Infinitesimal proper Euclidean motion ($V(x)=a\times x+b$)

The derivative at the identity of a smooth curve in $\mathrm{SE}(3)$ is a vector fieldAfter shifting the origin perpendicular to $a\ne 0$, it becomes rotation about the axis parallel to $a$ plus translation along that axis.

The integral curves of $V(x)=a\times x+b$ are lines when $a=0$, and helices around an axis when $a\ne 0$. The helix is closed exactly when its axial translation component vanishes, in which case every nonfixed orbit is a circle.

A simple closed curve in ${\mathbb{R}}^3$ preserved by a nonconstant smooth family of proper rigid motions is a circle. Differentiating the normalized family at the identity gives a nonzero infinitesimal proper Euclidean motion tangent to the curve. Compactness rules out its line and nonclosed helical orbits, leaving a circular rotation orbit.