What these proposals like to forget (even if addressing everything else) is that you need to slow down once you arrive if you want to have any time at all for useful observation once you reach your destination.
What's the point of reaching alpha centauri in 30 years if you're gonna zip past everything interesting in seconds? Will the sensors we can cram on tiny probes even be able to capture useful data at all under these conditions?
Jupiter is 43 lightminutes from the Sun.
If we shoot a thousand probes at 0.1c directly at the Alpha Centauri star, they should have several hours within a Jupiter-distance range of the star to capture data. Seems like enough sensors and time to synthesize an interesting image of the system when all that data gets back to Earth.
Could the probe just fire off some mass when it got there?
Any mass that it fires would have a starting velocity equal to that of the probe, and would need to be accelerated an equal velocity in the opposite direction. It would be a smaller mass, so it would require less fuel than decelerating the whole probe; but it's still a hard problem.
Be careful with the word "just". It often makes something hard sound simple.
Not trying to oversimplify. But suppose 95% of the probe's mass was intended to be jettisoned ahead of it on arrival by an explosive charge, and would then serve as a reflector. That might give enough time for the probe to be captured by the star's gravity...?
It seems to me that building a recording device that can survive in space, that it's very light, and that can not break apart after receiving the impact from an explosive charge strong enough to decelerate it from the speeds that would take it to Alpha Centauri is... maybe impossible.
We're talking about 0.2 light years. To reach it in 20 years, that's 1/10th of the speed of light. The forces to decelerate that are pretty high.
I did a quick napkin calculation (assuming the device weighs 1kg), that's close to 3000 kiloNewton, if it has 10 seconds to decelerate. The thrust of an F100 jet engine is around 130 kN.
IANan aerounatics engineer, so I could be totally wrong.
You’re just talking about a very inefficient rocket (bad ISP).
A rocket works the same way (accelerating mass to provide thrust), just far more efficiently and in a more controlled fashion.