What’s the minimum number of people you should send in a generational ship to Proxima Centauri?

[ravi_iitian]

“They then averaged the results of these simulations over 100 interstellar journeys based on these various factors and different values to determine the size of the minimum crew. In the end, Dr. Marin and Dr. Beluffi concluded that under conservative conditions, a minimum of 98 crew members would be needed to sustain a multi-generational voyage to the nearest star system with a potentially-habitable exoplanet.”

https://www.universetoday.com/139456/whats-the-minimum-number-of-people-you-should-send-in-a-generat...

[reinierb]

---

@Ravindra B wrote:

“They then averaged the results of these simulations over 100 interstellar journeys based on these various factors and different values to determine the size of the minimum crew. In the end, Dr. Marin and Dr. Beluffi concluded that under conservative conditions, a minimum of 98 crew members would be needed to sustain a multi-generational voyage to the nearest star system with a potentially-habitable exoplanet.”

https://www.universetoday.com/139456/whats-the-minimum-number-of-people-you-should-send-in-a-generat...

Humbug! There is no way a mathematical model can acount for all possibilities and eventualities, no matter how many times you "loop" it. Thus, there is no way to calculate the ideal crew size. 

A good starting point in my view would be to send as many people as you can fit into the biggest ship you can construct- simply because no computer or mathematical model can predict what would happen if one half of the crew decides it does not like the other half.

Sometimes it is hard enough for people to get along on a drive across town- imagine what might happen when people (who may or may not like each other) are forced to live together in close proximity for several hundred years.

No mathematical model can predict those sort of dynamics, so for the moment, this study is just so much pie in the sky.   

---

[ravi_iitian]

---

@Reinier B wrote:

---

@Ravindra B wrote:

“They then averaged the results of these simulations over 100 interstellar journeys based on these various factors and different values to determine the size of the minimum crew. In the end, Dr. Marin and Dr. Beluffi concluded that under conservative conditions, a minimum of 98 crew members would be needed to sustain a multi-generational voyage to the nearest star system with a potentially-habitable exoplanet.”

https://www.universetoday.com/139456/whats-the-minimum-number-of-people-you-should-send-in-a-generat...

Humbug! There is no way a mathematical model can acount for all possibilities and eventualities, no matter how many times you "loop" it. Thus, there is no way to calculate the ideal crew size. 

A good starting point in my view would be to send as many people as you can fit into the biggest ship you can construct- simply because no computer or mathematical model can predict what would happen if one half of the crew decides it does not like the other half.

Sometimes it is hard enough for people to get along on a drive across town- imagine what might happen when people (who may or may not like each other) are forced to live together in close proximity for several hundred years.

No mathematical model can predict those sort of dynamics, so for the moment, this study is just so much pie in the sky.   

---

---

Would a cannabis lab help? 

Or, perhaps, we can use Noah's idea of twosomes.

Wait! What about some polygamists. 

[resultsassoc]

I kind of agree with Rene. Current technology limits us to about 38,500 mph (about 61,000 kph). That would take longer than homo sapiens has existed.

On the other hand, a solar-sail-powered ship will eventually reach about 0.1c. It might take 45 years to reach that speed, meaning 45 years to decelerate at the other end. At 0.1c the period between acceleration and deceleration would take about 45 years. So, 135 years might be do-able.

Crew of 40 adults and 40 children, plus several thousand frozen embryos, would do it. Unthaw a batch of embryos every 25 years, say 40, and recycle everything including human waste and corpses. Now, 135 years hence, you are in orbit around a human-compatible planet in the Alpha Proxima system. If such a planet exists.

I have no clue how to land the thing.

[reinierb]

---

@Bill H wrote:

I kind of agree with Rene. Current technology limits us to about 38,500 mph (about 61,000 kph). That would take longer than homo sapiens has existed.

On the other hand, a solar-sail-powered ship will eventually reach about 0.1c. It might take 45 years to reach that speed, meaning 45 years to decelerate at the other end. At 0.1c the period between acceleration and deceleration would take about 45 years. So, 135 years might be do-able.

