You know, improving every part
of this process as much as I can.
But, well, yeah, we'll see where the this
research takes me. Well, that's great. Thanks so
much for coming on the podcast and talking
about that. And and also reassuring our listeners
that,
you know, at the moment, at least, there's
probably not much to worry about in terms
of large asteroid impact. I mean, when when
we say large, I mean, what what are
the what are the sizes of asteroids at
the moment that can sort of sneak through?
Is it the like, that that Russian asteroid,
for example? Did did we see that coming,
or is that just too small for us
to detect?
No. I think that with with Chelyabinsk specifically,
and I I wasn't in the field in
2013,
but, I think what happened with that one
is that it came from the sunward direction.
So none of the telescopes could point in
that direction because, obviously, you can't point the
telescope towards the sun.
And so we're we're sort of looking to
tackle this problem from a couple different avenues.
There's a a new telescope in the Southern
Hemisphere called the Vera Rubin Observatory
that's set to dramatically increase the rate of
asteroid discoveries.
There's also a mission called NEO surveyor, which
will be a space based telescope that NASA's
building right now and hoping to launch in
the next few years,
which will hopefully try to detect these sort
of sunward geometries from the Earth from a
different vantage point in space. So maybe we
can discover these jelly bins like objects.
And in terms of size in general, yeah,
it's this sort of tens of meters to
maybe a 150 meter size range that we
still haven't discovered,
most of them. So that's where the the
next discovery,
effort is is headed.
Well, that's great. Thanks, Rahil. Thanks for coming
on the podcast, and I'll put a link
to, Rahil's paper, in the notes for this
podcast. Best of luck with your research. Thank
you. Thank you so much.
That was the aerospace engineer,
Rahil Macadiah.
Thanks, Rahil, for a fascinating conversation.
Physics World will be at the APS Global
Physics Summit, which will take place in March
in Denver, Colorado
and online.
At the largest physics meeting in the world,
you can join thousands of physicists,
students, and policy leaders for a week of
connection and collaboration.
Explore cutting edge science
shaping our shared future,
and be part of the global physics community
driving innovation forward.
Explore the meeting at summit.aps.org.
And when you're in Denver,
look out for us at IOP Publishing's
booth at the conference exhibition.
I'm afraid that's all the time we have
for this week's podcast.
I'm Hamish Johnston, and our producer is Fred
Isles.
Our theme music is called one three seven,
and it was composed and performed by the
physicist
Philip Moriarty.
We'll be back again next week.