We spend a lot of time talking about how to get an interstellar probe up to speed. But what happens if we do achieve a cruise speed of 12 percent of the speed of light, as envisioned by the designers who put together Project Daedalus back in the 1970s? Daedalus called for a 3.8-year period of acceleration that would set up a 46-year cruise to its target, Barnard’s Star, some 5.9 light years away. That’s stretching mission duration out to the active career span of a researcher, but it’s a span we might accept if we could be sure we’d get good science out of it.
Maximizing the Science Return
But can we? Let’s assume we’re approaching a solar system at 12 percent of c and out there orbiting the target star is a terrestrial planet, just the sort of thing we’re hoping to find. Assume for the sake of argument that the probe crosses the path of this object at approximately ninety degrees to its orbital motion trajectory. As Kelvin Long shows in a recent post on the Project Icarus blog, the encounter time, during which serious observations could be made, is less than one second. A Jupiter-class world, much larger and observable from a greater distance, itself offers up something less than ten seconds at best for scientific scrutiny.
That’s a paltry return on decades of construction and flight time, not to mention the probable trillion or more dollars it would take to build such a probe, and it hardly compares well to what we’ll be able to achieve with even ground-based telescopes as the next generation of optics becomes available. What to do? Long is looking into these issues as part of the Project Icarus team, which is revisiting the Daedalus concept to see how changing technologies could alter the flight profile and produce a mission whose results would be substantially more useful.
Image: The Daedalus starship arrives in the Barnard’s Star system. Credit and copyright: Adrian Mann.
One option is to do the unthinkable. Instead of ramping up flight speed to get to the destination more quickly, perhaps a better alternative is to slow the mission down. There are two ways to do this: 1) Aim for a slower cruise speed in the first place and/or 2) attempt to decelerate the vehicle. The latter choice is a genuine conundrum for reasons Long makes clear:
Another option being examined [for deceleration] is reverse engine thrust, but the problem with this is that if we assume an equal acceleration-deceleration profile then the mass ratio scales as squared compared to a flyby mission and so requires an enormous amount of propellant; definitely a turn-off for a design team seeking efficient solutions.
What this boils down to is that if you want to carry enough propellant to turn your spacecraft around and decelerate, you have to carry that additional propellant with you from the start of the mission. The rocket equation yields a stubborn result — the requirement for propellant increases not proportionally but exponentially in relation to the final velocity required. The initial fuel mass becomes vast beyond comprehension when we apply the numbers to slowing an interstellar craft, which is why the Icarus team, as it looks into deceleration, is examining ideas like magsails, where the incoming vehicle can brake against the star’s stellar wind.
A magsail or, for that matter, various other sail possibilities (Robert Forward described decelerating a manned interstellar vehicle by lightsail in his novel Rocheworld) offers the unique advantage of leaving the fuel out of the spacecraft — you’re braking against a stellar particle flux, or against starlight itself. But whether or not such ideas prove feasible, they’re more likely to at least help if the spacecraft is traveling slower to begin with, making it easier to decelerate further. A slower transit also reduces stress on the vehicle’s engines and structure during the boost phase.
The Case Against Going Faster
Long notes that Project Icarus is far from having answers on just what cruise speed will be optimal — Icarus is a work in progress. But these issues are at the heart of the interstellar quest:
…all of this analysis goes to the heart of whether a flyby probe such as Daedalus is really useful given what it took to get there. The potential science return is massively amplified by performing a deceleration of the vehicle and although it is a significant engineering challenge this is why the Icarus team decided to address this problem; and it is a problem, even if you choose to just decelerate sub-probes. Coming up with a viable solution to the deceleration problem in itself would justify Project Icarus and the five years it took to complete the design process.
Supposing you gave up on trying to stop the probe in the destination system, but simply made your goal to slow it down enough to make protracted scientific observations as it passed through? It’s clearly an option, and again we’re considering a trade-off between the shortest travel time and the ability to maximize science return. Interstellar flight is a challenge so daunting that it makes us question all our assumptions, not the least of which has always been that faster is better. Not necessarily so, the Icarus team now speculates, and perhaps a fusion/magsail hybrid vehicle will emerge, a significant upgrade from the Daedalus design. And this reminds me of something I wrote about magsails back in 2004 in my Centauri Dreams book:
At destination, a magnetic sail is our best way to slow [the] probe down, with perhaps a separate solar sail deployment at the end that can brake the vessel into Centauri orbit. If you had to bet on the thing — if the human race decided a fast probe had to be launched and was willing to commit the resources to do so within the century — this is where the near-term technology exists to make it happen.
