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Theme 10 – Leftovers:
Comets
ASTR 101
Prof. Dave Hanes
Why
“Comet”?
From the Latin cometes,
(and the Greek κομητης )
meaning “wearing long hair.”
Some Early Interpretations
Aristotle (350 BC): gases rising up
into the atmosphere and igniting
Ancient Chinese: celestial omens
in the heavens, marking important
events such as the death of kings
Likewise in Europe until ~1600:
"When beggars die there are no comets seen;
The heavens themselves blaze forth the death of princes."
Shakespeare, in Julius Caesar
Historical Renderings
the Bayeux Tapestry,
1066 AD
Giotto’s “Adoration
of the Magi” (1305)
shortly after
Halley’s Scientific Contribution
Looked at historical records of many old comets, concluding that three
of them (1531, 1607, and 1682) were the same one, making return
visits.
Using Newton’s [new] laws, he boldly and correctly predicted its return
in 1758! (He did not live to see it; he would have been 102.)
https://www.youtube.com/watch?v=L1SycRpYH3k
Near
the Sun
The comet becomes bright because frozen material evaporates
outward from the nucleus. Then
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the huge coma (bigger than the Earth) and the big tail
(millions of km long) reflect a lot of sunlight, and
the gas also fluoresces, like a Neon lamp, absorbing energetic
solar ultraviolet radiation and re-emitting it in the visible.
The Nucleus
Whipple (in the 1950’s): “Dirty snowballs”
about 10km in diameter
Pebbles in a matrix
of frozen ices
(water, ammonia,
methane, ‘dry ice,’…)
Animal Tails vs
Comet Tails
Why Two?
One tail consists of ions (that is, charged particles
– single atoms or simple molecules). They are
low-mass, so they are readily pushed straight out,
directly away from the sun, at high speed.
The other tail is made of dust and pebbles, small
solid lumps. They are more massive, and move
more slowly. This often results in a curved tail
lagging behind as the comet moves along its orbit.
Discovering
Comets
These days, astronomers carry out automated searches for new objects
using telescopes equipped with sensitive electronic detectors. The role
of the dedicated amateur has been much diminished!
In the last 15 years, almost 2400 comets have been discovered – most
of them very faint, and only 72 of them by amateurs.
Famous Finders:
the Shoemakers, and David Levy
Gene Shoemaker was the first to persuade scientists that
lunar craters are from impacts, not volcanoes
The Fate of Comets:
1. Disruption by Sunlight
Every time a comet passes the sun, some of its icy material evaporates
and is lost into space (hence the coma and tail).
Eventually, all that icy material is gone and only bits of rubble (pebbles,
rocks) remain. The comet slowly breaks apart.
Clearly, comets cannot last forever! (Comet Halley has an expected
lifetime of ~30,000 years -- still many orbits to go!)
The Fate of Comets:
2. Orbits Perturbed by Gravity
This can lead to ejection from the Solar
System, or collisions with planets or the
Sun:
https://www.youtube.com/watch?v=J0d-mwJsgQs
A Tidally-Disrupted Comet
www.ing.iac.es
Here, the disrupted Comet Shoemaker-Levy – with its pieces
redirected to an eventual series of collisions with Jupiter.
Details Hard to Discern
The Jovian Impact (1994)
Puzzle:
Why do we still see comets??
Answer: there is steady replenishment, requiring
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a mechanism for injecting new comets;
and
a source (a ‘reservoir’) of primitive comets
The sources are:
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The Kuiper Belt, and
The Oort Cloud
1. The Kuiper Belt
[an artist’s rendition]
Note the tilted, eccentric orbit of Pluto – very
different from the orbits of the eight inner planets.
Properties of the Kuiper Belt
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beyond the orbit of Neptune
flattened (like the Solar
System itself)
contains many billions of tiny
ice lumps – pristine comets
that have never yet
approached the sun
also contains some larger
Kuiper Belt Objects (KBO’s)
the best-known of these is
Pluto!
2. The Oort Cloud
- much farther out, surrounding the Solar System
Comet Replenishment: a ‘Nudge’
Occasionally, Oort Cloud or Kuiper Belt comets may have their
orbits ‘perturbed’ by the gravity of a nearby passing star, an
outer planet, or one of the larger Kuiper Belt Objects. (A typical
comet is only about one trillionth of the mass of the Earth, so
they are easily tweaked!)
The object’s new orbit may, by chance, take it inward, towards
the Sun.
What Then?
The Influence of Jupiter
Birth to Death
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After formation in the original Solar System nebula, a
cometary nucleus may spend billions of years in the Oort
Cloud or the Kuiper Belt
Some small gravitational perturbation directs it inward
The gravity of an inner planet (most likely Jupiter) changes its
orbit, and it is captured into an orbit of much shorter period
Repeated passages around the sun fairly quickly lead to its
destruction
Late in a Comet’s Life:
Spaceplace.nasa.gov
The Composition of Comets
We can learn about the composition of comets by studying
the light they emit when they are heated by the Sun. This
spectrum, for instance, reveals the presence of compounds
of O, H, C, N -- the common elements.
Learning More
With the coming of the space age, we have
developed the ability to visit comets in the inner
Solar System, to explore their structure in detail.
We will consider three examples:
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‘fly-by’ missions to Comet Halley (1986)
the ‘Deep Impact’ mission to Comet Tempel (2005)
the Rosetta probe and Philae lander (2014)
1. Visiting Halley’s Comet (1986)
One Big Problem
Halley’s Comet has a retrograde orbit.
Limited Goals:
the Giotto Probe
The Image and Interpretation
(note the comet’s size!)
2. Deep Impact (2005)
The objectives:
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send a space probe out to Comet Tempel
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travel alongside it for a while, and send back pictures
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project a large solid object in the path of the comet so that
the comet runs into it
see what kind of material comes flying off the surface of the
comet to learn about its rigidity, structure and nature
Impact!
http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/Tempel.mp4
What Did We Learn?
Comet Tempel is much as we imagined, but may contain
somewhat more fluffy dust particles and a bit less icy
material than had been thought. The spectra revealed the
presence of silicates (like sand on Earth) plus the other
common elements.
In other words, no surprises, but rather a fine-tuning of our
understanding.
This experiment was a great success, but we really need to
land on and study a comet in detail: the Rosetta probe!
3. The Rosetta Project (2014)
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Send a probe out to Comet Churyumov-Gerasimenko
Put the probe into orbit around the comet, travelling along with it,
and survey its surface in detail
Send a lander (Philae) down for a ‘soft’ landing on the comet
Experiment on the material of which the comet is made
Artist’s impression: www.cbc.ca
Why “Rosetta”?
3.bp.blogspot.com
`
en.wikipedia.org
The Actual Ten-Year Path,
Beautifully Pre-Planned
(by design, the probe skims past other interesting objects en route!)
wctechblog.com
Closeup Images
on arrival
surface details (from the
orbiter)
The Size of the Comet
(compared to Toronto)
harrisonruess.com
Some Problems
The Philae lander indeed reached the surface.
Unfortunately, it bounced and eventually wound
up settling in a shaded area next to a vertical wall.
This limited the available solar power and thus the
time available for direct experiments (the batteries
ran down without recharging).
Still, a Great Success!
Visit the mission website for the latest news
http://www.esa.int/Our_Activities/Space_Science/Rosetta
A Later Portrait
Jets of gas are
produced as
the comet
slowly nears
the Sun and
heats up.
Philae came
back to life (more
sunlight!).