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32 votes
Accepted

Multiply Earth's orbital speed with square root of 2

It is the Solar System "escape speed" for an object in orbit around the Sun, at the distance of the Earth. The energy of an object in orbit is the sum of its kinetic and potential energy. ...
ProfRob's user avatar
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29 votes
Accepted

Why did the dust between the planets disappear during the birth of the solar system?

Dust happens in two ways. "Primordial dust" just condenses out of the protostellar material in the disc providing it gets cool enough and dense enough. "Second generation" dust is ...
ProfRob's user avatar
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28 votes

Does the Oort cloud have any large holes in it and if not does this mean nothing large has entered the solar system for a long time?

The Oort Cloud has not been directly observed. The Oort cloud is still theoretical. We're pretty sure it's there, but we don't have direct evidence. The objects that make it up are too small, too dark,...
Darth Pseudonym's user avatar
24 votes

Is it coincidence that the earth's rotation and revolution are in the same direction?

Not a coincidence, most other planets rotate the same way. It is a consequence of how planets were formed from a disc of gas and dust that was orbiting around the sun as it formed. The sun formed ...
James K's user avatar
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18 votes

What would happen if Venus and Earth collided?

The outcome of a Venus - Earth collision depends on the impact speed. At a relatively low speed, the matter is likely to condense to form a new planet. But if the speed is high enough the planets will ...
PM 2Ring's user avatar
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18 votes

What would happen if Venus and Earth collided?

This is the kind of thing that computer simulations are done for, but I am not aware of any that were specifically for a Venus-sized body hitting the earth. The specific details would depend very much ...
Mark Foskey's user avatar
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18 votes

Unreasonable ratios in astronomy as ways of remembering

The angular size of the Moon and the Sun, as viewed from the Earth is pretty much exactly $1/1$ with slight variations due to the eccentricity of the Moons orbit. Another coincidence is that the ...
ScienceSnake's user avatar
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17 votes
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Why is Mercury's orbit so unusual?

Mercury appears to be on the boundary between being stable and unstable in the Solar System (e.g. Lithwick & Wu 2011). Its orbit can be highly perturbed by interactions, principally with Venus and ...
ProfRob's user avatar
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17 votes
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Is it coincidence that the earth's rotation and revolution are in the same direction?

I think it is coincidence, apart from that there may be anthropic arguments for the spin axis inclination (the difference between the spin and orbital axes) to be small but non-zero. i.e. A small, but ...
ProfRob's user avatar
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16 votes

How close will Pluto come to Earth this year (2024)?

As far as I see, the separation between Earth and Pluto this year is always bigger than 34.045586 AU (5,093,147,172 km). This minimum separation is reached on 20th July 2024 around 21h UTC checked to ...
planetmaker's user avatar
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14 votes

Would be possible to detect Planet Nine as a microlensing event along the line of its projected orbit?

The planet would not be able to micro-lense light from distant stars, but it could in principle obscure a star along its path. In order for gravitational lensing to work, the light path from the ...
planetmaker's user avatar
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12 votes

Multiply Earth's orbital speed with square root of 2

This speed is the escape from the solar system speed, from the orbit of the Earth. If you had a rocket, and it was orbiting the sun at the same distance as the Earth it would be travelling at 29.7 km/...
James K's user avatar
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11 votes
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What's the reason for the hollow region in Oort cloud?

This is a fascinating question and I do not think there is yet an absolutely agreed consensus even on the likely distribution of the Oort cloud objects and it isn't clear to me how the pictures you ...
ProfRob's user avatar
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10 votes
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Voyager spacecraft distance from Earth

We can access trajectory data for the Voyagers via the Horizons system of JPL. For various reasons, Horizons didn't have great trajectory data for these spacecraft. However, in 2022, R. Jacobson (...
PM 2Ring's user avatar
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10 votes

How close will Pluto come to Earth this year (2024)?

In the interest of comparison, here is a small Python script to compute the moment of closest approach of Pluto to the geocenter using astropy.coordinates.get_body()...
Roy Smart's user avatar
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9 votes

What determines the edge/boundary of a star system?

