Interstellar Travel in Gwynne Shotwell's Lifetime

[RotoSequence]

https://twitter.com/thesheetztweetz/status/1187745445361180672

Shotwell: "I think we will have a propulsion breakthrough in my lifetime that we can then say we will build a ship and start the journey" to the next potentially habitable solar system.

What kind of technologies could Ms Shotwell be referring to? People who have done the hard math on the rocket equation, energy densities, and efficiencies have shown that a fission based nuclear engine can achieve up to 0.05c, fusion engines can achieve up to 0.1c, and antimatter engines could hit up to 0.9c. Unprecedented developments in reliability engineering are required for any of these propulsion schemes, and fission based rocketry doesn't strike me as providing interstellar voyages on a useful time scale. What could Gwynne have in mind?

[daedalus1]

Antimatter is out of the question as it's too expensive to produce even a few atoms. Also the storage is a big issue.
 It can realistically only be fusion, which would be a reasonable travel time of less than a century to the nearest systems.
The Daedalus starship study by the British Interplanetary Society in the 1970's is a good example of what is required.

[Cheapchips]

There enough private sector effort in fusion that something should pay off.  Fusion that's viable for spaceflight should happen in the next decade.  But an actual decade this time. 

If the solar system is truly opened up then anti matter harvesting is potentially viable. Musk's mention that.

Fusion propulsion seems easy enough once reactors are viable.  No idea how fast the path to antimatter propulsion is once we have an actual supply.  It certainly feels way out there.

Gwynne reiterating interstellar ambitions and Musk's "we have a lot of talented engineers, maybe too many. They should perhaps go work on something else" musings * do make me think they're seriously thinking about pivoting R & D that way after Starship.

Fusion and antimatter drives are really for Mars anyway.  Interstellar travel will just be residual capacity. 


* Can't remember the exact words he used at the BC update

[sanman]

Does SpaceX really even look at anything beyond chemical propulsion? It sounds too distant for the near/medium term time horizon.

[RotoSequence]

Does SpaceX really even look at anything beyond chemical propulsion? It sounds too distant for the near/medium term time horizon.

No one's going interstellar on chemical propulsion. Even nuclear fission doesn't have enough energy density to do the job quickly, and that exceeds the energy density of chemical propellants by many orders of magnitude.

[Cheapchips]

Musk's anti matter tweets. 

Exactly. Super money move is anti-matter drive, especially if you solve capturing antiprotons in deep space. 

(Context, relative to using nuclear propulsion)

https://twitter.com/elonmusk/status/1098651639248875520?

Anti-matter production would require vast amounts of energy. No need though. We have a super reliable, massive, free fusion reactor called the sun. 

https://twitter.com/elonmusk/status/1098652491883765761?

So how challenging is a space born antiproton capture device if you throw hundreds of ultrasmart and driven engineers at it? 

[sanman]

Musk's anti matter tweets. 

Exactly. Super money move is anti-matter drive, especially if you solve capturing antiprotons in deep space. 

(Context, relative to using nuclear propulsion)

https://twitter.com/elonmusk/status/1098651639248875520?



So how challenging is a space born antiproton capture device if you throw hundreds of ultrasmart and driven engineers at it? 

Is this supposed to be something like a Bussard ramjet, but capturing anti-protons as well as protons? In which case, you'd probably want to exploit the charge difference somehow, to segregate them.

How is he sure that there are enough anti-protons available to be captured from the interstellar medium? Is there any data to support that?

[rakaydos]

Musk's anti matter tweets. 

Exactly. Super money move is anti-matter drive, especially if you solve capturing antiprotons in deep space. 

(Context, relative to using nuclear propulsion)

https://twitter.com/elonmusk/status/1098651639248875520?



So how challenging is a space born antiproton capture device if you throw hundreds of ultrasmart and driven engineers at it? 

Is this supposed to be something like a Bussard ramjet, but capturing anti-protons as well as protons? In which case, you'd probably want to exploit the charge difference somehow, to segregate them.

How is he sure that there are enough anti-protons available to be captured from the interstellar medium? Is there any data to support that?

