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I recently came across Could pulsars really act as "lighthouses" to help in interstellar travel?, asked a week ago. The author was trying to figure out if pulsars could be useful for interstellar FTL travel, because they could be used to find a craft's position; the answer was, of course, yes. I had some reservations, though. My main issue was that it seems, from an engineering standpoint, not very feasible. Here's my thought process:

  • To be certain of successfully triangulating your position to a high degree of accuracy, you'd need to have probably half a dozen or more candidates to observe from any one location.
  • Observing a pulsar isn't easy. The issue is that if you make a random jump out of hyperspace, you won't know where any pulsar should appear unless you can quickly determine a general location. This means that you'd need to do a lot of guessing, and essentially discover pulsars all over again.
  • This in turn means that you'd need a large radio telescope, and that's not really easy to attach to a typical spacecraft. Optimally, the dish is over 100 meters in length.

Let's say that we overcome various other technical hurdles, and need to attach a 100-meter parabolic radio telescope (although I'm open to other designs, if you can make a very convincing argument otherwise) to an interstellar spacecraft, for the purposes of finding and monitoring pulsars for navigation. I'm concerned as to whether or not the telescope could survive sub-lightspeed travel for any extended period of time. By this I mean acceleration for perhaps two weeks to a speed of maybe $0.01c$, staying at constant speed for three months, then decelerating for two weeks. Propulsion would likely be from chemical rockets.

  • Will the telescope survive the harsh conditions of space, including micrometeoroid impacts?
  • Will there be any physical stresses from the motion of the ship that could damage it?
  • Are there any other potential dangers to the telescope itself, and can they be overcome?

So far, nobody's actually addressed the original scenario in as much detail as I'd like. I'd love answers that do that; it's why I asked the question. However, I wouldn't be totally opposed to answers that suggest different but related options, such as using a different type of telescope, or using pulsars a different way. But you'd have to make a really good case for doing so, and you'd still have to justify that this option would survive the spaceflight.


My motivation for asking this is that I've considered using pulsars for this purpose in several stories, but I've always gotten hung up on how to solve this sort of problem.

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    $\begingroup$ A side question - why one big and not many smaller ones? $\endgroup$ – Mołot Oct 19 '17 at 14:28
  • $\begingroup$ @Mołot I was a little worried about the difficulties of keeping them aligned. They'd be attached to an object moving quite fast and possibly shaking; on Earth, the ground typically doesn't move enough to jostle them. Plus, I just know more about single-dish radio telescopes than interferometers. $\endgroup$ – HDE 226868 Oct 19 '17 at 14:30
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    $\begingroup$ But you don't need to keep them aligned, do you? You just need to make them aligned when you are using them. Also, I don't see what would create shaking if your ship is under no acceleration or under constant one. There is a lot of things around me at $1g$ and nothing is shaking ;) $\endgroup$ – Mołot Oct 19 '17 at 14:35
  • $\begingroup$ @Mołot Sure, but that's my concern. The things are going to be operating for maybe half an hour for each pulsar. My concern for the shaking is a combination of any engine issues and potential warping of the craft itself. shrug Maybe I'm overestimating it; at any rate, I'd like to stick with the option I know more about, unless there are any really compelling arguments against doing so. $\endgroup$ – HDE 226868 Oct 19 '17 at 14:37
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    $\begingroup$ If the telescopes were only needed occasionally to take bearings, why not release a small number of independently flying units take your bearings and then recall them to the ship? Better still why not carry a complete star map catalogue or at least some of the nearest galaxies? If you can find Andromeda and Triangulum (both visible to the naked eye under good conditions) and one other local galaxy then you know where you are (unless you are jumping millions of light years). I would have thought navigation would not be a significant issue especially with computerised help. $\endgroup$ – Slarty Oct 19 '17 at 15:50
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A scintillation array + good computer = synthetic aperture

Pulsars are not exactly discrete and quiet things. And radio telescopes do not actually need to look like in the film Contact. In fact... the radio telescope that discovered the first pulsar today looks like this:

enter image description here

The Interplanetary Scintillation Array, Mullard Radio Astronomy Observatory , Cambridge, UK

Yes, those uneven poles and sagging wires are what first picked up the signals from the pulsar that affectionately became called LGM-1 (Little Green Men 1).

