ASTRONOMY INTRO: HUNTING ASTEROIDS
By Jennifer Weinberg-Wolfe, Undergraduate Physics Major, Harvard University
This activity is designed to orient the teacher and student to
using the microbservatory as well as learn some of the basics
about astronomy and observational sciences.
Many of the skills developed in this activity will be useful for the other
activities to be completed with the Microbservatory and may be used as
an introduction to the software.
There are many different short activities included in this general introduction...
they do not all need to be done at the same time. You may wish to perform
this activities, and design an answer "key" that will enable you to evaluate
students' work.
Background:
This file for most activities will consist of information specific to the
activityÕs topic. Provide a BRIEF history of astronomy and the
development of telescopes as well as how they work
(about reflecting v refracting and data collection by CCDÕs etc)
Pre Lab Sheet/ Questions to think about/ Some Activities
What is a telescope used for?
What types of information can be gathered by a telescope?
Where are the best locations for telescopes and why?
What factors limit your use of
telescopes?
The telescopes you will be able to use have a field of view of ___,
assuming the moon is X (how many?) degrees in diameter, how many
moons can the telescope see at one time?
How many galaxies of (?) degrees in diameter? How many stars?
What fraction of the night sky are you actually
looking at through the telescope?
Remember that light has a finite speed. In other words, it takes light
time to get from one location to another.
The farther away something is from us, the longer it will take
that light to get to us.
The light year is a distance measurement that tells how far
light travels in one year.
When you look at light from stars, you are actually looking back
in history at light that was sent out many years ago, depending on
how far away the star is.
If the telescope can see images up to ___ km away,
how old is the oldest light that the telescope can ÒseeÓ?
Imagine that the sky is a large ball and you are on the inside.
Because the surface is curved, it is very difficult to determine
how far two objects are from each other if they are on the
surface of the ball.
The same is true for stars in the sky.
It is even more difficult to determine
which stars are closer or father away, but for now we will
focus on determining how far away they appear from eachother.
To do this, astronomers use measurements based on angles.
Assuming the entire sphere is 360 degrees and that there are 60
minutes in each degree and 60 seconds in each minute, astronomers
commonly use degrees, minutes and seconds to describe relative distances.
For example, you already did any activity on the field of view of
the telescope. That information was quoted to you as a
certain amount of degrees. In the same way, degrees can be used
to show separation in the sky. How many degrees can be seen at
one time in the sky (from horizon to horizon)?
How many degrees are there from the horizon to directly over head.
Go outside one night and try to determine the amount of degrees between
some bright stars or constellations
that you recognize, use a star chart if you need help identifying
constellations or stars.
Parallax activity
If you hold your hand with your arm completely extended and
look at your thumb, alternating between looking with your right
and then left eye you should see an apparent shift in position of your thumb.
This is called parallax and is due to the relative distance between your
thumb and the background of what ever you are looking at which appears
to remain still. You can use the apparent shift of your thumb to
determine the length of your arm just as astronomers use the
apparent shift of closer stars in relation to the distant background
stars to determine the distance.
Of importance to the parallax measurement is the apparent
shift as well as the distance between measurements
(for your thumb, this is the distance between your eyes).
For astronomers, this is commonly the distance the earth travels if
two pictures of the same region are taken at different times.
Try to draw the trigonometry involved in these parallax calculations.
Remember that measurements of apparent motion in the sky are measured
in arc secs (see previous exercise if you are unfamiliar with this term).
Now fill your picture with the approximations about your thumbs movement
as well as the distance between your eyes. Solve, using trigonometry
and finding the missing component to the triangle for the length of your arm.
Measure your arm, were you close? What could account for some of
the differences between the expected and actual measurement?
Could you have improved your measurement? How would you change
the procedure to make it more accurate? Repeat the measurements
with your new procedure?
Were you closer this time?
Post your results on the information page so that others get
a chance to learn from what you have done. Read what is already there
about this activity.
Can you answer someone elseÕs question or did any of the
material make you think of something else you could do?
Vocabulary
Parallax
Light Year
Reflecting
Refracting
CCD
Astronomical Unit
Par Sec
Speed of Light
Field of View
Activity: Asteroid search
There should be some already gathered images for this part and the
students should have to learn to use the imaging software well and be
asked to do everything possible to the data they are given
(light curves, subtracting noise...etc). Each option of the imaging
software should be part of the activity with detailed instructions
and examples why that would be useful as well as many questions
throughout so that the students are
forced to think through the entire exercise.
Suggested links to supplement work on this assignment:
http://uranus.space.und.nodak.edu:443/lectures/Gaffey_4.12.96/asteroid1.html
http://www.star.le.ac.uk/edu/comets/asteroids.html
http://toucan.phy.bris.ac.uk/NinePlanets/nineplanets/asteroids.html
http://wwwflag.wr.usgs.gov/USGSFlag/Space/wall/asteroids.html
http://nssdc.gsfc.nasa.gov/planetary/planets/asteroidpage.html
http://nssdc.gsfc.nasa.gov/photo_gallery/PhotoGallery-Asteroids.html
http://www.uaeu.ac.ae/resources/SolarSystem/asteroid.html
http://www.ccsn.nevada.edu/Planetarium/asteroid.html
Questions to think about before undertaking your observations:
Since most asteroids are known to be located somewhere between Mars
and Jupiter, about how far away would that make them from the Earth?
