Asteroid Fly-By on August 18

ke4roh writes "An asteroid will fly near the planet and be visible with binoculars from the northern hemisphere August 18, so says this article. Astronomers say it will cross the sky at 8 degrees per hour and fade out of view as it approaches the sun and hence goes through its various phases - full, gibbous, half... down to nothing. Such a show only comes about twice a century, so take a look before it disappears!" Another reader sends in a few useful links: "Here's the complete article from the folks at NASA Space Science with extra links including details on the astreroid's trajectory."

Comments

Disc of asteroid? Will it show phases?

[hyacinthus]

I notice that the article doesn't say anything about whether the asteroid will show an apparent disc from Earth, but this is easy enough to calculate, I suppose--

Diameter of asteroid: 800 m
Perigee distance: "1.3 x distance of Moon"
Distance of moon: 384,000,000 m approx.
Thus, perigee distance: 500,000,000 m approx.

Angle subtended by asteroid: 800 / 500,000,000
= .0000016 radians approx
= .000092 degrees approx
= .33 seconds of arc.

And this is only at perigee, of course.

By comparison, the disc of Neptune subtends about 3 seconds of arc (don't remember exactly), and just shows a disc in larger amateur telescopes. I don't think anyone with a pair of binoculars is going to be able to discern phases on this asteroid.

hyacinthus.