Hot answers tagged

61

Sure, let's do the order of magnitude calculation! According to Wikipedia: In cool, dense regions of the ISM, matter is primarily in molecular form and reaches number densities of $10^6$ molecules per $\text{cm}^3$. In hot, diffuse regions of the ISM, matter is primarily ionized, and the density is [$10^{−4}$ to $10^{-2}$] ions per $\text{cm}^3$. Compare ...


30

Even on the shadowed face there are other sources of radiation that will control the temperature of the bar. For example, on the example that you read, the earth itself will radiate! You may think that if you remove the Earth and other bodies floating around, including the Sun then the temperature will drop down to absolute zero (0 K). But even then the bar ...


17

Strictly speaking vacuum is the state of lowest energy. That means no matter or radiation (photons or any other particles). Note that space is not a perfect vacuum. Also note that, technically, a gas of planets and comets etc. has a pressure (there is usually little reason to care about it though). There is also radiation pressure due to the photons. ...


14

Let's start by assuming you're in the shade, so you're not receiving any radiation (apart from the cosmic microwave background, which I think we can ignore). The amount of heat per unit area that you radiate is given by Stefan's law: $$ J = \varepsilon \sigma T^4 \tag{1} $$ The emissivity of human skin is allegedly 0.98, and the area of skin of an adult ...


14

$10^{26}$ kg is about 5% of the mass of Jupiter and much larger than the mass of the Earth. Gravitational contraction would ensure that your mass of "mud" would indeed form a (roughly) spherical planet. The timescale upon which it does so is entirely dependent on what sort of radius you began with. The "dynamical" freefall timescale is given approximately by ...


13

1) Most materials you use in everyday life contain far more moisture than you might believe. This is a major reason materials meant to be exposed to space are specially designed and tested. In a general vacuum, most fabrics and many plastic will outgas - all of the absorbed moisture and oils will work their way to the surface and boil off - which is a major ...


12

The container on Earth will be cooled by convection currents i.e. it transfers heat to the air around it, and also by black body radiation. By contrast the container in space can only cool by black body radiation, and obviously it will cool down more slowly. You can calculate the cooling in space using the Stefan-Boltzmann law assuming you know the ...


11

The intensity of the light from the Sun at the orbit of the Earth is around 1.4 kilowatts per square metre. For comparison, a domestic heater is usually around 3 kW, so a satellite with a 2m surface area (admittedly this is bigger than most satellites) facing the Sun needs to dissipate as much energy as used to heat your living room. This is in addition to ...


11

I prefer to think of it that the Earth and Sun actually orbit around their combined center of mass, which just so happens to be very deep inside the sun. The same can be said for the Earth-Moon system.


10

No, the "skipping" effect does not work like this. It is not like a stone skipping on water. The problem is that before re-entry the capsule is travelling at a high velocity relative to the Earth and this means it is in a highly elliptical orbit. Actually it's in something approximating a Hohmann transfer orbit. If you did nothing to change this velocity ...


9

Planets made mostly of water almost certainly exist, and at least one may have been detected. However, such bodies will not be made entirely of liquid. In fact they will be mostly solid, even if the temperature is very high. This is because water can form more than one form of ice. The kind we're familiar with forms at low temperatures and is less dense ...


9

I don't understand why we are able to see and measure curvature / warping of space at all. The Earth's surface is curved and this can be observed via the vast number of pictures of the Earth from space that now exist. However, the surface curvature can also be "seen" via measurements on the surface itself. For example, if one were start at the North ...


9

Unlike most depictions that you see in movies or heard of maybe, the human body can actually maintain its stability for a short while. I am not sure of exactly how long it takes for a permanent injury to occur or swellings to start appearing, but what is known is that there's no immediate injury. Meaning you do not explode, your blood doesn't boil, nor does ...


8

ZPM isn't the full answer. A combination of gyro and thrusters are used. Primarily they use Control Moment Gyroscopes (CMG) located in the Unity Module. Secondary options with more thrust are the Russian Control Thrusters on both Progress and Zvezda (means star) modules. The CMG's are quite heavy gyroscopes at about 600 lbs each. Inside the black casing ...


