Comets and Other
- Some history: Easily visible,
and known since antiquity; Often considered to be signs of important
events, sometimes bad omens and carriers of disease in their
tails; Up to Tycho Brahe, who measured the parallax of one in
1577, they were thought to be atmospheric phenomena.
- What are they? Dirty,
dark iceballs in elongated orbits; The ones
we see from Earth have a several-km wide dark nucleus, which usually
develops a much larger halo ("coma") and one or two tails
of dust and ions, many millions of km long (in addition to chunks
they leave behind...), when approaching the Sun.
- Sizes: In the outer Solar System
some are bigger than 100 km (there are perhaps 35,000 of these, and 1 billion
small ones!) Size is difficult to determine from Earth, estimates depend
on how much light they reflect (comet nuclei are very dark).
Special Comets and Exploration
- Halley's comet: The most famous
one; Has a very elongated orbit; Seen by the Chinese in 240 BC,
comes back every 76 yrs, the last time was 1986.
- Shoemaker-Levy 9: Broke apart
and smashed into Jupiter in 1994.
- Recently: Comet Hale-Bopp in
1997, Ikeya-Zhang in 2002,
then C/2002 V1 (NEAT), C/2004 F4 (Bradfield), and other ones that
we saw breaking apart; Comets McNaught in early 2007 and Holmes in
late 2007; Several new ones seen every year by amateurs; More than
1000 discovered by SOHO.
- Effects on Earth: Direct hits
can be very destructive, but past comet impacts were probably
responsible for bringing to Earth most of the water we see on
it today; What if we pass through a comet's tail?
- Exploration: Very interesting because
of what they tells us about the origin of the Solar System; The
first missions were to Halley [Giotto
flyby in 1986] and Borrelly [Deep
Space 1 in 2001]
- Recent missions: The
sample return mission Stardust flew by Wild
2 in Jan 2004,
collected dust samples and returned them to Earth in 2006; Deep
Impact blasted a hole in Tempel-1 in
2005; ESA's Rosetta, launched in 2004, will meet its target comet
- Short period comets: Those with
periods up to a few hundred years have orbits close to
the ecliptic and come from the Kuiper belt, 30–50 AU from the
Sun, where Pluto and other distant, frozen leftover planetesimals
- Long period comets: Comets with
long periods are more icy, have randomly oriented orbits, and
come from the Oort cloud, up to 50,000–100,000 AU; Probably formed
from objects ejected in random directions by close encounters
with solar system planets, and ground up into small pieces by
collisions. There may be a trillion of them, but they
probably only make up at most a few tens of Earth masses.
- Effect of the Sun: Each passage
near the Sun makes an ice layer evaporate away (especially the
Sun-grazing ones), and radiation damage causes them to
become dark; After 500 or so passes they may fall apart
(we saw several that did in recent years).
- Other changes: Even far from
the Sun, they are affected by radiation, collisions with other objects
and sandblasting from dust grains in the interstellar medium (plus
stars and supernovae...), so even "ancient" ones are
often not preserved in their original form; Some could be captured by
planets (Saturn's moon Phoebe?).
Beyond the Solar System
- Heliopause: The place where
the Sun's influence ends; The solar wind meets the external interstellar
wind and its bow shock, and real interstellar space begins.
- The galactic neighborhood: Patterns
of interstellar gas and molecular clouds, and stars from 4 light
page by luca bombelli <bombelli at olemiss.edu>,
modified 29 sep 2012