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This (amateur) question has always been on my mind. We send probes to leave the solar system, but when we reach there, their battery is almost empty, so when finally after ~30 years we reach there, we can hardly "see" anything.

What do we need? Higher speed. If we could reach where Voyager is right now in 5 years, we could collect much better and more useful information.

What can we do today with the current technology? Almost all rocket fuel is spent to escape the Earth's gravity, so the probe is not really accelerated in space after that.

Why isn't rocket/fuel sent to space first, instead of the probe (I know transport capacity is limited, so we need to send fuel in chunks), and when desirable amount of fuel is there (don't know, 10-20 launches?) we connect them, attach the probe, and ignite in space.

Couldn't we reach much much higher speed there?

Why isn't this method used? I know it's expensive, but that shouldn't be the blocking issue, should it?

--Edit-- I found this: Why can't we launch from space?

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    $\begingroup$ "Almost all rocket fuel is spent(?) to overtake the Earth's gravity.." Not when it comes to trips that escape the Sun's gravity to leave the Solar System.. That (so far) requires gravitational slingshots off planets in order to speed up the probe - after most of the fuel is spent. What you're talking about would require a huge amount of fuel. $\endgroup$
    – Andrew Thompson
    Commented Dec 28, 2015 at 9:47
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    $\begingroup$ If you spend all of your fuel at once, you reach the target earlier than if you save some of the acceleration until later. It is optimal to coast (with chemical rockets). The other part of the question is about the trade off between one big launcher or several smaller which has been addressed in other questions. $\endgroup$
    – LocalFluff
    Commented Dec 28, 2015 at 10:10
  • $\begingroup$ Related maybe to the point of being a duplicate: space.stackexchange.com/questions/741/… $\endgroup$
    – Rikki-Tikki-Tavi
    Commented Dec 28, 2015 at 14:16
  • $\begingroup$ "The other part of the question is about the trade off between one big launcher or several smaller which has been addressed in other questions" -- after a point, isn't cost / risk / efficiency increases too much for one big launch than several small? $\endgroup$
    – Zotyi
    Commented Dec 28, 2015 at 23:02
  • $\begingroup$ @Zoli Not necessarily, if there is enough demand. For example, cargo ships and oil tankers keep getting larger and larger. Issues specific to SLS should not be generalized to super heavy lift as a concept. $\endgroup$
    – LocalFluff
    Commented Dec 29, 2015 at 4:28

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This is the basis for several concepts of interstellar probes. The most awesome is the Bussard ramjet, aimed to exploit the 'fuel' already present in the interstellar space - protons (hydrogen ions) (or other ions, in some variants) for magnetically compressed fusion.

Other than that, gathering reaction mass (e.g. water, methane, or carbon dioxide for processing) on distant worlds is one of the most important shortcuts in return missions to avoid launching megarocket trains at exorbitant (no pun intended) cost. The complexity of such collection and processing is still one step beyond current technology (Bussard ramjets are at least seven+ steps beyond).

Fueling on the way to the destination is not done due to absence of maintained orbital fuel depots and lower mission success probability as soon as one provides for orbital rendezvous and refueling. This may become an option once these technologies are perfected, for low-thrust ion engine etc. exploration probes.

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  • $\begingroup$ I read several concepts, but these are currently beyond real implementations. But my "idea" looks realistic, well, at least possible right now to implement. I don't talk about interstellar travel, I mean, visiting closer stars, that is impossible currently, but leaving solar system in a few years, reaching kuiper belt, etc $\endgroup$
    – Zotyi
    Commented Dec 28, 2015 at 22:54
  • $\begingroup$ "But my "idea" looks realistic" To an audience of one. $\endgroup$
    – Andrew Thompson
    Commented Dec 29, 2015 at 2:50
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The needed fuel has to be launched into space either way, so a refueling option would add complexity without reducing the total launch mass requirement.

To get to where the Voyagers are today in only 5 years would require them to leave Earth going more than 115km/s. Because of the nonlinearity in the rocket equation, reaching that speed with chemical rockets would require about one trillion tons of fuel, which would take about 7 billion Saturn V launches to bring into Earth orbit (or 100 billion Falcon 9 reusables).

Using more fuel-efficient ion thrusters, the fuel requirement to reach 115km/s would be greatly lessened, but acceleration would be too low to reach that speed in less than decades.

In any case, the Voyager spacecraft aren't that severely compromised by the decay of their RTG power sources. They may not be able to use all their instruments at once, but they don't really need to.

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  • $\begingroup$ Less than half the $\Delta v$? With 9 km/s extra after getting to LEO, you are well above solar system escape velocity, even without gravity assist. $\endgroup$
    – SE - stop firing the good guys
    Commented Dec 28, 2015 at 19:02
  • $\begingroup$ Ah, misread a ∆v chart there, sorry. $\endgroup$
    – Russell Borogove
    Commented Dec 28, 2015 at 19:26
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    $\begingroup$ Yes, even if launched from space. Linear increases in ∆v require exponential increases in propellant mass. $\endgroup$
    – Russell Borogove
    Commented Dec 28, 2015 at 23:20
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    $\begingroup$ We sent the Voyager spacecraft to investigate the outer planets of the solar system. That mission was completed in 1989. There are many different instruments on the spacecraft, and some are expected to operate for as long as another decade. My point is that running e.g. the magnetometer and the cosmic ray detector simultaneously isn't dramatically more valuable that running them alternately. $\endgroup$
    – Russell Borogove
    Commented Dec 28, 2015 at 23:28
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    $\begingroup$ Let us continue this discussion in chat. $\endgroup$
    – Russell Borogove
    Commented Dec 29, 2015 at 17:33
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The main reason this isn't done, is cost. A single rocket launch costs $100-300 million, so it's very expensive to create a faster deep-space mission.

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  • $\begingroup$ spacex's re-usable rockets might make this option cheap enough to try. $\endgroup$
    – IHateToxicPlatforms
    Commented Dec 28, 2015 at 18:25
  • $\begingroup$ @dvdmn Let's go by the numbers in Russell's post. Let's say the cost of a single F9R launch is \$1 (yes, one dollar). Your total launch cost is now about \$100bn. Compared to, playing it nice, \$500M for a slower mission. Your proposed method is 200 times more expensive, or for the OP's suggested mission, costs \$99.5bn to save a few decades of trip. That's expensive no matter how you slice it. $\endgroup$
    – user
    Commented Dec 30, 2015 at 20:39

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