Consider an interstellar spaceship.

It is a rather classical design, somewhat similar to the ISS Venture Star from Avatar:


Although it is not in tractor configuration.

The ship is very long and skinny. At the nose there is a thick, actively cooled radiation shield to protect from collisions with neutral interstellar hydrogen. After that there are the passangers in cryosleep, along with life support equipment and maintenance robots. Than come the very large fuel tanks and another thick shield. At the end there is an antimatter beamed core engine, putting out charged pions almost with c.

The plan is to accelerate with the help of lasers (in the home system) and a foldable photon sail, then accelerate further to 0.7c, and then coast a while, before flipping and starting to decelerate (to achieve orbit in a nearby star system.)

But there is a problem: the ship has to change it's orientation by 180 degrees to start deceleration. Inevitably it would travel sideways for some time, while having the highest relative velocity to the interstellar medium.

Assuming that it takes 5 minutes to rotate the ship, could the radiation hitting the unshielded side cause structural damage and/or harm the passangers?

Should additional shielding used on the cabin's side walls? Or would the minimalist shielding against galactic cosmic rays suffice for a short period of time?

  • $\begingroup$ Is there a reason that it doesn't get hit by any radiation on the sides during regular space-flight/why doesn't it need radiation shielding before worrying about the rotation bit? $\endgroup$ Apr 6, 2017 at 21:15
  • 2
    $\begingroup$ @Mithrandir24601, at 0.7c, the majority of "background" radiation will come from ahead, and at the same time, "induced" radiation from encountering otherwise-ordinary interstellar particles at too high a speed will come exclusively from ahead. $\endgroup$
    – Mark
    Apr 7, 2017 at 1:17
  • $\begingroup$ So when the OP says 'could the radiation...' do they mean 'radiation' or 'neutral interstellar [particles]'? $\endgroup$ Apr 7, 2017 at 8:56
  • 4
    $\begingroup$ @Mithrandir24601 At that speed, "neutral interstellar particles" are no longer "neutral interstellar particles." From that reference frame, they are particle radiation. Or, at least they are close and might as well be considered such; is there a specific limit for the definition of particle radiation? 0.9c? 0.999c? Anyway, at that speed the distinction is a nitpick. OPs question is pretty obvious. "Are we safe during the flip?" $\endgroup$
    – Loduwijk
    May 2, 2017 at 21:00

2 Answers 2


No real threat to the passengers

The ISV has multiple measures to deal with the few threats in interstellar space. From your link:

Although intense magnetic fields are used to deflect stray gas molecules, the occasional dust grain requires a physical barrier. The shield is in multiple layers, spaced one hundred meters apart. Impact of a debris grain (traveling at a relative speed of 0.7C) with the first layer of the shield causes vaporization into a plasma. The spray of plasma particles strikes the second layer, and the impacts cause spalling from the back of the second layer. These particles are stopped by the third layer. A fourth layer acts as a backup in the unlikely event that something gets past the third layer. Once cruise speed is reached, this shield is detached and moved by small thrusters thousands of miles in front of the ship to improve survivability if a larger particle of debris is encountered.

This combination of safety measures would be sufficient to protect the ship during this maneuver, and the ship would not perform this maneuver at all if larger material was present.

Even without the shield the passengers would be relatively safe. The Breakthough Starshot proposes a postage stamp sized craft to be sent to Alpha Centauri at .2 c. According to this paper, the erosion on the unshielded surface is only .5 mm deep over a ~4 year journey. So 5 min at .7 c won't be a big issue.

With the shields in place little if anything would happen to the spacecraft. Without them, there will be very minor surface erosion, heating, and some charging of the spacecraft.


For those concerned by radiation touched off by relativistic impacts this states that the HAB sections of the ship are made of nonmetallic compounds which reduce this effect.

