# Tag Info

35

Let's assume mass of the person plus spacesuit to be $m_1$=100kg Asteroid density: $\rho=$2g/cm$^3$ (source) that is 2 000kg/m$^3$ 15km/hour is a good common run. That's roughly v=4m/s The orbital height is negligible comparing to the radius, assume 0 over surface. Linear to angular velocity (1): $$\omega = {v \over r }$$ Centripetal force (2):  F = ...

35

One thing to keep in mind is that objects that are bound gravitationally actually revolve around each other around a point called a barycenter. The fact that the earth looks like its revolving around the sun is because the sun is much more massive and its radius is large enough that it encompasses the barycenter. This is a similar situation with the Earth ...

31

Anything the mass of a star is going to get hot like a star and fuse hydrogen like a star. In other words it will be a star not a planet! While it's technically possible to have a rocky planet the mass of a star, in practice when stellar systems form there aren't enough metals available to build such a large object. Large objects are invariably built from ...

26

The answer kind of depends on how old you are. At a very introductory level, say, maybe middle school or younger, it's "okay" to refer to Jupiter as a failed star to get the idea across that a gas giant planet is sort of similar to a star in composition. But around middle school and above (where "middle school" refers to around 6-8 grade, or age ~12-14), I ...

24

Pluto is now classified as a dwarf planet. The main difference between a planet and a dwarf planet has to do with the requirement that a planet clear out the material in and near its orbit. Planets do this, dwarf planets do not. The reclassification was triggered by the discovery of many additional object (the Edgeworth-Kuiper Belt) out beyond the orbit ...

21

Correct. If you split the earth up into spherical shells, then the gravity from the shells "above" you cancels out, and you only feel the shells "below" you. When you are in the middle there is nothing "below" you. Refrence from Wikipedia Gauss & Shell Theorem. {I am using some simplistic terms, but I don't want to break out surface integrals and ...

17

A "Trojan" object is any smaller object that shares the same orbit as a larger body but leads or trails it by about 60 degrees in the orbit. These positions are the L4 and L5 Lagrange points (respectively) in the larger body's orbit about its parent object. The L4 and L5 Lagrange points are locations of stable gravitational equallibrium between the larger ...

16

The simplest way to think about it is that there is mass all around you in the center of the Earth so you get an equal gravitational "pull" from all directions. The pulls cancel out so you get no acceleration. If one assumes constant density for the Earth (which isn't strictly speaking true but it is close enough for this illustration) the gravitational ...

16

I think you are confused as to what the 'surface' of Jupiter or Saturn are. They have a large liquid hydrogen centre, but this is surrounded by an incredibly thick layer of atmosphere, which has clouds, gases, liquids etc. So you would first pass through the outer layers of atmosphere, falling through denser and denser gas until you float at a height which ...

16

There are a few things that keep Saturn's rings roughly the way they are. First, Saturn's D ring actually is "raining" down on Saturn currently. But, the phenomenon of shepherd moons prevents the vast majority of material from leaving the other rings: "The gravity of shepherd moons serves to maintain a sharply defined edge to the ring; material that ...

15

The leading theory is that at a distant point in its past, Uranus was struck by a very large object, which knocked it to its side, and current tilt. Imagine if you took a top, and smacked it with a rock. The top might be turning perfectly alright at first, but after it had been hit, the top would most likely be wobbling significantly. Similarly, after an ...

15

No, not by jumping. Jumping gives you an acceleration only from the location on the surface. As soon as you leave the surface, you have no way of adjusting your orbit. Either you reach escape velocity, or you will return to your initial location after exactly one orbit. The only way to prevent this would be to have an additional acceleration once you ...

14

This is a gravitational phenomenon known as tidal lock. It is closely related to the phenomenon of tides on Earth, hence the name. Tidal locking is an effect caused by the gravitational gradient from the near side to the far side of the moon. (That is, the continuous variation of the gravitational field strength across the Moon.) The end result is that the ...

14

To some extent the universe exhibits something called self-organized criticality where a dynamic, non-linear system with many degrees of freedom (the gas after the Big Bang but before the emergence of structure) eventually forms a system with a notable degree of scale invariance (moons orbiting planets, planets orbiting stars, stars orbiting galactic ...