Of course, I now look back on that passage and shudder at my use of the phrase ‘near-term’ to describe the vehicle in question, but maybe a very loose definition of ‘near-term’ to mean ‘within the next few centuries’ will suffice (hey, I’m an optimist). In any case, when we’re talking journeys of forty trillion kilometers (the distance to the nearest stellar system) and more, a century or two seems little enough to ask. And while I do believe this, I rejoice at the spirit of Project Icarus, whose team presses on to discover whether such a thing could be attempted in an even shorter time-frame.
The magsail concept may be applicable to all scales of craft. The needle probes, utilizing von Neumann concepts, are intended to have what we can call creative potential. I envision a flotilla of the needle probes, moving to become a large-enough connected ring enroute, and initiating the current which would flow on their outer conducting frames. Having only the mass of the ring to decelerate, this is bound to be a delicate operation in itself.
Couldn’t agree more. The original Daedalus starship project was conceived when mainframes were the last word in computers and Saturn V launches were as cool as it got. If one changes the perspective to information retrieval — not an easy thing to do! — instead of just getting there and waving as the probe whizzes past the star system, then all manner of possibilities begin to emerge.
In brief, we need to think in terms of not one, but (very) many, probes. Not large probes, but (very) small. Not ponderous and reinforced, but a cloud. Not single, but networked. In line with the previous post, we need to think in terms of needles and feathers. Obviously some people have been thinking along these lines, but as a coherent research program, I think exploration of the possibilities has just begun. We need to completely reverse the order of possibility exploration. The very last thing to be researched should be the propulsion mechanisms.
To be frank, given what we know about the kinematics of a large object (wasn’t the original original Daedalus as big as a battlecruiser?) traveling at 1/8th C, the odds of the thing reach its destination were almost nil. It was not cost effective by any measure. Elementary economics is as fatal as Einstein to the idea. It’s long past time to move beyond Project Daedalus.
Note: I was thinking about conceptual blinders recently when I was rereading a book from the late 70’s about the future of computers over the next couple of generations. While some of the stuff was good and still interesting, the author had completely missed that computing in the future would be based on the power of the network. “Network,” in fact, does not even appear in the book.
Please forgive the double post.
Just after I completed my original post in the fog of morning, I suddenly remembered Robert Forward’s wonderfully-named conceptual interstellar probe “Starwisp,” a design which incorporates many of the ideas being considered above for post-Daedalus (R.I.P) thinking. Here is the wikipedia article which brings the Starwisp up to date.
http://en.wikipedia.org/wiki/Starwisp
I don’t get why the encounter time is as short as 1 second for an Earth class planet. 12%c is about 20,000 mile/sec, therefore the probe will travel the Earth-Moon distance in about 12 seconds.
The useful observation time is measured in minutes, short yes, but not 1 second.
Presumably we would be aiming at a planet with an already- known orbit, and with judicious mid-flight course corrections a close fly-by could be arranged quite easily.
Maybe one option would be to forget about decelerating the probe and focus on developing instruments that could suck up as much data as possible in that split-second flyby.
Another possible solution might be to detach and decelerate the (comparatively) small science instruments from the main probe, leaving what’s left, the primary transmitter to Earth, speeding past the destination at 0.12c. That way, the science instruments would have more time to collect data and would only have to relay it to the main probe, which could then transmit their data back to Earth.
Just brainstorming…
Conversely, just what data could be obtained in a 1 second flyby, assuming massive data storage and relatively slow transmission?
I would imaging that with a couple of centuries, sensors would be very fast, with data storage might be at the atomic level, so that huge amounts of useful data could be acquired in the “snapshot” that is eventually transmitted back to the solar system.
Since the ship is still traveling fast, gravitational slingshot could be used to target the next target star and thus allow local data collection for a number of star systems for the same price.
I think I mentioned this before but we could slow down very small probes using mag/sol sail technology by dropping them into the exhaust plume of the main craft, if the probes are very light they will slow down very fast.
Deploy a cloud of silvery nanoprobes well before the target system like a fighter-jet dropping chaff. Let the target system’s starlight work on retarding the cloud’s motion and steer the particles into perpetual orbit around the star system. Internetworked and able to tie their antennae together, they’d each be cheap to slow down, and in aggregate, they’d be able to blanket the system with instruments orbiting for very long term study. Let the main probe sail on by, unimpeded. Or perhaps steer it toward another stellar destination or vaporize it entirely via impacting it into the star so that it won’t be an out of control missile careening off into the cosmos. Cause you know whoever survives the blast of a 12%c probe impacting their planet is gonna be really hacked off and it won’t take them long to plot the reverse trajectory right back to Earth. Then our great-grandchildren are gonna have some ‘splainin’ to do.
