# What are the likely first forms of relativistic spacecraft propulsion?

#### greswd

I've been interested in relativistic spacecraft since news of the Breakthrough Starshot project announcement a few years ago.
Breakthrough Starshot's method of laser propulsion still has many technical hurdles needed to be crossed.

So I'm wondering what you guys think the first forms of relativistic spacecraft propulsion are likely to be.
Whether Breakthrough Starshot's laser propulsion will come first, or whether other methods of propulsion will progress faster and give us those relativistic speeds of maybe even up to Lorentz factors of +10-20%.

Related Aerospace and Astronautics Engineering News on Phys.org

#### PeroK

Homework Helper
Gold Member
2018 Award
I've been interested in relativistic spacecraft since news of the Breakthrough Starshot project announcement a few years ago.
Breakthrough Starshot's method of laser propulsion still has many technical hurdles needed to be crossed.

So I'm wondering what you guys think the first forms of relativistic spacecraft propulsion are likely to be.
Whether Breakthrough Starshot's laser propulsion will come first, or whether other methods of propulsion will progress faster and give us those relativistic speeds of maybe even up to Lorentz factors of +10-20%.
Just an observation. The guy in the video says two things:

1) Describes what looks like a plausible plan to fire a tiny probe (one tiny computer and a 3m sail) to nearby stars in the next couple of decades.

2) That this is the century where we leave our solar system and become a galactic species.

It seems to me that, whether 1) is feasible or not, 2) is technologically so far in the future that it is impossible to estimate.

It's not even clear that we could safely get a single manned mission to Mars this century.

In addition, even if 1) is successful, the journey takes 20 years (about 4 light-years at 1/5 the speed of light) and the data takes 4 years to get back. If you launched the probe in 2040, say, you wouldn't know it was successful until 2064.

This illustrates that if and when we start exploring outside the solar system, the time of each mission itself will eat up the decades.

#### greswd

Just an observation. The guy in the video says two things:

1) Describes what looks like a plausible plan to fire a tiny probe (one tiny computer and a 3m sail) to nearby stars in the next couple of decades.

2) That this is the century where we leave our solar system and become a galactic species.

It seems to me that, whether 1) is feasible or not, 2) is technologically so far in the future that it is impossible to estimate.

It's not even clear that we could safely get a single manned mission to Mars this century.

In addition, even if 1) is successful, the journey takes 20 years (about 4 light-years at 1/5 the speed of light) and the data takes 4 years to get back. If you launched the probe in 2040, say, you wouldn't know it was successful until 2064.

This illustrates that if and when we start exploring outside the solar system, the time of each mission itself will eat up the decades.
yeah, that seems like the likely outcome.

do you think another method of relativistic propulsion will leapfrog to become the first in use?

#### gleem

do you think another method of relativistic propulsion will leapfrog to become the first in use?
Calculate the energy needed to accelerate a 1 KG mass to 0.1c, barely relativistic. Then examine sources that might provide this energy in a reasonable time. You will see why they want to use a very small craft that is accelerated by a a source on Earth.

#### mfb

Mentor
A momentum-limited Orion spacecraft is not too far away from current technology and it could reach ~3% the speed of light. If we don't need that much payload mass then smaller bombs might work, and an uncrewed spacecraft could handle much larger acceleration, reducing the number of bombs needed. The final speed depends critically on how much mass we need in the blast shield towards the end, sacrificing some efficiency in capturing the explosion but reducing the mass could boost the spacecraft further towards the end.
It would need dedicated R&D, of course, it would also need a much larger nuclear weapon program, but the main issue is the overall price.

A fission-fragment rocket could reach similar speeds, but it can be built much smaller. If the 1,000,000 Isp can be achieved, then even 10% the speed of light wouldn't be too difficult. Similar to above: Needs dedicated R&D, but we would know what to work on.

Breakthrough Starshot needs the incredibly lightweight spacecraft. While it is quite possible that we can build them in the future this looks like a more high risk option to me. If it works, however, then we can send spacecraft to many places at reasonably affordable cost.

#### anorlunda

Mentor
Gold Member
Breakthrough Starshot needs the incredibly lightweight spacecraft. While it is quite possible that we can build them in the future this looks like a more high risk option to me. If it works, however, then we can send spacecraft to many places at reasonably affordable cost.
The minimum masses for different spacecraft can be vastly different.
1. A round trip, plus lots of people and equipment for colonization.
2. Minimal manned mission, one way, one person.
3. An unmanned one way mission. We might call it a probe. Able to send messages back, "Wish you were here."
4. A probe without the ability to send messages back. Carries a flag, "Greetings from Earth."
It makes public discussions difficult because participants have such different mental models of spacecraft in mind. Scientists visualize the probe end, and romantics visualize Star Trek Enterprise or Battlestar Galactica size generation ships. The smallest hypothetical payload I've seen mentioned was three grams.

Gold Member
For grins, I wondered how long it would take to accelerate to $0.1c$ if you are accelerating at a constant acceleration of $a=g$. The answer is 35.4 days. You likely couldn't accelerate much faster than that and still be safe for humans.

"What are the likely first forms of relativistic spacecraft propulsion?"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving