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It seems like NASA wants to put someone on Mars by 2030, 61 years after we first landed on the moon. This date doesn't seem too far off. However, will it take another 60 years or more to reach the next planet in the solar system? When will we land on Venus? How about Neptune? Have any rough dates been said or is it just too far off to give a proper estimate?

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    $\begingroup$ Venus is too hot and the atmospheric pressure (92 bar) is too high for a human landing. Landing on gas giant without a surface is impossible. So the proper estimate is never. $\endgroup$
    – Uwe
    Commented Apr 29 at 19:30
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    $\begingroup$ Mars is the only practical option in the near future really. Pluto might be possible but is a very long way and not strictly a planet. After Mars I'd imagine asteroids or Jovian or Saturnian moons would be the next target $\endgroup$ Commented Apr 29 at 20:16
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    $\begingroup$ NASA will not put someone on Mars by 2030. $\endgroup$
    – gerrit
    Commented Apr 30 at 6:39
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    $\begingroup$ -1 for the lack of basic knowledge of environmental conditions on the other planets you named, I'm afraid. $\endgroup$
    – Graham
    Commented Apr 30 at 12:18
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    $\begingroup$ See meta for discussion on edit rollback. $\endgroup$ Commented Apr 30 at 14:29

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Landing on any other planet in our Solar System is practically impossible. Mercury is the only one that is even remotely possible, and then only at the day/night terminator, it gets too hot too fast otherwise. Venus is way too hot (Even a spacecraft can only survive a few minutes, but a high atmosphere balloon visit is possible), the gas giants have no surface, and thus only Mercury is even possible anytime relatively soon.

The more likely next step is landing on an asteroid or one of the moons of an outer planet. Ceres would be a particularly interesting target. How long that will take to happen, however, is very difficult to know. I think if we have the technology to go to Mars we very likely will be able to mine asteroids fairly soon thereafter, which will make them a likely target.

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  • $\begingroup$ I feel like it would be a good idea to point out why it is unfeasible to land on planets in our own system. As it stands, I'm tempted to post about it myself - but I feel like it would be a duplicate to your own answer. $\endgroup$
    – Zibbobz
    Commented Apr 30 at 13:01
  • $\begingroup$ Added a sentence explaining why. $\endgroup$
    – PearsonArtPhoto
    Commented Apr 30 at 13:02
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    $\begingroup$ Why Ceres, a largeish inert rock, and not Europa (which is still, AFAIK, the best candidate for unicellular life) or Enceladus (which is fascinating because of its geysers)? Is there something special in Ceres I don't know about (loads, I'm sure, but what are you thinking of)? $\endgroup$
    – terdon
    Commented Apr 30 at 13:35
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    $\begingroup$ We even if we aren’t going to be walking on Venus in the near term due to the thick and hot atmosphere, sending humans to Venus is still an option. There are plans to have zeppelins floating around 50km up with people on Venus $\endgroup$ Commented Apr 30 at 15:10
  • $\begingroup$ @terdon The radiation near Jupiter is insane. Its magnetosphere is far larger and more active than Earth's. Its radiation zones make our Van Allen belts look positively benign. When sending a robotic probe to the Jovian system we need to use significant shielding to minimise the damage to digital circuitry. See en.wikipedia.org/wiki/Juno_Radiation_Vault So it's tricky to send living organisms to Jupiter without them being cooked by the EM radiation and zapped by the particles. $\endgroup$
    – PM 2Ring
    Commented May 1 at 4:17
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So I happen to have given this question a lot of thought. It is a bit similar to (back in 50BC) Brutus asking Julius Caesar when Rome might have a colony on Greenland or New Zealand (assuming that somehow they knew that Greenland and New Zealand existed back then).

Have you ever played Civilization? In that game...

As time advances, new technologies are developed; these technologies are the primary way in which the game changes and grows. At the start, players choose from advances such as pottery, the wheel, and the alphabet to, near the end of the game, nuclear fission and spaceflight. (ref)

Applied to real life today, there are a few prerequisite technologies that we must develop before off-world colonies can happen.

Ultra-Long-Time-Horizon Decision Making

First, our economy must truly master the technology of large-scale ultra-long-time-horizon investing. Nation-states sometimes accomplish this and we end up benefitting from infrastructure projects that last for decades and sometimes even centuries (e.g. hydro dams, railways, highways, GPS, Internet). But we are nowhere near good enough at this for an idea like "I'm going to plant the seeds of a new human civilization on another world" to receive funding commensurate with the net present value of the proposal. It is not yet possible for people with power and influence to reach a consensus about such a complex value proposition. Today, momentous accomplishments are the result of a fish ladder of smaller accomplishments each with their own individual value propositions. As a society, we need to master the ability to make larger leaps.