To answer this question you will have to find the distances to different
planets, this information is most likely to be found in a text or reference book.
Are the asteroids much closer or farther away than most of the other
sources of light in the sky? What is the ratio of distances between the earth
and asteroids and the earth and nearby stars? If an asteroid was said to be
located one mile from your school, approximately how far away would the
nearest star be from your school on the same scale assuming it is night?
Which would appear to move more in a defined period of time,
against a distant background, asteroids or stars? Why? Hint: Think about
trigonometry to answer this question and the relationship between the angle
between an object's two positions and the distance the object is from the
observer. Refer back to your answer for questions 3 and 4 to help you
determine this.
Now...
Using information you have gleaned from a reliable resource on asteroids,
locate a region of the sky where you might expect to find an asteroid.
Request two images of the same area of the sky at different times
on the same day, where you might expect to find an asteroid .
Use the image processing software to subtract the noise from the
images: Line them up and subtract them.
Work carefully through the following calculations, showing your work and
explanations for why you did what you did whenever possible.
1) After you performed the subtraction between the two images, what
physical significance do the dark and light regions on the image have?
2) Can you find an asteroid on the image? What do you expect it to look
like? Why?
3) How large is the region of the sky that you are looking at? In other
words, what is the field of view of the telescope you are using? What
percentage of the total night sky is this field of view? In other words, how
many distinct images would you have to take with the telescope you are
using to have a picture of every part of the night sky. Remember that there
are 360 degrees on the full sphere, that this does not take into account the
number of square degrees or the fact that you only see half the sky at once.
Note, the field of view of a telescope is usually given in square degrees .
4) How many total known asteroids are there? You will have to use an
outside source to find the answer to this question.
5) Assuming that asteroids are evenly distributed on the night sky, how
many different asteroids should you be able to see in the partial picture of the
night sky that you have? Refer back to the second part of question three for
help on this question.
6) Is it a fair assumption to say that all asteroids are evenly distributed in
the night sky? Why or why not?
7) The asteroids can be thought of as concentrated in the same plane as
the planets since the majority of asteroids orbit the sun between Mars and
Jupiter in the Asteroid Belt. Assume that the ecliptic plane (the plane in
which most planets and asteroids orbit the earth) has a width of 10 degrees
and that most of the asteroids are concentrated in this region. How does this
affect your estimate in question 5? What is your new estimate for the
number of asteroids that you theoretically could see?
8) What other factors might further affect your new estimate? Consider
the brightness of all asteroids, their size, shape, orientation in space, location
in orbit in relation to the sun, etc. Do you think that your estimate is too high
or too low for the number of asteroids that should be in the telescope's field
of view? Why?
9) Did you look in the right direction to see asteroids? How would you
determine the best direction to look?
10) How could you change the experiment and your data collection to
improve your chances of finding an asteroid?
11) If you do have evidence of an asteroid answer the following questions.
Even if you have not found an asteroid, you can answer the questions by
making educated estimates about how fast an asteroid should be moving.
a) How far has it moved in the time between your pictures? Begin
by determining how far it has gone on the image in terms of the field of view
(make your measurement in arc minutes) and then take into account the
distance that the image is from the earth (you know it to be approximately
between Mars and Jupiter in the Asteroid Belt) to determine the actual
distance traveled by the asteroid.
b) Assuming it is somewhere in between Mars and Jupiter, about
how quickly is the asteroid moving? To answer this question remember that
you already know the distance that it has traveled (from part a) as well as how
long it took to go that far (the time between your images).
c) Should it be moving faster or slower than the earth around the
sun? Why? You will need to consider KeplerÕs Third Law for this question
which states that the period of revolution in years, P, (the time it takes to go
around the sun) is related to the distance from the sun in astronomical units,
a by the equation: P2 = a3 For example, in the most simple case, consider the
earth. Its period in years (P) is 1 and the distance from the sun to the earth is
also 1 AU. Clearly 12 =13. Use this relationship to think about how fast the
asteroid should be moving.
12) Now determine the speed at which the earth revolves around the sun.
You will have to use the distance between the sun and the earth as well as the
length of a year to determine this answer.
13) Compare the speed of the earth with that of the speed of the asteroid.
Do these numbers agree with your prediction from 11c? Why or why not?
14) What other information would you need to know to plot or determine
the actual orbit of the asteroid that you found?
15) In the space below design an experiment with detailed procedure to
gather the information you think would be useful to determine the orbit of
the asteroid. In other words, plan a way to get the things you said you needed
for question 12.
Activities Page
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