131

We can breathe pure oxygen for unlimited time if the pressure is not too high; about 0.4 bar is okay. Breathing pure oxygen at 1 bar is possible for some hours, but a longer time may damage the lungs. A mix of oxygen and helium is also breathable and is used for deep diving. Xenon cannot be used due to its narcotic effect on the body. Argon is less ...


107

(*) Jupiter, for all intents and purposes, doesn't have a solid surface to stand on. Not any more than you could say that Earth's atmosphere has it, before you hit Terra Firma. It's an enormous ball composed of mostly Hydrogen and Helium, but also other heavier elements in smaller parts, and it's so massive that its own gravity compresses these gases into ...


92

Given a pair of objects that are gravitationally bound to each other, they will orbit around their common barycenter (center of mass of the system). The object to be most logically deemed the moon will be the one of lesser mass because it will be further from the barycenter than its companion. For example, Pluto has a gravitationally bound companion named ...


82

You're observing shamefully bad journalism. The "protect Earth from aliens" bullet point in the "Highlights" section of the article was put there by an editor who either ignorantly or willfully distorted the actual role of the Planetary Protection Officer. The first paragraph of the story gets a little closer: The full-time role of "planetary ...


72

Gravity isn't just about mass, but about distance, too. Our moon has a surface gravity of about 1/6th of Earth, because it is small and less dense than the Earth is. Surface gravity of a body is inversely proportional to the square of its radius, holding mass constant. That means that if you compressed the moon such that it was $\frac{1}{\sqrt{6}}$th of its ...


50

The "gravity of Mars" is not a number but rather a complex field. The most recent is remarkably detailed, made up to spherical harmonics degree and order 120, described by 29,512 coefficients: These maps are made using orbiters (three orbiters in this case), not landers. A lander/rover can give just one local gravitational acceleration and direction, which ...


50

Starting out from Earth, you have the free 30 km/s from Earth's movement around the Sun, which is in the plane of the ecliptic. To get far out of the plane you either have to boost a similar amount "up" or "down" (which is beyond the capability of current rockets) or go via one of the gas giants, and use its gravity to change course. So at least to start ...


39

In July 2005, the Deep Impact mission released an impactor that excavated a crater, estimated to be 100 meters wide and 30 meters deep, into comet Tempel 1.


37

We've had 5 flyby missions to the outer solar system so far. All of them had primary missions at one or more planets. That set the main constraints for their trajectories. Anything after the last planetary encounter was secondary. For Voyager 2, for instance, the Neptune flyby was aimed at a close encounter with Triton, which reduced the possible exit ...


34

Yes, it is. Given two spherical, uniform, bodies one with mass $m_1$ and radius $r_1$ and the other with mass $m_2$ and radius $r_2$, then the surface acceleration due to gravity will be equal when $$r_2 = \sqrt{\frac{m_2}{m_1}} r_1$$ For the Moon to have the same surface gravity as the Earth, we can plug in suitable numbers, and you end up with a radius ...


33

It can keep an atmosphere, and in fact does. The atmosphere is something akin to a high grade Earth-based vacuum. But that's probably not what you are looking for. Okay, so what would happen with, say, an Earth sized atmosphere on the Moon? A lot of really interesting things would happen actually. First of all, in longest days of the night, the atmosphere ...


31

There are a few special things about Pluto, as compared to the other dwarf planets in the Kuiper belt. These include: It is the largest dwarf planet known, by diameter. (Note, this was only determined after the New Horizon's flyby) It orbits relatively close to the Sun, at times it is even closer than Neptune! It has the largest satellite system of any non-...


31

Pluto was discovered by a manual search of the sky using a blink comparator. This is an extremely laborious process. For Pluto, it made sense to go to all this trouble, because there were indications that a ninth planet had to be out there: Neptune's orbit was perturbed by the gravity of another planet, it seemed. Later, it turned out that that wasn't the ...


28

Simple. It was the easiest to land on. Titan has an atmosphere, which makes landing there quite a bit easier than landing on Europa, which does not. In addition, Europa has only been known as an object of interest since Galileo, which was the last mission that even had a chance of sending a lander there. It was suspected as an object in the Voyager flyby, ...


