Interstellar Travel?

Interstellar travel revolves around the answers to three major questions:

1) Where will we go?

2) What will we do when we get there?

3) How will it benefit folks back on Earth?

Far beyond the issue of whether interstellar travel is technologically possible is the very practical issue of answering these questions long before we turn the first screw in the hardware.

A few of the thousands of stars within 50 light years of Earth.(Credit: Atlas of the Universe)

In science fiction, finding new destinations is usually handled by either manned or unmanned expeditionary forces. Not surprisingly, the hazardous experiences of the manned expeditions are usually the exciting core of the story itself. When we create this technology ourselves, the first  trips are usually to a popular nearby star like Alpha Centauri (e.g Babylon 5). When we are co-opting alien technology, we often have to go to where the aliens previously had outposts often hundreds or thousands of light years away ( e.g Contact or Stargate). In any event, most stories assume that the travelers can select multiple destinations and hop-scotch their way around the local universe within a single human lifetime to find a habitable planet. Apparently, once you have convinced politicians to fund the first trip, the incremental cost of additional trips is very small!

The whole idea of interstellar travel was created by fiction writers, but because it has many elements of good science in it, it is a very persuasive idea. It is an idea located smack in the middle of the ‘gray area’ between fantasy and reality, and this is what goads people on to try to imagine ways to make it a reality. One thing we do know is that it will be an expensive venture requiring an investment at the level of many percent of an entire planet’s GDP.

By some estimates, the first interstellar voyage will cost many trillions of dollars, require decades to construct, and involve tens to hundreds of passengers and explorers. Assuming a project of this scope can even be sold to the bulk of humanity that will be left behind to pay the bills, how will the destination be selected? Will we just point the ship towards any star and commit these resources to a random journey and outcome, or will we know a LOT about where we are going before the fuel is loaded? Most of us will agree that the latter case for such an expensive ‘one of’ mission is more likely. By the way, let’s not talk about the human Manifest Destiny to explore the unknown. Even Christopher Columbus knew his destination in detail (India!) and traveled within a very benign biosphere, free breathable atmosphere and comfortable gravity to get there in a few months.

So…where will we go?

Contrary to popular ideas, we will know our destination in great detail long before we leave our solar system. We will know whether the star has any planets, and we will know it has at least one planet in its habitable zone (HZ) where temperature would allow liquid water to exist. We will know if the planet has an atmosphere or not. We will know its mass and size, and perhaps more importantly, whether the planet has a biosphere. We will not invest perhaps trillions of dollars to study a barren Mars or Venus-like planet. All of these issues will be worked out by astronomical remote-sensing research at far lower cost than traveling there. If a nearby star does not have detectable planets, we will most certainly NOT mount a trillion-dollar mission to just ‘go and see’!

The 10 nearest stars are: Proxima Centauri (4.24 lys), Alpha Centauri (4.36), Barnards Star (5.96), Luhman 16 (6.59), Wolf 359 (7.78), Lalande 21185 (8.29), Sirius (8.59), Luyten 726-8 (8.72), Ross 154 (9.68) and Ross 248 (10.32). This takes us out to a distance of just over 10 light years from Earth. The prospects for an interesting world to visit are not good.

Proxima Centauri has one recently-detected Earth-sized planet orbiting inside its HZ, making it a Venus-like world of no interest. Alpha Centauri B has one unverified Earth-sized planet, but not in the star’s liquid-water HZ. It orbits ten times closer than Mercury. There are no planets larger than Neptune orbiting this star closer than our planet Jupiter. Barnards Star has a no known planets, but a Jupiter-sized planet inside the orbit of Mars is exluded, so this is still a viable star for future searches for terrestrial planets in the star’s HZ. Luhman 16 is a binary system whose members orbit each other every 25 years at a distance of 3 AU. A possible companion orbits one of these stars every month at a distance closer than Mercury. As for the stars Wolf 359, Lalande 21185, Sirius, Luyten 726-8, Ross 154 and Ross 248, there have been searches for Jupiter-sized companions around these  stars, but none have ever been claimed.

So, our nearest stars within 10 light years are pretty bleak as destinations for expensive missions. There is no solid evidence for Earth-sized planets orbiting within the HZs of any of them. These would not be plausible targets because there is so little return on the high cost of getting there, even though that cost in terms of travel time is the smallest of all stars in our neighborhood. This also sets a scale for the technology required. It is not enough to visit our nearest star, but we have to trudge 2 to 3 times farther before we can find better destinations.

Better Destinations.

Let’s take a bigger step. Out to a distance of 16 light years there are 56 normal stars, which include some promising candidate targets.

Epsilon Eridani (10.52 ly) has one known giant planet outside its HZ. It also has two asteroid belts: one at about three times Earth’s distance from our sun (3 AU) and one at about 20 AU. No one would ever risk a priceless mission by sending it to a sparse planetary system with deadly asteroid belts and no HZ candidates!

Groombridge 34 (11.62 ly) –The only suspected planet has a mass of more than five Earths. No mission would be sent to such a planet for which an atmosphere would probably be crushingly dense and probably Jupiter-like even if it was in its HZ.