14

The angular size of the object can be calculated by basic trigonometry: $\theta=2\cdot \arctan(r/d)$, where $r$ is the radius of the object you're viewing, and $d$ is the distance between you and the object ($\theta$ is the angle). The average (volumetric) radius of Saturn is 58,232 km. The distance between Titan and Saturn is 1,221,830 km. Plugging the ...

13

A circle is a very difficult shape to maintain. Even the slightest deviation, and a circle is bypassed. Orbits are elliptical when any of the following things happen: Another object strikes the planet in such a way to change its orbit. It would have to be massive compared to the primary object, at least a sizable fraction. Gravitational interaction with ...

13

There are a lot of factors that go into whether or not a planet has an atmosphere. First, the mass and size of the planet. Really what it comes down to is the escape velocity. The higher the escape velocity (ve), the easier it is for a planet (or moon) to retain any atmosphere it gets as the gases that make up the atmosphere have to be moving faster to ...

13

@dmckee guessed correctly. From An excerpt from an address delivered before Section A of the American Association for the Advancement of Science, on August 23, 1882, by Prof. Win. Harkness, Chairman of the Section, and Vice President of the Association: (ref) He was destitute of what would now be regarded as the commonest instruments. The invention of ...

13

OK, I tried to do the math here. Something remotely resembling maths, at least. Assumptions: It is possible to reach an orbital/horizontal speed of $v_O = 5\textrm{ ms}^{-1}$, for example by running. The density of the object to orbit is similar to Earth's density, i.e. $\rho = 5500\textrm{ kgm}^{-3}$. We want to orbit at a height of $2\textrm{ m}$ above ...

12

The answer is absolutely not. There is nothing interesting/important/rare/weird/abnormal/whatever astronomically happening in December 2012. I have written at length on this topic and a year ago summarized my posts on the subject. In those, I've covered the vast majority of 2012 doomsday or whatever claims. Besides that, though, 2012 is likely not the ...

11

There are 5 points relative to an orbiting body in a mostly circular orbit which are gravitationally stable, meaning that a small body placed in such a location would remain there. These are called Lagrangian points. There are 3 such points along the axis between the planet and star called L1 (between), L2 (behind the smaller body), and L3 (opposite the ...

11

There are two elements to why the universe appears to be so orderly: the physical laws of that govern the universe are the same everywhere, and astronomical objects are very, very, very far from each other. Consider two objects, one much larger than the other, and both very far from anything else. Because of gravity (which works the same everywhere), the ...

10

Begin by imagining that the moon isn't quite a perfect sphere. One side is just a little bigger than the other. As the moon rotates, the heavier face will swing around towards the earth a little faster, and it will swing away from the earth a little slower, since it feels a stronger gravitational attraction via its larger mass. Since gravity is a ...

10

Habitable by whom? There are conditions that are uninhabitable by humans, however, many "extremophiles" survive perfectly happy. Although, if you are talking about humans, here is a small list (all the rest are probably more "nice to have" requisites): Approximately 20% oxygen (more or less depending on the pressure) Temperatures that allow for liquid ...

10

I don't think the situation you mentioned is possible. You're describing two planets, each of which being in each others L3 points (a lagrangian point is a point in an orbit with special gravitational properties, where an object will remain somewhat stationary relative to the body whose orbit it's in). Even our comparatively tiny spacecraft which sit in ...

10

The definition of planet set in 2006 by the International Astronomical Union (IAU) states that, in the Solar System, a planet is a celestial body which: Is in orbit around the Sun, Has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and Has "cleared the neighbourhood" around its orbit. A non-satellite body ...

10

Officially, no -- but there is a weak case to be made that the Moon orbits the Sun rather than the Earth. If you trace the Moon's path in a Sun-centric frame of reference, that path is completely convex. Quoting this Wikipedia article: Unlike most other moons in the Solar System, the trajectory of the Moon is very similar to that of its planet. The ...

10

If they're sitting still, and are very bright, they are planets. Install Stellarium on a computer or a smartphone. First time you run it on a computer, enter your location in the settings (no need to do that again after the first time); on the smartphone, it deduces the location automatically each time. The program will show you what planets are visible at ...

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