Hook: Some comets orbit the Sun on a regular basis,
but others come in from deep space, a region known as the Oort Cloud.
What causes them to make this journey, and will we ever be able to explore the Oort Cloud?
Introduction: Before I get into this episode, I want to
remind everyone that it's been several decades since I've been able to enjoy a bright comet
in the night sky.
I've seen mind blowing auroras, and witnessed a total solar eclipse with my own eyeballs.
The Universe needs to deliver this bright comet for me, and it needs to do it soon.
By recording this video now, I will summon it.
I will create a video that'll be hilariously out of date in a few months, when that bright
comet shows up.
Like that time we totally discovered a supernova in the Virtual Star Party, by saying there
wasn't a supernova in that galaxy, but there was, and we didn't get to make the discovery.
Anyway, on to the episode.
Let's talk about comets.
Comets are awesome.
They're made of gas, dust, rock, and organic materials, smashed together, and existing
mostly unchanged since the formation of the Solar System 4.5 billion years ago.
Every now and then, some gravitational interaction kicks a comet into an orbit that brings it
closer to the Sun.
Because of the increased radiation, the comet's volatile gas and dust sublimates off the surface,
leaving behind a long tail of ice.
And this is how we discover them.
In fact, comets are one of the objects in the night sky regularly found by amateurs.
And by discovering a comet, you get to have it named after you.
Of course many of the comets are named after robotic observatories, just another way the
robots are taking human jobs.
Address the question: The source of comets was originally proposed
by Gerard Kuiper in 1951, when he theorized that there must be a vast disk of gas and
dust surrounding the Solar System, out beyond the orbit of Pluto.
This "Kuiper Belt", contains millions of objects, which orbit the Sun, jostling
each other with their gravity.
These interactions kick these Kuiper Belt comets into orbits that bring them closer
to the Sun, where they get their characteristic tails.
Astronomers call these short period comets, since they orbit the Sun relatively often.
They're given names and designations, and astronomers can calculate when the comet will
pass near to the Sun and flare up again.
The famous Halley's Comet is a good example, which was known to antiquity, but had its
orbit first calculated in 1705 by Edmond Halley.
Every 74 to 79 years, Halley's Comet swings near the Sun, flares up and we get a view
of this amazing object.
It last passed our area in 1986, and it's not due to return until 2061.
I should be in my third robot body by then.
The long period comets are much more mysterious.
These objects come out of nowhere, pass through the inner Solar System or smash into the Sun,
and then zip back out into deep space.
Now, where do they come from?
The Dutch astronomer Jan Oort calculated that there must be an even vaster cloud of ice
even farther out beyond the Kuiper Belt - between 5,000 and 100,000 astronomical units from
the Sun.
Just a reminder, 1 astronomical unit is the distance from the Earth to the Sun, so we're
talking really really far away.
Like, the Voyager 1 spacecraft, which is the most distant and fastest object ever sent
out by humanity, will still need about 300 years to reach the edge of the Oort Cloud.
Astronomers think that occasional gravitational nudges in the Oort Cloud cause these long
period comets to fall down into the inner Solar System and make their rare appearances.
It could take a comet like this hundreds of thousands or even millions of years to complete
an orbit around the Sun.
I'll need a few dozen robot bodies for that repeat observation.
Check out this cool picture of Comet C/2017 K2 PANSTARRS, taken by the Hubble Space Telescope.
This is a great example of a long-period comet, which is visiting our neighbourhood for the
first time in the 4.5 billion-year history of the Solar System.
This is the dimmest, farthest comet ever discovered, first seen when it was out beyond the orbit
of Saturn.
This cloud of material around the comet is probably the sublimation of frozen volatile
gases, like oxygen, nitrogen, carbon dioxide and carbon monoxide.
Astronomers think it started to become active about 4 years ago, and they just discovered
it now.
As it gets closer to the Sun and warms up, it'll become a true comet, when its hard-as-rock
water ice structure starts to sublimate and earns its tail.
It should make its closest approach in 2022 when it gets about as close to the Sun as
Mars.
And this is why we can't detect out into the Oort Cloud yet.