So you do not need to cover your array with a big metal sheets like you see on parabolic antennas. What your ship needs is not a big dish, but a number of "whiskers". They do not need to be massive, they just need to be long, say up to 100 meters for a nice round number.

Further helping this is the fact that you can "cheat" by having a computer compare the signals from each different whisker. By running some fast and fancy math on the incoming signals, you create a fake antenna that is just as large as if it had been a massive dish that has a hundred meter radius. This is known as a synthetic aperture. And even if the gathered energy of the pulsar signals do not become as plentiful as it would be with a "solid" disk, the extreme angular resolution is such that it becomes easy to pick out the relevant signals from noise.

So can a radio telescope that can listen in on "beacon" pulsars survive a space trip? Oh yes, it can. You just eject the drogue weights that un-spool the "whiskers", and soon you will have the bearings of all the pulsars you need to make an accurate estimate of your position.

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    $\begingroup$ You know, I've actually seen one of these but completely forgot about it for this. I still would rather have a parabolic dish, but your idea does seem to solve some of the problems (e.g. micrometeoroid impacts) that I was worried about. It's also much better for storage when not being used. +1. $\endgroup$ – HDE 226868 Oct 19 '17 at 14:44
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    $\begingroup$ Thank you for the reference and the research. I knew the receivier did not have to be solid but I did not have the background. $\endgroup$ – P Chapman Oct 19 '17 at 14:54
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    $\begingroup$ @HDE226868. In reality, if whiskers started to be used because they are more technologically cheaper, whisker technology would quickly be improved to the point that it was at least as good if not better than dish technology, despite any apparent initial physical limitations. $\endgroup$ – Mad Physicist Oct 19 '17 at 16:09
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    $\begingroup$ @Mark. That is absolutely true. I am just saying that if dishes gave excellent performance but incurred a huge cost in manufacture, transportation and storage, etc, while whiskers gave adequate-ish performance for a significantly low cost and more robustness, the sheer demand for whiskers would drive development in materials, array density, signal processing software and whatever else was necessary to make a decent set of whiskers comparable to all but the most excellent dishes. This happens often enough in the real world. Compare early intel to sparc vs now for an example. $\endgroup$ – Mad Physicist Oct 19 '17 at 23:22
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    $\begingroup$ Using pulsars to find your position with high accuracy would presumably be easier if you knew roughly where you were, by some combination of knowing where you were supposed to be and mapping visible stars (which can be scanned quickly) against your map of the stellar neighborhood. The bigger a computer you've put on the ship, the more stars it will be able to consider in trying to find your location. $\endgroup$ – Rupert Morrish Oct 20 '17 at 2:31
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You do not want your dish hanging in the breeze as you zoom about. You want it when you want it, and not before or after. You want an Inflatable dish.

inflatable GATR dish

from http://www.gatr.com/products/4m-antenna-system

GATR has introduced an inflatable 4 meter-class communications hub. GATR’s unique shape and design has enabled this high-capacity 4.0m antenna that is 80+% less volume and weight than comparable sized deployable rigid antennas (4 cases, less than 400 pounds total).

Your sleek, suave spaceship slips thru space. On emerging and desiring a fix on pulsars, the GATR-type inflatable radiotelescopes are deployed. They inflate to very large size, enabling rapid detection of the pulsars in question.

After listening to the pulses (they sound good), you deflate the GATR radiotelescopes, pumping the inflation gas back into cylinders. The deflated GATRS are packed away and your ship corrects course and streaks away.

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    $\begingroup$ I have an issue with this: In an atmosphere, as you pull air from the inflatable thingy, the outside atmosphere will push against the outside. Without an atmosphere, the inflatable thingy will stay inflated even without inside air. Also, a very large size means a very large surface. I'm not sure this would be lighter compared to a whisker system much larger in size but at a similar weight. $\endgroup$ – Calin Ceteras Oct 20 '17 at 6:14
  • $\begingroup$ @CalinCeteras: Not to mention that you need to carry a huge amount of matter to push into it, wouldn't you? $\endgroup$ – Zaibis Oct 20 '17 at 6:34
  • $\begingroup$ You have to fill it in an almost absolute void. To fill a 100 meters diameter sphere at a pressure of 1kg/square meter, or 1/5 pounds/square foot (1/1000 of Earth atmospheric pressure) you need some 600 kg of air. However, in a void this could be too much pressure. Just as a reference, that 100m diameter sphere would have an area of 30 000 square meters, or 300 000 square feet, several football fields. $\endgroup$ – Calin Ceteras Oct 20 '17 at 6:46
  • $\begingroup$ Deflated dish could be retracted using something like windlass. Why not? This still is an option. $\endgroup$ – Mołot Oct 20 '17 at 14:50
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    $\begingroup$ @Calin Ceteras - filling a balloon to turgor absent atmospheric pressure will take much less gas than filling on earth, as you suggest. The question of whether pumping gas out of a space balloon will deflate it is an interesting one. I am not sure. But even if it does not collapse as deflated, you can reel it in and fold it up once the gas is out. $\endgroup$ – Willk Oct 20 '17 at 18:25
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Ignoring the amount of likely technological advancement between then and now

