A potential method of starship deceleration that does not strain the limits of contemporary technology does, however, exist. The principle of this device - a form of magnetic braking called the 'magsail', - is illustrated in Figure 7.2.
The magsail is derived from early approaches to the collection of fuel for interstellar ramjets, which are described in a following chapter. In 1974, Matloff and Fennelly suggested that the magnetic field generated by a superconducting solenoid could extend for thousands of kilometres in the interstellar medium. Gyrating around the ship generated magnetic field lines, interstellar ions would approach the ship closely. But Matloff and Fennelly were unable to conclude whether the ions would be collected into the scoop or reflected back into space.
In 1990 this concept was reinvestigated by Andrews and Zubrin. Using sophisticated computer codes, they demonstrated that a solenoidal field would tend to reflect interstellar ions elastically, decelerating the ship by linear momentum conservation. Their analysis also reveals that a simple supercurrent ring is superior as a braking mechanism to a superconducting solenoid.
The magsail principle works as follows. Magnetic field lines emerge from the supercurrent ring, as shown in Figure 7.2. Interstellar ions gyrate around the field lines, and therefore approach the starship. Close to the starship, the magsail generated magnetic field lines are so close together that a magnetic mirror effect is created. The ions are reflected back into space, in the direction of starship velocity, and the spacecraft decelerates.
In interplanetary space the effective field radius of the magsail is about ten times its physical radius. The magsail's mass can be estimated using equation (9) of Zubrin and Andrews (1989):
Magsail generated magnetic field lines
Magsail generated magnetic field lines
Spacecraft velocity direction
Spacecraft deceleration direction Figure 7.2. The magsail as an interstellar braking mechanism.
where Rmag is the magsail's physical radius, 7mag is the magsail supercurrent, and
, is the magsail's current/mass density ration in amp-m kg.
To obtain an expression for magsail induced deceleration DECmag, we next modify equation (10) of Zubrin and Andrews (1989) to include the ratio of magsail mass to ship mass Ms:
0-59 MfsPin Vs
where ^ is the permeability of free space (4n x 10~7Namp2), pin is the interstellarion mass density, and Vs is the ship velocity. Integration of equation (7.5) yields a non-relativistic expression for ship velocity Vmag at time t during magsail deceleration as a function of ship velocity during interstellar cruise Vcr, at the start of magsail deceleration:
i Rmag mag
M t mag
From Zubrin and Andrews (1989), magsail mass = 8.7 x 104kg and magsail supercurrent = 159 kiloamperes. If we adopt a current/density mass ratio of 5 x 106 amp-mkg-1, a compromise between Zubrin and Andrews near-term and optimistic projections, equations (7.4) yields a magsail physical radius of about 420 km.
In operation, the effective magsail physical radius will be reduced because of the necessity to store supercurrent to compensate for variations in interstellar ion density. Since this might be done by counter-winding some coils to the main magsail, we assume that Rmag is reduced to 120 km. Selecting a conservative value of local interstellar medium ion density (0.05 protons cm3, from Matloff and Fennelly (1974)), and assuming a total ship mass of 3.8 x 105 kg, we can solve equations (7.5) and (7.6).
Figure 7.3 presents a magsail deceleration profile for the parameters described above, and an interstellar cruise velocity of 0.03 c, obtained by the solution of equations (7.5) and (7.6). The ship requires about 50 years to decelerate from 0.03 c to 0.0022 c, during which time it traverses about 0.46 light years. Deceleration from 0.03 c to 0.001 6 c requires about 60 years, during which time the ship traverses about 0.48 light years. Deceleration from 0.04 c adds just a few years to the deceleration process.
Note that magsail deceleration is much more efficient at high velocities. For this reason the terminal stage of starship deceleration will use the light sail directed at the target star, as described in a previous chapter. The above analysis may, however, actually underestimate magsail performance, since a magsail could be pointed into the solar wind of the destination star, thereby enhancing its performance. Also, neutral interstellar atoms encountering the rapidly varying magsail magnetic field might be ionised.
As reviewed by Cocks et al. (1997), many inner Solar System applications of superconducting magsails have been suggested. But as discussed by Vulpetti and Pecchioli (1991), these may be difficult to implement, because present-day superconductors operated within the orbit of Mars tend to 'go normal' and lose superconductivity unless massive thermal shielding is employed.
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