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  • [INTRO ♪]

  • Over the last 60 years, we've visited a lot of places in the solar system.

  • The Sun, the Moon, all eight planets, some comets, even Pluto!

  • Every time it seems like we've been everywhere worth going,

  • some new and amazing object shows up.

  • And these days, many of those objects are asteroids.

  • Astronomers have been discovering asteroids

  • since the early 1800s, but modern technology

  • has turned them into the hottest new frontier

  • of solar system exploration.

  • So let's look at three exciting discoveries that marked firsts

  • in asteroid research and gave us great reasons

  • to keep exploring the asteroid world.

  • First up is asteroid 24 Themis, discovered in 1853

  • by astronomer Annibale de Gasparis.

  • It's the parent body of one of the largest families of asteroids,

  • made up of thousands of objects that all seem to have been

  • knocked off Themis by ancient collisions.

  • So, it's got that going for it.

  • But more recent telescope observations made in the mid-2000s

  • gave astronomers even more reason to be interested in this rock.

  • Their observations suggested that asteroids like Themis

  • could be responsible for something pretty important: Earth's water.

  • For a long time, planetary scientists had imagined

  • a clear distinction between asteroids and comets.

  • They thought asteroids were bone-dry, and only comets,

  • which orbit farther from the Sun, were covered with ice and snow

  • and therefore, could have hit Earth and left behind water molecules.

  • The only problem was, they didn't see that clean division in their observations.

  • Some meteorites (which mostly come from asteroids)

  • show evidence of having formed in the presence of water.

  • And a few objects in the Themis family also had features

  • more typical of comets than asteroids.

  • So that made it natural to check out the parent body in more detail.

  • When two separate teams used NASA's Infrared Telescope Facility

  • in Hawai'i to study Themis in the mid-2000s,

  • they found something pretty surprising.

  • A thin layer of frost seemed to be covering its surface.

  • And embedded in that frost were carbon-based molecules

  • the building blocks of life here on Earth.

  • Themis seems to orbit just far enough from the Sun to keep it all stable.

  • The fact that it's possible for an asteroid to hold onto ice is pretty fascinating,

  • because planets like ours probably formed from asteroids like Themis.

  • And as all that rock clumped together to form planets,

  • the water and organic molecules must have come along for the ride.

  • Themis isn't the only asteroid where we've discovered something

  • hiding in plain sight.

  • And for 87 Sylvia, thatsomethingwas pretty big:

  • not just one, but two of its very own moons!

  • Asteroids!

  • With little tiny moons!

  • It sounds pretty cute!

  • Astronomers first spotted Sylvia orbiting in the main asteroid belt

  • way back in 1866, but it took until the 21st century

  • for anyone to spot its companions.

  • Scientists noticed its first moon in 2001

  • and its second four years later.

  • We'd found lots of double-asteroids by then,

  • but no one had found a triple system.

  • Now we know they existand they can be pretty useful!

  • By tracking the paths of the smaller moons, astronomers

  • were able to pin down details about Sylvia's mass and density.

  • Those gave them clues to its composition,

  • which can be tricky to figure out in an asteroid.

  • But of course, the first order of business

  • was to give the moons cool names.

  • Sylvia was named after Rhea Sylvia, the woman ancient Romans

  • believed gave birth to Rome's founders.

  • So, of course, the moons are named for her sons Romulus and Remus.

  • But the names are more than just some good, clean historical fun.

  • The evidence suggests that the moons might literally be

  • from Sylvia itself, knocked off during an ancient collision.

  • In fact, researchers think there could be even more moons,

  • because Sylvia is what astronomers call a rubble pile.

  • Basically, instead of being one big, solid object,

  • it's more like a collection of rocks held together by gravity.

  • That means it would be especially easy to knock off

  • bits and pieces during a collision.

  • Research suggests that up to 6% of asteroids

  • might have at least one companion, but Sylvia was the first one

  • spotted with multiple.

  • Not bad for a pile of rocks!

  • Our final nifty asteroid is 163693 Atira,

  • named for the Pawnee goddess of Earth.

  • And when it was discovered in 2003, Atira was the first known asteroid

  • that was always orbiting closer to the Sun than Earth.

  • Since then, astronomers have found around 20 more,

  • but these objects remain really tough to study.

  • For one, Atira is small, with a diameter around five kilometers.

  • But more importantly, it can be tricky to study objects

  • closer to the Sun than us.

  • Because, you know what else is in that direction?

  • The Sun, which is definitely not an astronomer's friend.

  • But these objects are definitely worth studying,

  • because it's sort of surprising that asteroids like Atira can stick around at all.

  • Atira's orbit is really elongatedsometimes it nearly reaches Earth,

  • while at other points it can be just half as far from the Sun.

  • That means that, while it never crosses Earth's orbit,

  • it does pass through the part of space occupied by Venus.

  • Usually, crossing paths with a planet is bad newsover time,

  • it's pretty likely that its enormous gravity will fling you around.

  • So how come Atira and its family are still here?

  • One hint comes from their inclination.

  • Atira's orbit isn't just elongated, it's also tilted

  • more than 25 degrees relative to the plane of the solar system.

  • Objects that are both tilted and elongated can experience

  • what's called the Kozai-Lidov resonance.

  • Over time, they can basically trade elongation

  • and tilt back and forth in a way that helps stabilize their orbits.

  • Most of the time, we see this effect with planets orbiting other stars,

  • so it's pretty exciting to see it up close.

  • No wonder some researchers are proposing that Atira

  • and its friends would be a good target for a future spacecraft to visit.

  • Really, any of these unusual asteroids would be great places to check out.

  • And thanks to these three discoveries,

  • we have a much better idea what to look for.

  • Thanks for watching this episode of SciShow Space!

  • And if you like learning about asteroids, you might like our video

  • about the Asteroid Belt, where most asteroids come from.

  • Chances are, it's pretty different from what you might have imagined.

  • [OUTRO ♪]

[INTRO ♪]

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小惑星探査における3つの歴史的ファースト (3 Historic Firsts in Asteroid Exploration)

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    林宜悉 に公開 2021 年 01 月 14 日
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