"Getting past the detection phase, I proposed designing a small asteroid-landing probe that could be launched when a potential threat is discovered. It would have two functions: tracking (a transmitter on the surface would allow very precise positioning, even without onboard navigation systems) and science (detect the composition and other characteristics of the asteroid). The smaller the probe, the cheaper it would be and the faster it could be delivered.

Ideally, you'd want such trackers on as many NEOs as possible over time, even ones that aren't an imminent problem. Perhaps a large ion vehicle could make a multi-year mission of delivering several of these on a single mission."

How small is small?
Less than 1 gram, a gram, a kg, 10 kg, 100 kg, or more than 100 kg?

If we start with up to "18,000 potentially hazardous objects", we could want 18,000 or more probes.

How much money could each probe cost to make?
Negative dollars, less than $100, $1000, $10,000, $100,000, or more than $1,000,000

How does the probe get to an object and land safely?
Is it fired from some gun? And does it "land" at around 2 km/sec?
Does it, if launch from Earth L-1, and solar "sail" to the object and lightly land? Is it directed to an object? Does it hunt for objects. Can the probe "talk" to us. Can they "talk" to each other? Could there be various mother ships that tasked with particular zones which have tens or hundreds of probes. And could launch something with 1/2 dozen mothership. Or could a mothership hitchhike an other space missions.
Are probes passive like tracers put in ammunition- are they radioactive contamination, are the "spray painted" white dots, are they "like" laser reflectors on the Moon, or do they echo radar?