More like slowly farming and storing naturally produced antiprotons until you have enough to fuel a rocket.

[Donosauro]

How is he sure that there are enough anti-protons available to be captured from the interstellar medium? Is there any data to support that?

More like slowly farming and storing naturally produced antiprotons until you have enough to fuel a rocket.

Centauri Dreams on the topic of collecting antimatter in space:
https://www.centauri-dreams.org/2016/08/03/antimatter-production-harvesting-in-space/

In short, it doesn't seem to be a reasonable approach. 

[edzieba]

How is he sure that there are enough anti-protons available to be captured from the interstellar medium? Is there any data to support that?

More like slowly farming and storing naturally produced antiprotons until you have enough to fuel a rocket.

Centauri Dreams on the topic of collecting antimatter in space:
https://www.centauri-dreams.org/2016/08/03/antimatter-production-harvesting-in-space/

In short, it doesn't seem to be a reasonable approach.

250 micrograms/year from Saturn = 45MJ/year

But that's for raw annihilation propulsion. Using antimatter to catalyse fission or fusion gets you much better performance per gram of antimatter carried.

[Robotbeat]

Beamed propulsion methods (particularly those that are beaming something below the speed of light) seem a promising approach for visiting the nearest star systems. Particularly when coupled with magnetosail braking. The nice thing about beaming is that the power source stays in the solar system, so you can send multiple ships per year with the same investment. Enables a stream of probes, cargo, and even settlers. A one-way trip is not nearly so lonesome when you know more people will arrive every few months.

[TrevorMonty]

Beamed propulsion methods (particularly those that are beaming something below the speed of light) seem a promising approach for visiting the nearest star systems. Particularly when coupled with magnetosail braking. The nice thing about beaming is that the power source stays in the solar system, so you can send multiple ships per year with the same investment. Enables a stream of probes, cargo, and even settlers. A one-way trip is not nearly so lonesome when you know more people will arrive every few months.

Good idea for travelling within this solar system and there are no technical show stoppers.

[scienceguy]

So I did some research as to how feasible it is to harvest antimatter from the interstellar medium. Apparently antiprotons are 10^-4 times as abundant as protons in the interstellar medium (Bambi and Dolgov, 2007). There is about 1 atom per cm^3 in the interstellar medium. Thus, there is about 0.1 antiproton per cubic meter in the interstellar medium.
 
What are the chances that this antiproton will hit a hydrogen molecule in outer space?

Mean free path of a fast moving particle in a gas: lambda = 1/(pi)ND^2 where N is the density of the molecules in molecules/cm^3, and D is the diameter of the molecules (McDaniel, 1989, p. 50).

Radius of a hydrogen molecule: 10^-10 m = 10^-8 cm

Density of interstellar medium: 1 atom/cm^3

Mean free path = lambda = 1/(pi)ND^2
 
Lambda = 1/(3.14)(1 molecule/cm^3)(10^-8 cm)^2 = 3.18 x 10^15 cm = 3.18 x 10^13 m

1 light year is 3600 s x 24 hr x 365 days x 3 x 10^8 m/s = 9.46 x 10^15 m

Thus these antiprotons will on average travel 0.3 % of a light year until they hit a hydrogen.
Of course, that is on average. Many will go much farther before they hit matter.

So let’s say that 1% of the interstellar antiprotons survive. Let’s say this SpaceX spaceship has a scoop that collects 10^4 m^2 of what they pass by. Say the craft is travelling at 1000 m/s to start.

10^4 m^2 x 10^-3 antiprotons/m^3 = 10 antiprotons per m scooped.

10 antiprotons/m x 1000 m/s = 10^4 antiprotons/s scooped.

