9 ms·
What are planes but slow satellites on a suborbital trajectory?
by tokai 4y ago
What are planes but slow satellites on a suborbital trajectory?
- ddalex 4y agoPlane needs to burn energy to stay up there, the sattelite just ... sits in a curvature of space time...
- lovich 4y agoGoing down the captain pedant conversation path here, but technically all satellites also need to burn energy to stay in orbit or will eventually fall. The only ones who don’t have achieved escape velocity
- adastra22 4y agoNo, they don’t. In the absence of drag, which only the lowest satellites have, they just stay up there forever. The fuel is needed for orbit changes and correcting drift due to gravitational instabilities.
- drdeca 4y agoAre any of them truly free of drag? Like, are there 0 molecules of atmosphere at some height, or just entirely negligible amounts of atmosphere for all practical purposes?
- denkmoon 4y agoNone. In theory there are parts of space that are a true vacuum but not everywhere, and definitely not near planetary bodies. There's a reason our satellites need station-keeping fuel. https://en.wikipedia.org/wiki/Orbital_station-keeping https://en.wikipedia.org/wiki/Orbital_station-keeping
- lmm 4y agoEven in deep space there are something like two atoms per cubic meter just "there". But atmospheric density drops exponentially with altitude.
- adastra22 4y agoYeah anything above about ~1000km or so is essentially zero drag.
- lovich 4y agoAnd all the dragless satellites around the earth are focusing on documenting as all the frictionless spherical cows on the earth
- DFHippie 4y agoIANAP ("I am not a physicist"), but any two objects in orbit around their common center of gravity are slowly radiating energy into space in the form of gravity waves. This is why LIGO reports its chirps. Of course, this isn't very much energy, but given enough time all should orbits collapse.
- adastra22 4y agoI am a physicist. The gravitational energy loss from planet + satellite scale orbiting bodies is so small as to be orders of magnitude less than, say, the influence of gravitational anomalies like the Himalayas, or the tidal pull of the Moon.
- antognini 4y agoTo further add some (maybe helpful) pedantry, the boundary between an airplane and a satellite is usually taken to be the point at which the velocity required to remain aloft via aerodynamic lift exceeds the orbital velocity if there were no atmosphere.
- Kye 4y agoThis thread is like that meme/joke where a programmer's code gets more and less elaborate for a basic Hello World as their career advances.
- lelandfe 4y agoReminds me of this old classic: https://github.com/EnterpriseQualityCoding/FizzBuzzEnterpriseEdition https://github.com/EnterpriseQualityCoding/FizzBuzzEnterpris...
- deleted 4y ago[deleted]
- mLuby 4y agohttps://www.smart-jokes.org/programmer-evolution.html https://www.smart-jokes.org/programmer-evolution.html
- trelane 4y ago> The only [satellites] who don’t have [to expend energy to stay in orbit] achieved escape velocity ... and are therefore no longer satellites. https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-a-satellite-58.html https://www.nasa.gov/audience/forstudents/5-8/features/nasa-... https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-orbit-58.html https://www.nasa.gov/audience/forstudents/5-8/features/nasa-... Some Lagrange points are stable and therefore will not decay toward the Earth outside of other factors. (Because these systems are never sufficiently isolated, in the eternal view, therefore, also still require energy, though much, much less.) Though, of course, an object at a Lagrange point may still not technically be a satellite of earth. https://solarsystem.nasa.gov/faq/88/what-are-lagrange-points https://solarsystem.nasa.gov/faq/88/what-are-lagrange-points (though by the NASA definition above I'd argue that they are)
- hoseja 4y agoLagrange points L1,2,3 are metastable and all are also perturbed by all the reality of not being result of two point masses in perfect vacuum.
- xypage 4y agoSatellites also need to use some energy otherwise their orbit will eventually [0] bring them into atmosphere [0]: https://en.wikipedia.org/wiki/Orbital_decay https://en.wikipedia.org/wiki/Orbital_decay
- brianpan 4y agoWe're all sitting in a curvature of space time, man. -_-
- inopinatus 4y agoAn object on a suborbital trajectory is by definition not a satellite. As a practical matter, there's a differing relationship with atmosphere. Planes depend on air to produce lift and sustain flight, but satellites are either inconvenienced by air, or entirely unaffected by it.
- amanaplanacanal 4y agoBut from a photography perspective, how does any of that matter?
- inopinatus 4y agoAltitude, atmospheric effects, and relative angular velocity are all factors in photography. Imaging from orbital platforms is also cheaper than airborne reconnaissance (per square meter, although the up-front capital investment is greater), covers a wider variety of purposes, and you don't have to worry about airspace violations; however, it may provide poorer definition, especially the more affordable commercial satellite imagery, and cannot compensate for cloud cover. So the distinction is significant on technical, operational, financial, and political levels.
- kavalg 4y agoCompensation for cloud cover in the visible spectrum is achieved by just picking an image from some of the next satellite passes. Also, there are active illumination imaging instruments (e.g. SAR) that can penetrate through clouds and see at night. Atmospheric correction however is really an issue and often results in distinct patches on the "satellite" view
- HideousKojima 4y agoThe further you get from the object you're photographing, the closer your photo gets to an orthographic projection instead of a perspective projection.
- kragen 4y agoWell, with the same camera, you get 100 times higher resolution from 1 km than from 100 km. But a satellite in a polar orbit overflies the whole Earth, typically, every few weeks (though keyhole-type satellites only photograph a very narrow track) while in many cases the only available aerial imagery is years or decades old. And a satellite can fly over restricted airspace (the only way to stop it from doing so, even for its owner, would be to blow it out of orbit) while doing that in an airplane is likely to get you thoroughly murdered and possibly result in a diplomatic incident. The result is that satellite photographs are much more frequent and have much better coverage, while aerial photographs have much higher resolution. The dishonest naming of the Google Maps feature has given people extremely unrealistic expectations of what satellites can do, which results in difficulty in selling actual satellite photography products when they don't match what people have come to expect from GMaps.
- Karellen 4y agoAren't satellites, by definition, orbital?
- grayclhn 4y agoWhen I jump, do I become a satellite?
- giantrobot 4y agoPlanes are just satellaren'ts.
- tempestn 4y agoI love that much like the engineers at Google, the HN audience couldn't help but take the bait here.
- otikik 4y agoEarth orbits around the Sun and the Sun orbits around the center of the galaxy so from a certain perspective all pictures humans have ever taken are satellite pictures.