Theme 10 – Leftovers: Comets

ASTR 101Prof. 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,

shortly after 1066 AD

Giotto’s “Adoration of the Magi” (1305)

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

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 matrixof 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

a mechanism for injecting new comets;and

a source (a ‘reservoir’) of primitive comets

The sources are: 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 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

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: ‘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:

send a space probe out to Comet Tempel

travel alongside it for a while, and send back pictures

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)

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 cameback to life (more sunlight!).