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Comment by Symmetry | Hacker News Reader

Which planet happens to be the closest to us right now is the metric most congruent to people's notion of closest and is often important for things like time delays in radio communication. By that metric it's currently Mercury, closely followed by Venus[1]. But it will change and keep changing.

In terms of how hard it is to get places you really want a delt-v map[2]. By that metric Venus is the closest at 640 m/s from Earth intercept to Venus intercept.

It's sort of interesting that, over an indefinite period of time, Mercury is closest on average but that doesn't really correspond to our intuitive notion of "closest" nor is it a particularly useful metric for anything that comes to mind. So the whole gotcha here is really pretty silly.

[1]https://www.theplanetstoday.com/

[2]https://i.imgur.com/AAGJvD1.png

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Basically, the average distance between one planet and another will always be greater than the distance between the respective planet and the sun. (Note that in the table on the last page, all average planet distances for earth are > 1 AU.) Thus whichever planet is closest to the sun will always be the closest on average to any other planet.

This is true for the Solar System because all our planets have near-circular orbits. Suppose though if you had two planets with highly elliptical orbits and similar arguments of perigee. They will spend most of their time far away from their star but relatively close to one another.

I'm not sure if such an elliptical orbit would be possible while still classifying them as planets and not dwarf planets though.

IIRC we've found some big exoplanets in wild orbits. They tend to form in a circular-ish orbit because that's how you get a big enough chunk of the dense bits of an accretion disk, but interactions with other planets/stars can put eg gas giants into wild orbits after formation.

EG https://astronomynow.com/2019/08/28/exoplanet-found-in-unusu...

Outside of binary planets orbiting each other, is this achievable as a (mostly) stable configuration?

Intuitively, if the two planets have orbiting periods that are not basically identical, then after long enough they will also have long stretches of time where they are on opposite sides of the star (with a slight caveat if the periods are rational multiples of each other, but in either case their positions will be asymptotically uncorrelated). On the other hand, if they are close and have the same period, I'd expect their gravitational pulls would eventually merge them together unless they become a binary planet system.

But I have no physics/astrophysics background so this could easily all be stupid.

Almost true. Good thinking though. There are exceptions to this rule, so it's more of a guidance than the actual rule. For example you could have two planets orbiting the Sun at the relatively similar distance from the Sun and a small distance from each other.

You couldn't actually have two separate planets with similar orbital distances like that though.

Right. They would necessarily have different orbital periods because the size of an orbit is directly proportional to the time it takes to complete one orbit: https://en.wikipedia.org/wiki/Orbital_period#/media/File:Sol...

This is why objects in geostationary orbit can only exist at a particular distance from the earth: https://en.wikipedia.org/wiki/Geostationary_orbit

Well, that's not the issue here. The issue is two planets can't exist near each other because they can't possibly have formed that way 4.5B years ago and kept existing at that point since then. Their gravitational influences would have long since caused them to collide into each other and form a single planet. There's a reason there's big gaps in the orbital distances of every planet; it's simply not possible to pack them in too tightly.

That's still the same scenario, just slower to become apparent. You need to change other things, like they orbit each other as they go around the Sun, to change this.

Which planetary orbit is closest to Earth’s? That’s what I think people are meaningfully thinking.

Venus by that measure.

Yup, that's equivalent to the delta-v definition afaik. (Venus)

It's close equivalent. It's easier to go inwards than outwards in terms of delta-v because orbits further out move more slowly. So if things were a little different the orbit of Mars could have been closer to Earth in terms of distance while Venus would still be closer in terms of delta-v.

Only if you want one-way trip. In case of the two-way trip, Mars could be still closer due to lesser Sun gravity influence that needs to be compensated.

Did you read to the end? The point of the paper is not the gotcha about which planet is really closest. Their point is that their PCM method allows you to quickly estimate distances between groups of planetary bodies in a novel way. They aren't trying to be "gotcha" about it, they're introducing a new model for estimating solar distances.

