  The recent progress in laser propulsion research has advanced substantially
the prospects to realize interstellar spaceflight within a few decades. Here we
examine passive deceleration via momentum braking from ionized interstellar
media. The very large area to mass relations needed as a consequence of the low
interstellar densities, of the order of 0.1 particles per $\mathrm{cm}^{3}$, or
lower, are potentially realizable with magnetic sails generated by
superconducting coils. Integrating the equations of motion for interstellar
protons hitting a Biot Savart loop we evaluate the effective reflection area
$A(v)$ in terms of the velocity $v$ of the craft. We find that the numerical
data is fitted over two orders of magnitude by the scaling relation $A(v)\ =\
0.081A_R\log^3(I/(\beta I_c))$, where $A_R=\pi R^2$ is the bare sail area, $I$
the current and $\beta=v/c$. The critical current $I_c$ is $1.55\cdot10^6$
Ampere. The resulting universal deceleration profile can be evaluated
analytically and mission parameters optimized for a minimal craft mass.
  For the case of a sample high speed transit to Proxima Centauri we find that
magnetic momentum braking would involve daunting mass requirements of the order
of $10^3$ tons. A low speed mission to the Trappist-1 system could be realized
on the other side already with a 1.5 ton spacecraft, which would be furthermore
compatible with the specifications of currently envisioned directed energy
launch systems. The extended cruising times of the order of $10^4$ years imply
however that a mission to the Trappist-1 system would be viable only for
mission concepts for which time constrains are not relevant.