8

People don't immediately compress because the body is more or less a pressure vessel. It's not a very good pressure vessel for dealing with vacuum, but it's something. It's your body's resistance to pressure that lets you do things like spray bodily fluids (from your mouth, or from your bladder, or from your arteries). When my wife was in labor with my son, ...


8

I don't think the question can be answered because you don't say how the orbital energy is to be dissipated. However it's quite interesting to compare the orbital energy with the energy required to boil the ice. Let's suppose our ice supplied is aboard the International Space Station, so they are at an altitude of $h = 300\ \mathrm{km}$ and moving at an ...


8

Reaction wheels, momentum wheels, and control moment gyros are three somewhat distinct ways of controlling the rotation and orientation of a spacecraft. Reaction wheels are the easiest to understand, at least in their simplest form. Consider a spacecraft such as a space telescope that is nominally not rotating with respect to inertial space. The reaction ...


8

People say the Earth rotates around the sun and not vice versa because a reference frame attached to the center of the Sun more closely approximates an inertial reference frame than a reference frame attached to the center of the Earth. Yes, there is the issue of the path of the other planets, but even if the Earth were the only planet, we would still say ...


8

If you are out of the sunlight, the main source of cooling will be radiation. The amount of net heat you radiate away depends on the temperature of your skin $T_{skin}$ and the ambient temperature of the surrounding environment $T_{ambient}$: $\frac{Q}{t} = e \sigma A (T^{4}_{skin}-T^{4}_{ambient})$ where $Q$ = heat loss in Joules $t$ = time in seconds ...


7

Some instruments would require modifications in order to facilitate playing them. Large instruments would need to be strapped down, and something like a double bass or drums would probably require its player to be strapped into a harness in order to prevent them from pushing themselves off as they played and floating away from the instrument. I can imagine ...


7

There are two phenomena at work here: the inverse-square law and extinction. Picture a sphere centered on a star. At time $t_0$, a star emits $n$ photons, spread in random directions. At some time $t_1$, the photons are arranged in a sphere of radius $R_1$1. At some time $t_2$, the photons are arranged in a sphere of radius $R_2$, where $R_2>R_1$. This ...


7

Light years and parsecs have been used since long before SI existed, so a lot of it is tradition. But using light years also makes it very obvious how long the light has traveled to get here, and thus which era of the universe we are seeing the object in. Something that is 11 billion light years away dates from the era of early galaxies, for example. If you ...


7

Mind if I pick a few nits before telling you why this won't work to make you move? Not hearing any objections, so I'll nitpick away. Note that the center of gravity (center of mass is also a better term) is a point, the axis of spin passes through it but the axis is a line, so avoid saying "the axis is this point"; it just sounds weird. But all of that is ...


6

There is gravity on the moon. The gravity on the moon is about 1/6 the amount we have on Earth, but it is there. Anything with mass, anything has a gravitational field. For something like a baseball, this field is minute enough that you will never notice it. For something like the moon with a mass of 7.3459×10^22 kg (From Wolfram Alpha) the gravitational ...


6

There are two concepts here that may be getting mixed together, namely, conservation of linear momentum and conservation of angular momentum. Newtons laws state that an object in motion will say in motion unless acted upon by an external force. So unless interstellar friction is a problem, the spaceship will keep travelling linearly in the same direction ...


6

A good way to get some perspective about a lightyear is to scale everything down to everyday lengths. Start with the definition of a lightyear: it is the distance that light travels in 1 Julian year of 365.25 days = 31,557,600 seconds. This picture shows the distance between the Earth and the Moon to scale. It would take light 1.29 seconds to cross this ...


6

There is no day and night in space! Day and night on earth are defined as times when the earth is whether we are facing towards the sun or opposite to the sun. When we are facing the sun, the light from the sun scatters due to the atmosphere and makes the sky appear blue. On moon there is very little atmosphere, so even during the day, the sky appears ...


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