Before morning coffee:

Over an indefinite period of time, I'd expect that all planets are on average placed in the center of the Sun, and equally far away.

Now:

Oh, right. It's not the distance to the average, it's the average of the distance.

And the error in your approximation goes up as the orbit gets bigger, which recovers the original result.

This exact reasoning is why I've always been kind of annoyed by the song Bitch Don't Kill My Vibe by Kendrick Lamar. In the chorus he goes "I can feel your energy from two planets away" and even though I know art doesn't have to conform to scientific reality, poetry and music almost especially, it ALWAYS bugged me from the first time I heard it til today. Like what does 2 planets away even mean? that's a hugely variable distance.

    We're heading for Venus
    And still, we stand tall
    'Cause maybe they've seen us
    And welcome us all, yeah
    With so many lightyears to go
    And things to be found
    I'm sure that we'll all miss her, so

    It’s the final countdown

Someone’s science teacher knows who was too busy writing lyrics in class to learn about outer space…

Venus is just the first stop, duh.

Heh, reminds me of the Simon Singh criticism of Katie Melua's "Nine Million Bicycles".

Which she followed up on https://m.youtube.com/watch?v=21iUUe-W8L4

It means that is a huge distance, in human terms. Hugely variable, but huge at the end.

Haha that's an interesting question. I'm figuring that the closest "2 planets away" would be the canonical distance between Earth and Mercury, ie 57 million miles (92 million km).

But it's the other extreme that's more interesting.

Let's say that for a planet to be the "next" planet over at any given time, it has to be the closest one... by direct line measurement, not by orbit. So for Earth, the next planet over could be at any given time Mercury, Venus or Mars. The most isolated Earth can be from all other planets would be when the planet that is closest to us at a given moment is as far away is it can be. That turns out to be Mercury, whose maximal distance is about 138 million miles away (222 million km). There's always going to be a planet that is that distance to us or closer.

So imagine that Mars happens to be 137 million miles from us while Mercury and Venus are both at least 138 million miles away. That would make Mars the "next" planet over. Then the "next" planet over from Mars could be either Venus or Mercury. If we're assuming Mars is as isolated as possible than the next closest planet besides Earth would be Mercury which at its furtherst could be at most 198 million miles away (319 million km). Thus, ignoring trigonometry which would put a constraint on the Mercury-Mars leg of the triangle, two planets away from us could be at most 336 million miles (540 million km) away.

So between 57 and 336 million miles is your answer.

I always assumed this line uses the traditional order of planets from the sun and referenced the common trope that "Men are from Mars, Women from Venus".

This is the first thing I've heard that makes any real artistic sense. I'm gonna give it another listen, thank you.

Yup silly.. And people consider the closest mostly in the sense of the closest distance between orbits not distance between planets...

> In terms of how hard it is to get places you really want a delt-v map[2]. By that metric Venus is the closest at 640 m/s from Earth intercept to Venus intercept.

Well, that's also a minimum that changes over time. It might be closest at a particular moment but still not "on average"

> By that metric Venus is the closest at 640 m/s from Earth intercept to Venus intercept

By the way, I think there is a typo on that delta-v map. I doubt low Venus orbit to Venus is 27km/s, vs 9.4 for the earth, when Venus gravity is just 90% that of Earth.

Depends on whether you take atmospheric drag into account. If you do, then you'll be fighting Venus' thick atmosphere all the way up, and that 27km/s figure could well be accurate.

I don't like it when delta-v maps include atmospheric drag, because the numbers depend on how aerodynamic your rocket is, in contrast to the other manoeuvres where the amount of delta-v doesn't depend on the type of rocket you have at all.

Here is the OG source of those numbers: https://old.reddit.com/r/space/comments/1ktjfi/deltav_map_of....

Skimming the thread, they made "assumptions" for cases like taking off from bodies with atmosphere.

