Four drivers fall away
And the fifth takes over.
This site’s hazard framework has five drivers: solar radiation and wind, planetary rotation and magnetic fields, tidal flexing, internal heat, and galactic cosmic rays with the interstellar medium. Applied to nine destinations inside the heliosphere, the first four do most of the work.
Past the heliopause, four of the five fall away entirely. There is no solar wind, no planetary magnetosphere, no tidal flexing and no internal heat. What remains is the fifth, and at 0.2c it stops being a background and becomes the dominant design load.
That inversion is worth holding onto, because it is why the cruise phase is a materials problem rather than an environmental one. A craft in interstellar cruise is not being weathered. It is being eroded at a rate set by how fast it is going.
Dust becomes the design load
Kinetic energy scales with the square of speed.
A dust grain that would be harmless at 30 kilometers per second is not harmless at 60,000. Kinetic energy scales with the square of relative velocity, so the same grain arrives with four million times the energy. Over a 21-year cruise, cumulative exposure rather than any single impact is what sets the design.
Two consequences. The radiation environment destroys unshielded silicon over that interval, with no maintenance and no reboot from the ground — which is part of why full autonomy is the only operating mode and why that specification is the least doubtful thing in the interstellar case. And the sail must still be intact at the far end if it is going to serve as the transmit aperture, after 21 years of proton bombardment and dust erosion.
The mitigation is not structural. It is the swarm: roughly 5,000 craft over a 15-year campaign, with a 90 percent loss rate accepted, because the marginal craft costs a day of sunlight. This is the one regime in the architecture where attrition beats redundancy.
Target selection interacts with this. Tau Ceti’s debris disk holds roughly 10 times the dust of the solar system, so a flyby there at 0.2c is a materially different risk problem from a flyby through a clean system. The dust that signals a dynamically active system is a habitability discount and a survivability discount at once — the hazard and the signal are the same physical fact. See the target set.
The zodiacal crossing
10 to 35 percent of the total column, before leaving home.
A craft accelerated in the inner system crosses the zodiacal dust cloud on its way out, and that crossing alone accounts for between 10 and 35 percent of the entire interstellar dust column it will encounter over the whole mission — the spread depending on the departure asymptote’s ecliptic latitude, since a shot steeply out of the ecliptic plane crosses much less of it than one along the plane.
A craft accelerated at 40 AU crosses essentially none of it.
This is a real second-order advantage for outer-system departure and it should be sized honestly. It reduces total dust exposure by 10 to 35 percent, which moves the swarm attrition rate proportionally. It does not by itself justify relocating a beam array through four orders of magnitude of collector penalty — the arithmetic for that is on the departure corridor page. It does contribute to the case for the nuclear branch, whose vehicles are single and expensive rather than numerous and cheap, and for which a third less dust exposure over a 60-year cruise is worth having.
It also argues, cheaply, for choosing a departure asymptote out of the ecliptic where the target geometry permits. That costs nothing and nobody has priced it.
The wilderness is unmeasured
Two spacecraft, one location, a few decades.
The local interstellar medium density, on which the entire magsail deceleration profile depends, has been measured in situ at exactly one location, by two spacecraft, over a few decades. Voyager 1 crossed the heliopause in 2012 and Voyager 2 in 2018, and both are sampling a single direction through a medium known to be inhomogeneous on parsec scales.
The medium past the heliopause is as poorly characterized as anywhere in this architecture, and it is the medium the entire braking argument rests on.
That is the argument for the relay chain on arrival and deceleration: a node at 550 AU is not only an astronomical instrument, it is the first in-situ characterization of the departure corridor at a density and a distance that matter, decades before any craft has to survive it.
This page is the thinnest in the section, and that reflects the source rather than the topic. The medium is treated in a few paragraphs in the current draft, with its numbers cross-referenced from the dust-path and swarm arguments rather than derived here. Dust flux as a function of galactic direction, shielding geometry for a gram-class craft, and erosion rates on a patterned dielectric membrane are all unresolved, and none of them is hard to estimate.