Over 15 years ago New Scientist had an article about micro probes to Alpha Centauri. They were not too hard to make and launch. When I mentioned the article in a newsgroup, the general reply was : “yes, and how are you going to get any data back ?”. The idea is that micro-probe => micro-power to transmit data back. A network of micro-probes could do better. Better still if the probe(s), passed the star, then reached the area where the star itself could work as a gravitational lens. In this case the transmission energy could be beamed back much more efficiently. I think it would be difficult to get the right geometry though as it would be hard to steer a probe at that speed micro or otherwise.
Personally, as I wrote before, my preference is for an interstellar mission is to travel to the sun focal point and image the target system from there.
Traveling at just 0.1% of the speed of light it takes about 1 year to get at 550 AU and beyond. You get plenty of observation time and you don’t need to think about deceleration. Best of all, you get to see the system in your lifetime for a small fraction of the price.
The plan would be to survey the nearby systems, locate the one most promising and launch a mission to each of them 550 AU in the opposite direction. If what I have heard about the amazing power of the sun as gravitational lens is true, then you’d be able to explore star systems hundreds of light years away.
For example, I remember reading that a mission using the sun as gravitational lens “could see cars on a planet around Alpha Centauri”.
I’d say this corresponds roughly to 1 m/pixel resolution at 4 ly. I assume that this also means 100 m at 400 ly. Consider that Viking’s map of Mars was around 250 m/pixels and it was excellent.
Sorry I meant 1% of the speed of light to reach 550 AU in about 1 year, not 0.1%. About 10 times slower than Daedalus.
Hi kzb,
distance to moon at perigee ~225,000 miles.
12%c ~22,000 miles/s
time to cross Moon-Earth orbit ~250,000 miles/22,000 miles/s ~ 11 s.
Earth radius 6371km ~3960 miles–>7920 miles diameter.
Earth diameter 1/28th Earth-Moon orbit,
11s/28 <1 s.
Obviously crude analysis just based on earth diameter, in reality any probe approaching an object will make observations on approach and on de-approach, an assumption I stated in the original icarus article.
Kelvin
No problem once we learn how to burn dark matter. :>
I have come across the idea that if the probe deployed a very large lightsail ahead of itself that focused starlight back onto a smaller sail carrying a mini probe, the mini probe could be brought to a stop before to main sail was fried by close approach to the star.
A mainsail area of 1000 square km and mass of a few hundred tonnes could perhaps bring a 100 kg probe with a 1 square km sail to a stop.
Pray for WISE to dredge out a brown dwarf + planets at 1 light year!
:)
P
All,
From the above as well as previously presented analysis it is clear we need a complete top to bottom rethink of the entire Interstellar travel paradigm, especially if we want to do anything before ~2200 CE. Barring new physics that radically changes the Rocket Equation as well as the Economic viability of Interstellar Travel we need to formulate some basic operating principles to guide research on the viability of Interstellar Travel between now and ~2200. Beyond that we can speculate all we want, but it is pure Science Fiction.
First, it is increasingly clear that we need some sort of fundamental breakthrough or “gamechanger” either in physics or in how we approach Interstellar Travel conceptually to make it a viable propositon, at least over the next couple of Centuries. Beyond 2200 CE things are fundamentally “unknowable”. While it is prudent to be humble about what we currently know, we must start somewhere, and this means figuring out where and how to get there based on “known physics”. Given known physics we may be able to push out the travel radius from Sol/Terra to ~12 lyrs, provided that we find something interesting to go explore. which may be a big if.
Beyond that things are simply to far away given the various practical constraints imposed by known physics, and this assessment even assumes that some sort of advanced Matter/Anti-matter propulsion system is available and we have become a Type-1 Civilization. Even with all that it is clear we are not going to some place beyond ~12 lyrs anytime soon, and even the ~12 lyrs is highly optimistic since it may be Alpha Centauri or nothing at all for many Centuries to come. The key first step over the next ~50 years is to conduct a comprehensive Interstellar survey out to ~60 lyrs with a detailed focus on ~12 lyrs. If something interesting turns up great. However, if not then it will become clear we need new physics to make Interstellar practical (unless there is a Human Emergency to preserve the species and build a Generation Ship) and probably must become a Type-2 Civilization to practically travel beyond ~12 lyrs in a period of interest to people on Earth.
