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Comment by tokai | Hacker News Reader

What are planes but slow satellites on a suborbital trajectory?

An 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.

But from a photography perspective, how does any of that matter?

Altitude, 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.

Compensation 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

The further you get from the object you're photographing, the closer your photo gets to an orthographic projection instead of a perspective projection.

Well, 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.

is this still true tho? can't they just stick a big telescope lens on a satellite and get pretty detailed imagery?

You can and there were / are 7-8 Hubble sized telescopes in orbit, with somewhat other optics and sensors looking in the other direction. Most likely the same is true for siblings of JWT.

There is a whole lot of pesky athmospheric interference limiting how much detail you can actually resolve.

You can, and satellite optics typically are a lot bigger than aerial photography optics, but the wavelength of light and the sizes of satellites you can afford to launch still impose a practical limit. For US companies, laws impose another limit.

The first incarnation of Google maps used low resolution Landsat imagery for most of the US. Massachusetts stood out distinctly with a different color palette because they had a public dataset of higher quality aerial imagery for the whole state.

I love that much like the engineers at Google, the HN audience couldn't help but take the bait here.

Plane needs to burn energy to stay up there, the sattelite just ... sits in a curvature of space time...

Going 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

No, 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.

Are 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?

None. 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

Even in deep space there are something like two atoms per cubic meter just "there". But atmospheric density drops exponentially with altitude.

Yeah anything above about ~1000km or so is essentially zero drag.

It adds up. Also, Kessler syndrome potential is not negligible.

Kessler syndrome is important but somewhat unrelated.

Beyond LEO the drag is really negligible in the sense that other factors (e.g. gravitational abnormalities, the moon) have larger effects.

And all the dragless satellites around the earth are focusing on documenting as all the frictionless spherical cows on the earth

IANAP ("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.

I 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.

To 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.

This 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.

Reminds me of this old classic: https://github.com/EnterpriseQualityCoding/FizzBuzzEnterpris...

https://www.smart-jokes.org/programmer-evolution.html

> 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-... 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 (though by the NASA definition above I'd argue that they are)

Lagrange 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.

Satellites also need to use some energy otherwise their orbit will eventually [0] bring them into atmosphere

[0]: https://en.wikipedia.org/wiki/Orbital_decay

We're all sitting in a curvature of space time, man. -_-

Aren't satellites, by definition, orbital?

Earth 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.

Planes are just satellaren'ts.

When I jump, do I become a satellite?

Yes, but you are falling down instead around.

You do if you jump both high enough and fast enough sideways such that when you come back down you miss the Earth.

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