NEOSSat: one telescope, several kinds of moving and changing targets

NEOSSat launched on February 25, 2013, as a Canadian microsatellite carrying a telescope. The Canadian Space Agency leads its space astronomy work, while Defence Research and Development Canada is responsible for the High Earth Orbit Space Surveillance mission, or HEOSS. The shared spacecraft supports distinct questions about natural objects, artificial objects and changing starlight.

Astronomy observations include near-Earth asteroids and comets, especially targets difficult to observe close to the Sun’s direction from the ground. Surveillance observations track satellites and debris. These activities can both measure positions against background stars, but their interpretation differs: an asteroid follows a solar-system trajectory, while a piece of Earth-orbiting debris belongs to a different dynamical and operational context.

A position in an image is a direction on the sky, not a complete three-dimensional orbit. To connect it with a physical trajectory, analysts need accurate times, the observing spacecraft’s location and additional observations. A short observed arc can permit multiple candidate paths. Further measurements test those paths and reduce uncertainty; they do not merely add more pictures to a collection.

Imagine two predicted tracks that both pass through a target’s first measured position. An observation several hours later can help distinguish them if their predicted directions have separated by more than the measurement uncertainty. An object’s brightness alone would not resolve this geometric ambiguity. For collision research, the useful result is a trajectory with characterized uncertainty, rather than a dramatic-looking dot or streak.

Choose the measurement for the question

NEOSSat also supports studies of exoplanet transits by following subtle dimming of stars. Here the main question concerns brightness versus time rather than the target’s apparent track. Observations can refine a transit’s timing and help test a proposed orbital period. They complement other observatories; a single dimming event does not by itself identify every physical cause of a brightness change.

For a fictional target, dips recorded on day 2 and day 7 are five days apart. That spacing makes a five-day period a candidate, but gaps in the observations might hide additional dips. A planned observation at the predicted next event tests the interpretation. In both asteroid tracking and transit work, the telescope becomes most useful when measurements are placed in a time sequence and compared with explicit predictions.