Here is more on how they came up with the number for Venus, including some actual math: https://old.reddit.com/r/space/comments/1ktjfi/deltav_map_of....

The OG image mentions that there are assumptions being made. The image linked by GP is a derivative work, improving on and crediting the work of /u/CuriousMetaphor, however it omits some of the caveats in the legend.

As other replies are saying, that does include the atmospheric drag. But that number wouldn't actually happen with rocket thrust. You'd never actually do that, launch a rocket from the surface of Venus - you'd first take advantage of the atmosphere to do aerodynamic flight, first carry the rocket to much higher altitude with an airplane and then ignite it from there.

Didn't work out so well for Branson.

It would be much more of an advantage on Venus than it is on Earth.

Worked just fine for Orbital/ATK Pegasus.

Thank you for pointing this out.

> I don't like it when delta-v maps include atmospheric drag

Yes, I find it quite unintuitive, especially as the map is now asymmetric: if you take into account drag on liftoff, you would also take into account aerobraking for reentry. It means that the map can't really be used for body-to-body calculations, as it assumes "rocket liftoff" for the low orbit<->surface transition.

Ideally, atmospheric parameters should be specified some other way on the map, or it could branch to show both liftoff and reentry costs on each body (and possibly delta-v due only to gravity).

It's just the first segment on the trip.

Reentry delta-V isn't really well-posed. The delta V that would enter orbit, or even less, with a somewhat different angle reenters. So the "reentry delta V" might very possibly be negative, in that you could go Earth LEO to body surface with less velocity change.

> possibly delta-v due only to gravity)

Now, that's more useful to have around.

The numbers for reaching LEO include typical atmospheric drag losses. That's only a km/s or so on Earth but on Venus with it's very thick atmosphere the losses are much higher.

From the surface? It’s unlikely that anything would every launch from the surface of Venus.

Probably more likely than a launch from the surface of Jupiter.

It's not only unlikely, it's unpossible.

If you're using chemical propulsion, you're not going to get much more than 20km/sec even with a whole lot of stages.

You'd certainly need something beyond a chemical rocket but there are options. At that density a turborocket[1] would certainly be worth the extra mass but I think still wouldn't be enough of an advantage. In the realm of roughly existing technology, a nuclear ramjet[2] could get you to the upper reaches of the atmosphere and give you a nice little initial boost as well to your speed. And in the realm of SF, a nuclear saltwater rocket[3] would still be easily capable of making it out of Venus in one stage.

[1]https://en.wikipedia.org/wiki/Air_turborocket

[2]https://en.wikipedia.org/wiki/Project_Pluto

[3]https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket

> At that density a turborocket[1] would certainly be worth the extra mass but I think still wouldn't be enough of an advantage.

How's a turborocket work without free oxygen gas in the atmosphere? I mean, maybe fluorine, but I doubt you come out ahead that way.

Density effects, though, make balloon-launched rockets, etc, conceivable.

To get a turborocket to work in an atmosphere without oxygen you just have a classic rocket engine, with its own fuel and oxidizer, use its exhaust to drive a turbine the same way a jet is used to drive a turbojet. You have to leave off the afterburner stage that many existing turborockets have where you inject more fuel to burn after the final turbine.

If you're using air for just working mass and not for an oxidizer, it's my belief that this does not improve specific impulse but only improves peak thrust/engine weight.

Because the energy lost to the movement of the propellant goes up as the square of the velocity but the thrust goes up linearly. Bulking out your propellant by a factor of ten while reducing the exhaust velocity by a factor of ten doesn't change the resulting thrust, but reduces the energy you need for that thrust by a factor of 10.

The above elides a lot of complexity but the Wikipedia summary of the situation is pretty good. https://en.wikipedia.org/wiki/Rocket#Energy_efficiency

I don’t understand. Two objects are on average closest, but that’s unintuitive because another object is closer based on some astronomical measurement which isn’t distance, which is more intuitive how?

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