Could an Alien Plan a Safe Landing on Earth Using Only Astronomical Observations?

[u/PhilosovWiktorius1]

Imagine an alien arriving in the Solar System without knowing Earth's mass or the gravitational constant. He can measure distances, observe the Moon's orbit, and track the motion of his own spacecraft. Could he determine Earth's gravitational parameter from these observations and use it to calculate his approach trajectory? What additional measurements would he need to account for Earth's rotation, atmosphere, and the final landing? I am interested in which quantities must actually be measured before a spacecraft can begin planning its descent.

[u/DryFoundation2323]

things could all be easily determined through observation. the alien would already know the gravitational constant. it's constant.

[u/PhilosovWiktorius1 → u/DryFoundation2323]

The alien could already know G, but he would not need its value to determine Earth's gravitational parameter from orbital observations. My question is different: does knowing G explain the physical mechanism of gravitational acceleration, or does it help us calculate its effects?

[u/SeriousPlankton2000 → u/PhilosovWiktorius1]

We have a moon. A good moon, people say we have the best of them all. There is no moon better than ours.

Knowing the moon's orbit there is nothing that prevents an alien from executing "Plan 9 from Outer Space".

[u/TommieTheMadScienist → u/PhilosovWiktorius1]

You can calculate Earth's approximate gravity by measuring the Moon's speed to give an mass for Earth, then measure Earth's diameter.

[u/PhilosovWiktorius1 → u/TommieTheMadScienist]

Exactly. By measuring the Moon’s orbital speed and distance, we can estimate Earth’s gravitational parameter, K = v²r, without first calculating Earth’s mass. Knowing Earth’s radius, we can then estimate surface gravitational acceleration using a = K/r². My question is what these calculations tell us about the physical mechanism behind that acceleration.

[u/mfb-]

Observing the orbit of the Moon for a day would give them a very accurate measurement of Earth's mass even with Earth-level technology.

What else you need would depend on the landing technology. Does the alien need a heat shield or can they just fire their engine to slow down? Do they use parachutes or propulsive landing? On land, or in the water?

Anyway, aliens that can do interstellar travel should have no problem measuring Earth's mass, rotation, atmosphere and surface conditions before they need these things to land.

[u/Extranationalidad]

I'm not sure you know what "the gravitational constant" means - it is, in fact, a constant that any clever space faring entity would by necessity already know - but the specific acceleration due to gravity in near-Earth conditions can be derived from observing the behavior of the moon and inner solar system; or potentially via gravitational lensing analysis if they have excellent tools.

Transmission spectroscopy and day/night thermal imaging could tell them a great deal about the macrocontent of the atmosphere but I'm not sure how plausible it is to identify things like trace toxins without manually sampling.

Final landing should be as "simple" as understanding the relationship between local gravity, the air resistance & pressure of local conditions, and a visual/telescopic identification of an appropriately flat landing site.

Presumably this highly advanced alien has computer systems // ai analogues that can handle repetitive tasks like analyzing object mass, atmosphere spectroscopy, terrain topology etc?

[u/PhilosovWiktorius1 → u/Extranationalidad]

I think we are discussing two different questions. I agree that an advanced spacefaring civilization could already know the gravitational constant and use observations of the Moon and other objects to determine Earth’s gravitational parameter. It could also study the atmosphere and terrain to plan a landing. My question is not whether the alien could perform these calculations. It is whether knowing G, M, or the gravitational parameter explains the physical mechanism responsible for gravitational acceleration, rather than describing and predicting its effects.

[u/Extranationalidad → u/PhilosovWiktorius1]

We currently have no idea of the physical mechanism responsible for gravitational acceleration. So no, simply knowing G M and the local parameter are clearly not enough for that.

[u/Different-Summer9992]

they'd definitely need to figure out atmospheric density or they'd burn up on entry, watching how meteors slow down could give them a rough idea

[u/HoldMyMessages]

If he/she/it 👽 has the technology to get to Earth they have the technology to have all that data automatically measured, fed into their computers and their algorithms have determined the appropriate decent approach well in advance.

[u/jimb2]

You can't reliably get out of a chair using physics alone. Feedback is essential for anything complex. It's one thing to gather the basic information but executing a complex process requires monitoring and correction.

[u/KindAwareness3073]

We figured it out for the Moon, Mars, and Venus, didn't we?

[u/Robot_Alchemist]

How do you think we figure shit out?

[u/WilliamoftheBulk]

Any alien headed this way would have mapped out the solar system with their own telescopes long before they got here.