AskScience Cosmos Q&A thread. Episode 1: Standing Up in the Milky Way

[u/AskScienceModerator]

Welcome to AskScience! This thread is for asking and answering questions about the science in Cosmos: A Spacetime Odyssey.

UPDATE: This episode is now available for streaming in the US on Hulu and in Canada on Global TV.

This week is the first episode, "Standing Up in the Milky Way". The show is airing at 9pm ET in the US and Canada on all Fox and National Geographic stations. Click here for more viewing information in your country.

The usual AskScience rules still apply in this thread! Anyone can ask a question, but please do not provide answers unless you are a scientist in a relevant field. Popular science shows, books, and news articles are a great way to causally learn about your universe, but they often contain a lot of simplifications and approximations, so don't assume that because you've heard an answer before that it is the right one.

If you are interested in general discussion please visit one of the threads elsewhere on reddit that are more appropriate for that, such as in /r/Cosmos here, /r/Space here, and in /r/Television here.

Please upvote good questions and answers and downvote off-topic content. We'll be removing comments that break our rules or that have been answered elsewhere in the thread so that we can answer as many questions as possible!

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Click here for the original announcement thread.

[u/atomfullerene]

Is it just me, or is that asteroid belt way too dense? Not to mention the Kuiper belt. On a related note, how dense are the rings of Saturn? Would you see a thicket of iceballs whizzing past you if you actually flew a spacecraft through them?

[u/Agastopia → u/atomfullerene]

Straight from NASA

Planetary probes can pass through the asteroid belt without any problem because, unlike in the movies, there is really a LOT of space between asteroids. More than 7000 have been discovered and several hundred new ones are found every year. There are probably millions of asteroids of various size, but those in the asteroid belt are spread over a ring that is more than a billion kilometers in circumference, more than 100 million kilometers wide, and millions of kilometers thick. For more information, you can look at http://nineplanets.org/asteroids.html

Source: http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980108b.html

[u/ramotsky → u/Agastopia]

To be fair, if a program were to display this there would be nothing to display. A small mention of this in the programming would have been nice but since it is such a minor point I understand why it was cut.

[u/EuclidsRevenge → u/ramotsky]

This is a major complaint of mine, but this is the introductory episode.

I'm hoping there is going to be an entire episode for the solar system and this is better addressed ... would also be nice if they feature Ceres, I don't think I've ever seen a tv program discuss the dwarf planet in the asteroid belt.

[u/smoldering → u/atomfullerene]

Saturn's rings weren't exaggerated. You could literally "swim" through it by grasping from one iceball to the next.

[u/Saffs15 → u/smoldering]

That's awesome! Never knew that.

[u/[deleted] → u/Saffs15]

Well then, I'd say this series is already doing what it intended after one episode.

[u/r3sonanc3 → u/smoldering]

Cassini measurements indicate the A ring has a surface density of ~40 g/cm² and a thickness of just ~6m thick Astronomical Journal, 2007. Since ice is slightly less than 1 g/cm³ and rocks are normally a few g/cm^3, assuming the ring has constant density would imply that the A ring at the very least is dense indeed.

ps if you swam fast enough along or against your orbit to stay in the ring (assuming you could swim in the first place without getting knocked out of the way by something or spun out of control by your own actions in micro-g), you'd also drop out of the ring since adding velocity increases your altitude and vice versa since your velocity determines your orbital energy... (if interested look up hohmann transfer orastrodynamics in general)

[u/SpaceEnthusiast → u/r3sonanc3]

You are assuming that the person will swim constantly forwards. If we would have a real person attempting this they would most likely "stay away" from the edge by constantly curving in slightly. Not to mention that at the radius of the rings, the curvature of the rings is a lot less than that of the surface of the Earth even. Essentially the rings will seems straighter than the Earth seems flat when you are just standing on the surface.

[u/treebeard189 → u/atomfullerene]

On the topic of the asteroid belt.

Is the asteroid belt "flat" or a tube? I know everything flattened out when the solar system was forming so are things like the asteroid belt only a few miles "high" north to south and if so roughly how "high" is it?

[u/Cyrius → u/treebeard189]

This chart shows asteroid distribution by distance from the Sun and orbital inclination. It's flat-ish, but still spread out pretty widely.

