5 ms·

I think we might be able to design something that would be able to absorb most of the energy as well as possibly decelerate the probe impact like an explosive c

I think we might be able to design something that would be able to absorb most of the energy as well as possibly decelerate the probe impact like an explosive charge that won’t work that well as an impulse engine in vacuum but would work against an object. Some of these objects are also not very dense so some ablative enclosure might also work.

sbierwagen 7y ago
30 kilometers per second is 67,108 miles per hour. That's a lot of probe impact energy to absorb.
- tomrod 7y ago
  Something like a Newton's cradle with different radii? One to deform on impact, another to take a sizable portion of the force?
- jsjohnst 7y ago
  What if it “landed” (more like impacted) at a 90° angle to the comet’s direction of travel? Discounting the extreme difficulty of timing that right (like trying to hit a bullet with a smaller bullet, whilst wearing a blindfold, riding a horse — haha), wouldn’t it dramatically decrease the impact energy?
  - Pfhreak 7y ago
    Think about a car travelling 60mph, and you want to climb aboard. You can either jump on the hood (and likely get made into paste) or jump onto the side and immediately have a 60mph 'jerk' forward. In either case, you are going from 0 to 60 more or less instantly and the results on your fleshy meat body will be the same. You could try to run real fast in front of the car, away from it, but you are limited by your meat body's technology to ~10mph. You still hit the car at a difference of 50mph. Meat paste once again. To avoid a painful collision, you'd need to get up to speeds of about 57mph, which is pretty hard for your body to do. And if you could get up to 57mph, why do you need tha car? There's no friction in space, so you'd just keep going until something bumped you.
    - dogma1138 7y ago
      If you lasso the car with a deceleration rig or an elastic cord you can accelerate safely tho because it adds delay to how fast the energy is transferred to you.
      - Pfhreak 7y ago
        Yep, the trick is that the lasso needs to be able to stretch enough to accelerate more slowly. You'd also need whatever harpoon you fired into the comet to be able to survive thousands of times its own mass in acceleration.
ncmncm 7y ago
This is not a ridiculous approach. Forget about crashing into it. Instead, imagine a very large, very stretchy net. Large meaning many, many kilometers long. On impact, only the nearly massless front bit of the net needs to accelerate instantly. The acceleration gradually spreads back to the probe, which is then accelerated gradually to match the object. After it matches, there is still a great deal of energy in the stretched-out net, which in principle could accelerate your probe to be as much faster than the object as it started out slower than. Another way to think of this is with a non-stretchy tether perpendicular to the object's path. The object hits one end, and the probe swings around behind it, gaining 2x before it gets to the other side. It could let go at various other points to head off in some other direction at less than 2x. This is not too different from what we do when we fly probes past planets to give them a speed boost. In that case, the tether is gravity.
- colanderman 7y ago
  So if my back of the envelope math is correct (and I've probably screwed something up as it's past midnight here), to accelerate a 100 kg probe to the speed of the object (30 km/s) at a maximum of 1000 m/s² using a steel spring would require a spring of modulus about 0.11 kg/s². (For reference, a slinky is less "stretchy" at 0.8 kg/s².) It would take about 47 seconds, and the spring would elongate by a maximum of 530 km. To withstand this tension (100 kN), the steel wire (ultimate tensile strength ~400 MPa) would need to have a cross-sectional area of 250 mm², or diameter of 16 mm. Given this wire diameter, and a Young's modulus for steel of 200 GPa, the spring itself would require a diameter of ~42 cm and ~200,000 turns, if we allow it to stretch completely during acceleration. Fully relaxed, the spring would be 3.2 km in length. This works out to at least 130 m³ of steel, or 1000 metric tons, which is somewhere around USD$500k. Eminently affordable. Of course, that's a drop in the bucket compared to the cost of launching a 3.2 km long, 1000 metric ton slinky into outer space.
  - ncmncm 7y ago
    Steel is not usually chosen when elasticity or tensile strength per unit mass are goals. I am actually more worried about the propagation of tension in the tether being restricted to the speed of sound in the material. It seems like the end would just snap off, for any realizable material.