Crew of 40 adults and 40 children, plus several thousand frozen embryos, would do it. Unthaw a batch of embryos every 25 years, say 40, and recycle everything including human waste and corpses. Now, 135 years hence, you are in orbit around a human-compatible planet in the Alpha Proxima system. If such a planet exists.

I have no clue how to land the thing.

---

 A solar sail might work as  apropulsion method if the solar wind were constant (which it is not), but even if it were, the problem is how to limit the ship's initial acceleration to under to 2 G's, and to keep it at under 2 G's to make it possible for people to move round the ship while performing routine tasks such as doing maintenance, producing food, cooking, driving the ship, and such. Since accelerations in space are cumulative, the people on board will become too massive to move even their eyelids long before the ship reaches even a fraction of 0.1c. 

Then the problem repeats itself as the ship slows down, presumably with the combined solar winds from the Alpha Centauri system stars. Only this time, the crews' bodies have to adapt to their decreasing mass, which according to research done by NASA, can have unpredictable, if not always fatal consequences. 

For the moment, we are stuck on Earth, and we won't be leaving it any time soon.   

[resultsassoc]

Controlling acceleration isn't, pardon the word, rocket science. At a steady acceleration of 1G it is my undersanding that one has reached c by the time one exits the solar system. Therefore, when one has reached 0.1c, fold the sail. Redeploy about the same distance from destination.

[reinierb]

---

wrote:

Controlling acceleration isn't, pardon the word, rocket science. At a steady acceleration of 1G it is my undersanding that one has reached c by the time one exits the solar system. Therefore, when one has reached 0.1c, fold the sail. Redeploy about the same distance from destination.

---

 Assuming of course, that the solar wind blowing off your target star is as fast and dense as the Sun's. In practice, you'll need a lot of rocket science to stop the ship if this is not the case.  

[richard_wein]

---

@Bill H wrote:

Controlling acceleration isn't, pardon the word, rocket science. At a steady acceleration of 1G it is my undersanding that one has reached c by the time one exits the solar system. Therefore, when one has reached 0.1c, fold the sail. Redeploy about the same distance from destination.

---

Actually, it would take approximately a year of 1g acceleration to approach a velocity of c, and in that time you would have travelled about half a light year. For 0.1c divide by 10.

ETA. What was I thinking? Divide the time by 10. Divide the distance by 100.

An acceleration of 1g would be comfortable, but maintaining that acceleration for any length of time would be incredibly hard to do (even ignoring relativistic effects, which we can do in the case 0.1c).

[richard_wein]

I think Reinier's original point was that solar wind is only effective while you're still relatively close to the sun. So you would need to achieve extremely high accelerations (far more than 1g) while you're close to the sun, or else you'll never reach 0.1c.

[richard_wein]

I just found this table of figures for solar wind scenarios:

The last row is the one closest to the scenario we're talking about (0.13c instead of 0.1). You would start off heading towards the sun, and go round the sun, passing as close as 0.019 AUs. Then open your gigantic ultrathin sail and achieve some body-crushing acceleration (not stated), which would fall to a mere 100m/s^2 (10g) by the time you're back to Earth, getting you to Pluto in just 1.7 days. Sounds like fun!

Wimps might prefer the 0.042c scenario.

[reinierb]

---

@Richard W wrote:

I just found this table of figures for solar wind scenarios:

The last row is the one closest to the scenario we're talking about (0.13c instead of 0.1). You would start off heading towards the sun, and go round the sun, passing as close as 0.019 AUs. Then open your gigantic ultrathin sail and achieve some body-crushing acceleration (not stated), which would fall to a mere 100m/s^2 (10g) by the time you're back to Earth, getting you to Pluto in just 1.7 days. Sounds like fun!

Wimps might prefer the 0.042c scenario.

---

 Lots of rocket science here to discuss, so to those who might have thought that using the solar wind for anything useful is no more complicated than deploying and furling a sail, you are welcome to join in.  

