ONCE, asteroids were nothing but tiny dots of light in a telescope. Then, about 50 years ago, we began to pass that light through “BVR” filters – glasses tinted blue, “visual” (green) and red – to see if these asteroids come in different colours. Turns out, they do. Most of the brighter ones are somewhat reddish (they’d look brown to our eyes), while the darker ones are essentially colourless. Those colours give us a language to sort and classify these objects.

At an upcoming planetary sciences meeting, Fr Jean-Baptiste Kikwaya of the Vatican Observatory will be reporting on his latest observations of “near-Earth asteroids”, a particular class of asteroid often only a few tens of metres large that sometimes come closer to us than our own Moon. Curiously, though, he’s using the same technique we used back then: BVR colour filters. But with the Vatican’s modern telescope and detectors, he can measure bodies so tiny and faint that we didn’t even know they existed 50 years ago. And by using the same filters as back then, he can make a direct comparison of these metre-sized boulders to the 100-kilometre asteroids out in the main belt that we first observed so long ago.