NASA’s Hubble Space Telescope (HST) has discovered the smallest object ever seen (in visible light) in the Kuiper Belt, a region of the Solar System beyond the planets extending from the orbit of Neptune (at 30 AU) to approximately 55 AU from the Sun. The discovered object is estimated to be only 1 km across. The smallest Kuiper Belt Object (KBO) seen previously in reflected sunlight had the size of roughly 50 km. Observed at the distance of 6.76 billion km (45.2 AU), an object 1 km in diameter is about 100 times dimmer than those the HST can detect directly.
This particular KBO was actually discovered indirectly, in the data from Hubble’s Fine Guidance Sensors (FGS), which provides high-precision navigational information to the observatory’s attitude control systems by looking at select guide stars for pointing. The extreme precision of FGS instruments can reveal the effects of a small object passing in front of a star. This would cause a brief occultation and diffraction signature in the FGS data as the light from the background guide star is bent around the intervening foreground KBO.
Hilke Schlichting of the California Institute of Technology and her collaborators reported the discovery of such a signature after an analysis of FGS data from 50,000 guide starts taken over the period of 4.5 years. They discovered a single 0.3 second long occultation event. The duration of the occultation was used to estimate the object’s distance and the amount of dimming provided means for determining its size.