However, there may be a viable alternate path if the problem is constrained to ~12 lyrs. Perhaps the “Needles and Feathers” nano-technology can be used not just to gather data, but to take resources from various Interstellar locations and build some highly specialized infrastructure as we adopt a Lilly pad strategy towards Alpha Centauri and other close in Star systems. If we get real lucky there may even be a Brown Dwarf between Sol/Terra and Alpha Centuari that we could build around. So the question becomes what to construct as we start the process, perhaps as early as 2050-2075 of throwing ourselves a long forward pass?
Given the breakthroughs in Teleportation technology over the past decade it is probably safe to assume that barring catastrophe it will be possible to teleport large objects, but not living tissue by the end of the 21st Century if not far sooner. This in turn means that once we can create a teleportation receiver of some type around another star system we can begin to teleport objects there including probes that in turn can create larger receivers and larger objects. Therefore, what we need to send to targeted star systems are not probes since Advanced Telescopes of various types including some that use the Suns gravitational lensing effect should tell us what we need to know, but tiny manufacturing devices that can create teleportation receivers. Once this is done we can start shipping the heavy stuff in bulk. Given nano-technology based “Needles and Feathers” and Beamed Propulsion we may be able to start creating the teleportation receivers around the nearest star systems as early as the first decades of the 22nd Century.
As for direct Human travel this will be a direct function of how close, need and available technology, although in a pinch if we really had to and “money was no object” we could go with something like Orion or improved Orion technology. In fact, given known physics and Economic constraints likely to prevail through 2200 CE it may turn out that for non Human Interstellar exploration nano-technology driven “Needles and Feathers” Beamed Propulsion and Teleportation are the key technologies for dealing with the constraints in the Rocket Equation. The only time when large ships will be created is when Humans are going to take the step towards Interstellar travel unless Human teleportation becomes viable in the future. In essence, it is not at all clear if given their likely cost, large and expensive unmanned probes to the Stars have a real future given likely technical advances over the next ~200 years.
How feasible (in terms of fuel economy) would a craft be that accelerated by firing slugs from a rail gun powered by antimatter @ 0.9c or so? Seems like you could do a lot with a craft like that, although I guess the gun would rapidly degrade.
“Supposing you gave up on trying to stop the probe in the destination system, but simply made your goal to slow it down enough to make protracted scientific observations as it passed through?”
One problem is that escape velocity for a Sun-sized star is around 600 km/s, or about 0.2% of the speed of light.
So even if you decelerated down to just 1% of lightspeed, you’d still fly through the target system. But you would only extend your “close” flyby to 12 seconds. (You’d spend around a day within 1 au of the target planet, and about three minutes closer than the Moon is to Earth.)
Doug M.
I sincerely hope that we can pull off the construction and launch of an interstellar probe well before the ‘next few several centuries.’ Though chastised by some for over-stepping his intellectual turf and being an alarmist, I couldn’t agree with Stephen Hawking more: if humanity does not begin to spread out and leave Earth very soon, we and all of our scientific and cultural achievements will disappear when the extinction event finally over takes us. Space colonization is our only guarantee of long term survival and yet the vast majority of us have seemed to have turned a deaf ear to this irrifutable logic. Perhaps the discovery of a genuinely habitable planet within 30 ly will be enough to wake us from our complacent slumber. In the meantime, for those of you who remain unconvinced of the threats to our survival as a species, here is a non-exhaustive list:
1. Biological weaponry
2. Deadly plague arises from the natural world
3. Nuclear holocaust
4. An impact catastrophe
5. Ecological collapse
6. Some combination of the above
The benefit of not going extinction coupled with the promise of essentially endless adventure—sounds like a win-win prospect to me.
Kelvin: I don’t see why the useful observation time depends on the diameter of the planet. We can make useful and detailed observations of the moon from here, and as you say that is about 11 seconds distant. So I still say 1 second is too pessimistic.
However this article has made me seriously question the mission profile. Quite probably it will not be cost-effective in competition with solar-system based observatories. Some way of braking will have to be found !
“How feasible (in terms of fuel economy) would a craft be that accelerated by firing slugs from a rail gun powered by antimatter @ 0.9c or so?”
The maximum velocity ever demonstrated by a railgun in the lab is about 6 km/s, or 0.00002c.
There are significant and fundamental challenges to getting high speeds out of railguns.
The rocket equation is NOT the fundamental barrier that some are saying. Given (!) the efficient use of nuclear fusion, an exhaust velocity approaching 10%c is possible. This means a probe with a mass ratio of 2.72 (and excellent efficiency) could reach 10%c. To modify the mission profile to include slowing back down from that velocity we need a mass ratio of just 7.4.
