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[sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9)
Section - E.17.3 I heard that an asteroid was going to hit the Earth?!

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These such questions typically occur after a news report of a future
close encounter between the Earth and an asteroid.  To date, all such
reports have resulted from (1) Astronomers did not yet know well
enough the orbit of a newly-discovered asteroid to say with any
certainty that it would not hit the Earth; (2) Reporters not checking
their stories or misunderstanding what they were told; or (3) both.

Objects that can potentially come close to the Earth are referred to
as Near-Earth Objects (NEOs).  The International Astronomical Union
maintains lists of such objects. About 100 asteroids are classified as
"Potentially Hazardous Asteroids" (PHAs), at
<URL:http://cfa-www.harvard.edu/iau/lists/Dangerous.html>; they all
have a projected closest distance to Earth of less than 0.05 AU (7.5
million km).  A list of closest approaches to the Earth by PHAs
between 1999 and 2099 is available at
<URL:http://cfa-www.harvard.edu/iau/lists/PHACloseApp.html>.  A list
of moderately close (to within 0.2 AU) approaches to the Earth by
asteroids and comets between 1999 and 2032 is available at
<URL:http://cfa-www.harvard.edu/iau/lists/CloseApp.html>.  At the
moment, NONE of these encounters is thought to pose a serious risk.

The "potential hazard" of PHAs lies in their orbits and the
perturbations on those orbits from the planets and the Moon currently
not being known with sufficient accuracy to completely exclude the
possibility of a collision, but, generally, labeling these asteroids
as PHAs is erring on the side of extreme caution.  It is not worth
losing any sleep over them.

User Contributions:

1
Keith Phemister
⚠
Sep 13, 2024 @ 11:23 pm
Copied from above: If the Universe were infinitely old, infinite in extent, and filled
with stars, then every direction you looked would eventually end on
the surface of a star, and the whole sky would be as bright as the
surface of the Sun.
Why would anyone assume this? Certainly, we have directions where we look that are dark because something that does not emit light (is not a star) is between us and the light. A close example is in our own solar system. When we look at the Sun (a star) during a solar eclipse the Moon blocks the light. When we look at the inner planets of our solar system (Mercury and Venus) as they pass between us and the Sun, do we not get the same effect, i.e. in the direction of the planet we see no light from the Sun? Those planets simply look like dark spots on the Sun.
Olbers' paradox seems to assume that only stars exist in the universe, but what about the planets? Aren't there more planets than stars, thus more obstructions to light than sources of light?
What may be more interesting is why can we see certain stars seemingly continuously. Are there no planets or other obstructions between them and us? Or is the twinkle in stars just caused by the movement of obstructions across the path of light between the stars and us? I was always told the twinkle defines a star while the steady light reflected by our planets defines a planet. Is that because the planets of our solar system don't have the obstructions between Earth and them to cause a twinkle effect?
9-14-2024 KP

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