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We've been mocking Starship because it's not going to fly to another star, but according to this solar system delta v map it only takes 5.63 km/s from a Moon transfer orbit to Sun escape velocity. A 130 t dry (with 10 t payload) and 1330 t wet Starship with 380 s Isp has 8666 m/s, so it'd have 3036 m/s excess velocity. It could also fly by Jupiter for extra speed and/or a plane change.

Which star could Starship fly to soonest? Not just pass through its Hill sphere, but like fly by at 1 AU (or theoretically collide with it). Proxima centauri is about 4.2 ly away. At 3 km/s it would take 420 ka (kiloanni) to travel 4.2 ly. In 420 ka any currently near stars are long gone (nearest stars flying by). So the question is, which star could Starship realistically fly to, be there when Starship is there. Bonus question: What's the relative speed in the encounter?

PS: Someone suggested retanking starship with expendable tankers en route to Jupiter and gain 20 km/s with a burn at Jupiter. Much more expensive, but this is just theoretical and it's within specs of Starship and it seems to get us within the time frame of the passing stars diagram, so we can also consider this variant.

darsie
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    +1 for an unexpectedly interesting question! – Russell Borogove Jul 28 '21 at 00:33
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    Rats! I'd never thought to mock Starship, and now it's too late! +1 :-) – uhoh Jul 28 '21 at 01:05
  • New Horizons was launched on January 19th 2006 with the largest C3 ever at the time. It gained about 2 more miles/sec after a gravity assist from Jupiter. Still, it didn't reach Pluto until July 15th 2015. At that time, Pluto was 4.5 light hours from the Sun. So 9.5 years to go 4.5 light hours. The closest star, Proxima Centari, is a little less that 4.3 light YEARS away, so the travel time (~ 83,000 years) is about four times longer than recorded history. We need a lot faster mode of travel before we can think about travel to the stars. – Vince 49 Jul 28 '21 at 02:04
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    @uhoh: Maybe you think "Tomorrow Doesn't Matter Tonight", but Elon thinks "Nothing's Gonna Stop Us Now". "It's Not Enough" to just reach orbit; you need the delta-V to "Get Out Again" to escape velocity. All so Elon can build a colony and say, "We Built This City". – DrSheldon Jul 28 '21 at 02:49
  • @DrSheldon love it! – uhoh Jul 28 '21 at 03:14
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    I'll be the contrarian. Robert Zubrin has advocated very strongly, and multiple times, for sending humans to Mars. However, he has also advocated very strongly, and multiple times, that if something done in space will take longer than 50 years to accomplish, don't do it (now). The problem is that future technology will overtake whatever we do now 50 years into the future. 420000 years from now? Stealing from a tennis shoe company: Just don't do it. – David Hammen Jul 28 '21 at 03:31
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    With that 5.63km/s you would get to the nearest star the day after the universe ended. Say, 14.3 infinity years + a day. If you manage to tack on an extra 1km/s, the journey will be shortened to "only" 300 000 years. If you manage to tack on an extra 200 km/s, the journey will still take 1500 years. And if you want to go to proxima centauri, you first need to catch up to the tune of 22.2 km/s, which is the rate it is leaving the local scene. – CuteKItty_pleaseStopBArking Jul 28 '21 at 07:23
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    Let's rephrase the question: Using a bicycle, which airliner will you be able to board in flight? The problem of airliners flying around changing positions is not the problem, the problem is that they all move faster than your maximum speed, in locations you cannot reach in the first place. – CuteKItty_pleaseStopBArking Jul 28 '21 at 07:29
  • @PcMan, if I read it correctly, the graph provided in OP shows alpha centauri is currently getting closer and won't start moving away from the sun for the next ~27000 years. What or when are those 22.2km/s leaving rate you mention? – user721108 Jul 28 '21 at 09:35
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    @DavidHammen there's a name for the "when to launch?" problem and I think there a few posts here that refer to it, but I can't figure out what it's called and can't find them right now. update: Found one! Research paper on 'Wait calculation' for interstellar travel – uhoh Jul 28 '21 at 10:02
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    @DavidHammen I agree, don't do this. It's just a thought experiment to legitimate the name "Starship". It very much looks possible, so figuring out which star to encounter is just curiosity. – darsie Jul 28 '21 at 10:45
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    @PcMan, If you want an analogy I suggest you launch a blimp well ahead of the aircraft departure to be at the encounter when the aircraft comes there, too. This is a close fly by mission, so we don't board the aircraft, we just take pics of it. – darsie Jul 28 '21 at 10:49
  • @qqjkztd elementary kinematics. it does not matter if something is racing towards you, or away from you, you have to exert equal excess acceleration to match velocities with it. – CuteKItty_pleaseStopBArking Jul 28 '21 at 12:27
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    @PcMan ok but the whole thing is about intercepting trajectories, not matching velocities. – user721108 Jul 28 '21 at 14:24
  • @PcMan OP is aware of this, and asks about the relative velocity at the encounter as a bonus question. – Russell Borogove Jul 28 '21 at 15:48
  • Other side of the coin: How much additional time needed for the SV to send a message back to Earth, and who on Earth will remember to listen for it (or even, still be alive to listen for it?) – Solomon Slow Jul 28 '21 at 22:46
  • Its only flown 12km straight up so far. Taking about visiting other stars is a true giant leap. – Innovine Jul 29 '21 at 10:17

1 Answers1

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I think the comments have basically summed up the answer, but I would like to add a few numbers to the situation.

