This morning I want to pick up on the ‘problem of arrival’ theme I began writing about on Friday, and we’ll look at interstellar deceleration issues a good bit this week. But I can’t let Monday start without reference to the Icarus results from Gran Sasso that finds neutrinos traveling at precisely the speed of light. All of this adds credence to the growing belief that the earlier Opera experiment was compromised by equipment problems. The news is all over the place (you might begin with this BBC account) and while we’ll keep an eye on it, I don’t plan to spend much time this week on neutrinos. We still have much to get done on the subject of slowing down.
Magsails and Local Resources
When you begin to unlock the deceleration issue, the options quickly multiply, and you find yourself looking into areas that weren’t remotely the subject of your earlier research. As we saw on Friday, the concept of magnetic sails grew organically out of Robert Bussard’s idea of an interstellar ramjet. Bussard didn’t want to slow down — he wanted to go very fast indeed. Read the comments on that post and you’ll find Al Jackson’s entertaining reminiscences of a dinner with Bussard (Tau Zero author Poul Anderson was present too), and a reminder that the scientist always claimed to have come upon the ramjet idea because of an encounter with Mexican food. The usual story has it that it was a burrito which, bitten down upon, suddenly opened for Bussard the splendors of matter being forced into a cylinder at high speeds.
Or maybe he was eating huevos rancheros — the story seems to have varied a bit over the years. Whatever the case, the idea of scooping up interstellar hydrogen and fusing it turned into a 1960 paper for Acta Astronautica and, along the way, into a critique by Robert Zubrin and Dana Andrews that showed just how much drag an electromagnetic scoop could generate. Andrews was working for Boeing at the time, and had grown interested in using Bussard concepts right here in the Solar System, thinking that a big enough scoop could gather hydrogen for use in an ion engine that could be powered up by an onboard nuclear reactor. A self-fueling ion drive might not be adaptable for interstellar missions, but for interplanetary work it seemed worth a look.
But the numbers were intractable. The magnetic scoop Andrews hoped to deploy created more drag than the ion engines produced thrust. The two researchers quickly found that the scoop’s best function was as a magnetic sail, and their work on the idea appeared in the literature in the early 1990s. In his 1999 book Entering Space, Zubrin recalls that the time was right for the magsail given that Paul Chu (University of Houston) had just invented the first high-temperature superconductors, which a magsail could theoretically use to create the magnetic field that would allow it to ride on the solar wind. Practical high-temperature superconducting wire born out of this work might one day allow magsails to achieve higher thrust-to-weight ratios than solar sails.
Magsails have clear propulsion implications, but Zubrin states the obvious about their most effective uses:
…the most interesting and important thing about the magsail is not what it can do to speed up a spacecraft — what’s important is its capability for slowing one down. The magsail is the ideal interstellar mission brake! No matter how fast a spaceship is going, all it has to do to stop is deploy and turn on a magsail, and the drag generated against the interstellar plasma will do the rest. Just as in the case of a parachute deployed by a drag racer, the faster the ship is going, the more ‘wind’ is felt, and the better it works.
Which takes us to the idea of using in-situ resources to tackle the deceleration problem. If your goal is to launch a starship that can decelerate in the destination system to explore it, the magsail lets you do the job without carrying the deceleration fuel aboard the vehicle. Play around with the numbers long enough and you’ll see what a huge boost this would be, for otherwise you’re carrying all the fuel needed to slow down a starship (moving, perhaps, at .10 c!), and that means you’ve got to get all of that fuel up to cruise in the first place. The idea of creating drag against the interstellar medium and a destination stellar wind thus has a powerful appeal.
Rise of the Superconductor
When Bussard studied how his ramjet could operate in a region of interstellar space where the density of hydrogen was roughly 1 hydrogen atom per cubic centimeter, he saw that he would need a collecting area of 10,000 square kilometers. This is so vast that even if it were made of 0.1-centimeter mylar, a physical scoop would weigh something on the order of 250,000 tons. But a much smaller collector generating a magnetic field seems practical given the advances in superconducting alluded to above, with a loop of superconducting wire deployed from the spacecraft, the current applied to it cycling continuously to generate the magnetic field. Here’s how Zubrin and Andrews described it in a paper based on their presentation at the 1990 Vision-21 symposium at NASA’s Lewis Research Center (now Glenn Research Center):
The magnetic sail, or Magsail, is a device which can be used to accelerate or decelerate a spacecraft by using a magnetic field to accelerate/deflect the plasma naturally found in the solar wind and interstellar medium. Its principle of operation is as follows: A loop of superconducting cable hundreds of kilometers in diameter is stored on a drum attached to a payload spacecraft. When the time comes for operation the cable is played out into space and a current is initiated in the loop. This current once initiated, will be maintained indefinitely in the superconductor without further power. The magnetic field created by the current will impart a hoop stress to the loop aiding the deployment and eventually forcing it to a rigid circular shape.
Image: A space probe surrounded by a magnetic sail. Early work on these concepts has taken place at the University of Washington under Robert Winglee, with reports available at NASA’s Institute for Advanced Concepts site. Credit: NASA/University of Washington.
Thus the hybrid concept Andrews and Zubrin came up with in the Vision-21 work, extending ideas they had first presented in a 1988 paper: Use laser beaming technology to push a sail to interstellar cruise speeds, then deploy a magsail upon arrival to reduce deceleration time. The authors looked at the numbers and worked out 0.8 years for acceleration, 17.4 years of coasting at almost half the speed of light, and 18.8 years for deceleration. This gets you about 10 light years out in around 37 years, a mind-bending pace that uses a huge sail and some generous assumptions about laser power that we’ll look at tomorrow. For there are other ways to use lasers, even for deceleration, and other ways, too, to exploit the local interstellar medium.
Zubrin and Andrews’ paper from Vision-21 is “Use of Magnetic Sails for Advanced Exploration Missions,” in the proceedings of Vision-21: Space Travel for the Next Millennium” (NASA Conference Publication 10059. The citation for their 1988 work is given in yesterday’s Centauri Dreams post.