  • 1
    $\begingroup$ I have read that about 25% of interstellar medium is neutral, and therefore can not be effectively deflected by magnetic fields: en.wikipedia.org/wiki/Interstellar_medium However, the ejection of the shield actually solves the problem. $\endgroup$
    – b.Lorenz
    Apr 7, 2017 at 14:15
  • 1
    $\begingroup$ Even then the erosion rate from uncharged particles will be minor for that 5 min. The biggest threat out there is still cosmic rays. $\endgroup$ Apr 7, 2017 at 14:26

So, I'm a bit late to this one, but seeing as the question popped up as being related to some other ones I've looked at lately, and the accepted answer is wrong, I thought I may as well chime in.


could the radiation hitting the unshielded side cause structural damage and/or harm the passangers?

No structural damage, but it will kill everyone on board and probably trash all the electronics, too.

Should additional shielding used on the cabin's side walls?

You could, but it would make more sense to use a slightly different ship design that avoids the flipover. That shielding is heavy!

Or would the minimalist shielding against galactic cosmic rays suffice for a short period of time?

It doesn't make sense to talk about "minimalist shielding" for GCRs: they're too hazardous for that. They are, however, amenable to magnetic deflection in a way that neutral gas particles in the interstellar medium are not. Your GCR protection probably won't save you.

The problem is that although the interstellar vacuum can be pretty vacuuous, it ain't empty. Space around our star is unusually low density thanks to the explosion of the Geminga supernova blowing must of the interstellar medium away. You might find one hydrogen atom per 100cm3. A ship travelling at .7c, therefore, can expect to have 2.1x108 hydrogen atoms hit every square centimetre of its cross-section every second. Note that these are mostly neutral atoms, so magnetic shielding ain't gonna save you from them.

The paper Radiation Hazard of Relativistic Interstellar Flight looks into more detail at what this rain of high-energy particles will do to unshielded humans. It ain't pretty: the radiation dose is about 104REM per second. That's a lethal dose in less than a tenth of a second, and more than enough to ruin electronic systems throughout the ship. A five minute turnaround guarantees that the ship will be sterilised and that all computers will be thoroughly wrecked.

Shielding against this radiation isn't impossible... somewhere between 10 and 30cm of titanium should do the job, or a metre or two of water. That's a lot of shielding though, shielding which isn't obviously present on any part of the ship other than the big stack of debris shields at the end:

ISV Venture Star

and remember that every part of the ship that contains vulnerable things (people, other living things, electronics, antimatter) would need to be shielded. All that dead mass for a tiny moment during flipover. What a waste!

One of the technical advisors for the design, Charles Pellegrino, should have known better. He'd been involved with a previous design, Project Valkyrie, which similarly used a tension structure instead of a rigid hull, and antimatter rockets. The major difference though is that the Valkyrie design had two engines at opposite ends of the ship, one for boosting, one for braking. This gives some redundancy, but also means that you don't have to flip around and irradiate yourself at the midpoint of your journey.

Really, all the Venture Star needs is to dock the laser sail at the debris shield end, and to have an extra bit of laser shielding at the other end to protect the rockets during the laser boost phase. It isn't like the debris shield is big enough to protect the laser sail anyway, so there's no loss of performance, and it comes with the benefit that everyone gets to live.

  • $\begingroup$ I thought the current understanding was that several supernovas associated with the Pleiades Moving Group are thought to have caused the local bubble, not Geminga. $\endgroup$ Nov 6, 2019 at 14:59
  • $\begingroup$ Hello, we meet again! (I also noticed this post, and have a pretty good idea of how you found it, too :P) Unless I'm misunderstanding something, the debris shield is designed to detach and join up again at that spire on the opposite side. If the shield was detached and made to hover about a km out, staying centered in front of the crew module for the entire flip, couldn't that protect them during those 5 minutes? $\endgroup$ Nov 6, 2019 at 23:06
  • $\begingroup$ @GiladM it can't protect the habitation section and propulsion section at the same time, and nor would it be able to shield any other bits of electronics on the ship that lay outside those areas. $\endgroup$ Nov 7, 2019 at 8:12

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .