Oumuamua may be a hydrogen iceberg • Hacker News Board

Re: Oumuamua may be a hydrogen iceberg

#91

Post by TeMPOraL » Thu, Jun 04, 2020, 4:15 PM UTC

manquer wrote: post #80

TeMPOraL wrote: post #47

Earlier quoted context omitted.

Rule 1 of orbital mechanics: if you're about to dock with something, you're already on the same trajectory as that thing. Might make a good radiation shield, though.

You could always impact/land violently with it without having to match its trajectory. If your instruments are designed to survive the impact , it could work.

You can, but you only save as much velocity as the difference during the impact. Can't think of the case where that is, at the same time, both a useful saving and not fast enough to melt your equipment from a high-velocity impact.

Re: Oumuamua may be a hydrogen iceberg

#92

Post by theandrewbailey » Thu, Jun 04, 2020, 4:36 PM UTC

phkahler wrote: post #18

sq_ wrote: post #9

Earlier quoted context omitted.

That's really cool. Definitely helpful in order to drop deceleration fuel requirements if we ever manage to put together a relativistic interstellar spacecraft.

In a bit over 1M years another star is expected to pass around 1000 AU from the sun. Chemical rockets should be able to get there in a decade or two. It's a huge stretch, but we have a million years to prepare.

Gilese 710?

https://en.wikipedia.org/wiki/Gliese_710

Re: Oumuamua may be a hydrogen iceberg

#93

Post by btilly » Thu, Jun 04, 2020, 5:47 PM UTC

valuearb wrote: post #86

btilly wrote: post #27

Earlier quoted context omitted.

Matching speeds with it for this operation would take much better rockets than we have available.

A Starship sized vehicle using the well tested NERVA engine would be able to add around 20 km/sec to the velocity it already has in earth orbit. So pretty close and gravity assists would increase that. Once you dock with Oumuamu, you can run it’s material through at an 890 ISP and gradually accelerate to much higher speeds.

You underestimate the problem.

First you have to add something like 40 km/sec to go from Earth orbit to escape into interstellar space, and then you have to add another 20 km/sec to match speeds with Oumuanu. That's 60 km/sec, which is 3 times the speed and takes 9 times the energy.

Pioneer and Voyager got to interstellar space with gravity assists. But those work best when you barely get to a planet's orbit, then it picks you up and you go close to 180 around it. (Thereby adding 2x its velocity to your own.) Once you start getting to the velocities needed for Oumuamu, you just shoot by the planet and don't get much of an assist. (Assuming, that is, that there is an alignment between the planets and a random interstellar visitor. Which is extremely unlikely.)

I stand by my statement. We do not have good enough rockets for this maneuver.

Re: Oumuamua may be a hydrogen iceberg

#94

Post by manquer » Thu, Jun 04, 2020, 7:20 PM UTC

TeMPOraL wrote: post #91

manquer wrote: post #80

Earlier quoted context omitted.

You could always impact/land violently with it without having to match its trajectory. If your instruments are designed to survive the impact , it could work.

You can, but you only save as much velocity as the difference during the impact. Can't think of the case where that is, at the same time, both a useful saving and not fast enough to melt your equipment from a high-velocity impact.

Well is it not cheaper(by payload mass) to have higher protection to your equipment than more fuel ? Any benefit is useful , and this could be substantial depending on the strength of materials used

Re: Oumuamua may be a hydrogen iceberg

#95

Post by TeMPOraL » Thu, Jun 04, 2020, 8:00 PM UTC

manquer wrote: post #94

TeMPOraL wrote: post #91

Earlier quoted context omitted.

You can, but you only save as much velocity as the difference during the impact. Can't think of the case where that is, at the same time, both a useful saving and not fast enough to melt your equipment from a high-velocity impact.

Well is it not cheaper(by payload mass) to have higher protection to your equipment than more fuel ? Any benefit is useful , and this could be substantial depending on the strength of materials used

I imagine that, mass for mass, it isn't (of the extra fuel you carry, each bit you expend means the rest has that much less mass to push). Also, I can't think of means of protection that would withstand a high velocity collision with space gravel, that wouldn't be at the same time prohibitively heavy.

Re: Oumuamua may be a hydrogen iceberg

#96

Post by yencabulator » Sat, Jun 06, 2020, 1:59 AM UTC

motoboi wrote: post #42

fenwick67 wrote: post #3

This is great sci-fi fodder, imagine if you could land a tiny craft on that thing and then use the hydrogen as fuel without having to haul all that fuel into deep space (very unlikely to be practical but the idea is exciting).

Imagine you just land a tiny craft on that thing and let the thing take you to interestelar space without you having to burn no more fuel. Unfortunately, if you can't catch it, you don't need it for that, as you'll be traveling faster that it to be able to intercept it and would just burn fuel trying to break enough to match its speed.

If you match its speed, sure you'd be already independently traveling to interstellar space at the same speed as it was.

But landing on it and using the free large fuel tank of hydrogen it has now became you can 1) accelerate more 2) decelerate at your destination 3) steer (at least a bit).