sbierwagen 7y ago

30 kilometers per second is 67,108 miles per hour. That's a lot of probe impact energy to absorb.

tomrod 7y ago
Something like a Newton's cradle with different radii? One to deform on impact, another to take a sizable portion of the force?
jsjohnst 7y ago
What if it “landed” (more like impacted) at a 90° angle to the comet’s direction of travel? Discounting the extreme difficulty of timing that right (like trying to hit a bullet with a smaller bullet, whilst wearing a blindfold, riding a horse — haha), wouldn’t it dramatically decrease the impact energy?
- Pfhreak 7y ago
  Think about a car travelling 60mph, and you want to climb aboard. You can either jump on the hood (and likely get made into paste) or jump onto the side and immediately have a 60mph 'jerk' forward. In either case, you are going from 0 to 60 more or less instantly and the results on your fleshy meat body will be the same. You could try to run real fast in front of the car, away from it, but you are limited by your meat body's technology to ~10mph. You still hit the car at a difference of 50mph. Meat paste once again. To avoid a painful collision, you'd need to get up to speeds of about 57mph, which is pretty hard for your body to do. And if you could get up to 57mph, why do you need tha car? There's no friction in space, so you'd just keep going until something bumped you.
  - dogma1138 7y ago
    If you lasso the car with a deceleration rig or an elastic cord you can accelerate safely tho because it adds delay to how fast the energy is transferred to you.
    - Pfhreak 7y ago
      Yep, the trick is that the lasso needs to be able to stretch enough to accelerate more slowly. You'd also need whatever harpoon you fired into the comet to be able to survive thousands of times its own mass in acceleration.

ncmncm 7y ago

This is not a ridiculous approach. Forget about crashing into it. Instead, imagine a very large, very stretchy net. Large meaning many, many kilometers long. On impact, only the nearly massless front bit of the net needs to accelerate instantly. The acceleration gradually spreads back to the probe, which is then accelerated gradually to match the object. After it matches, there is still a great deal of energy in the stretched-out net, which in principle could accelerate your probe to be as much faster than the object as it started out slower than. Another way to think of this is with a non-stretchy tether perpendicular to the object's path. The object hits one end, and the probe swings around behind it, gaining 2x before it gets to the other side. It could let go at various other points to head off in some other direction at less than 2x. This is not too different from what we do when we fly probes past planets to give them a speed boost. In that case, the tether is gravity.

colanderman 7y ago
So if my back of the envelope math is correct (and I've probably screwed something up as it's past midnight here), to accelerate a 100 kg probe to the speed of the object (30 km/s) at a maximum of 1000 m/s² using a steel spring would require a spring of modulus about 0.11 kg/s². (For reference, a slinky is less "stretchy" at 0.8 kg/s².) It would take about 47 seconds, and the spring would elongate by a maximum of 530 km. To withstand this tension (100 kN), the steel wire (ultimate tensile strength ~400 MPa) would need to have a cross-sectional area of 250 mm², or diameter of 16 mm. Given this wire diameter, and a Young's modulus for steel of 200 GPa, the spring itself would require a diameter of ~42 cm and ~200,000 turns, if we allow it to stretch completely during acceleration. Fully relaxed, the spring would be 3.2 km in length. This works out to at least 130 m³ of steel, or 1000 metric tons, which is somewhere around USD$500k. Eminently affordable. Of course, that's a drop in the bucket compared to the cost of launching a 3.2 km long, 1000 metric ton slinky into outer space.
- ncmncm 7y ago
  Steel is not usually chosen when elasticity or tensile strength per unit mass are goals. I am actually more worried about the propagation of tension in the tether being restricted to the speed of sound in the material. It seems like the end would just snap off, for any realizable material.