28

It is important to realize that space probes aren't really useful for finding objects in deep space. Space is so empty that a probe sent in a random "exploratory" direction would have a negligible chance of detecting an object orbiting the sun. The best way to find objects outside the ecliptic is to look for them using really large Earth-based or orbital ...


27

Fundamentally, water is water. In its purest form, it is the same anywhere, except perhaps for the isotopes. However, one of the wonderful things about water is the fact that it's a good solvent, and in fact has many things in it that aren't water. For instance, one could not survive off of ocean water: we humans require fresh water. The one potential ...


26

Yes, it is possible. As James K observed in a comment, the surface gravity of Uranus is slightly less than that of Earth, but its mass is 14 times larger. If Earth were orbiting Uranus, it would be a very large moon, but it would still be considered a moon, and thus a moon with a higher surface gravity than its planet. The reason this is possible is that ...


23

The radius of Jupiter and the other gas giants is defined, somewhat arbitrarily, to be the radius at which the atmosphere has a pressure of 1 bar. As your question points out, they had to pick something. So that's what they picked. This convention is used for all of the gas giants in our solar system. For a visible boundary definition, you would need to ...


21

Nobody really knows for sure. And there's two, nay three more odd things about it's rotation on its own axis, namely: It is the only planet in the Solar system that rotates retrograde, i.e. clockwise, when all other planets rotate prograde, or anticlockwise, on their axes, Latest findings (merely a good month ago as of writing this answer) revealed that ...


21

The shielding of the near side of the Moon by the Earth is in all reality quite insignificant, with merely 0.01% of the Moon's sky obstructed by the Earth. If we look at this animation of the speed of light from the Earth to the Moon that is in scale, we can appreciate to what degree the Earth shields it from asteroid impacts:      &...


19

Could they be harvested? Sure. Could they be harvested in an economically profitable way? Probably not, for all the reasons you listed. There's a lot of unknowns to this question — while the chemistry they mention certainly makes sense, there's lots of stuff about the atmosphere we don't know. A harvesting ship would need to find exactly what ...


18

The lineup occurs once every 175 years. The launch window the last time this alignment occurred was from 1976 to 1980 [1], so the next time it would open would be around 2151-2154. Another Voyager-like mission could be mounted at any time really. We have the technology to do it, but the cost of mounting such a mission outside of the Grand tour launch window ...


17

Curiosity photographs thousands of rocks every week. It does that for years. You will find rocks with almost any shape you can think of. This one happens to be somewhat round in one direction. From the shadows and the way it stays on what is apparently a slope, I would infer that it is not a sphere, but is actually ragged on the other side. I'm no geologist ...


17

As called2voyage♦ pointed out in a comment, this looks like a concretion. It is not the first concretion observed on Mars, and it was most likely formed back when Mars had liquid water. More information from an article on Discovery News, dated 24 September: According to MSL scientists based at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., ...


17

The traditional method (as used in the Apollo project) was to crash used SIVB stages into the Moon.


16

It is not coincidence and it does not apply to just the Earth. The Sun, Earth, Jupiter, Saturn, Uranus, and Neptune all have sizable magnetic fields. Mercury and Ganymede have smaller but still noticeable magnetic fields. All of these bodies have one thing in common: They have a sizable amount of rotating, electrically conductive fluid somewhere beneath the ...


16

You are correct about some of the sources of moon quakes. NASA states there are are least four types of moon quakes: There are at least four different kinds of moonquakes: (1) deep moonquakes about 700 km below the surface, probably caused by tides; (2) vibrations from the impact of meteorites; (3) thermal quakes caused by the expansion of the frigid ...


16

There is a map of lunar pits, created by R. V. Wagner and M. S. Robinson of the School of Earth and Space Exploration, Arizona State University, in 2014. From Distribution, Age, and Formation Mechanisms of Lunar Pits (PDF) by mentioned authors: Map of the locations of all currently-known pits. Orange stars indicate mare or highland pits, and blue ...


16

Canals on Mars has quite an interesting history, starting with Giovanni Schiaparell. He produced this map of Mars in 1877 It is interesting to compare this map to a more modern map Note that the same general features are labeled, however, the lines that run between them don't show up at the modern image. Giovanni was Italian, and called the lines on these ...


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