KNOWLEDGE TO “GET”

FROM TODAY’S CLASS MEETING

ASTR 105G Class Meeting #35 Monday, April 18th

COMETS: Section 12.2 (pg 347-353) 1) What are they? 2) What are their orbit characteristics? 3) What are their ‘PARTS’ when: a) far from the Sun (NUCLEUS only) b) near to the Sun (nucleus, coma, dust tail, gas ion tail) 4) Why must comets have large OSAs and large orbit eccentricities to ‘survive’?

Wednesday’s & Thursday’s LAB

Build a Comet Lab

BuildaCometLab-Apr20-21.pdf

(numbered pages 219-231 in the Lab Manual…)

in the LAB-MANUAL folder on the class web page *************************************

There is NOT a Take Home assignment due this from last week’s In Class Volcanoes on Io Lab

Grade Results of QUIZ #6

SCHEDULE FOR THE REMAINING SEMESTER:

HOMEWORK #4 is due Wed, April 27th

QUIZ #7 Friday, April 29th

Final Exam: Wednesday, May 11th 10:30 AM -12:30 PM, here in BX 102

COMETS The word COMET is derived from the Greek word

“KOME”, which translates to “Hair” Comets are seen to have ‘hair’ tails trailing them…

Comet Hale-Bopp (1997)

The background stars are EMITTING radiant energy The whitish color associated with the COMET is SUNLIGHT reflected from DUST PARTICLES that the comet nucleus has shed The BLUE streamers of light are EMITTED radiant energy from gas-atom IONS (also shed from the comet nucleus) which have had their electrons excited by their absorbing ultraviolet-wavelength radiant energy from the Sun Hale-Boob was a ‘classic looking’ comet..

Comets are:

Small & Icy / dirty-dusty objects (solid nucleus < ~20 km in diameter)

which ORBIT AROUND THE SUN on orbits that have

LARGE Orbit Eccentricity values and

Generally LARGE (> 5 AU) OSAs

AND …their ‘far from the Sun’ appearance differs from their ‘near to the Sun’ appearance..

Comet PanSTARRS, March 2013

Let’s think about: i) What would happen to Pluto, or to Saturn ring

particles, if they neared the Sun:

i) how would they change and what would they look like? leads us to the ‘parts’ of a comet

ii) What would happen if these objects frequently neared the Sun ? [leads us to the lifetime of a comet]

iii) Where in the Solar System can dirty ice balls survive for very long (billions of years) time periods? iv) What orbit shape must comets travel to both SURVIVE yet periodically form their distinctive PARTS?

We worked through on Fri, April 15th an In-Class Exercise

imagining what physical processes affect a :

10-km diameter ICY sphere with DIRT and DUST mixed in with the ice …..

when that SPHERE is located FAR FROM and

then NEAR TO the Sun…..

A SOLUTION for this Exercise is available inline in the lecture-EXERCISES folder

Not all comets are ‘Classic’ looking

Comet C2001-Q4

Some comets exhibit only a single tail….

Comet Bradfield

The structure of a COMET when near the Sun:

The structure & size of a COMET when near the Sun:

Structure & size of a COMET when FAR from the Sun:

Nucleus (1-10 km)

A comet’s Gas/Ion Tail points away from the Sun…the Dust Tail lags behind in the comet’s

orbit path

We have seen a nucleus of a comet ‘up close’ Comet Halley’s Nucleus (Giotto spacecraft image)

image obtained in 1986….

Jets of evaporating ice, which carry away dust particles

15 km

…we have also attempted to see what is inside a comet nucleus by ‘whacking’ its surface with a

spacecraft to excavate an impact crater… The DEEP IMPACT Mission (July 2005)

The impact explosion of a 1-meter diameter copper sphere impacting the surface of Comet Tempel 1 This time-sequence of images to-the-left was obtained by the DEEP IMPACT spacecraft, which released the copper sphere that whacked in to the nucleus

~6 km

BEFORE AFTER Impact Impact

We have even returned to Earth pieces of material obtained from within a comet’s COMA

The 5-km nucleus of Comet Wild 2, from a camera onboard the STARDUST spacecraft, which passed near to Wild-2 and ‘captured’ pieces of its COMA

CAPTURED and subsequently RETURNED-TO-EARTH pieces of dust from the COMA of Comet WILD-2

After ‘grabbing’ the Wild-2 samples, the STARDUST spacecraft returned to Earth,

landing in the Utah desert

As the STARDUST return capsule plunged through Earth’s atmosphere, it looked like a Meteor…. The re-entry vehicle landed, under parachutes, in the Utah desert in January 2006. Samples from the comet are being analyzed in labs here on Earth,

SHORT-PERIOD vs LONG-PERIOD COMETS

Comets are generally divided in to TWO orbit-characteristic groups:

