When talking about interstellar probes in the mainstream, the matter of speed always come up. Usually that speed is the speed of light, and how at the present we can’t obtain that with our current state of rocket technology.
But at Paul Gilster’s Centauri Dreams blog, ideas flow like clear, clean water. And the optimism in the comments column is forever present:
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.
One commenter especially had a rather unique take on interstellar sample taking that reminded me of the late Mac Tonnies’ theory of UFOs (..like a cat chasing after a flash-light beam)…
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.”
Wouldn’t a native observer from that system see strange, glowing objects flitting around like so many fire-flies, breaking the laws of physics and disappearing magically?
Sure I might be stretching the comparison some, but you get the idea.
In the meantime, Gilster’s post might have some merit, if the endgame is parking the probe in the target system for good. I just don’t see the merits of a fly-by, however slow.
If a fly-by is the goal, we might as well build super-duper telescopes at the Sun’s maximum gravitational lensing effect and just observe the target system.
And manipulate the photons enough to grab a few alien atoms.
It’ll be cheaper.