You can round that up to 10 to allow for some inefficiencies and it is still quite do-able in principle.
I’m a little surprised that nobody’s mentioned the Voyagers. They’re currently in their 34th year of operation, and are expected to have about a decade left before they run out of power. That’s a total mission lifespan of ~44 years, which is pretty much exactly what was projected for Project Daedalus back when.
Missions with lifespans in decades are becoming familiar. Cassini launched in 1997 and should keep going until 2017; New Horizon launched in 2006 and should still be active into the 2030s. Even orbiters like Mars Odyssey and Magellan are now expected to last a decade or more.
So an interstellar mission lasting up to 50 years would seem plausible, especially if it kept providing a steady trickle of science along the way.
Doug M.
If the claimed resolution for gravitational lensing given above is correct, I think there’s little point in sending probes unless they can stop and thoroughly explore the target system, so perhaps we should accept that fusion powered probes are really only suited to that type of mission.
It would require only a tiny fraction of the initial propellant load to be held for traveling throughout the target system, so once it got there the probe could spend decades exploring.
Stopping does mean halving the interstellar speed for the same propellant ratio, so a delta v for Icarus of at least 0.2c should be the design goal.
I can’t shake the feeling we are missing something obvious….. something ‘the sun is bright’ obvious that doesn’t get thought about because well, its that obvious…. Can’t put my finger on it!
What follows is probably vain attempt at out of the box thinking:
Does a probe need to be a spaceship, in the conventional sense? We are aiming at returning data from another star system after all, and based on much of the above analysis it looks as though a solid matter space ship may not be the best way to go about this, even if future technologies bring it to the point of plausibility.
There is the idea of using the solar focus as a huge telescope lens, but this has the drawback of being an entirely passive system. To boot it sufers the great disadvantage of telescopes over probes: It sees only the surfaces nature illuminates, and cannot return any physical samples.
Is there anyway to have an active sensing solar focus telescope? How big would a radio transmitter (for example) have to be to perform a scan at that distance using the sloar focus to amplify the return signal?
Ok we still have no sample return, but we don’t get that with a flyby either. However one thing a probe could still do better than an active sensing system is learn more about its environment by interactin with it. Its had to concieve of a system that would allow anything but the very crudest interaction from a distance over interstellar distances.
So, moving on…. There is the idea of a giant ‘space tentacle’. Rather than sending one microprobe, send one microprobe followwed by a string of micro relay stations, to get around the problem of data return to Earth. If it is concevable to decellarate a microprobe then perhabs a bridge of tiny relay stations, like a tentacle with sense organs at the tip, could be strung between our solar system and the target star? However we are talking about a huge distance, and each relay station will have limited power due to their size…. we may need a hell of a lot of them.
Perhaps we can progress by rethinking the physical nature of the probe. The structure and functions of a probe are essentially information, stored by the matter that makes up the probe in its shape and composition. Dusty plasmas have been theoretically shown to support behavoir complex enough to qualify as simple life (here: http://www.sciencedaily.com/releases/2007/08/070814150630.htm )…if it can be used to make simple life then why not a space probe? I don’t exactly have a design plan for one, but that involves no fundamental breakthroughs in physics, only breakthroughs in plasma control. The mass of such a construction could be close to negligable – its a cloud of well organised plasma with a few dust particles in. And if the mass is near negligable it may be possible to carry out a nearly (but not quite) negligable sample return mission – a few thousand atoms from a planets upper atmosphere, or a few particles of space dust from the target system perhaps. Enough maybe to provide a ‘ground truth’ for our telescopic observations. Of course I have no idea how such a thing could be built, powered, or how it wold function…..
What else could we build an exotic probe from? Nothing not totally sci fi (and I realise I’m already stretching the concept of non-scifi to breaking point) springs immediatly to mind. Which doesn’t mean there arent possibilities, just that they elude my cold adled brain right now.
Ok, how else could we get a ‘ground truth’?
Is there anyway we could, in a tiny way, get another star system to come to us? Well that may be a promising avenue of thought, as in a way this has already been accomplished: the stardust mission collected several grains believed to be of interstellar origin. We know that bits of our solar system are gettng blown into interstellar space all the time ; tiny particles of dust produced by collisions, scraps of planetary atmopsheres. Perhaps if we understand and map the interstellar medium well enough (a big job in itself) we could select points on our own solar systems outer edge to place collectors for material that we know is likely to have originated in a specific star system? Given the sparsity of material out there it would have to be a big collector, or a long term hunt – but even a one particle from, lets say, Bernards star could change our understanding of that system. Maybe something like a sensitive tracking station on the heliopause could look for solid particles, then ionise them with a well aimed laser pulse and bring them in with a magnetic field? Total speculation, and possibly total tosh to, I admit.