LONG PERIOD Comets

SHORT PERIOD Comets

with the dividing orbit period being 200 years

(From Kepler’s 3rd Law, P2 = OSA3, an orbit period of 200 years has an OSA = 34 AU)

HALLEY’S COMET is the best known SHORT PERIOD comet

[it has an orbit period of 76 years; OSA= 18 AU]

Halley’s comet will next be traveling through the

inner Solar System in the year 2061

Halley’s Comet has a large eccentricity (~0.97), so it approaches within < 1 AU of the Sun and as far as almost Pluto’s OSA

Earth’s orbit

Neptune’s orbit

Halley’s Comet orbit

HALE-BOPP is a LONG PERIOD comet; it was one of the 20th century’s brightest comets

[its orbit period is ~2500 years; OSA= ~168 AU]

Hale-Bopp will next be traveling through the inner

Solar System in the year 4400 (+/- a few years..) .. its Orbit Plane is perpendicular to the Earth’s orbit plane

Hale-Bopp has a large eccentricity (~0.995), so it approaches within < 1 AU of the Sun, and as far as 9 times Pluto’s OSA

Pluto’s orbit

Hale-Bopp’s orbit

Comet’s travel on VERY ECCENTRIC orbit paths .. and these comet orbit paths are not necessarily

in the same plane as most of the planets

In fact, comet orbit paths are in all directions… below is a ‘looking at the Sun’s equator’ view

Comet orbit paths

When a comet is near to the Sun, it possesses the following components:

ION TAIL

DUST TAIL can be 1 AU in length!

COMA Up to 2,000,000 miles across

NUCLEUS 0.1-20 km in size

Direction to the Sun

The COMA is a temporary atmosphere that develops as the NUCLEUS ice sublimes

(‘evaporates’) when the nucleus is near the Sun

Direction of comet motion

When a comet is far from the Sun, it possesses ONLY the following components:

NUCLEUS

When the comet is far from the Sun, no nucleus ice sublimes (‘evaporates’), so no COMA forms and therefore no ION or DUST tails are active

Direction of comet motion

RESERVOIRS of small, icy objects that could become comets if they neared the Sun are present in:

THE KUIPER BELT is located near and beyond the orbit of Neptune

The OORT CLOUD extends to ~100,000 AU from the Sun

Objects in the OORT CLOUD were formed in the vicinity of the planets, BUT the gravitational interactions between those small objects and the Gas Giant planets ‘scattered’ those small objects in to high eccentricity orbits that extend as far as 100,000 AU from the Sun and as near to the Sun as grazing the Sun’s surface, or plunging in to the Sun

How might individual comets ‘END’ their existence?

Some comets plunge in to the Sun,

some comets ‘evaporate away’ (leaving maybe a

rocky ‘nucleus’ with no icy material), some comets strike solid surfaces and produce craters,

and some comets leave the Solar System entirely…

WHY WOULD WE NOT EXPECT A COMET TO HAVE AN OSA = 2 AU?

WHAT PHYSICAL REASONS CAN BE INVOKED TO EXPLAIN COMET NUCLEI

BEING NO BIGGER THAN ~10 km?

Comets do NOT rapidly pass across our sky during just one night, .. but because they ARE generally traveling fast when

their come and tails are visible (think Kepler’s 2nd Law) comets DO exhibit observable motion (changed position) from night-

to-night

When will the next great comet appear?

We’ll know it when we see it! … it was thought

that this could occur in late 2013 / early

2014…..but alas this did not occur…

COMET ISON:

Predictions for COMET ISON, first

detected in Sept 2012, suggested that it could

be as BRIGHT AS VENUS

or JUPITER… IF it survived passing very near to (or even just within) the Sun’s

outer layers… ‘close pass’ would occur in Nov 2013..

and thereafter, if ISON survived, it could have been amazingly bright, and even visible during the daytime, during December 2013

..and there was much News

Coverage of ISON as it approached the Sun on Thanksgiving weekend 2013…

But, ISON did not survive its close encounter

with the Sun, and ‘fell apart’ very soon after it’s Sun encounter and a few days later was not at all

visible… so, NO SPECTACULAR Comet was observed during December 2013

http://www.space.com/23780-comet-ison-survival-sun-roasting.html

… one interesting COMET OCCURRENCE during 2014 was Comet SIDING SPRINGS

which passed 130,000 km from MARS (in October)

What might a comet that has lost all of its ice look like and be composed of? ….what might we call such an object?.. we’ll discuss this during our

next class meeting

BRIGHT COMETS during the year 2016 are catalogued at the website:

http://www.aerith.net/comet/weekly/current.html

… comets (even before their true physical nature was known) have intrigued humans since they were first observed…. there is a long record of comet observations in Chinese archives