Physics 101 - Astronomy - Spring 2019
Class notes for day 17, March 21, 2019
Ch. 13 - Asteroids
Asteroids and meteoroids are small “rocky” objects. The main difference
between the two is size. Asteroids can be divided into three major groups
1. The Asteroid belt between the orbits of Mars and Jupiter. Ceres, the
largest asteroid, is also a dwarf planet. Asteroids are often seen as streaks in
photos. Tens of thousands of belt asteroids with diameters larger than a
kilometer are known to orbit the Sun between the orbits of Mars and Jupiter. The
gravitational attraction of Jupiter depletes certain orbits within the asteroid
belt. The resulting Kirkwood gaps occur at simple fractions of Jupiter’s orbital
period.
2. Trojan Asteroids are in the same orbit as Jupiter, but about 60
degrees ahead or behind Jupiter in its orbit. Lagrange points are places where
these Trojan asteroids are trapped in the orbit of Jupiter.
3. Near-Earth Objects are objects that can get close to the Earth.
Asteroid Icarus comes close to Earth and to the Sun. The Apollo asteroids move
in highly elliptical orbits that cross the orbit of Earth. Many of these
asteroids will eventually strike the inner planets.
I showed some photos of Ceres and Vesta from these websites:
http://www.nasa.gov/mission_pages/dawn/main/index.html
http://dawn.jpl.nasa.gov/
http://en.wikipedia.org/wiki/4_Vesta
There is a sheet with some facts about the Dawn mission here, if you are
interested:
http://dawn.jpl.nasa.gov/mission/dawn_fact_sheet.pdf
The DAWN spacecraft was in orbit around Vesta, the second-largest of the
asteroids in the Asteroid Belt, for almost a year, and is now orbiting Ceres.
You can see a Ceres movie at:
http://dawn.jpl.nasa.gov/news/news-detail.html?id=4677
and also
http://www.jpl.nasa.gov/video/details.php?id=1392
The first mapping orbit was used to make this video:
http://www.jpl.nasa.gov/video/details.php?id=1380
Some asteroids have been studied up close, and I showed a few pictures taken by
spacecraft.
Matilde is darker than a briquette of charcoal.
Itokawa was visited by the Japanese spacecraft Hayabusa, which brought back some
material.
Asteroid Eros was imaged by a spacecraft called NEAR which then landed on it and
sent back data on its composition. This is described on the NEAR spacecraft
site: http://near.jhuapl.edu/
For a simulation of the trajectory of an asteroid see:
http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=433;orb=1
Ch. 13 – Comets
Halley’s Comet was last seen in 1986 but was not very bright. A previous
appearance was in 1910. The orbital period is 76 years, so it will be back in
2062. This comet was the first one to be recognized as a periodic object in
orbit around the Sun. Edmond Halley found historical records of a comet
appearance every 75 or 76 years, for hundreds of years, and so he concluded that
it was an object in an orbit with that period. The comet was named after him for
that reason.
There are two types of comet tails: an ion tail and a dust tail.
Comet Hale-Bopp (seen in 1997) had two distinctive tails:
http://apod.nasa.gov/apod/ap050522.html
A comet trajectory is usually a very narrow ellipse, with eccentricity near 1.0.
Some comets appear to come from almost 100,000 A.U. from the Sun, in a region
called the Oort cloud. These are the long-period comets, with periods of
thousands or even millions of years.
Short-period comets result when a comet is deviated from a highly-eccentric
orbit by the influence of a Jovian planet. For comets, short-period means less
than about 200 years.
Fragmentation of a comet can occur as
it passes Earth or another planet. Meteor Showers are usually due to old comets that broke up. There is definite
proof that some meteor showers are due to known comets. Others are probably
fragments of asteroids.
For a list of meteor showers:
http://en.wikipedia.org/wiki/Meteor_showers#Famous_meteor_showers
A Meteorite is a meteor which reaches the ground. There are various types:
stony, iron-nickel, and carbonaceous. See the book or my powerpoint for some
pictures of these.
Before class I showed a set of video clips from an event that occured a couple years ago in Russia.
A large "Fireball Event" occurred over a city in Russia (Chelyabinsk) on Feb.
15, 2013. For those of you who like technical detail, see
http://neo.jpl.nasa.gov/news/fireball_130301.html
There are a number of YouTube videos of the Chelyabinsk meteor (some might contain ads). See these if you are curious about the largest meteor in recent history:
Videos of the Chelyabinsk event of 2013
Video taken by a cellphone, looking up from sidewalk, boom, running and chaos
after first minute (4:45)
(There is little of interest after the first minute, but it shows the complete
confusion of many people who didn't know what had happened. The Russion military
got on TV and assured everyone that it was not the beginning of a war.)
http://www.youtube.com/watch?v=b7mLUIDGqmw
A 10 minute newscast (in English) with Neil deGrasse Tyson
https://www.youtube.com/watch?v=_szd9B3FU8Y
Videos of blast hitting various locations, mostly taken by indoor cameras (2:46)
http://www.youtube.com/watch?v=tq02C_3FvFo
Mostly outdoor views, from cars and traffic cam (duration 1:24):
(Turn down volume, music added is just a distraction)
http://www.youtube.com/watch?v=90Omh7_I8vI&feature=player_embedded
An 8 minute newscast (in Russian)
http://www.youtube.com/watch?v=t3vCUdnsFs0
A 6 minute newscast (in English)
http://www.youtube.com/watch?v=uA_o28xF_48
10 minutes of raw video (including most of the above footage)
http://www.youtube.com/watch?v=dpmXyJrs7iU
Some of these will have ads before the video, or on bubbles on top of the video.
Near-Earth Objects (NEOs) are of concern since they could devastate a city
(or worse). Here is more information on these (optional).
There is an automated system to scan the sky for near-Earth objects and report
them to a computer system.
These and other reports are fed into a computer program called the JPL Sentry
system:
https://cneos.jpl.nasa.gov/sentry/
More information can be found on the home page of the Center for Near-Earth
Object Studies:
https://cneos.jpl.nasa.gov/
Extra material:
The Rosetta mission has arrived at a comet and returned lots of new information
about comets:
http://rosetta.esa.int/
There is an intermediate region between the Kuiper belt and the Oort cloud,
called the inner Oort cloud. We know very little about this region. Only two large objects have been seen in this
region, but more are expected to be discovered in the future. A popular article
discusses the issues with this region of the Solar System:
http://www.nature.com/news/dwarf-planet-stretches-solar-system-s-edge-1.14921