[richard_wein]

Reinier B wrote:

 Lots of rocket science here to discuss, so to those who might have thought that using the solar wind for anything useful is no more complicated than deploying and furling a sail, you are welcome to join in.  

---

Here's the report that the table came from…

http://www.niac.usra.edu/files/studies/final_report/333Christensen.pdf

That's based on solar radiation pressure, not solar wind. But I also found an article about using solar wind:

https://www.scientificamerican.com/article/spacecraft-solar-sail/

It seems like NASA is taking these ideas seriously, but only for small unmanned missions. A multigenerational ark would probably need a sail with an area of millions of square kilometres.

[resultsassoc]

0.04c accelerates at 1g and makes the trip in 101 years, shorter than my estimated 135 years. Juvenile Ocean Quahogs would just be entering their adolesence.

[richard_wein]

---

@Bill H wrote:

0.04c accelerates at 1g and makes the trip in 101 years, shorter than my estimated 135 years. Juvenile Ocean Quahogs would just be entering their adolesence.

---

Unfortunately, 1g is just the acceleration at the moment the ship gets back to 1 AU distance from the sun (the distance of the Earth from the sun). The intial acceleration would have to be much higher. I'm no physicist, but it seems to me that the solar pressure--and therefore the acceleration--would fall off with the square of the distance from the sun. So the intial acceleration, at 0.019 AU, would be over 25g.

(There seems to be an inconsistency between the table and the accompanying text, which implies that the final two scenarios involve approaching the sun to within 4 solar radiuses. That's about 10 times closer!)

Another problem is that this scenario doesn't allow for slowing down at the other end of the journey. The sail is a one-use thing, so you couldn't use it again at the other end. Taking another sail with you would be equivalent to doubling the mass of the sail, which would halve the acceleration, so you wouldn't be able to reach 0.042c. (According to the report, halving the acceleration would increase the trip length by a factor of 1.4, which presumably means that the terminal velocity would be reduced by approximately that factor, to 0.03c.)

ETA. I suppose in principle you could leave the sail in place throughout the journey, and so use it again at the other end. Since we're assuming that this 2 nm thick sail could remain intact throught the acceleration period, then I guess we might as well assume it can remain intact throughout the journey.

[reinierb]

---

@Richard W wrote:

---

@Bill H wrote:

0.04c accelerates at 1g and makes the trip in 101 years, shorter than my estimated 135 years. Juvenile Ocean Quahogs would just be entering their adolesence.

---

Unfortunately, 1g is just the acceleration at the moment the ship gets back to 1 AU distance from the sun (the distance of the Earth from the sun). The intial acceleration would have to be much higher. I'm no physicist, but it seems to me that the solar pressure--and therefore the acceleration--would fall off with the square of the distance from the sun. So the intial acceleration, at 0.019 AU, would be over 25g.

(There seems to be an inconsistency between the table and the accompanying text, which implies that the final two scenarios involve approaching the sun to within 4 solar radiuses. That's about 10 times closer!)

Another problem is that this scenario doesn't allow for slowing down at the other end of the journey. The sail is a one-use thing, so you couldn't use it again at the other end. Taking another sail with you would be equivalent to doubling the mass of the sail, which would halve the acceleration, so you wouldn't be able to reach 0.042c. (According to the report, halving the acceleration would increase the trip length by a factor of 1.4, which presumably means that the terminal velocity would be reduced by approximately that factor, to 0.03c.)

ETA. I suppose in principle you could leave the sail in place throughout the journey, and so use it again at the other end. Since we're assuming that this 2 nm thick sail could remain intact throught the acceleration period, then I guess we might as well assume it can remain intact throughout the journey.

Another issue is that most, if not all calculations of this sort assume that space is a vacuum. It is not, which means that impacts on the sail by gas molecules and dust grains from opposite the direction of travel would have a cumulative braking effect on the ship.

Thus, radiation pressure during the first half of the journey must be consistently high enough to overcome all opposite accelerations, which seems unlikely to be possible beyond the inner solar system.    

---