Since 1972, reprints have included a foreword by Asimov explaining that advancing knowledge of conditions on Mercury has rendered some of the novel's descriptions of that world inaccurate.

A character notes that there are places on Mercury's sunside where it is hot enough to melt lead and boil sulfur, while the nightside is the only planetary surface in the Solar System that never sees the Sun.

Most of the novel's action takes place in and around an astronomical observatory located at the planet's north pole, where libration results in a half-mile movement of the terminator.

Starr and Bigman meet Mindes' friend Dr. Karl Gardoma, the observatory's physician; Jonathan Urteil, who works for a political opponent of the Council of Science named Senator Swenson; Dr. Lance Peverale, the head of the observatory; and Dr. Hanley Cook, Pevarale's chief assistant, who wants to succeed Peverale.

On the return journey to Earth, Starr admits to Bigman that the present quarrel between Senator Swenson and the Council of Science was a draw, in that Jonathan Urteil was not able to manufacture any scandal against Project Light, but the two top men at the Mercury observatory were exposed as felons.

Although the Sirians were first introduced in Lucky Starr and the Pirates of the Asteroids as the villains of the series, they remained nothing more than a vague external menace until this particular book.