Defeating the Tyranny of the Rocket Equation

With chemical rockets, the amount of delta-v that we need to travel between planets causes the payload ratio of present-day rockets to be tiny. This makes the journey too expensive and it fundamentally prevents any proposal involving people emigrating from Earth to another world from penciling out.

Artificial Biospheres

While a lot of progress has been made on closed ecological systems, we're in a situation where lots of people remain skeptical about whether this is something we can do when such a closed system is located far from Earth. We are still lacking a pudding with a compelling proof in it.

Ok, this is not an exhaustive list of the technologies that our civilization will need to master, but these are representative of some of the important ones.

Which Worlds First?

I don't think we'll colonize the outer gas giants themselves for a long time, although Saturn, Uranus, and Neptune all have Earth-like gravity at their surfaces. If we could somehow pave them over with a solid shell, they would greatly increase the amount of one-gravity surface area that humans could inhabit. However, we are likely to see colonies on their moons well before that happens.

enter image description here (ref) Surface gravity levels in our solar system (Y axis: Earth gravities)

"Near term", self-sustaining civilizations on the Moon, Mars, and even in the upper atmosphere of Venus (ref, ref) are all possible. If we allocated a few billion dollars to mastering each of the technologies listed above, we would see breakthroughs in all three fields. In that case, we could have one or two space-colonization success stories by the end of this century.

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    $\begingroup$ Could you explain what the image is showing? What is the X axis, what is the Y axis? How are the celestial bodies arranged there? Polar and Equatorial what? $\endgroup$
    – terdon
    Commented Apr 30 at 11:21
  • $\begingroup$ @terdon It's a bar chart, so there's no X axis as such; it simply lists the values for the most significant bodies in our solar system (except for the Sun itself, of course).  And it seems highly likely that the Y axis is the force of gravity at the surface of each body, relative to Earth's (i.e. in multiples of ɡ₀).  Where the body is significantly aspherical, it shows values for both the equator and the poles.  (Those bodies are all oblate, i.e. flattened at the poles, and so the latter values are larger.)  … $\endgroup$
    – gidds
    Commented Apr 30 at 23:10
  • $\begingroup$ …The Y axis is clearly logarithmic, and the horizontal lines are spaced by a factor of e; the values seem to cluster around them. $\endgroup$
    – gidds
    Commented Apr 30 at 23:11
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However, will it take another 60 years or more to reach the next planet in the solar system?

Much more, if ever. As Pearsonartphoto has pointed out: it is nearly impossible to land on any other planet. It might never happen. Even for Mars the case is very hard to make, and more governed by exploration and prestige than by science or economics.

The cost difference between staffed and unstaffed missions is very large, and the quality of robots is increasing, so the economic or scientific case for staffed missions diminishes over time.

We'll probably get humans to Mars eventually, but personally, I don't believe there will be humans on the surface Mars before 2050.

When will we land on Venus?

We will never land with humans on Venus. The surface of Venus is too hot for humans or their current technology. The surface of Venus is much harder to survive on than the Moon, Mars, or Deep Space.

How about Neptune?

We will never land with humans on Neptune.

Have any rough dates been said or is it just too far off to give a proper estimate?

Anyone who sets a date on landing on Venus or Neptune or even Ceres should do so only when writing a science fiction story and not when involved in actual space exploration.

Even for Mars, the target dates for landing keep shifting. Personally, I would be surprised if humans landed on Mars in the next 20 years. A round trip to Mars is difficult and expensive, as you have to bring (or locally build) the entire launch system to get back to Earth. A one-way trip might be more feasible, but I am skeptical if agencies are willing to invest in a one-way journey.

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    $\begingroup$ And I would say a local-build option really is "one-way until proven". $\endgroup$ Commented Apr 30 at 7:16
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    $\begingroup$ I would not use the word never. Do you know how long never is? It is such a long time so I would not bet on it. $\endgroup$ Commented Apr 30 at 11:06
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    $\begingroup$ We “only” need more or less automated and self-replicating asteroid mining to really get the ball rolling for huge craft. When spacecraft mass is no longer a concern lots of things become possible. $\endgroup$
    – Michael
    Commented Apr 30 at 11:31
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    $\begingroup$ @gerrit: Well, if you can do it with minimal cost (because it’s done by robots), why not? Doesn’t have to be necessary for our survival or even comfort. $\endgroup$
    – Michael
    Commented Apr 30 at 12:40
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    $\begingroup$ @gerrit: Somebody would certainly volunteer. Even if it’s just to be the first. Heck … we probably wouldn’t even have trouble finding volunteers for a one-way trip on which you die after a few hours on the surface. $\endgroup$
    – Michael
    Commented Apr 30 at 13:10