2025AJ....170...22L

As NASA's New Horizons spacecraft exits the solar system bound for interstellar space, it has traveled so far that the nearest stars have shifted markedly from their positions seen from Earth. We demonstrated this by imaging the Proxima Centauri and Wolf 359 fields from Earth and New Horizons on 2020 April 23, when the spacecraft was 47.1 au distant. The observed parallaxes for Proxima Centauri and Wolf 359 are $32\mathop{.}\limits^{^{\prime\prime} }4$ and $15\mathop{.}\limits^{^{\prime\prime} }7$, respectively. These measurements are not of research grade, but directly seeing large stellar parallaxes between two widely separated simultaneous observers is vividly educational. Using the New Horizons positions of the two stars alone, referenced to the three-dimensional model (3D) of the solar neighborhood constructed from Gaia DR3 astrometry, further provides the spacecraft spatial position relative to nearby stars with 0.44 au accuracy. The range to New Horizons from the solar system barycenter is recovered to 0.27 au accuracy, and its angular direction to $0\mathop{.}\limits^{^\circ }4$ accuracy, when compared to the precise values from NASA Deep Space Network tracking. This is the first time optical stellar astrometry has been used to determine the 3D location of a spacecraft with respect to nearby stars and the first time any method of interstellar navigation has been demonstrated for a spacecraft on an interstellar trajectory. We conclude that the best astrometric approach to navigating spacecraft on their departures to interstellar space is to use a single pair of the closest stars as references, rather than a large sample of more distant stars.