# How do we know what percentage of NEOs we've discovered?

I stumbled upon this answer: https://space.stackexchange.com/a/41535/10334

It states the percentage of NEOs we have already discovered.

Seeing this I've asked myself: How do we know what percentage is already discovered? For that one would have to know, how many objects there are. But how can you if you haven't discovered them?

• I don't know for sure, but I suspect it's based on the number of NEOs we expect versus the number we've actually tracked. How that expectation came to be though, I don't know. If the expectation is based on the number we've tracked so far, I smell a rat.
– Mast
Feb 20, 2020 at 13:20
• I'm really curious what's going on with the dip around 4km. Either there's something off with our expectation of how many there should be, or that size is really good at hiding for some reason. Feb 20, 2020 at 14:30
• Looking more closely at the rest of the chart, the 4km dip might not be a problem with the model or our detection at that size. The expected total for that size bucket is only a few dozen; so if the percentage found line calculated on number found/number expected it wouldn't take a large absolute variance for it to just mean our solar system is weird at that datapoint. Feb 20, 2020 at 20:57
• you seem to forget that we have - thanks to Kepler, Einstein and other Astronomers - quite exact models of our solar system. We can calculate what mass must circulate a certain orbital to get the model match the reality i.e. paths of known objects. Also we have models for the known bodies - down to their mass and hence forth expected gravitational influence on other bodies. We can therefore with a good measure say "there need to be more bodies of x mass to make the model match the known paths". We know the densities of those bodies and their variance so we can predict their size Feb 21, 2020 at 11:06