We can barely detect comets outside the orbit of Saturn, not to mention hundreds of times
farther than that.
Patreon Bumper In a second, I'm going to talk about some
interesting new research about the stellar interactions that Sun is going to be having
that could cause some of these interactions, but first I'd like to thank:
Shoki Kwabe Twin3
Avi Mousavi
And the rest of our 777 patrons for their generous support.
If you love what we're doing and want to get in on the action, head over to patreon.com/universetoday.
Bend their minds: Our Sun isn't alone in the Milky Way, obviously.
It's a vast swirling storm of hundreds of billions of stars, and over the tens of thousand
of years, other stars come much closer to the Sun than we see today.
The European Space Agency's Gaia spacecraft recently released one of the most detailed
of stellar positions and motions, and gave us a much better picture of where our Sun
is going, and what it's going to be interacting with in the future.
In order to interact with the Oort Cloud, astronomers have calculated that a star needs
to get within about 6.5 light years before it can interact gravitationally, depending
on its mass.
Based on data gathered by the Gaia spacecraft, astronomers charted out the motions of 300,000
stars in our vicinity of the Milky Way in the next 5 million years or so.
Of those stars, 97 will come within 15 light-years of the Sun, and 16 will get closer than 6.5.
The most interesting of these is Gliese 710.
In 1.3 million years, it'll pass less than 2.5 light-years away from the Sun, plunging
right through the Oort Cloud.
Gliese 710 has about 60% the mass of the Sun, and it's going about half the speed that
stars normally go as they sweep past the Solar System.
Which means that it's going to stick around for a long time, pushing comets around with
its mass, and send showers of comets down into the Solar System.
On average, it seems like a star passes within 15 light-years every 50,000 years or so, jostling
up our collection of comets.
This is important, because comet impacts could be a cause of past extinction events on Earth.
By tracking the movements of stars in our region, astronomers could try to match up
past events with times that stars jostled up the Oort Cloud, and predict future events.
Could we ever reach the Oort Cloud and explore it?
A few years ago, a space observatory was proposed that could attempt to observe objects as distant
as the Oort Cloud.
Known as the Whipple Mission, it would orbit in the Sun-Earth L2 point, and watch the sky
with a wide field of view.
It would try to detect transiting events when objects as small as a kilometer across passed
in front of a more distant star.
In theory, the mission would be capable of spotting these transits out as far as 22,000
astronomical units or nearly half a light year.
Unfortunately, it hasn't gotten past the proposal stage.
Another intriguing idea is known as the FOCAL mission, which involves sending a space telescope
out to a distance of 550 light-years away from the Sun.
At this point, the telescope can use the gravity of the Sun itself as an enormous lens, focusing
the light from more distant objects.
Actually, you'd need to go farther.
At 550 light-years, the sunlight drowns out anything the space telescope might try to
see.
Instead, it needs to go out to a distance of more than 2,000 light-years from Earth,
when the light focused by the Sun turns into an Einstein Ring around it.
What could you do with a telescope like this?
If an exoplanet were to pass behind the Sun, perfectly lined up, you could resolve features
as small as 1 kilometer across on a world 35 light-years away.
A telescope like this gives us a very good reason to learn to travel out and explore
the Oort Cloud.
Conclusion: The Gaia spacecraft is still hard at work
gathering data, and astronomers are expecting another massive data dump in April, 2018.
Over time, the spacecraft will map out the position and movements of a billion stars
in the Milky Way.
Comets are awesome, and I'd like to see a visible comet in the night sky, but I'd
like them to keep their distance.
Ask for Comments: Have you ever seen a comet with your own eyeballs?
Let me know your story in the comments below.
Next Episode: In our next episode, we talk about electric
sails as an exotic way to explore the Solar System.
Not solar sails, electric sails.
That's next time.
Suggested Playlist Time for your playlist.
We've got an episode of TMRO about how long period comets arrive more often, an explainer
from the OSIRIS-REx mission, Crash Course Astronomy tackles the Oort Cloud, a lecture
on the Kuiper Belt and its implications, and finally, Mike Brown teaches you how the Oort
Cloud might have formed.
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