Fun with numbers for Sep 27, 2019

Near-miss asteroid? Pah!

Asteroid 2017 KP27 just missed Earth on Sep 26th; well, missed by almost 1 1/2 million km, but considering how big space is, that's a close-ish call.

Whenever one of these close calls happens there's a lot of Tweeting and Facebooking and Instagramming, not to mention the ignorant bleating from the absurdly ignorant of science official voice of the establishment traditional media, about how we could have all died just-like-that. Where are Ben Affleck and Bruce Willis when we need them?

Not so fast. (Literally, that's the issue: velocity squared.)

Asteroid 2017 KP27 relative speed in this pass was 4,805 m/s and its diameter is between 19 and 42 meters. There's no estimate of its mass in the Nasa database, but we can make a few approximate calculations. Generally these asteroids fall into three types: carbonaceous (think giant snowball, only made of methane and carbon dioxide), silicate (big rock), or metallic.

Data from Krasinsky et al. (July 2002) "Hidden Mass in the Asteroid Belt," Icarus, 158 (1): 98–105, puts densities for these three types at 1.38, 2.71, and 5.32 g/cm$^3$ respectively.  (Source: Wikipedia for the reference and the actual article for the numbers, viva la public library proxy server.)

Assuming as reference cases the two extremes in diameter and using the geometry of a sphere (we're just doing order-of-magnitude calculations, really), we can compute the kinetic energy KE for both this close call and the next time we pass asteroid 2017 KP27 (Sep 15, 2020; closest approach will be 44 million km at a relative speed of 13,178 m/s):

As is clear from the table, even at its higher speed in 2020, this puny little pebble is at best a single city-buster, while at its recent speed it carried less energy than a theater-wide tactical nuke.

So, not the ELE some people feared. Still, could ruin your day if it landed within the same neighborhood.

Visible from space? Double Pah!

From Orson Scott Card's The Swarm:

Visible from space isn't really that distinctive, in fact it's positively common, as long as there's enough color and light contrast. For purposes of this analysis, space starts at an altitude of 100 km (could also be 80 km, but, what the hay, I like round numbers in metric).

I can see objects 1 m tall at a distance of 1 km, and I'm not exactly hawk-eyed, so an object more than 100 m along its smallest dimension should be visible from space by someone like me (possibly much smaller, for people with better vision who didn't spend most of their life reading, coding, writing, or doing math), as long as there's enough contrast with the surroundings.

My heavy-reader eyes have a resolution of at least 1/1000 radians per pixel (actually more, since I can see some detail in 1 m objects at 1 km, but let's keep it simple), which is 3 minutes and 26.3 seconds of angle [in degrees].

For comparison, the Cheyenne Tactical M200 Intervention marksmanship rifle has sub-MOA (Minute Of Angle) accuracy at over 2000 m. (A pair of these rifles are important in Red Metal by Mark Greaney, a [mostly] good piece of MilFic I recently read. And, no, it's not a Grey Man novel, it's a different storyline.)