The Milky Way galaxy is an expansive giant.
Home to hundreds of billions of stars
and at least 100 billion planets,
many of which shine brightly in our own night sky.
But there are others hiding in plain sight.
If we gaze at the stars hard enough,
we can even get lucky and catch a glimpse
of the fluting remnants of solar system bodies.
What are these near-Earth objects,
and could they have played a role in our evolution?
I look at these objects as tracers
of the very earliest stages
of planet formation in the solar system.
We can study the planets and try to understand
how they form and evolve.
But the planets are complicated bodies.
They have weather and wind and water.
They have plate tectonics.
They have volcanism.
All of these things act to erase
and reset the surfaces of the planet.
And that makes it very difficult
to learn about how the Earth formed
four and a half billion years ago.
In contrast, the asteroids are leftover fragments
of the planet formation process,
and they've been sitting out in the solar system
for the past four and a half billion years
relatively undisturbed.
And so, by studying these objects,
we're really able to peer back into the past,
and understand what the conditions were like
as planets were forming, both chemically and physically.
So we have 600,000 of these things to study.
We can't send a spacecraft to all of them,
but we can use telescopes here on Earth.
For a long time, the traditional definition
of an asteroid and a comet,
is that an asteroid's a rocky body,
and that a comet's an icy body that shows activity.
A comet injects a halo,
those types of things that we associate with a comet.
The past 10 or 15 years or so,
we've started to discover asteroids
that, for whatever reason, all of a sudden turn on.
Just like a comet would, they start to show activity.
They start to show evidence of outgassing,
loss of volatiles, loss of dust.
Just like a comet.
And we now think that there are quite a few processes
probably responsible for taking some of these objects,
that we would have classified as asteroid,
and turning them into comets.
Collision is one possible way
of making an asteroid look like a comet.
If you have an asteroid and collide a body onto its surface,
that's gonna kick up a big dust cloud
and, as viewed from Earth,
we will see an asteroid with a big dust cloud around it
that looks just like a comet.
They can be a hazard,
and we certainly know that near-Earth asteroids
can and do hit the Earth.
We have numerous pieces of evidence all over the Earth,
in the form of impact craters,
that, in some cases, very large objects have hit the Earth
and we think that there's a pretty good correlation
between extinction events
and impacts from asteroids and comets.
While the most famous has been the impact
that potentially killed off the dinosaurs,
we have evidence of that impact
off the coast of the Yucatan Peninsula in Mexico,
but there are numerous others throughout history
that we think correlate with extinction events
that we see in the fossil record.
With that said, though, those events are exceedingly rare.
Exceedingly rare.
And they're so rare that I don't stay up at night
worrying about the next killer asteroid coming along.
While impact events here on Earth
are few and far between,
parts of asteroids and comets
have reached our planet in the form of meteorites.
There's a whole host of terminology
associated with meteors.
There's meteors, there's meteoroids, there's meteorites.
Meteors come from a variety of sources.
Some of them are particles
that have been released by comets.
Some of them are dust particles released by asteroids,
generally formed through collisions.
So two asteroids collide, and produce a shower of fragments.
Some of those fragments can make their way to the Earth.
Before that passage through the atmosphere,
you could refer to that particle or that rock
as a meteoroid.
If that meteor makes its way to the ground,
then that is when it becomes a meteorite.
Generally what happens with meteorites,
is that they will impact the atmosphere
and as that body passes through the atmosphere
it undergoes extreme heating and fragmentation effects.
It fragments into many pieces and those pieces get scattered
all over what we call strewn field.
Those pieces can range in size from little dust particles
all the way up to large blocks.
The biggest meteorites in the world
can be the size of rooms.
In February of 2013, over Chelyabinsk, Russia,
a modest sized meteoroid or asteroid hit the atmosphere,
passed through the atmosphere as a meteoroid,
actually blew up at a relatively high altitude
and showered down fragments of that material
onto the surface of the Earth.
That was not predicted and part of the reason for that
is that it was a relatively small object,
about 15 meters in size or so.
Something that small is hard to detect
with ground-based telescopes.
But perhaps more importantly,
is that it approached from the direction of the Sun.
If something of that size
is coming from the direction of the Sun,
you're not gonna know about it ahead of time,
unless it's been discovered years previously.
This amazing fireball, a streak of light
that appeared in the sky,
it left behind a vapor trail or a gas trail,
and people ran to their windows to look at this.
Several minutes after the fireball,
the shockwave from that event
made its way down to the surface,
and that shockwave blew out windows
all over this region in Russia
and it was the shattering of windows
onto people's faces that were looking out the window
that caused the greatest amount of injuries in that event.
So it wasn't necessary there was a large impact crater
and flash heating or anything like that.
It was the shock of the fireball.
This is actually a relatively small piece.
The largest piece that was recovered
was maybe half a meter in size.
It was actually at the bottom of a lake.
This is actually what we refer to as an ordinary chondrite,
which is unsurprisingly a fairly ordinary meteorite.
It's the most common type of meteorite
that falls on the Earth.
About 80% of all meteorites fall into this category.
Well, this meteorite itself formed
about four and a half billion years ago in the solar system,
and has been floating around in space since then.
It probably lived part of its life
on the surface of a main-belt asteroid.
At some point that main-belt asteroid was probably impacted
and fragments were removed from that surface,
and those fragments then made their way to Earth.
It impacted and delivered the meteorites to the ground.
So there's nothing terribly extraordinary about it,
except for the fact that it has a very specific
and very well documented impact event associated it with it,
which is fairly unusual.
Most of the time these things fall in remote regions,
where people don't witness the fall.
But this is the most documented impact in history.
This is one of the more popular type of meteorites.
It's called a pallasite.
This is a mix of gem-quality peridot, or olivine,
in a matrix of metal.
And so it's a really striking, beautiful piece,
and we think that something like this
probably formed around the core-mantle boundary
of a large protoplanetary body.
So just like the Earth has a core, a mantle and a crust
we think planetary bodies in the solar system
some of them may have undergone
a similar differentiation process.
And so this is a chunk of a core-mantle boundary
of one of these differentiated protoplanets,
and obviously that protoplanet has been disrupted
so that it has been collisionally fragmented into a piece
that we have now collected as this meteorite.
Many of them are cut into these slices
so that you can see the sort of appearance
and internal texture of the meteorite.
The last piece here that I'll show
is a fascinating meteorite.
This is an iron meteorite.
It a slice of the iron meteorite.
Its surface has been etched
so that you can see these crystallized,
interleaved pattern of crystals on the surface of it.
This particular iron meteorite
has the oldest date of any rock in the solar system,
and that 4.567 billion years old.
This rock was probably the first thing
that formed in the solar system,
the first planetary body that formed in the solar system
four and a half billion years ago.
And there's a really interesting history
about how we think something like this
could have formed so quickly.
It requires very rapid accretion,
so the body accreted very quickly,
and because we're looking at iron,
this is a chunk of iron-nickel metal,
this is almost certainly the core
of the differentiated body.
So you're looking at rapid accretion and differentiation
in the first few million years of solar system history,
which is staggering to think
that such complicated geological processes
were happening so early in the solar system.
And it's those bodies that this meteorite represents
that were the precursors to the Earth
and the planets in the solar system.
So it was objects like this
that were colliding and sticking together
that ultimately formed and built up the planets.
So by studying these objects
we can understand the building blocks and the ingredients
that went in to form the Earth and set up the conditions
that make the Earth habitable and sustainable for life.