astro 101

ASTRO 101

Principles of Astronomy

Instructor: Jerome A. Orosz (rhymes with “boris”)Contact:• Telephone: 594-7118

• E-mail: [email protected]

• WWW: http://mintaka.sdsu.edu/faculty/orosz/web/

• Office: Physics 241, hours T TH 3:30-5:00

Homework/Announcements

• Homework due Tuesday, March 19: Question 6, Chapter 7 (What role did Charon play in enabling astronomers to determine Pluto’s mass?).

• Tuesday, March 26: Wrap-up, review

• Thursday, March 28: Exam #2

Next:

Chapter 7

(Dwarf Planets and Small Solar System Bodies)

Pluto

• Essentially nothing was known about Pluto until the 1970s. It is far from the Sun (more than 40 times the

Earth-Sun distance), hence it receives little sunlight. In turn, we see very little light reflected back.

The planet looks nearly pointlike owing to its large distance from us, so it was not possible to see surface features.

Pluto’s Orbit

• Pluto’s orbit is highly inclined relative to the orbits of the other planets.

Pluto’s Orbit

• Pluto’s orbit is also more eccentric than the orbits of the other planets. The period is 247.7 years

Pluto’s First Moon

• Pluto’s first moon was discovered in 1978.

• The orientation of the moon’s orbit is such that during certain times, Pluto and its moon appear to pass in front of each other every 3.2 days as seen from Earth.


• Pluto and Charon passed in front of each other in the late 1980s.


• Pluto and Charon passed in front of each other in the late 1980s.

• Studies of these events allowed astronomers to measure the masses and radii of Pluto and Charon.

Images from Courtney Seligman (http://www.cseligman.com)


• The masses can be deduced from Kepler’s Laws.

• The radii can be deduced by looking at the total brightness over time.


• Pluto’s mass is 1/500 of the Earth’s mass.• Pluto’s diameter is 2300 km.


• Pluto’s mass is 1/500 of the Earth’s mass.• Pluto’s diameter is 2300 km. This is smaller than

Jupiter’s large moons, and also Earth’s moon.

More Moons for Pluto

• Two additional moons were found using the Hubble Space Telescope in 2005.

Pluto “Demoted”!• The definition of a “planet” was changed recently:

– Planets: The eight worlds from Mercury to Neptune.

– Dwarf Planets: Pluto and any other round object that "has not cleared the neighborhood around its orbit, and is not a satellite."•

– Small Solar System Bodies: All other objects orbiting the Sun.

http://www.space.com/scienceastronomy/060824_planet_definition.html

New Classification Scheme for the Solar System

• Many catagories overlap. For example, some asteroids are dwarf planets.

• In most cases, the distinctions between the catagories are not important for us.

Dwarf Planets• Ceres was discovered

in 1801 by Piazzi.• Its orbit lies between

Mars and Jupiter.• Thousands of

asteroids are now known to be in the “asteroid belt”.

• Ceres was reclassified as a dwarf planet in 2006.

Dwarf Planets• Since 2003, three

additional dwarf planets have been discovered beyond the orbit of Pluto.

• Eris, the largest one, is apparently composed largely of water ice.

• Haumea appears to be slightly oval shaped.

Dwarf Planets• Since 2003, three

additional dwarf planets have been discovered beyond the orbit of Pluto.

• Eris has a very eccentric and inclined orbit. The period is 557 years.

Next: Small Solar System Bodies

• Asteroids

• Kuiper Belt Objects

• Comets

Small Solar System Bodies

• The first asteroid, Ceres, was discovered in 1800 using visual observations through a telescope.

• By the late 1800s, a total of about 300 asteroids were found.

• The big breakthrough came in 1891 when photography was used...


• As of September 2008, roughly 190,000 asteroids have been cataloged.

• Only three asteroids have diameters larger than 300 km. The smaller you go in diameter, the more asteroids there are.


• Most of the asteroids orbit between Mars and Jupiter.• Note the deceptive scale: the average spacing is

1,000,000 km between objects!


• Most of the asteroids orbit between Mars and Jupiter.• Jupiter’s gravity causes the asteroids to gather into

certain orbits.


• Most of the asteroids orbit between Mars and Jupiter.• Some cross the orbit of the Earth. Asteroid 1994

XM1 was closer than the Moon on its nearest approach.


• Most of the asteroids orbit between Mars and Jupiter.• Some have the same orbit as Jupiter, but either lead

or lag the planet by 60 degrees.