An instrument as specialized and large as a radio telescope wouldn't be kept open unless it was needed. The thing about space is don't you have to worry as much about structural engineering against gravity. So you can have very thin loosely supported structures that can sprawl out on demand.

Currently in space this is taken advantage of all the time. When a satellite is put into orbit in most cases its solar panels are rolled out. When a probe reaches its destination its antennae is deployed. A radio telescope isn't much different from an antennae dish. This reduces the chance it will get damaged.

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  • $\begingroup$ This seems . . . kinda complicated. Dishes need to be precise, and trying to get everything into alignment each time seems like it could be problematic. $\endgroup$ – HDE 226868 Oct 19 '17 at 14:32
  • $\begingroup$ It really isn't, there are probes that deploy radio dishes all the time. It's only rocket science. $\endgroup$ – anon Oct 19 '17 at 14:37
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    $\begingroup$ My point is, we are no where near having the technical basis for a FTL engine, we already have the technical basis to do this. That is why this is likely. $\endgroup$ – anon Oct 19 '17 at 14:53
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    $\begingroup$ @HDE226868. Look up the IR dish on JWST: 6.5m across (not 100m, but also needs to be much more precise). It is made out of 18 panels, each one of which can be focused separately. The back is a fairly complex, but lightweight frame. It would not be a huge stretch to have a completely collapsible frame capable of supporting dozens or hundreds of small dish segments given the existence of FTL technology. $\endgroup$ – Mad Physicist Oct 19 '17 at 16:12
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    $\begingroup$ @HDE226868, a dish antenna needs its curvature to be correct to within about a tenth of a wavelength. For a 1GHz signal, that corresponds to no deviations of more than about 30 mm, which is well within the capabilities of current engineering. $\endgroup$ – Mark Oct 19 '17 at 20:51
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X-ray pulsars

From https://www.seeker.com/how-to-use-a-pulsar-to-find-starbucks-discovery-news-1766499711.html

Apparenly x-ray pulsars are easier to see

Or, you could look for pulsars that emit X-rays, a much brighter signal. X-ray antennas are also smaller and lighter, says physicist Richard Matzner at the University of Texas at Austin. Their drawback is oversensitivity to electrons surrounding the Earth.

But an X-ray–based positioning system could pinpoint an object to within 10 meters, an improvement on the 100-meter or so accuracy of the radio pulsar system.

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  • $\begingroup$ It's an interesting thought. AFAIK, this would reduce the number of pulsars available, but I couldn't say by how much. I do have one question: What sort of instrument would the ship need? My knowledge of X-ray telescopes is not fantastic. $\endgroup$ – HDE 226868 Oct 19 '17 at 14:55
  • $\begingroup$ @HDE226868 - It looks like many x-ray telescopes are put in satellites for the for the atmospheric electron problem. Two reference I found quick are: imagine.gsfc.nasa.gov/science/toolbox/xray_telescopes1.html imagine.gsfc.nasa.gov/science/toolbox/… Disclaimer: I am not an expert. $\endgroup$ – P Chapman Oct 19 '17 at 15:00
  • $\begingroup$ @HDE226868 -- the NICER HW package should be able to handle the challenge (and it's already spaceflight worthy) $\endgroup$ – Shalvenay Oct 20 '17 at 4:04
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The physical stress of motion won't be a problem.

Every telescope on earth is built to sustain 9.8 m/s2 acceleration indefinitely.

Accelerating up to 0.01 c (299,792,4.58 m/s) over the course of 2 weeks (1209600s) can be accomplished with a constant acceleration of 2.48 m/s2.