From what numbers I can find on Wikipedia, Starship will have a dry mass of 120 t and a payload capacity of ~ 100 t.

According to a tweet from Musk back in July 2019, a starship in orbit with a 100 t payload will have a remaining delta-v of 6900 m/s. I don't know if that's still the case, and the numbers you see floating about in various forums aren't detailed enough for me to really check that with my own math.

https://twitter.com/elonmusk/status/1151300180148252674

So let's assume that 100 t of payload is also fuel. Let's just ignore the plumbing for the moment. How much further does that boatload of methane and LOx get us? Well according to the rocket equation, a mass ratio of 120/220 and an exhaust velocity of 3700 m/s, you get a further 2242 m/s of delta-V, for a total of 8142 m/s. If you add a further 5 - 10 km/s from a Jupiter gravity assist, you get around 13,000 - 18,000 m/s of delta-V

18,000 m/s is not enough.

The picture you provided actually illustrates this fact quite well: Star trajectories over the next 100000 years

This diagram of distances expresses the very minimum you have to do to get to another star. The slope of each line represents its velocity. If you want to get to a star, your spaceship line has to intersect with one of the star lines.

As you can see, Voyager 2, at 17,000 m/s is nowhere close to having its line be steep enough to intersect with any of the stars, apart from Ross 154 possibly, if it remains on the same course. Given that the sated velocity for Voyager is similar to the calculated Starship velocity above, it's safe to say that Starship will never reach any of these stars.

What about other stars? Sure, it's possible that another star will make a close approach in the far future that is within Starship's reach, but after a certain point in time the trajectories of these stars becomes harder and harder to predict.

Ingolifs
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  • "So let's assume that 100 t of payload is also fuel. Let's just ignore the plumbing for the moment." – No need. What you need for this mission is essentially a merger of the tanker variant (all payload is additional fuel) and the deep space variant (no flaps, legs, or other recovery hardware, no header tanks, no heat shield, no sea-level engines, possibly nuclear power generation instead of solar panels), so the plumbing has already been figured out or at least thought about by SpaceX. – Jörg W Mittag Jul 29 '21 at 06:55
  • Does a flyby of a 2nd planet like Saturn help? Candidates for new stars that enter our neighborhood in the future may not be very hard to predict as much as it is just simply work to sort through a larger table of candidates, and that should probably be done in Astronomy SE anyway. – uhoh Jul 29 '21 at 08:34
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    @uhoh, flying by both Jupiter and Saturn restricts the trajectory to a narrow angle, so it would only be worth it, if the encounter were in that direction. – darsie Jul 29 '21 at 11:35
  • @ingolifs, Ok, so you're saying even the new Gaia catalog doesn't have precise enough data that we could simulate the chaotic motion of stars for a 1 AU flyby. How about the scenario where we refuel Starship and do a burn rather than a ballisticy flyby at Jupiter? PS: I got 8666 m/s for a Starship with 10 t payload. That's without stripping it of fins, legs, etc or using more fuel. – darsie Jul 29 '21 at 11:55
  • @darsie you can fly over the top or bottom of Jupiter as well as the side so it is a narrow slice of solid angle, and as long as the year is not constrained that slice isn't so narrow either. I'm just curious if it helps significantly with velocity or not. – uhoh Jul 29 '21 at 12:06
  • @darsie I meant Jupiter or Saturn, whichever planet is last in a given scenario. – uhoh Jul 30 '21 at 01:02
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    @uhoh Jupiter has 12 year orbital period and Saturn 29. Over 12 years we get 12 solid angles covered by a Jupiter flyby but only one of those will also include a Saturn flyby. The angle covered by the Saturn flyby is narrower. Also there are probably key holes to fly through at Jupiter that give us more delta v than the one that gets us a Saturn encounter and I'm not sure if Saturn makes up for that. Jupiter has 3.33 Saturn masses. Starship may be outdated in 50 years, so the possible flybys are limited. – darsie Jul 30 '21 at 08:42
  • @darsie I think that can be better summarized in an answer so I'll post a question about this. Thanks! – uhoh Jul 30 '21 at 11:10