I can’t track down my ‘too obvious to notice’ thought….. although I like the ideas of building an ‘exotic’ probe from something other than solid (or conventional?) matter, or hunting the edge of our solar system for extrasolar samples. Whether anything I’ve just said actually has any merit I do think that if we are going to think about interstellar exploration with ‘near term’ technologies, barring new physics, out of the box is where its got be at.
I wonder if the ‘dusty plasma probe’ and ‘space tentacle’ ideas could be combined? Plasma structures in space can reach huge sizes, could the tendril be made of plasma with a collection of conventional micro sensors at the far end sending information along the tendril as vibrations transmitted between electrostatically connected dust particles. Perhaps the microsensors could leave a trail of dusty plasma as they travel, manufactred out of the existing interstellar medium? If only I had more idea about dusty plasma physics…..
That figure is relevant at the star’s surface, probably a more interesting value is the escape velocity at the orbital separations of the planets – presumably a collision with the target star is not a desirable outcome of the mission! At 1 AU the escape velocity of the Sun is roughly 42 km/s.
Leads to the question of how useful gravity assist manoeuvres in a binary star system or using gas giants would be for such a mission. (That might also impose interesting constraints on when to launch a mission to Alpha Centauri to make the best use of gravity-assisted deceleration in the binary)
Any ideas on what zodiacal dust would do to a spacecraft passing through the system at these kind of speeds?
Kzb,
The observation doesn’t just depend on the diameter of the object, it was just BOE crude estimates. I stated that in reality you would approach and de-approach the object. Also you would have long distance telescopes. Daedalus carried several if I recall.
Even if I re-do the analysis, which I won’t for now, the encouter times at these flyby speeds are still way too low to justify the mission when you compare it to the ground/orbit/lunar based deep space observations as you suggested (another trade study we are doing by the way). I entirely agree with you that to justify such a mission deceleration has to be done. Hence why this is a project requirement for Icarus, separating it from the less harsh requirements of the Daedalus study. Although we have a total mission duration of 100 years (compared to ~40-50 for Daedalus) to allow for this.
The real reason I wrote the article is because there is the expectation that Icarus should go faster than Daedalus, from members of the public and also members of the design team. It’s sort of seen as a cool thing to do if we can say get it up to a cruise velocity of 0.15-0.2c, though a real engineering challenge. I also thought at the outset that perhaps 0.2c was our upper velocity bound and we should do our best to achieve this, perhaps by using high gain systems like antimatter catalyzed fusion. With a few years boost thrown in a flyby probe travelling at 0.2c would get to nearest stars in ~25 years.
However, given that we have to decelerate, I wrote the blog article to start a provocative debate on whether ‘faster is better’, given that we may have to trade cruise speed for encouter time, thus making the conditions for deceleration more achievable and perhaps not using what some see as an unrealistic 250 Hz pulse frequency. Next generation fusion demonstrators are talking about repitition rates of ~1-10 perhaps. So I also argue that if Icarus was to similarly choose an ICF based drive, then perhaps we should consider lowering the pulse frequency (~50-100), elongating the boost phase period and perhaps going for a cruise velocity between 0.06-0.1c, instead of the 0.12c for Daedalus. Nothing decided of course, this is all just healthy debate and we welcome external input and the reason for writing such articles.
I don’t understand your later point about mass ratio. Daedalus mass ratio in the two-stage configuration is ~40, so if want to decelerate from 12.2% of light speed back down to a solar insertion velocity then mass ratio is the squared of this, assuming equivalent boost/deboost phases. Where do you get the 7.4 from?
Obviously, its another goal of the design team to bring that huge monster of a mass (~53,000 tons) down to something more reasonable. A question for us is what is the minimum practical size of a Daedalus fusion engine – something we are looking at.
For intereste the Daedalus 1st stage and 2nd stage engine was in the ratio 3.1. I have done some calculations with 3-4-stage Daedalus variants but apart from hitting the performance gains from adding more stages you also hit a practical limit to the size of the technology. One thing we should (and probably will) do is to consider all of the Daedalus engineering components, have small we can shrink them down to, giving our 100 year mission requirement and then estimate the infrastructure requirements for this smaller version. This would also place an estimate on the time from when such a probe could be launched. Hopefully it would be sooner than the current figure floating around which appears to be ~200 years hence – reference Dyson, Millis, Cassenti who looked at economic, energy and velocity scaling requirements/trends.
