why does predicting things in space (outside the planet) seem to be a lot easier than predicting weather on Earth?
In: Physics
Space is easy; it’s empty. The overwhelming majority of interactions in space are just gravity pulling things around.
Weather is the sum total of the 10^44 molecules in Earth’s atmosphere all bumping into each other. That gets hard to calculate with precision beyond short-term predictions.
So compare a system with 10^44 objects interacting vs. a system with 11 objects interacting (i.e. the Sun, the planets, and whatever individual object you care about).
How predicable something is depends on the number of factors, not the size of the system. In regards to the solar system, there are few bodies large enough to affect the whole and they all follow simple, easy to predict rules (Newtons laws of motion, taught to middle schoolers). To predict the weather perfectly, you’d have to know the location and energy of every single air molecule in the atmosphere, which follows much more complicated fluid dynamic rules (something people put on the level of relativity and quantum mechanics in terms of years of study needed to understand it)
Earth’s enviro-sphere is pretty complex. Interactions between ground temperature, planetary albedo, minutes of sunlight, time of year, snow, water, foliage cover, water temperature, ocean currents, the magnetosphere, ionosphere, im-probably-forgetting-a-sphere, wind speed, air pressure, air _quality_, all are one big non-linear complicated mess that we _think_ we know how to simulate (obviously, our forecasts are pretty good), but probably have only just scratched the surface on. I mean, there’s a reason why more accurate climate and weather change prediction models need to run on supercomputers.
Stuff in space on the other hand, by and large, follows Newtonian physics. At least as far as most practical purposes. An object at motion stays in motion etc. Once something is orbiting something larger, it either continues to orbit, the orbit decays or is perturbed, or something accelerates or perturbs it out of that orbit.
For most purposes, every body in our solar system can be thought to be as part of one or two two-body systems. Earth orbits the Sun. The moon orbits Earth. Apollo capsule orbits the Moon. Is it true that the sun affects the orbit of a capsule in orbit around the Moon? Absolutely it does. So does the Earth. BUT, since gravity is inversely proportional to the distance, the effect of the closer body has a much more significant effect; often to the point that the other bodies’ effects are negligible.
Having said that, we do have a good idea of how say Jupiter’s gravity affects Earth’s orbit around the sun (exactly how and how much of an effect it is I don’t know, but its there), but its impact is somewhat negligible compared to the math of us flying to the Moon or Mars or whatever.
What do you mean by “predicting things in space”?
Weather is mathematically chaotic, which means it outcomes change dramatically with only tiny differences in input. Ergo the ability to predict accurately degrades significantly with increased time.
By comparison motion of objects in our solar system are (for the most part) two-body systems, which are mathematically stable.
There really isn’t any link between these two things, and I have no idea why you’ve decided to group them together in this question. You’ve basically asked why is “hard-thing” hard to do when this completely unrelated “easy-thing” is easy.
There aren’t that many variables in deep space. There’s only a few nearby objects massive enough to exert meaningful gravitational influence and so things move very much like clockwork. Eclipses and transits can be predicted with extreme accuracy many years in advance.
Weather is a much more complicated system driven by fluid dynamics and thermodynamics in gases and liquids on a planetary scale. It’s not a dozen cosmic billiard balls moving in circles, it’s a nearly infinite number of molecules sloshing around unpredictably.
If you throw two balls to a table on Earth it’s fairly easy to predict if they will bump into each other.
If you throw 10 balls to a table it’s not so easy anymore because balls start getting in the way of eachother but you can make a rough estimate.
If you throw a billion balls at the table you can no longer predict anything anymore.
Things in space are super easy to predict. Whatever speed and direction they’re moving right now? They’ll continue to move that speed and direction *forever* (unless they get too close to something else, of course). Simply plot something’s current motion, and now you can predict it for the next billion years.
Weather is a hugely complex system, with lots of unpredictable things that factor into it, and unpredictable results even when you account for the current state of everything. All you can do is make an educated guess based on historical data and your skill.
Those two things are pretty much *exact opposites* in terms of prediction.
In terms of modelling and learning (am a CS guy) space is a simple calculation of a Handful of variables, like momentum/velocity, gravity, etc. Very little in the way of complex interaction, the equations for orbit are pretty simple and can be solved with algebra and or some light calculus. The future state of a spacecraft can be calculated far in advance, barring some deviation like debris or getting hit by something.
Weather on the other hand involves rippling changes throughout the world. Sea temperatures, season, pressures, temperatures, wind speeds etc, which all continually evolve over time and and affect each other. If we use a knot as the analogy, weather is far more tangled. You pull one strand to loosen it but may tighten another. Things that change themselves also change other factors in strange ways. IIRC weather predictions are a fairly complicated differential equation, aka heavier calculus and harder simulations that have to be consistently re-evaluted. Additionally, the farther out the weather prediction is, the less accurate it is. Predicting weather five minutes from now has few variables, but predicting the weather a month from now means predicting every interaction and variation in the system. With spacecraft , again, it’s not unreasonable to plot the entire trajectory to say, the moon, very accurately and in advance.
Hope this helps .
Latest Answers