Oumuamua may be a hydrogen iceberg • Hacker News Board

Re: Oumuamua may be a hydrogen iceberg

#81

Post by perihelions » Thu, Jun 04, 2020, 1:05 AM UTC

monadic2 wrote: post #2

Makes me wonder if you could slingshot around the sun towards relativistic speeds with thermal power. At the very least it could save fuel on the egress, if not the second half. Can you aerobrake around a star?

"if you could slingshot around the sun towards relativistic speeds with thermal power"

Unfortunately, no. The boost you can steal from a powered flyby is only of the same order as the escape velocity of the massive object. For the sun, this is 617 km/s at its surface, or 0.002 c. The term you're interested in is "Oberth effect".

To get relativistic speeds on the table, you'd need exotic, compact objects whose escape velocities are relativistic. I.e. white dwarfs, neutron stars & black holes.

No one asked, but: Freeman Dyson pointed out [0] some astounding properties of a short-period white dwarf binary. (Particularly: there are two stars; both are extremely dense; and they are orbiting around each other at an extremely high relative velocity). If you could perform a close slingshot maneuver around one of these stars, you would be accelerated to relativistic speeds in mere seconds. What's more, this would be an entirely unpowered, propellantless manuevuer (stealing orbital momentum from the binary system). And finally, as an incredible example of the equivalence principle, human passengers wouldn't even notice the insane ~10^4 g's of acceleration: it's equivalent to free-fall.

[0] This is a PDF https://www.ifa.hawaii.edu/~barnes/ast242_s14/Dyson_Machines... (1963)

Re: Oumuamua may be a hydrogen iceberg

#82

Post by manquer » Thu, Jun 04, 2020, 1:11 AM UTC

jamestimmins wrote: post #33

"It has now passed beyond Saturn’s orbit and will travel another 10,000 years before exiting the system". How big is our solar system? I always assumed it referred to everything between the sun and Pluto, but 10,000 years suggests it's far larger.

There are many definitions on the limit . Depends on how you see it, one definition not commonly mentioned is the size of the hill sphere [1] . Depending on the direction you look at the size of hill sphere changes .

- [1] https://en.wikipedia.org/wiki/Hill_sphere

Re: Oumuamua may be a hydrogen iceberg

#83

Post by andrewflnr » Thu, Jun 04, 2020, 1:32 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.

Right, you want to wait until the next one is inbound and catch it while it's still coming towards you. You just have to design the probe to survive a "landing" at several kilometers per second. I'm sure we can figure something out.

Re: Oumuamua may be a hydrogen iceberg

#84

Post by marcosdumay » Thu, Jun 04, 2020, 2:05 AM UTC

dmurray wrote: post #77

ddxxdd wrote: post #63

Earlier quoted context omitted.

Thermal power requires a reservoir of cold temperature; your car's engine would do nothing if the incoming air was just as hot as the post-ignited air.

I'm sure this is true in practice, but I can't wrap my head round why it's correct in theory. Let's say I hook up my exhaust to my intake, I can still squirt some gasoline into the engine, make it explode and expand and thereby drive a piston, at the cost of having even hotter air the next time round. If I repeat this long enough, my engine will melt, but that's a minor matter of materials science. Maybe I can win my…

If you heat it more each time, yeah, the old air can act as a cold reservoir and make things run.

If you do draw it with enough detail, you'll notice that if you do not heat it more at each turn, you won't be able to get any pressure difference at your "explosion".

Re: Oumuamua may be a hydrogen iceberg

#85

Post by monadic2 » Thu, Jun 04, 2020, 2:25 AM UTC

ddxxdd wrote: post #63

monadic2 wrote: post #2

Makes me wonder if you could slingshot around the sun towards relativistic speeds with thermal power. At the very least it could save fuel on the egress, if not the second half. Can you aerobrake around a star?

Thermal power requires a reservoir of cold temperature; your car's engine would do nothing if the incoming air was just as hot as the post-ignited air.

Yes, I was envisioning solid hydrogen fuel powering by outgassing as envisioned in the article.

Re: Oumuamua may be a hydrogen iceberg

#86

Post by valuearb » Thu, Jun 04, 2020, 5:14 AM UTC

btilly wrote: post #27

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).

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.

Re: Oumuamua may be a hydrogen iceberg

#87

Post by ajuc » Thu, Jun 04, 2020, 6:05 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 it's speed you already burned all the fuel you'd have to burn to get where it's going.

Re: Oumuamua may be a hydrogen iceberg

#88

Post by monadic2 » Thu, Jun 04, 2020, 7:31 AM UTC

DarmokJalad1701 wrote: post #71

monadic2 wrote: post #2

Makes me wonder if you could slingshot around the sun towards relativistic speeds with thermal power. At the very least it could save fuel on the egress, if not the second half. Can you aerobrake around a star?

> slingshot around the sun You can only slingshot around the sun if you were already in a hyperbolic orbit w.r.t the sun. An object in a closed helio centric orbit cannot gain any momentum by "slingshotting" around the sun.

It can if it loses mass, correct? Energy conservation would result in increased momentum.

Re: Oumuamua may be a hydrogen iceberg

#89

Post by [deleted] » Thu, Jun 04, 2020, 12:28 PM UTC

DarmokJalad1701 wrote: post #71

monadic2 wrote: post #2

Makes me wonder if you could slingshot around the sun towards relativistic speeds with thermal power. At the very least it could save fuel on the egress, if not the second half. Can you aerobrake around a star?

> slingshot around the sun You can only slingshot around the sun if you were already in a hyperbolic orbit w.r.t the sun. An object in a closed helio centric orbit cannot gain any momentum by "slingshotting" around the sun.

[deleted]

Re: Oumuamua may be a hydrogen iceberg

#90

Post by fennecfoxen » Thu, Jun 04, 2020, 12:29 PM UTC

monadic2 wrote: post #88

DarmokJalad1701 wrote: post #71

Earlier quoted context omitted.

> slingshot around the sun You can only slingshot around the sun if you were already in a hyperbolic orbit w.r.t the sun. An object in a closed helio centric orbit cannot gain any momentum by "slingshotting" around the sun.

It can if it loses mass, correct? Energy conservation would result in increased momentum.

Yes. Your mass in low Sol orbit has a much higher specific orbital energy, so thrusting a rocket or ejecting mass while at or near perihelion makes more of a difference than doing so in deep space.