Are more of these things showing up or do we spot them better? Inside the orbit of the moon seems one hell of a close shave!
We spot them better. Recently we weren’t looking for anything this small.
Until about 25 years ago, there was almost no detection system at all; objects of this kind were found quite by accident during surveys of the sky in search of comets. Until 1994, there was a really stubbornly stupid streak among many astronomers, who insisted that looking for these was a waste of time, because the consequences of impact were very nearly zero due to the small size.
In 1994 the SL-9 comet fragments smacked one after another into Jupiter, leaving Earth-sized scars in that planet’s atmosphere. That sobered people up something wonderfully. I’d be surprised if some of those old blowhards didn’t have nervous breakdowns and have to retire from that experience, just as happened after the ascendancy of the Alvarez model for mass extinctions became widely accepted (except in Darwin-mired England).
The SL-9 discovery was a small independent project consisting of three people (the Shoemakers, and David Levy) using a largish small reflector (or maybe it’s a Schmidt-Cassegrain) up on Mount Palomar. They spent night after night searching for (and finding) Earth-crossing asteroids; Carolyn Shoemaker still has the record I believe for most comet discoveries by a single, live person (the overall record is held by an automated orbital telescope).
Spacewatch (or is it called Spaceguard?) started sometime prior to that, I think in Australia, and it started as a small operation, gradually bringing other low-budget independent operations around the world into a network, and all searching for nearby potential threats. Budgets rose after the SL-9 impacts.
The short answer is, both — more efforts are made to find them, and therefore there’s an apparent increase in the number coming by. The fact is, any fluctuations in number of arrivals is quite random, and the big increase over the last couple of decades is due to the systematic search for them. They were there before, just like Antarctica’s ozone hole or extrasolar planets, but no one had ever noticed them before.
There was one last year that was inside the orbit of Geostationary satellites, less then 25000 miles.