Energy in 1 antiproton is E = mc^2 = 1.67 x 10^-27 kg x 9.0 x 10^16 m^2/s^2 = 1.5 x 10^-10 J

Energy in 10^4 antiprotons is then 10^4 x 1.5 x 10^-10 J = 1.5 x 10^-6 J

Translating this directly into velocity for a 10^5 kg ship:

KE = (1/2)mv^2              v = sqrt(2KE/m) = sqrt(2 x 1.5 x 10^-6 J/10^5 kg) = 5.5 x 10^-6 m/s

Momentum imparted is then p = mv = 10^5 kg x 5.5 x 10^-6 m/s = 0.55 kgm/s

Force on the ship is then F = dp/dt = (0.55 kgm/s)/(1 s ) = 0.55 N

a = F/m = 0.55 N/10^5 kg = 5.5 x 10^-6 m/s^2

However, this is only when the ship is going at 1000 m/s. Once it’s going much faster than that this will improve because the magnetic scoop will scoop more antiprotons per second.
 
References

http://www-ssg.sr.unh.edu/ism/what1.html

Bambi, C. and Dolgov, A.D. (2007) Antimatter in the Milky Way. Nuclear Physics B. 784: 132-150

McDaniel, E. W. (1989) Atomic collisions: electron and photon projectiles. Wiley.

[RotoSequence]

Isn't that close to the amount of velocity loss from drag caused by the interstellar medium?

[scienceguy]

Isn't that close to the amount of velocity loss from drag caused by the interstellar medium?

probably

[Unwieldy Bob]

She's most likely talking about beamed propulsion of some kind, perhaps the PROCSIMA laser coupled cold matter beam idea, if that turns out to be feasible.

[DistantTemple]

I haven't researched the details, but Anti-protons are produced at CERN by shooting particles into a block of metal. I don't know if this required the Large Hadron Collider or another cyclotron etc. Slowing down the Anti-protons in order to contain and transport them requires a much smaller cyclotron.

So if SX has 200 Starships in 2030,  with 500 people living on Mars, and a base on the moon.... And they are ready to build infrastructure to produce antiprotons. What wuold they do and what would they need?

Well ideally a place to build an immense cyclatron, where there is a natural vacuum, and a temperature of about 4degrees Kelvin. They would need access to vast electrical energy resources, and this would be best in a location where it is easy to transfer the antimatter to a ship.

Well The Moon's surface would be a good place for a moderate sized prototype, if they care abour having astronauts walk on a planetary body. But it has shortcomings - dust - 14 days shadow/night. (Phobos and Demios are possible alternative locations.)

I expect that they will just build one in space. They might even be able to avoid having a vacuum tube, and have a circular lattice to mount the magnets, if the vacuum of space is good enough. Super conducting magnets only require shielding from sunlight, which will be achieved using several square miles of solar panels. Since the main structures are rings with items massively repeated, they will be perfect for automated manufacture. HABS/a space station will be attached.

So SpaceX will revolutionise Antimatter production and reduce the cost of particle physics experiments by 4 to 6 orders of magnitude!!!
Solved - Antimatter is no longer too expensive!

[edzieba]

Accelerator production of Antimatter is not a great route to go down. An absolute magical 100% efficiency converter would require 180 Terajoules/gram. The LHC currently required 9000 Terajoules to produce ~2 nanograms (4,500,000,000,000 Tj/gram), or a 0.00000000004% efficiency. Or to match Saturn collection's 250 micrograms/year that needs 112,500,000 Tj (AKA 3.125*10^15 kWh, or 26.9 Gigatons TNT).

A mere "4-6 orders of magnitude" is nowhere close to sufficient for that to make sense.

[Cheapchips]

Dr. Gerald Jackson was on the Space Show recently.  He's done a fair amount of research on antimatter.  It's an interesting listen:


https://www.thespaceshow.com/show/20-oct-2019/broadcast-3394-dr.-gerald-jackson

[daedalus1]

Accelerator production of Antimatter is not a great route to go down. An absolute magical 100% efficiency converter would require 180 Terajoules/gram. The LHC currently required 9000 Terajoules to produce ~2 nanograms (4,500,000,000,000 Tj/gram), or a 0.00000000004% efficiency. Or to match Saturn collection's 250 micrograms/year that needs 112,500,000 Tj (AKA 3.125*10^15 kWh, or 26.9 Gigatons TNT).

A mere "4-6 orders of magnitude" is nowhere close to sufficient for that to make sense.

Also storing it is a big problem. You need a 100% hard vacuum, just about impossible.