4 ms·

Matching DeltaV for a short duration is easier than for a long duration, no? I’m not an astrophysicist, but that seems to make sense to me.

sigstoat 7y ago
> Matching DeltaV for a short duration is easier than for a long duration, no? you don't match deltav, you'd match trajectory. you may be able to do that at the cost of more or less deltav, depending on how clever and patient you are. once you've matched trajectory, you'll (basically) stay matched, as you're in space and there's nothing to disturb you.
SiempreViernes 7y ago
No, that doesn't make much sense does it? Delta V is a change in velocity, but comets don't change velocity very much. That remark makes more sense in the context of missile defence than when talking about space travel.
- jsjohnst 7y ago
  Sorry, yeah, definitely was very poorly worded. I meant to say: From what I know from reading on Helios-A and Helios-B, we already have a probe which can go over twice the speed of what I thought I saw the comet is traveling at. So couldn’t the probe match its velocity (dunno why I said Delta V before) to the comet, if even just for long enough to land without total destruction?
  - SiempreViernes 7y ago
    We probably could if it's travelling at low enough velocity. But if you match its orbit you're just going the same place at it is anyway, at which point "piggybacking" is just code for "getting some company". And if you don't match it well enough that you end up roughly the same place you're going to have a fast, and therefore violent, encounter.
dragonwriter 7y ago
You match velocity, not deltaV. Since natural objects accelerate only under environmental forces (mainly gravity for something like this), once you match velocity and are subject to essentially the same gravitational fields, you will stay matched (roughly).
- jsjohnst 7y ago
  I shouldn’t have asked the question while commuting, clearly didn’t explain myself well. Elsewhere I saw what I thought was the speed of the comet, which was around 69k mph. Didn’t Helios 1 and 2 do something like ~150k mph[0]? Assuming those numbers are correct (please say so if not), then what would stop an intercept from being technically possible (even if very very very hard)? [0] https://en.m.wikipedia.org/wiki/Helios_(spacecraft) https://en.m.wikipedia.org/wiki/Helios_(spacecraft)
  - bzbarsky 7y ago
    The Helios probes reached those speeds by starting out much slower than that and then falling toward the sun, speeding up in the process. To actually hitch a ride with another object you have to match their velocity _and_ their location at the same time. If the object you are trying to catch started further from the sun than you, and was already moving faster than you, then you can't match its speed and location by falling toward the sun: when you arrive at the same location, it will have fallen further than you and hence picked up more kinetic energy per unit mass than you did, and it started off moving faster than you to start with, so it's still moving faster.
jsjohnst 7y ago
s/DeltaV/velocity and trajectory/ Dunno why I said DeltaV, but clearly it was inappropriate.

sigstoat 7y ago

> Matching DeltaV for a short duration is easier than for a long duration, no? you don't match deltav, you'd match trajectory. you may be able to do that at the cost of more or less deltav, depending on how clever and patient you are. once you've matched trajectory, you'll (basically) stay matched, as you're in space and there's nothing to disturb you.

SiempreViernes 7y ago

No, that doesn't make much sense does it? Delta V is a change in velocity, but comets don't change velocity very much. That remark makes more sense in the context of missile defence than when talking about space travel.

jsjohnst 7y ago
Sorry, yeah, definitely was very poorly worded. I meant to say: From what I know from reading on Helios-A and Helios-B, we already have a probe which can go over twice the speed of what I thought I saw the comet is traveling at. So couldn’t the probe match its velocity (dunno why I said Delta V before) to the comet, if even just for long enough to land without total destruction?
- SiempreViernes 7y ago
  We probably could if it's travelling at low enough velocity. But if you match its orbit you're just going the same place at it is anyway, at which point "piggybacking" is just code for "getting some company". And if you don't match it well enough that you end up roughly the same place you're going to have a fast, and therefore violent, encounter.

dragonwriter 7y ago

You match velocity, not deltaV. Since natural objects accelerate only under environmental forces (mainly gravity for something like this), once you match velocity and are subject to essentially the same gravitational fields, you will stay matched (roughly).

jsjohnst 7y ago
I shouldn’t have asked the question while commuting, clearly didn’t explain myself well. Elsewhere I saw what I thought was the speed of the comet, which was around 69k mph. Didn’t Helios 1 and 2 do something like ~150k mph[0]? Assuming those numbers are correct (please say so if not), then what would stop an intercept from being technically possible (even if very very very hard)? [0] https://en.m.wikipedia.org/wiki/Helios_(spacecraft) https://en.m.wikipedia.org/wiki/Helios_(spacecraft)
- bzbarsky 7y ago
  The Helios probes reached those speeds by starting out much slower than that and then falling toward the sun, speeding up in the process. To actually hitch a ride with another object you have to match their velocity _and_ their location at the same time. If the object you are trying to catch started further from the sun than you, and was already moving faster than you, then you can't match its speed and location by falling toward the sun: when you arrive at the same location, it will have fallen further than you and hence picked up more kinetic energy per unit mass than you did, and it started off moving faster than you to start with, so it's still moving faster.

jsjohnst 7y ago

s/DeltaV/velocity and trajectory/ Dunno why I said DeltaV, but clearly it was inappropriate.