94 votes
Accepted

Why would space fleets be aligned?

The crews have evolved on worlds with orientation, so their psychology and conventions reflect the reality that they are planetary creatures. Orson Scott Card captured the importance of orientation ...
  • 9,662
65 votes

Killing a star safely

Problem: even if you could just stick a blanket over the sun, it is probably already too late. The solar system formed more than 4 billion years ago, and for all that time anyone who was watching and ...
64 votes
Accepted

Is stealing the moon actually possible?

Plans for stealing the moon. Shrink it Well, this seems like the obvious solution. Just invent a shrink ray, zap the moon so it becomes the size of a basketball, then carry it home with you and ...
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42 votes

Could gravitational lensing be used to protect a spaceship from a laser?

There are several problems with this. First of all, when someone fires a laser at you, you aren't going to know it until it hits you, so this would only work if Ship A were CONTINUOUSLY creating a ...
38 votes
Accepted

How much damage would a cupful of neutron star matter do to the Earth?

This is more or less my best guess. We're talking about a mass of about one hundred billion tons, composed of neutrons, previously held together by a terrifying gravitational field - and now ...
  • 52.5k
37 votes
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A planet illuminated by a black hole?

This scenario is quite problematic for two main reasons: evaporation and peak wavelength. The black hole's lifetime is too short We can make a rough estimate of the properties of the Hawking radiation ...
  • 97.8k
35 votes

Is stealing the moon actually possible?

It's perfectly easy to steal the moon. Just register a claim on it in every country and start issuing legal notices. It's been done before: Spanish woman claims ownership of the sun
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35 votes
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Black hole at the center of the planet

Your planet needs to be about 722500 times more massive than Earth for its core to undergo collapse into a black hole. Leaving aside the small detail that at this point your "planet" would ...
35 votes

A 1 kilometre wide sphere of U-235 appears in an orbit around our planet. What happens?

Normally, only the innermost kilogram of uranium in an implosion-type bomb actually undergoes fission (and only 0.6 to about 5g of mass are ultimately converted to energy - the sources don't agree on ...
  • 52.5k
33 votes

How can solar sailed ships be protected from space debris?

Generously spread the sail surface with self repair nanobots. Any impact with dust or sub-dust size object (which is more likely to happen) will pierce a hole through the sail (and leave a cloud of ...
  • 248k
32 votes
Accepted

What elements are useful for humans, but rare in our galaxy?

People need boron, because plants need boron. https://en.wikipedia.org/wiki/Boron Boron is a chemical element with the symbol B and atomic number 5. Produced entirely by cosmic ray spallation and ...
  • 287k
32 votes
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Making A Geosynchronous Orbit Impossible

Easy: Put a moon right above geostationary orbit. If you have a 24h day, and a moon that takes 25h to circle the planet, any satellite orbit at geostationary height would be totally unstable. The ...
31 votes

Why would space fleets be aligned?

In our world When you watch a movie, the ships are usually aligned. This is because of reasons that are most likely not based on our experience in space battle. I'm pretty sure I read about this and I ...
  • 4,544
30 votes

How much damage would a cupful of neutron star matter do to the Earth?

The answer isn't entirely clear what the final state of the Neutron Star matter would be, but it would most definitely completely destroy the "Totally Normal Office Building", and most of the country.....
  • 5,552
29 votes

How can solar sailed ships be protected from space debris?

A tiny piece of space debris is dangerous to the ship, as it may hit people, mechanisms, or fuel. But the damage to the hull itself would be negligible. It could simply be patched. The hull is the ...
  • 25k
29 votes

If Earth had a smaller radius, but was rotating at the same speed, would its day be shorter?

The length of the day is strictly a function of the time earth takes to rotate. If your planet takes 24 hours to rotate then regardless of its radius, it will have a 24 hour day. You can develop an ...
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28 votes
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How massive would a planet need to be to sustain negligible damage from impact with the Earth?

That really depends on what you consider 'negligible'. Is it 'Sterilization of all life, but planet is still there in one piece in the same orbit'? Or is it 'Everyone in the direct impact zone gets ...
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27 votes
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Could the moon crash into the earth if we colonised it and increased its mass?

As mentioned in comments, I don't have the full stability answer to hand (although see edit below). But I do have a practical answer. The practical answer is that no feasible human effort could ...
27 votes

Black hole at the center of the planet

No A natural occurring black hole that comes into existence due to mass collapsing onto itself must have more mass than the Tolman–Oppenheimer–Volkoff limit, which has been estimated to be around 2.17 ...
27 votes

Earth is accelerated out of the solar system - do we keep the Moon?

You're going to lose the Moon. At the Moon's current distance, the Earth's gravity can only change its velocity by $0.002m/s^2$ The mentioned acceleration of about $0.25m/s^2$ dwarfs that, and if at ...
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26 votes
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What is the minimum size for the Sun?

Black body radiation The Sun is, approximately, a black body. That means that the light it emits follows a particular spectrum according to Planck's law, with the shape of the spectrum determined ...
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24 votes
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Moons that can't see each other

In theory if the two moons were in the exact same orbit on opposite sides of the planet then yes. Having the moons closer to the planet and smaller also makes that easier. For example geostationary ...
  • 76.2k
23 votes

Killing a star safely

A star shines because it has mass... You put enough mass together, it gets a dense core, heats up, and voila, solar fusion. Yes, that's an oversimplification, but, fundamentally, making a star not ...
  • 25k
22 votes

Is stealing the moon actually possible?

You can't move the Moon, it just requires too much energy. But maybe we can deny everyone else the Moon... and ALL OF SPACE!!! Energy Required To Move the Moon out of the Earth's Orbit Let's say "...
  • 29.8k
20 votes

A planet illuminated by a black hole?

From Hawking radiation? No. The Hawking radiation emitted is inversely proportional to the black hole's size. To make the black hole glow with enough light to be as bright as a star from Hawking ...
  • 5,631
20 votes

What elements are useful for humans, but rare in our galaxy?

Lithium, like boron (covered in another answer), is relatively rare because few processes have produced it since the Big Bang created a tiny percentage -- but it's useful both as a chemical and as a ...
20 votes

Where could we add a new planet to the inner solar system?

There is a book about the possibility of human habitable exo planets around other stars. Habitable Planets for Man, Stephen H. Dole, 1964. https://www.rand.org/content/dam/rand/pubs/commercial_books/...
19 votes
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Aliens to play a mischievous space-time practical joke on Earth

Nothing we know of could achieve that. There are two things that distort time: velocity and gravity 18 minutes per 24 hours is a dilation factor of 1.0125. ($\frac{24hours + 18minutes}{24 hours} = 1....
19 votes

Why would space fleets be aligned?

Are you asking from the POV of TV shows and Movies? TV shows and movies always show fleets approaching one another in the same orientation and on the same plane. Why? For the convenience of the ...
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19 votes
Accepted

Could a planet's tidal forces break an asteroid passing very close to it?

Yes, what you are asking is exactly the definition of the Roche limit In celestial mechanics, the Roche limit, also called Roche radius, is the distance from a celestial body within which a second ...
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