Icarus is not required to completely decelerate. The exact wording of our project requirement is:
” The spacecraft mission must be designed so as to allow some deceleration for increased encounter time at the destination.
”
We can just bring it down by even a few percent to increase the encouter time, but this has to be balanced with ‘justifications for launch’ in the first place. The team are looking at various deceleration options, so far including the following:
1. Magsails
2. Orion
3. Medusa Sails
4. Reverse engine thrust
5. orbital slingshots
6. x-ray pumped lasers to decelerate a deployed sail probe.
7. just decelerate sub-probes, e.g. Starwisp probes.
I may be missing a few off our list. Any other suggestions from people and we would be entirely grateful.
We have some excellent people in the team looking at some of these areas. Like Adam Crowl looking at MagSails and Jim Benford looking at microwave beaming concepts. I am optimistic we can come up with something that is credible. But keep those ideas coming.
Kelvin
What about a few nuclear explosions to slow down the probe upon its arrival? Uranium is light in weight compared to the sudden braking energy that can be released – without demolishing the probe, of course.
That one-second encounter time is a show-stopper. It just isn’t possible to do enough in that time to justify the mission. Better instruments are not a solution either: the most an instrument can do is register every last photon that reaches it.
Nor is partial deceleration an answer. Even if it were possible to shed 90% of the velocity at no cost, the mission is still a non-starter. From the point of view of, say, climatology, or even meteorology, a 10-second look at a planet is no better than a 1-second look.
On the other hand, a probe that decelerates to rest within the target system could potentially operate there for decades. On arrival, it would turn into something like our present-day planetary spacecraft, but with the propulsion capability to visit all the planets, one by one, spending years studying each of them from low orbit. This phase of the mission might last even longer than the interstellar cruise – long enough, in fact, to allow two-way communication with researchers on Earth. That sounds more like a mission that could justify the effort put into launching it.
In a previous thread on FOCAL Jonathan Burns replied that the resolution FOCAL could achieve was: “In a nutshell, the same resolution we could obtain for bodies orbiting close to the Sun, if we were observing them from
550 AU away, or further.”
https://centauri-dreams.org/?p=785#comment-6003
So the extremely high resolution Enzo thought he’d heard about might not be correct.
Can someone clarify further?
Nathan,
Yes we are considering this, look up Medusa sail.
Geoff, I don’t agree with you that partial deceleration isn’t an answer. We have to start somewhere. If we can work out how to completely decelerate the probe then we will, but our calculations have to be credible.
Interesting suggestion about the probe remaining in the system as a communications link. You have given me some ideas here along the lines of the interplanetary/interstellar internet. The probe could also become a semi-permanant comms/navigation beacon for later missions.
Kelvin
Andrew W writes:
Andrew, the electromagnetic radiation from an object occulted by the Sun (i.e., on the other side of the Sun from the spacecraft), would be amplified by a factor of 108, according to Claudio Maccone’s figures.
“would be amplified by a factor of 10^8, according to Claudio Maccone’s figures.”
Thanks Paul.
So an object at a distance of 4.3 ly (271,209 AU) would at best appear as if it were actually only 27 AU away? So a Hubble telescope on FOCAL could at best get photos of a Neptune in the Centauri system as good as the Hubble photos of Neptune that we have?
http://hubblesite.org/gallery/album/solar_system/neptune/
Yes, it seems any proposal that does not include slowing down is pretty unviable.
What about Beam Propulsion? Wikipedia seems to think this is the most realistic chance we have of sending a probe to the stars. Then again, I am very wary of anything I read on Wiki. I’m just asking you experts if you agree.
BP is not without it’s drawbacks, however, though it does neatly eliminate the need for onboard fuel. What about way-stations? We would have to send them first. I develop my thoughts in more detail here, if you would care to look and comment it would be appreciated.
This is a very nice website. Thank you very much for starting and maintaining this. You guys rock.
I wonder, is there any reason the solar focus would not work for particles with mass traveling at near light speed, for example, nutrinos?
Why bother braking? Why not just design a Von Neumann machine which can survive hard-landing at that velocity (if there’s a gas giant in the system we can do aerobraking instead). Then have it build the necessary communications devices to send data back to Earth. In-situ resource utilization! All the kids are doing it!
This article here (pg. 17) :
http://www.cesr.fr/~g-wave05/presentations/optics/Koechlin.pdf
Gives a resolution of 3 nano arcoseconds at 789 AUs.
