# I Questions about nano spacecraft

1. May 11, 2016

### Nick Martin

2 types of nano spacecraft are planned to travel to Alpha Centauri , 4.2 light years away. One powered by an Earthbound laser, another self powered.

- The report I read talked about an average speed of 25% of light speed. However, it is constantly accelerating during the trip won't the average be skewed as it gets towards a significant fraction of light speed?

- If it does achieve this speed how long will the voyage seem to the spacecraft? From our point of view we are told 16 years, but won't it take the craft a lot less from ITS OWN point of view?

- What gain in mass will it have at 25% light speed?

- What will happen if it hits a planet at that relative velocity ? Most Solar System meteorites are going at only 15k or 20k on impact . What happens if 1 or 2kg hit a planet at 30million miles per hour?

- Its signals back to us are expected in 20 years - 16 years for travel and 4 for the return journey of the message. I assume the frequency of the waves will need massive compensation, as it will be receding from us when it sends. (the message still arrives at light speed, of course, just a change in frequency)

- The earth bound laser will need to have constant adjustment as the Earth turns and orbits the sun. HOw will it know if it is on target when the signals back are minutes, hours, days and years away?

I am hoping this is not another Elon Musk spoof, and we can all have loads of fun while they try to make it happen!!

Last edited by a moderator: May 11, 2016
2. May 11, 2016

### phyzguy

The probe would be accelerated rather quickly up to 25% of the speed of light, then would spend most of the journey coasting.

A little bit less. The time dilation factor $\gamma = \frac{1}{\sqrt{1-\frac{v^2}{c^2}}}$ is actually pretty small at this speed. Try calculating it.
Again it is small, see the previous answer
It would be instantly vaporized. But the odds of there being something in the way are quite small.
That's correct. But this is easy to calculate and compensate the receiver here on Earth for.
I think the laser beam would be larger than the solar system when it arrives back here. So no need for fine pointing, and we would just be picking up a small part of the signal.

3. May 11, 2016

### Nick Martin

"It would be instantly vaporized. But the odds of there being something in the way are quite small."

Yes but what would happen to the planet it hit?
And the odds are that it will hit something eventually before the end of the universe!!

"think the laser beam would be larger than the solar system when it arrives back here. So no need for fine pointing, and we would just be picking up a small part of the signal."

Sorry I didn't make this very clear - What I meant in the question was suppose the laser beam propelling the craft starts to miss its target - there is no feedback to the laser team for minutes/houors/days/years depending on how far it has travelled, and when they make the next adjustment it might have veered off again.

4. May 11, 2016

### phyzguy

It doesn't have all that much energy. A 1kg object going 25% of the speed of light has about the same kinetic energy as a small asteroid about 20 meters in diameter going at 10 km/sec. There are millions of these objects in our solar system, and planets get hit all the time.

I misunderstood your last question. I don't know the answer, but I think it's possible to design the light sail so that it stays in the middle of the beam, either by shaping the light sail or by using active feedback to keep it in the beam. So you don't rely on the ground to keep it perfectly pointed. Remember that the acceleration happens fairly quickly (I think in a matter of minutes) so that the probe isn't that far away when the acceleration is done.

5. May 12, 2016

### Nick Martin

Thanks for this. Clearly it's not going to spoil anybody's day in Alpha Centauri.
I don't think the acceleration happens quickly, I think it happens over months if not years. It has to get to 300m MPH which even at 1g would take 347 days. I think the acceleration is a lot less than 1G. If it was only 10 mins it would need to accelerate at 500,000G!!!
Nick

6. May 12, 2016

### phyzguy

This web site says the probes would reach 20% of the speed of light in 2 minutes, which is 60,000 g. As a check, 20% of c is 6E7 m/s, so a = 6E7m/s / 100 s = 6E5 m/s^2 = 60,000g