What Asteroids Look Like

• Asteroids have irregular shapes, and typically have craters and other features.


• A probe crashed into Eros on February 12, 2001.


• A probe crashed into Eros on February 12, 2001.• The chemical composition of Eros is similar to that of

old meteorites, indicating Eros contains “primitive” material.

Comets and Kuiper Belt Objects

• There are two main regions where the comets live: the Kuiper belt and the Oort Cloud.

• The Kuiper belt extends roughly 30 to 50 AU.

• The Oort Cloud may extend as far as 50,000 AU.


• There are two main regions where the comets live: the Kuiper belt and the Oort Cloud.

• There are roughly 1500 known KBOs and counting. Most are a few tens of km in diameter.


• KBOs are extremely faint and must be observed with the largest telescopes.


• A few KBOs have diameters approaching 1500 km. Most are much smaller.


• Sedna has an orbital period of about 10,500 years. It is one of the most distant known solar system objects.

Comets

• When a comet comes close to the Sun, things get interesting…

Comets

• Comets are bright, fuzzy objects that appear relatively suddenly.

• Most usually have a rather long, diffuse “tail”.

Comets

• Historically, comets were considered to be “bad omens.”• This tapestry depicts the appearance of Halley’s comet

during the coronation of Harold as King of Britain on January 6, 1066.

Comets

• Historically, comets were considered to be “bad omens.”• This tapestry depicts the appearance of Halley’s comet

during the coronation of Harold as King of Britain on January 6, 1066. The Norman invasion came later that year.

• See http://www.bayeuxtapestry.org.uk/Index.htm

Comets as Omens

• The English word “disaster” has two roots:

Comets as Omens

• The English word “disaster” has two roots:1) “dis”, a Latin term for “bad”.

Comets as Omens

• The English word “disaster” has two roots:1) “dis”, a Latin term for “bad”.

2) “astro”, the Latin term for “star”.

The Nature of Comets

• In 1705, Edmund Halley applied Newton’s new theories of gravity and motion to the orbits of bright comets. He suggested that the bright comets that appeared in 1531, 1607, and 1682 were the same object. Also, the comet of 1066 is now known to be the same object.


• Comets are objects orbiting the Sun.


• Comets are gravitationally bound to the Sun. The orbits can be highly elliptical (recall the

orbits of the planets are nearly circular). The orbital planes of the comets can be highly

inclined with respect to the Earth’s orbital plane (i.e. the ecliptic).


• The 8 planets orbit in nearly the same plane in space





The orbital periods range from a few years to thousands of years.


• Observationally, comets have A bright head, which consists of a small

nucleus (a few km across) and a coma made up of glowing diffuse gas, which may be 100,000 km across.

A dust tail, blown off by the radiation from the Sun.

A gas tail, blown off by particles from the Sun. The tails always point away from the Sun.

The Nature of Comets• Sunlight and the solar

wind blow the comet’s dust particles away from the Sun.

• As a result, the tails always point away from the Sun.


• The dust tail is seen by reflected sunlight.

• The gas tail is seen by its own emission (i.e. it is a cloud of hot gas).

• The dust tail can be up to 150,000,000 km long.


• A summary of the main parts. Note the difference in the size of the actual nucleus and the visible coma.


• The most popular model for a comet is the “dirty snowball” model.

• The nucleus is made up of icy material, including water, CO2, NH3, and CH4. The total mass is typically about 10-9 of the Earth’s mass.

The Nature of Comets• In 2005, NASA

sent the Deep Impact probe to the Comet Temple 1.

• The probe crashed into the nucleus at a velocity of 22,000 mph.

The Nature of Comets• In 2005, NASA

sent the Deep Impact probe to the Comet Temple 1.

• The probe crashed into the nucleus at a velocity of 22,000 mph.

• Seconds after impact, hot debris was spotted.

The Nature of Comets• This experiment

provided evidence that the nucleus is composed of weakly bound fragments of rock and ice.

• Many organic molecules were observed in the debris.

The Nature of Comets• This experiment

provided evidence that the nucleus is composed of weakly bound fragments of rock and ice.

• Other comets have been observed to fragment when they pass too close to the Sun or Jupiter.


• A comet is a very small, dirty snowball.

• When it is far from the Sun, it is very faint, and usually impossible to see from Earth.

• When a comet comes close to the Sun (and hence relatively close to the Earth), the Sun vaporizes the material, giving rise to the coma and the tail(s). Comets generally disappear after many trips near the Sun.

