I don’t want to move past Gregory and James Benford’s interesting sail ideas without pausing to examine another paper that ran in the preceding issue of JBIS. It’s a look at what we might do in the near-term with solar sails, written by Gregory Matloff (CUNY), Travis Taylor (BAE Systems) and collaborators. And it focuses on what inspired Centauri Dreams (the book) in the first place, the question of where we stand right now in terms of deep space propulsion.
In other words, never mind the politics or the economics. If we had to launch a mission soon (obviously with a robotic rather than a human payload), how far could we go and how fast? Matloff and Taylor lay out the near-term possibilities for reaching the heliopause (roughly 200 AU) and the Sun’s inner gravity focus (550 AU) using both sails and other propulsion options. The reference sail used here is a 100 meter disc massing some 157 kilograms, with structure and payload adding up to 100 kg for a total mass of 257 kg.
Let me just quote the paper’s conclusion:
A number of options exist for near-term interstellar exploration using robotic solar-photon sailcraft including sail unfurlment at the perihelion of a parabolic solar orbit, sail unfurlment at the perihelion of an elliptical solar orbit, and Jupiter-gravity-assist after sail unfurlment from an elliptical solar orbit. Although all of these techniques can propel a 257 kg sailcraft with an areal mass thickness of 0.0082 kg/m2 to the heliopause (200 AU) within a human working lifetime, [and] to the Sun’s inner gravitational focus at 550 AU from the Sun in a human lifetime, they are all slower than equivalent missions launched using higher technology sailcraft.
Note what Matloff and Taylor are saying about mission times — this defines the human reach at present using a mix of solar sail and solar-electric technologies. Reaching 200 AU takes about a human working lifetime (i.e., some 40 years), while the gravity focus takes a full lifespan, averaged here at 80 years. A critical factor is the areal mass thickness of the sail. The authors draw on Italian theorist Giovanni Vulpetti, who developed calculations based on areal mass thicknesses that were considerably thinner (0.001-0.002 kg/m2). As you might expect, such vehicles are capable of higher cruise velocities.
The paper is Matloff et al., “Near-Term Interstellar Sailing,” Journal of the British Interplanetary Society Vol. 59 No. 2 (February 2006), pp. 59-62.
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Once you get your telescope to 550 AU, won’t you want to stop it? Or can you send it with variable corrections to the poor focus before and after the 550 AU spot? In astrophotography (at the ameteur level, anyway) the first three things to get right for good photos is 1) focus, 2) focus, and 3) focus.
Then, too, once you’re out at 550 AU, and there’s some object on the other side of the Sun that you wanted to look at in detail, pointing to another object requires moving at more or less right angles to where the Sun is. How do you do that? OK, how do you do that without reaction mass?
I want one of these telescopes up as soon as an exo-Earth is detected. Yes, i know that Mars is hard to see when it’s on the other side of the Sun. But for an exo-Earth, looking through the Sun is ideal.
How bright is the Sun from there? Well, it’s about mag -26.7, except in Michigan. From 550 AU, it’s only 13.7 magnitudes dimmer, or mag -13. The full moon is about mag -12.7. So the Sun from 550 AU would be slightly brighter than the full moon from Earth. Not just a bright star. The Moon punches through clouds, a feature we like here in Michigan.
No need to stop the gravity focus spacecraft at 550 AU. Here’s an older story that explains this:
Matloff goes into this in Deep Space Probes: “Unlike optical lenses, in which the light diverges after the focus, the gravity-focused radiation remains along the focal axis for solar separations greater than 550 AU.” More there pp. 27-28. The real source on gravity focus missions is Claudio Maccone’s The Sun as a Gravitational Lens: Proposed Space Missions (Aurora, CO: IPI Press, 2002).
I wonder about using the supermassive black hole at the center of our galaxy as a focus point. How far is the distance from the Earth in this case?
Somebody help me with Hiro’s question. Off the top of my head I make the distance to galactic center at about 26,000 light years. Is my memory right?
Sorry, I mean the distance from the Earth to the focal lens of the supermassive black hole at the center of our galaxy.
No problem, that was my mis-reading. This one is far trickier, especially since the center and the probable black hole there are shrouded by interstellar gas and dust. But it’s an interesting question, and I’m hoping some of our readers will have an answer that’s at least approximate. I think that 26,000 light year distance to the core is about right (though I’m drawing on my poor memory), but just where we’ve been able to position the possible location of the black hole that may be there is something I don’t know.
Hi Paul & Hiro
Well the radii involved are related to space curvature by the square root of the mass, so I’d say roughly (3×10^6)^0.5 times the Sun’s = 1,732 times bigger. Thus the focus begins some 15 light years away from the black hole. Roughly.
Here’s a quick and easy summary…
With a solar sail passing .01 au from the sun we can accelerate a
small probe to about 1% of light velocity using a solar light sail
for propulsion. That is enough to go .40 light years from the sun
in 40 years.
If a light sail passes close enough to the surface of the sun it can
reach about 12% of light velocity if it can stand the heat . Also
for a short time accelerations of 10-1000 gs may result . Those
are fine for robot probes ,but humans can not with stand that .
That paper by Matloff et.al. is available online:
“Near-Term Interstellar Sailing
Another paper by Zubrin and Speith argues that solar sails to the stars might be a near term possibility:
Ultra-Thin Solar Sails for Interstellar Travel.
Phase I Final Report
Dr. Robert Zubrin
NASA Institute for Advanced Concepts
555-A 14th St., NW
Atlanta, GA 30318
They argue a solar sail driven craft could reach Alpha Centauri within 32 years using special ultra light weight materials for the sails.
What you need is very lightweight yet strong material for the sail. The single molecular layer material known as graphene might work:
Radical fabric is one atom thick.
Last Updated: Friday, 22 October, 2004, 13:18 GMT 14:18 UK
“A new class of material, which brings computer chips made from a single molecule a step closer, has been discovered by scientists.”
Graphene has the same strength in two dimensional form that carbon nanotubes have in 1-dimension.
However, if you read the Zubrin/Speith paper you see a problem with using very thin material is that part of the light energy just passes through, not imparting momentum to the craft. That’s the purpose of the calculations in their paper of finding an optimum thickness for reflectivity and lightness.
It might work to use just a few layers of the graphene sheets so that most of the light is reflected.
Graphene so far has only been made in sizes a few microns across. Still this might be enough to test the feasibility of using this for solar sail material in lab tests.
You might then just be able to tie very many of the micron size sheets together to get a solar sail of usable size.
Graphene, unfortunately, is transparent so its utility as dielectric sail material is doubtful. Quarter-wave nanotube sails might serve quite well instead, with their expected low areal density. Deploying such will be very tricky, though 1% lightspeed should be reasonable, except for the 400 gees or so expected. Another option is a detachable concentrator mirror that directs a beam at the probe sail. That might get the sail up to 5% lightspeed.