Editing Magnetic sail (section)

=== Deceleration in interstellar medium (ISM) ===
A spacecraft accelerated to very high velocities by other means, such as a fusion rocket or laser pushed lightsail, can decelerate even from relativistic velocities without onboard propellant by using a magnetic sail to create thrust (drag) against the interstellar medium plasma environment.  As shown in the section on [[#Magsail kinematic model (MKM)|Magsail kinematic model (MKM)]], feasible uses of this involve maximum velocities below 10% [[speed of light|c]], taking decades to decelerate, for total travel times on the order of a century as described in the [[#Specific designs and mission profiles|magsail specific designs]] section.<!-- DM: Not sure if this is synthesis. "Hence this would work best for very long duration missions." Need to find a reference that actually states this. --> [[File:The Local Interstellar Cloud and neighboring G-cloud complex.svg|thumb|upright=1.35]]
Only the magsail references consider deceleration in the ISM on approach to Alpha (<math>\alpha</math>) Centauri, which as shown in the figure is separated by the [[Local Bubble|local bubble]] and the [[G-Cloud|G-clouds]] and the [[Solar System]], which is moving at velocity <math>v_{sun} </math> and the local cloud is moving at velocity <math>v_{L|C} </math>. Estimates of the number of protons range between 0.005 and 0.5&nbsp;cm<sup>−3</sup> resulting in a plasma mass density <math>9\times10^{-24}<\rho_{im}<3\times10^{-22}</math> kg/m<sup>3</sup>, which covers the range used by references in the magsail specific designs section. As summarized in the magsail specific design section, Gros cited references indicating that regions of the G-clouds may be colder and have a low ion density. A typical value assumed for approach to Alpha Centauri is a proton [[number density]] <math>n_{i}</math> of 0.1 protons per cm<sup>3</sup><ref name="Gry A58" /> corresponding to <math>\rho_{im}\approx 10^{-22}</math> kg/m<sup>3</sup>.

The spacecraft velocity <math>v_{sc}</math> is much greater than the ISM velocity at the beginning of a deceleration maneuver so the apparent plasma wind velocity from the spacecraft's viewpoint s approximately <math> u_{im} \approx - v_{sc} </math>.

Radio emissions of [[cyclotron radiation]] due to interaction of charged particles in the interstellar medium as they spiral around the magnetic field lines of a magnetic sail would have a frequency of approximately <math>120 \, v_{sc}/c</math> kHz.<ref>{{Cite journal |last=Zubrin |first=Robert |date=1994-07-01 |title=Detection of Extraterrestrial Civilizations via the Spectral Signature of Advanced Interstellar Spacecraft |url=https://aip.scitation.org/doi/abs/10.1063/1.2950156 |journal=AIP Conference Proceedings |volume=301 |issue=1 |pages=1407–1413 |doi=10.1063/1.2950156 |bibcode=1994AIPC..301.1407Z |issn=0094-243X}}</ref> The Earth's ionosphere would prevent detection on the surface, but a space-based antenna could detect such emissions up to several thousands of light years away.  Detection of such radiation could indicate activity of advanced extraterrestrial civilizations.