I don't think there is a formal definition, but a working one would be that the system is gravitationally bound. This does impose an outer edge because of the tidal field of the Galaxy and other ...
ProfRob's user avatar
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9 votes

How is a Planet's Moment of Inertia Measured Remotely?

I dug around quite a bit and was unable to find anyone making measurements of the type that would lead one to determine the moment of inertia of Uranus or Neptune without first making assumptions ...
Paul T.'s user avatar
  • 727
9 votes
Accepted

How did the temperature of the solar system evolve?

There is no "nuclear flash". The approach to a stable, hydrogen-burning star is rather smooth and in fact the Sun was more luminous during its first few million years as a protostar than it ...
ProfRob's user avatar
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9 votes

Unreasonable ratios in astronomy as ways of remembering

1 km/s is roughly 1 pc per million years. This is roughly a consequence of there being $\pi \times 10^7$ seconds in a year and roughly $\pi$ light years in a parsec. Useful.
ProfRob's user avatar
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8 votes

What determines the edge/boundary of a star system?

Additional, or rather complementary, to the important 'gravitationally bound' criterion which applies to solid bodies like comets, asteroids and other outer solar system small bodies, one can go a ...
planetmaker's user avatar
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8 votes
Accepted

Would be possible to detect Planet Nine as a microlensing event along the line of its projected orbit?

With the caveat that it is easy to make an arithmetic blunder, I calculate that there is around a 1 in 25 chance per 6-month interval that planet 9 might cause a significant microlensing amplification ...
ProfRob's user avatar
  • 155k
8 votes

What happens to planets as they cross the Galactic equator?

Nothing whatsoever. To be honest I find it a little hard to imagine why you think anything could happen. The galactic equator is a plane that is roughly in the middle of the disc of stars that compose ...
James K's user avatar
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7 votes
Accepted

Does the Sun have different unique orbits around barycenters?

There's only one barycenter for the solar system, which includes the impact from every planet, asteroid, comet, and space probe in the solar system. The path the sun takes around that barycenter is ...
Darth Pseudonym's user avatar
7 votes

Difference in results between JPL Horizons and cspice (rust-spice)

The issue is with how you have configured HORIZONS to show the output. You have apparent RA, Dec selected as output columns which includes the precession and nutation, in addition to the light time ...
astrosnapper's user avatar
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7 votes
Accepted

Would it be practical to map out the Kuiper Belt and Oort Cloud via Radar?

An answer can perhaps be given in terms of the capabilities of the now defunct Arecibo radar facility. This was the most powerful radar system, as summarised in Venditti et al. (2023). Arecibo had a ...
ProfRob's user avatar
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6 votes

A solar system of solar systems

You don't have to look very far to find such a system. Proxima Centauri, the closest star apart from our Sun, has at least two exoplanets orbiting it, while this red dwarf star itself orbits a ...
Oscar Lanzi's user avatar
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6 votes

Does the Oort Cloud lose mass with time?

The Oort cloud objects are only very loosely bound to the Sun. That is, their total energy (kinetic plus gravitational potential) is only just negative. That means you only need a slightly positive ...
ProfRob's user avatar
  • 155k
6 votes

Does the Oort cloud have any large holes in it and if not does this mean nothing large has entered the solar system for a long time?

Maybe, but not likely. Let's look at something similar - the Asteroid Belt. The Asteroid Belt between Mars and Jupiter (not really "between" since some of it extends past Jupiter, and ...
MikeP's user avatar
  • 161
6 votes
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How much mass did the Late Heavy Bombardment add to Mars?

Alessandro Morbidelli has kindly consulted me on this topic. He said: "The late veneer on Mars is approximately equivalent to that of the Earth, from the abundance of highly siderophile elements ...
Michael_1812's user avatar
  • 1,456
6 votes

How close will Pluto come to Earth this year (2024)?

Distance and time are not simple concepts in astrodynamics! Checking Horizons, I found that if we calculate vectors to Pluto using Earth's geocenter as the origin, we can get several answers. First, ...
uhoh's user avatar
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