How do we know what the level of gravity is on other planets without having been there and/or landed rovers.
In: Earth Science
Gravity is a force two massive objects exert on each other (yes you also pull the earth towards you it’s just that the earth is roughly 10²³ times, read a 1 with 23 zeros, heavier and thus it’s pull is A LOT stronger than yours). This is true for large and small objects though it’s a rather weak force if you consider how massive the earth is and how you could still jump.
As a formula gravity is given by F = – gamma * m*M /r²
F is the force (gravity), gamma is a constant value that you can calculate once and that stays the same no matter the objects you’re comparing. Small m is the mass of the small object, and large M the mass of the bigger object and r is the distance between the two, it’s called r for radius, because the force works in all directions hence points on a circle with the same distance towards the center of mass experience an equal amount of force, but as we just look at one object it’s basically the distance between the two centers of mass. Oh and the minus sign is just there as a direction, in that case it’s an attractive force that pulls towards instead of pushing away (positive sign).
So what you could do is just look at the trajectory of the planet compared to other huge objects of known mass and distance, like the sun or one of it’s moons and so on. If you know one of these masses and the force you can calculate the other mass.
And so if you have the mass of that planet you essentially assume the center of mass if at the core of the planet (sounds reasonable for a ball shaped object), making r the distance between the core and it’s surface or the radius of that ball. The mass of your object (in that case the rover can be measured on earth). And so you got all you need.
Now as the rover is mostly staying on the surface and as the planet is likely not gaining or losing mass, you can also do what we do with the formula on earth and simplify it by putting gamma, M/r² all in one new constant called **g** and simply say F = m*g. And then you compare g_earth to g_mars for example and you can say how bigger or smaller it is.
For planets that have moons, the distance of a moon from the planet and the time it takes the moon to orbit give us a very accurate measure of the planets’ mass and gravity. If you want to know the acceleration due to gravity on a planet’s surface, you also need to know the size of the planet.
Mercury and Venus have no moons so their gravity was harder to work out. Initially their mass estimates were based on guessing their density based on what we though they were made of, and also knowing their size. Later we could do better by measuring the slight gravitational effect they had on each other and on the earth. Really good measurements did have to wait until we sent spacecraft there.
Gravity is a function of mass, and we can work out the mass of a distant star using the laws of relativity – by measuring the deflection of light passing by it. The bigger the mass, the more light (well, spacetime, in fact) will be “bent” by it.
Although gravity is classed as a _weak force_, its effects can be felt in a very large area. Simply by observing how said planets affect their moons, meteors, comets etc, we can estimate the gravitational pull of said planets
This estimation may not be 100% accurate, but with continued observations we can get very close
For distant stars and black holes etc, we are too far away to observe how they are affecting planets and other astronomical objects in their vicinity. Here we rely on observations of how these distant stars and black holes bend light – gravity bends light, and the more the bend, the higher the gravity
For many planets in the solar system we can get a good measurement by observing how fast their moons orbit, so we have had a good understanding of the gravity of Jupiter, Saturn, Mars etc. for hundreds of years. Where there is no moon it’s quite a lot harder but we were able make some guesses based on how big they are and what they seem to be made of. As soon as we can get a probe to fly by, we get a direct measurement by how much the probe’s trajectory is changed.*
For extrasolar planets (those around other stars), we can get a measurement of a planet’s mass by how much it wobbles its star as it goes around. In fact, for many extrasolar planets this is one of the few things we know: its mass, its orbital period and orbital radius. We’ve been able to measure the wobble of the star by the subtle red and blue shifts of its light as it wobbles towards us and away from us. More recently we’ve been able to directly observe their wobbles across the sky.
*Edit: Also in theory maybe by observing comets making close passes – although I don’t remember if anyone was ever able to do this in the pre-spaceprobe age.
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