This is only about 600 m/pixel at 4.3 ly
Paul, maybe you can find some more information on the real resolution from Claudio and put this to rest.
Enzo writes:
Yes, that’s my intention, Enzo. In fact, I’m sending this off to Claudio this afternoon, and will publish what he has to say to clarify this.
Steven Colyer writes:
Many thanks for the kind words! Glad to have you with us.
Thanks Paul.
By the way, while searching for FOCAL resolution, I stumbled on another
type of gravitational lens. The “transparent” one. Just briefly skimming through a couple of articles, in the case of the sun the focus seems to be between 20 and 50 AUs .
Of course the sun is not transparent, except to neutrinos. Maybe it’s possible to build a great neutrino telescope using the ice of a KBO :-) One that changes its aiming point as the KBO orbits.
Hi John Freeman
No reason at all. In fact because the Sun is mostly transparent to neutrinos their gravitational focus region starts at just 23 AU from the Sun, just outside the orbit of Uranus. Similarly for gravity waves – if they exist.
Hi Steven Colyer
There are a number of beamed propulsion concepts being explored for “Icarus”, but they’re secondary to the primary propulsion system which has to be fusion, because of the Project’s Terms of Reference. There have been several papers published over time which use a hybrid approach – using a beam system to send fuel pellets to the main vehicle. One such concept is being explored for “Icarus”. The main problem is that lasers and particle beams require either very high acceleration (and high power) or elaborate efforts to mitigate beam-divergence over longer acceleration ranges.
Thanks! So….. I can dream of a nutrino/gravity wave observatory using the transparent solar focus…its a nice thought. I assume the only observational targets for the nutrino observatory would be the fusing cores of stars? Not that a good ‘x-ray’ of the cores of nearby stars wouldn’t be extremely cool!
Thanks Kelvin, you sure have started a “provocative debate” !
The mass ratio of 7.4 is idealised, it does not come from this project. It is the ratio required for delta-v of twice the exhaust velocity for a single-stage rocket. Given 100% efficiency of energy in the propellant, fusion can theoretically give exhaust velocity around 10%c.
However, you are assuming much lower efficiency than this, and you are entirely correct, we need to stay focussed on what may be realistically achievable, not go off into fantasy.
My personal vote would be for a full deceleration, enabling planetary orbit to be achieved. I also think it is important for results to be received back home in less than a human lifetime, so 6%c is too slow.
The REAL goal of this study is political, to show the possibilities. A lot of people assume interstellar probes are impossible at foreseeable technology levels. A properly thought-out project, giving all the details, breaks that mindset. However people will want a realistic chance of living long enough to see the pictures.
Like Author C Clarke said: once you have mastered fusion-based propulsion, it is merely a case of burning enough fuel !
I like Cambias’s idea of aerobraking… I think that’s really thinking outside the box — since, who would actually consider it for a probe moving at such a high speed (0.12c)? It’s one of those ideas that seem totally ludicrous at first, but may open a new line of thought to slowing this thing down at its destination…
For example, what if the main probe ejected a few extremely durable sub-probes — i.e. solid state electronics, able to withstand a 100g acceleration, and encased in something like a tungsten-carbide shell (this is all pure speculation, by the way) — and attempted to aerobrake them in the star’s atmosphere? You think it’d be possible for to survive such an encounter and actually slow down enough to be captured in the system??
(Of course it’s an extreme idea that’s probably impractical, but everything about an interstellar probe seems to be extreme in nature.)
Scott G, yes the aerobraking idea was something that occurred to me also, but I more or less dismissed the idea. I’ve not tried to calculate it, but I think the heat generation would be shocking.
I remember calculating the heat generation passing through interstellar space, and at relativistic speeds that is considerable (megawatts). Yet the amount of hydrogen between here and Alpha Centauri would be a layer less than 1mm at atmospheric pressure.
Perhaps we need to calculate the possibilities. It’s entirely possible that a combination of propulsive braking followed by aerobraking at lower velocity could give a useful reduction in flight time. For example, if we have 12% delta-v available from thrust, the maximum velocity is 6%, if we have to brake by thrust alone. If 1%c aerobraking is feasible, the maximum velocity becomes higher (it’s not 7% though !)
A stars atmosphere is huge and has a range of density from whatever the inerplanetary mediums density is to whatever the photosphere density is. If we were looking at aerobraking a probe from 6% c I suspect that a stellar atmosphere will be the only option. Certainly a terestrial planets atmosphere wouldn’t be the best!