Epsilon Indi (11.82 ly) – has a possible Jupiter-sized planet with a period of more than 20 years. No known smaller planets.

Artist rendering of the planets Tau Ceti e and f (Credit: PHL @ UPR Arecibo)

Tau Ceti (11.88 ly) probably has five planets between two and six times Earth’s mass, and with periods from 14 to 640 days. Planet Tau Ceti f is colder than Mars and is at the outer limit to the star’s HZ. Its atmosphere might be dense enough for greenhouse heating, so the world might be habitable after all. But this is guesswork not certainty.

Kapteyn’s Star (12.77 ly) – It has two planets, Kapteyn b and Kapteyn c, that are 5 to 8 times the mass of Earth. Kapteyn b has a period of 120 days and is a potentially habitable planet estimated to be 11 billion years old. Again, a massive planet whose surface you could never visit, so what is the point of the interstellar expedition?

Gliese 876 (15.2 ly) has four planets. All have more than 6 times the mass of Earth and orbit closer than the planet Mercury. Gliese 876 c is a giant planet like Jupiter in the star’s habitable zone. Would you bet the entire mission that 876c has habitable ‘Galilean moons’ like our Jupiter? This would be an unacceptable shot in the dark, though a tantalizing one.

So, out to 15 light years we have some interesting prospects but no confirmed Earth-sized planet in its star’s HZ whose surface you could actually visit. We also have no solid data on the atmospheres of any of these worlds. None of these candidates seem worth investing the resources of a trillion-dollar mission to reach and study. We can study them all from Earth at far less cost.

Best Destinations.

If we take an even bigger step and consider stars closer than 50 light years we have a sample of potentially 2000 stars but not all of them have been discovered and cataloged. About 130 are bright enough to be seen with the naked eye. The majority are dim and cool red dwarf stars, which are still good candidates for planetary systems. In this sample we encounter among the known planetary candidates several that would be intriguing targets:

61 Virginis (11.41 ly) – It has three planets with masses between 5 and 25 times our Earth, crowded inside the orbit of Venus. The asteroidal debris disk has at least 10 times as many comets as our solar system. There are no detected planets more massive than Saturn within 6 AU. An Earth-mass planet in the star’s habitable zone remains a possiblity, but the asteroid belts make this an unacceptable high risk target.

Gliese 667 planets (Credit: ESO )

Gliese 667 (23.2 ly) –As many as seven planets may orbit this star, but have not been confirmed. All have masses between that of Earth and Uranus. All but one are huddled inside the orbit of Mercury. Planets c and d are in the star’s HZ and are at least 3 times the mass of Earth. Their hypothetical moons may be habitable.
55 Cancri (40.3 ly)- All five planets orbiting this star are more than five times the mass of Earth. Only 55 Cancri e is located at the inner edge of the star’s HZ and its hypothetical moons could be habitable. More planets are possible within the stable zone between 0.9 to 3.8 AU if their orbits are circular. This is a system we still need to study.

HD 69830 (40.7 ly) has a debris disk produced by an asteroid belt twenty times more massive than that in our own solar system. Three detected planets have masses between 10 to 18 times that of Earth. The debris disk makes this a high-risk prospect even if there are habitable moons.

HD 40307 (41.8 ly) Five of the six planets orbit very close to the star inside the orbit of Mercury. The fifth planet orbits at a distance similar to Venus and is in the system’s habitable zone. The planets range in mass from three to ten times Earth. Again, is a planet in its HZ with a mass too great for a direct human visit a good candidate? I don’t think so.

Upsilon Andromedae (44.25 ly) The two outer planets are in orbits more elliptical than any of the planets in the Solar System. Upsilon Andromedae d is in the system’s habitable zone, has three times the mass of Jupiter, with huge temperature swings. Its hypothetical moons may be habitable.

47 Ursa Majoris (45.9 ly) The only known planet 47 Ursae Majoris b is more than twice the mass of Jupiter and orbits between Mars and Jupiter. The inner part of the habitable zone could host a terrestrial planet in a stable orbit. None yet detected.

There are still many more stars in this sample to detect, catalog and study so it is possible that a Goldilocks Planet could be found eventually. But we are now looking at destinations more than 20 light years away at a minimum. This will considerably increase the cost and duration of any interstellar mission by factors of five to ten times a simple jaunt to Alpha Centauri.

Other issues.

Would you really consider a planet with two to five times Earth’s gravity to be a candidate? Who would want to live under that crushing weight? Many of the candidates we have found so far are massive Earth’s that few colonists would consider standing upon. Their surfaces are also technologically expensive to get to and leave. But perhaps these worlds might have moons with more comfortable gravities? There is always hope, but will that be enough to risk a multi-trillion-dollar mission?