Next…

Meteors: Things That Fall From the Sky

Meteors: Things That Fall From the Sky

• Large impacts have been observed recently…

Comet Shoemaker-Levy 9

• This comet was discovered in 1993, subsequent photographs showed a “string of pearls”.



• It was found to be orbiting Jupiter rather than the Sun.



• It was found to be orbiting Jupiter rather than the Sun.• It was on its final pass around Jupiter, and would hit

Jupiter in July of 1994.

Comet Shoemaker-Levy 9• Comet Shoemaker Levy 9 gave us a chance to study a

comet in detail:


comet in detail:• The collision tells us about the structure of the comet (e.g.

tightly or loosely packed).




• We can see material from deep in Jupiter’s atmosphere that has been ejected.





• We can see cometary material that has been ejected.





• We can see cometary material that has been ejected.

• The collision tells about the structure of Jupiter’s atmosphere.


• The colliding fragements made dark spots that were easily seen from Earth.

• The typical collision released 100,000 times more energy than an H-bomb.

Meteors

• Looking closer to home…

Meteors

• There are many small chunks of matter orbiting the Sun. A piece that is in space is a meteoroid. A piece that burns up in the Earth’s atmosphere

is a meteor (a bright streak of light). A piece that lands on Earth is a meteorite.

Meteors

• Many “meteor showers” are associated with comets.

Dust from Comets

• The dust tail contains small particles evaporated from the comet.

• These particles remain in orbit about the Sun.

• If the Earth passes through the “dust cloud”, then several meteors may be seen.

Meteor Showers

• During periods of high meteor activity, most of the events appear to come from one spot on the sky.

Meteor Showers


• This point is roughly where the comet’s tail was.

Meteor Showers


• This point is roughly where the comet’s tail was.

Dust particles enter the atmosphere and burn up, causing astreak of light.

•

Rocks from Space

• Some early cultures were aware that rocks sometimes fell from the sky. These items had great religious value, e.g. the Black Stone of Ka’aba.

Rocks from Space

• Some early cultures were aware that rocks sometimes fell from the sky. These items had great religious value, e.g. the Black Stone of Ka’aba.

• “Enlightened” scientists in the 18th and 19th centuries declared that stones cannot possibly fall from space. It was all primitive superstition.

Rocks from Space

• Thomas Jefferson said: “It is easier to believe that two Yankee professors [Profs. Silliman and Kingsley of Yale] would lie than that stones would fall from the sky.”

Rocks from Space

• Thomas Jefferson said: “It is easier to believe that two Yankee professors [Profs. Silliman and Kingsley of Yale] would lie than that stones would fall from the sky.”

• Jefferson was wrong: stones do fall from the sky.

Rocks from Space

• Evidence that rocks fall from space:

Rocks from Space

• Evidence that rocks fall from space: There have been eyewitness accounts of

impacts.

Rocks from Space


impacts. In many cases, the mineral composition of

samples indicates the material cannot be native to Earth.

Rocks from Space


impacts. In many cases, the mineral composition of

samples indicates the material cannot be native to Earth.

Most older samples are iron, most “fresh” samples are stony material.

Rocks from Space

Rocks from Space

• Occasionally rocks from space do damage…

• http://www.newsinhistory.com/blog/meteorite-smashes-through-roof-hits-napping-woman

• http://news.discovery.com/space/asteroids-meteors-meteorites/russian-meteor-blast-bigger-nasa-130217.htm

Where to Find Meteorites

• Antarctica is one of the best places to find meteorites on Earth, owing to the high contrast (black rocks on white snow).

http://www-curator.jsc.nasa.gov/curator/antmet/program.htm

Where to Find Meteorites

• Over time, meteorites tend to get concentrated in certain areas because of large-scale ice flows.

http://www-curator.jsc.nasa.gov/curator/antmet/program.htm

Meteorites

• Most older samples are iron. Iron is dense and not easily weathered.

• Most “fresh” samples are composed of stony materials. This material is easily weathered and does not

last long on the Earth’s surface.

Rocks from Space

• Why is are meteorites useful?

• They are material samples from outside the Earth that can be analyzed in the laboratory.

• We can measure the age of the solar system by studying meteorites.

Next

• Meteorites and the age of the solar system

astro 101

Documents

orbit of pluto

additional dwarf planets

plutos orbitplutos orbit

moons orbit

plutos diameter

dwarf planetssince

dwarf planetsceres

inclined orbit