3 days ago, it was first seen. About a dozen observations. F***, it's not fun that these pass with so little forewarning.

   ZTm0038* C2023 08 12.49542 06 32 33.23 +15 57 35.5          18.86rUNEOCPI41

Is the first entry, and asterisk-marked, on https://cneos.jpl.nasa.gov/scout/#/object/ZTm0038 at the bottom, in the Observations section.

Unusual (new to me) format. Appears to me, first guess to read:

   [object] [yr] [m] [day.time-decimal] [position] [elevation-angle]   [observatory]

Likely oriented relative to the plane of the ecliptic, and absolute direction relative to Earth at time of observation.

Not to spoil your lunch or anything:

https://www.youtube.com/watch?v=bU1QPtOZQZU

Of course 500 Km is a big difference with 400 meters but to those near the point of impact it wouldn't matter and the global effects would still be beyond anything in our history.

This is one interpretation of what this could look like:

https://youtu.be/ZyyrfB8s5cY?t=152

The difference between the 500km asteroid impact in the video, and a 400m asteroid impact is the difference between a man jumping, and a man jumping to the moon.

Yes, it is the difference between a sterilization level event and one that is probably survivable. But it would still be the most significant impact in all of recorded history and to get to a larger one you'd have to go back a long, long time.

Even a 400m land hit sounds pretty survivable for whole humanity, significant climate impact is expected beyond the 1km range?

Yes, it would be survivable for humanity. But depending on where it would happen, how dense the impactor is and at what speed it is traveling the effects would nevertheless be quite serious. Imagine NYC or SF taken out like this (of course the chances of that happening are really small given their area relative to the total of the planetary surface):

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/201...

Here's a visualization of its orbit, Earth in green, object in teal: http://orbitsimulator.com/gravitySimulatorCloud/yr/gsim2023....

According to https://www.projectpluto.com/neocp2/mpecs/ZTm0038.htm#elemen... the observations were taken a few days ago, August 12-13. I don't know when someone calculated the future trajectory and impact probability though.