There is also the issue of atmosphere. None of the candidate planets we have discussed transit their stars, so we cannot detect their atmospheres and figure out if they have atmospheres and if their  trace gases would be  lethal. The perfect destination worth the expense of a trip would have a breathable atmosphere with oxygen. Since free oxygen is only produced by living systems, our target planet would have a biosphere. We can only hope that as the surveys of the nearby stars continue, we will find one of these. But statistics suggests we will have to search much farther than 50 light years and a few thousand stars before we encounter one. That makes the interstellar voyage even more costly, not by factors of five and ten, but potentially hundreds of times. But if the trip were to a world with a known biosphere, THAT might be worth the effort, but possibly nothing less than this would be worth the cost, the risk, and the scientific return.

So the bottom line is that the only interstellar destination worth the expense is either one in which colonists can live comfortably on the planet with a lethal atmosphere, hermetically sealed under a dome, or a similar planet with a breathable oxygen atmosphere and a biosphere. Statistically, we will find far more examples of the first kind of target than the second. But in the majority of the cases, we will not be able to detect the atmosphere of an Earth-sized world in its habitable zone before we start the trip, and will have to ‘guess’ whether it even has an atmosphere at all!

The enormous cost of an interstellar trip to a target tens or even hundreds of light years away will preclude any guess work about what we will find when we get there. Consider this: Investing $100 billion to travel to Mars, a low-risk planet we thoroughly understand in detail, is still considered a political pipe dream even with existing technology! What would we call a trip that costs perhaps 100 times as much?

For more on this topic, have a look at my book ‘Interstellar Travel:An astronomer’s guide’. Available at Amazon.com.

 

Return here for my next blog posting on Friday, February   3

4 thoughts on “Interstellar Travel?”

  1. Obviously, we don’t know, but I found two papers in Nature vol 541 pp521 – 527 inclusive interesting, because they indicate that the collisional theory of planetary formation cannot be right. Specifically, the papers showed that, given that isotope distributions are functions of radial distance, Earth and Moon accreted from a narrow zone. If accretion was through planetesimals/embryos colliding, then the rocky planet zone would be well-mixed. As it happens, my theory of planetary formation requires the results of those papers, and because in that theory accretion starts as a chemical phenomenon, what you get is temperature dependent, which in turn is dependent on the mass of the star and the rate of stellar accretion. What that means is that to get a planet with a composition like Earth (granite, water, nitrogen etc) in the habitable zone, you need a star about the size of the sun, i.e. G or heavy K. Red dwarfs won’t do, and the two planets in the habitable zone of Kapteyn’s star would be Jupiter and Saturn type cores, hence presumably water worlds.

    On the other hand, it may be a bit early to write off Epsilon eridani. An Earth-like planet – if there were one there, would be expected to be about 0.5 AU from the star, and would you find that with current technology, bearing in mind it presumably would not eclipse?

    1. Thanks Ian, always nice to hear your points-of-view. I’m not a planetary astronomer so I really won’t comment on your ideas, however as an astronomer who has spent a lot of time doing observational work on young stellar objects in the infrared, I can’t really tell just how common our solar system isotopic variations are given that similar measurements for extrasolar planetary systems are difficult-to-impossible at the same resolution. The theoretical basis for protoplanetary disks is pretty well established in its overall details, but of course at the scale of planet-formation we are still a long way off from even seeing jovian-sized objects forming out of their surrounding material. We will not be able to see this process at the scale of the core formation event, but will only be able to resolve it when it reaches the gravitational accretion scale when gas flows are the dominant mass accretion material rather than planetessimals and asteroids. Getting the formation timescales to work out right and solving the planetary migration problem are where we are right now. But at some point the collision theory must be correct because we see heavily cratered asteroids of all sizes up to Vesta and Ceres (planetessimals) that vouch for collisions being important in growing them. The details are of course important and can be tested in our solar system. Pretty exciting stuff!!!!

      1. Of course there were collisions, and what I should have said, perhaps, was oligarchic collisions, e.g. of embryos, which is in the standard model. There is at least one report of a gas giant accreting, namely LkCa 15b (Salum et al., 2015. Nature 527, p342) and what I find interesting about this planet is that it is about 3 times as far from a sun-sized star as Jupiter, it is five times as big as Jupiter, and the star is only about 3 My old. Assuming the data are correct there is simply no time by standard modelling for that to be there so it had to form some other way. Of course if something large can form by monarchic growth, there will be smaller stuff as well in zones that are not accreted until later, and the craters on asteroids show that.

        Also, if LkCa 15b can become that big, that quickly, and Jupiter, in a more favourable zone (higher concentration of matter) is relatively small, then presumably the accretion disk around our star must been removed much more quickly. That would also make our system somewhat rare. It also suggests the reason for all the “Neptunes” and “super-earths” is that accretion lasted longer than in our system. So suitable planets for interstellar travel may be much rarer than we would like. Of course there are so many stars that there will be others, but as you point out, you would want to know it was there before setting off.

        1. I have always been fascinated by the gravitational instability method of forming really large objects out of gaseous disks. Some of these disks that have been observed seem to have spiral density waves, and so it seems plausible to me that super-jovians can form pretty quickly without collisions at all. I am not as sure of there is a distance limit to this process because 1- you have to create the instability some how (spiral galaxy waves are created by galaxy collisions and companion cannibalism) and also it is pretty cold in the distant disk regions….but instabilities don’t care about local gas temperature I suspect.

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