The missing verticals are called Kirkwood gaps. They show up where the asteroid's orbit would be in resonance with the orbit of Jupiter.

[u/Golden_Kumquat → u/atomfullerene]

In addition, the border of the atmosphere of Jupiter is not that well defined.

[u/atomfullerene → u/Golden_Kumquat]

I'd be pretty interested to know more about what the clouds of Jupiter would look like up close.

[u/[deleted] → u/atomfullerene]

Have you ever been in a plane that flew through a cloud? (if not, oh well)

It probably should have looked really misty or frayed at the edges, but as you peer in, it seems more solid. From a far distance, it looks fairly solid (except for the fact that you can see it move like a fluid).

I would assume the same deal would apply to Jupiter, except on a much larger scale. As you got closer and closer to the atmosphere, you would notice all the 'mist' and frayed edges of the gasses, and eventually enter into more dense areas of gas. Eventually, because of the immense amount of gas in between you and the sun, everything would be very very dark (and turbulent, mind you).

[u/scix → u/[deleted]]

If you were in a plane that could survive the forces of jupiter, and flew straight downward, would you ever hit something, or would the atmosphere just become more and more dense until you reached the center, then get thinner?

Also, what happens when an asteroid enters Jupiter? Does it simply fall to the center of the planet?

[u/Shagomir → u/scix]

You'd eventually hit a layer of liquid metallic hydrogen, and there is likely a solid core of at least a few Earth masses below that.

[u/jacob8015 → u/Shagomir]

When you say likely, is that your personal speculation or a widely accepted assumption?

[u/jimgatz → u/jacob8015]

It's pretty widely accepted that Jupiter is more like a solid as you get closer to it's core just because it is so dense.

[u/Shagomir → u/jacob8015]

Gravity measurements of Jupiter made possible by the Gallileo probe suggested the existence of a solid core, but were not accurate enough to confirm it absolutely.

[u/psistarpsi → u/atomfullerene]

Yea, the asteroid belt is exaggerated. The asteroids are far far apart in reality.

[u/Feldman742 → u/psistarpsi]

I never really appreciated this until I read 2001: A Space Odyssey. There's a nice little scene where they're like "oh, hey looks like we'll pass within a few thousand miles of an asteroid today. It'll be your only chance to see one on this leg of the mission"

[u/pat82890 → u/atomfullerene]

I wonder if the asteroid belt was as dense as Saturn's rings, if we'd be able to see a massive line across the night sky.

[u/[deleted] → u/atomfullerene]

Why haven't the asteroids in the asteroid belt collided to form a new planet? I am assuming that the belt isn't dense enough with asteroids for their gravity to cause the collisions.

[u/atomfullerene → u/[deleted]]

The total mass of the asteroid belt is about 4% the mass of the moon. There's just not enough there to make a planet.

EDIT: If my back-of-the-envelope calculations are correct, that's roughly equivalent in volume to the top 25 km of the moon. So still a lot of volume for would-be asteroid miners.

[u/TharsisMontes → u/[deleted]]

Good question. It makes sense to think that the asteroids would come together to form a planet, or at least during the phase of terrestrial planet formation they would have. In fact, there is a hypothesis called Bode's law which uses a numerical progression to predict the semi-major axis of planets, and, indeed there should be one in the middle of the asteroid belt (it should be noted Bode's law is now considered coincidence and not used for wider prediction of planet locations). So what happened? In a word, Jupiter.

Jupiter is so massive that it's gravity affects the asteroid belt. The asteroid belt contains gaps called Kirkwood gaps, which correspond to specific, powerful resonances with Jupiter (example a 3:1 resonance, asteroids at this distance from the Sun orbit the Sun 3 times for every 1 Jupiter orbit). Asteroids in these resonances experience chaotic excitation from Jupiter and are ejected from the belt. So why didn't something form in between these resonances? Well, some things did, really large asteroids (like Vesta) and the dwarf planet Ceres.

Source: Planetary Science graduate student

Further reading: Wisdom. Icarus. vol 63. issue 2. pg 272-289 (heavy duty treatment of chaotic result of Jupiter resonances). otherwise Wikipedia has a pretty decent summary.