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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 de

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

MayeulC 3y ago
> 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.
- mnw21cam 3y ago
  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.
  - TeMPOraL 3y ago
    Here is the OG source of those numbers: https://old.reddit.com/r/space/comments/1ktjfi/deltav_map_of_the_solar_system/ 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_solar_system/cbsq64k/ 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.
  - vikingerik 3y ago
    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.
    - ooboe 3y ago
      Didn't work out so well for Branson.
      - mnw21cam 3y ago
        It would be much more of an advantage on Venus than it is on Earth.
      - mlyle 3y ago
        Worked just fine for Orbital/ATK Pegasus.
  - MayeulC 3y ago
    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).
    - mlyle 3y ago
      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.
- Symmetry 3y ago
  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.
  - mr_toad 3y ago
    From the surface? It’s unlikely that anything would every launch from the surface of Venus.
    - jjk166 3y ago
      Probably more likely than a launch from the surface of Jupiter.
    - mlyle 3y ago
      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.
      - Symmetry 3y ago
        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 https://en.wikipedia.org/wiki/Air_turborocket [2]https://en.wikipedia.org/wiki/Project_Pluto https://en.wikipedia.org/wiki/Project_Pluto [3]https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket
        mlyle 3y ago
        > 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.
akolbe 3y ago
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.
- Asraelite 3y ago
  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.
  - nickparker 3y ago
    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-unusually-long-elliptical-orbit/ https://astronomynow.com/2019/08/28/exoplanet-found-in-unusu...
  - CaptainNegative 3y ago
    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.
- 3y ago
BurningFrog 3y ago
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.
- madcaptenor 3y ago
  And the error in your approximation goes up as the orbit gets bigger, which recovers the original result.
dr_dshiv 3y ago
Which planetary orbit is closest to Earth’s? That’s what I think people are meaningfully thinking.
- ianai 3y ago
  Venus by that measure.
- svachalek 3y ago
  Yup, that's equivalent to the delta-v definition afaik. (Venus)
  - Symmetry 3y ago
    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.
    - xvilka 3y ago
      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.
ryanmcbride 3y ago
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.
- hinkley 3y ago
  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…
  - VincentEvans 3y ago
    Venus is just the first stop, duh.
- olddustytrail 3y ago
  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 https://m.youtube.com/watch?v=21iUUe-W8L4
- yazantapuz 3y ago
  It means that is a huge distance, in human terms. Hugely variable, but huge at the end.
- zuminator 3y ago
  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.
adql 3y ago
> 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"
distortionfield 3y ago
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.
nuncanada 3y ago
Yup silly.. And people consider the closest mostly in the sense of the closest distance between orbits not distance between planets...
eyelidlessness 3y ago
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?

MayeulC 3y ago

> 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.

mnw21cam 3y ago
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.
- TeMPOraL 3y ago
  Here is the OG source of those numbers: https://old.reddit.com/r/space/comments/1ktjfi/deltav_map_of_the_solar_system/ 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_solar_system/cbsq64k/ 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.
- vikingerik 3y ago
  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.
  - ooboe 3y ago
    Didn't work out so well for Branson.
    - mnw21cam 3y ago
      It would be much more of an advantage on Venus than it is on Earth.
    - mlyle 3y ago
      Worked just fine for Orbital/ATK Pegasus.
- MayeulC 3y ago
  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).
  - mlyle 3y ago
    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.
Symmetry 3y ago
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.
- mr_toad 3y ago
  From the surface? It’s unlikely that anything would every launch from the surface of Venus.
  - jjk166 3y ago
    Probably more likely than a launch from the surface of Jupiter.
  - mlyle 3y ago
    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.
    - Symmetry 3y ago
      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 https://en.wikipedia.org/wiki/Air_turborocket [2]https://en.wikipedia.org/wiki/Project_Pluto https://en.wikipedia.org/wiki/Project_Pluto [3]https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket
      - mlyle 3y ago
        > 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.

akolbe 3y ago

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.

Asraelite 3y ago
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.
- nickparker 3y ago
  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-unusually-long-elliptical-orbit/ https://astronomynow.com/2019/08/28/exoplanet-found-in-unusu...
- CaptainNegative 3y ago
  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.
3y ago

BurningFrog 3y ago

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.

madcaptenor 3y ago
And the error in your approximation goes up as the orbit gets bigger, which recovers the original result.

dr_dshiv 3y ago

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

ianai 3y ago
Venus by that measure.
svachalek 3y ago
Yup, that's equivalent to the delta-v definition afaik. (Venus)
- Symmetry 3y ago
  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.
  - xvilka 3y ago
    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.

ryanmcbride 3y ago

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.

hinkley 3y ago
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…
- VincentEvans 3y ago
  Venus is just the first stop, duh.
olddustytrail 3y ago
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 https://m.youtube.com/watch?v=21iUUe-W8L4
yazantapuz 3y ago
It means that is a huge distance, in human terms. Hugely variable, but huge at the end.
zuminator 3y ago
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.

adql 3y ago

> 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"

distortionfield 3y ago

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.

nuncanada 3y ago

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

eyelidlessness 3y ago

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?