Since this is much lower than what we engineer telescopes on earth to it should be easy to engineer a telescope to withstand that level of acceleration.

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Can a telescope survive micrometeoroid impacts?

Data on micrometeoroid density is pretty sparse, and I wasn't able to find any. However, I think it is reasonable to use particle density as a proxy. We can get various particle density figures from this post on Space.SE, this paper from NASA/Goddard, and a selection of quotes about interstellar space here. The key here is going to be converting units. Lets put everything in terms of kg/m$^3$.

From the Space.SE graph, we have about $2\times10^{-13}$ kg/m$^3$ at 550 km orbit, which is where Hubble telescope is. Solar wind density at the distance of Earth can be converted from particles per cm$^3$ to density by assuming a particle mass of $.002 / 6.02\times10^{23} kg$. The 0.002 estimate is due to most particles in space being hydrogen or helium. This gives us a near-Earth particle density of $3\times10^{-14} $ kg/m$^3$. This also broadly agrees with the numbers from Space.SE out at 1000 km altitude. Finally, for the interstellar density estimates of 0.1-1000 atoms / cm$^3$, we convert via the same method to a range of $1\times10^{-16}$-$1\times10^{-12}$ kg/m$^3$. Note that we shouldn't be seeing the higher end of the range except in molecular clouds. Assuming we can steer our telescope clear of them, we should be traveling through space on the lower end of the spectrum.

Mass isn't the only thing driving the potential for collisions; velocity is as well. What we should be really measuring is mass flux, the mass of particles we encounter per unit area per second. Now here it will be difficult to know for sure the velocity of particles in the direction of an object as it is hurtling through space, since this is a vector problem. So we will make some assumptions for best case for Hubble, and worst case for our telescope.

Hubble is moving at roughly 8 km/s. Supposing the particles are not moving, we multiply Hubble's velocity by LEO particle mass density to get $8000\cdot2\times10^{-13}=2\times10^{-9} \frac{\text{kg}}{\text{m}^2\text{s}}$. For our telescope standing still in solar wind in Earth orbit, where solar wind has a velocity about 500 km/s, for a flux of $500000\cdot3\times10^{-14}=2\times10^{-8} \frac{\text{kg}}{\text{m}^2\text{s}}$. For our telescope moving at 0.01c relative to the interstellar medium (low estimate, since we are watching where we go), flux is $3000000\cdot1\times10^{-16}=3\times10^{-10}\frac{\text{kg}}{\text{m}^2\text{s}}$.

So our telescope is seeing within an order of magnitude the particle flux that Hubble has been seeing since 1993. So the operative question is: has Hubble been damaged by micrometeroids? Well, Hubble did have a mirror replacement in 1993, but since then, as far as I can tell, the mirror was not repaired or replaced by any subsequent servicing mission, and Hubble is evidently working fine today. Which means that Hubble has had its fine optical equipment not (significantly) damaged in space for almost 25 years. From this, it seems the particulate threat to a giant space mirror is not significantly higher than it is for Hubble, and we can expect a 25 year life span, at least.

Can the telescope survive the acceleration?

0.01c is 3,000,000 m/s. Divided by two weeks gets you 2.5 m/s$^2$. Since this is less than 1g, it goes without saying that anything that can structurally survive being on planet Earth will also survive this acceleration. There are plenty of structures that at least 100m across. Perhaps the most applicable, some Airbus A380 variants have a wingspan of 90m, so if it is doable with aerospace materials (i.e. aluminum), then it is practicable in space. Without any turbulence in space, I don't see any way that a telescope would need to be stronger than a large aircraft wing.

Any other considerations?

I can't think of any. In general, I think we should consider the fate our of our outer planet probes. Of Pioneer 10/11, Voyager 1/2, Galileo, Cassini, and New Horizons, exactly none of them hit anything. From what I can tell, the primary difficulties (with Voyager 2, and Galileo, if I'm not mistaken) were radiation related. But those difficulties happened in proximity to giant radiation fields. Good solar system mapping should allow you to send your telescope into deep space without running into any unexpected radiation.

Conclusion

From the evidence I have, I conclude that a large radio telescope could be moved into deep space without significant damage, and could be expected to operate for decades, at least, not even counting various improvements in space technology that could be expected in the near future.

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