# Which star could Starship reach soonest?

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.

• +1 for an unexpectedly interesting question! Jul 28, 2021 at 0:33
• Rats! I'd never thought to mock Starship, and now it's too late! +1 :-)
– uhoh
Jul 28, 2021 at 1:05
• @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". Jul 28, 2021 at 2:49
• 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. Jul 28, 2021 at 3:31
• 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. Jul 28, 2021 at 7:29

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.

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:

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.

• "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. Jul 29, 2021 at 6:55
• 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, 2021 at 8:34
• @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. Jul 29, 2021 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. Jul 29, 2021 at 11:55
• @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. Jul 30, 2021 at 8:42