Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!


  1. Dec 4, 2005 #1
    1. If a comet has an eccentricity of 0.98 and a semimajor axis of 12 AU, what are the apehlion and perihelion distances? What is the period? How does the velocity at perihelion compare to the velocity at aphelion?

    In this problem, I used the perihelion and aphelion equations to solve for them, where P=(1-e)a and A=(1=e)a. Perihelion = 0.24 and Aphelion = 23.76. I don't know how to calcuate the velocity, and am confused by that. And since I have the eccentricity and semimajor axis, do I use the equation P(years) = R(A.U.)^3/2?

    2. If this comet appeared to have a brightness of one at its nearest point to Earth, how bright would it appear at its maximum distance? Assume incorrectly that is does not change in luminosity as it approaches the Sun and use the inverse square law.

    I know to use the inverse square law on this one, but how can I relate that to brightness and distances?

    3. If an asteroid is in a 5:2 resonance with Jupiter, what is its semimajor axis?

    The next two problems are the same and I don't have any equations for resonance but I will keep looking.

    4. If an asteroid is in a 2:5 resonance with mars, what is its semimajor axis?
  2. jcsd
  3. Dec 4, 2005 #2


    User Avatar
    Science Advisor
    Gold Member

    1. Your answers for app and per are correct. Notice how apphelion and perihelon values are just (sma x ecc) +/- sma. That's basically what your formula says too. For velocities, you could just say that velocity at perihelion is faster than velocity at aphelion, which is correct for any orbit. It doesn't actually tell you to compute it, just to compare it. Off the top of my head, I forget the formula. See if you can find it.

    2. This question doesn't really make sense. What is it's nearest point to Earth? All we're given is its solar elements. This comet could have high inclination, meaning that it's nowhere near Earth's orbit as it passes the 1AU point in its orbit. It's closest point to Earth could be a brush with our outer atmosphere (~100km altitude, 6470 km distance). It's furthest point from Earth would be roughly its furthest point from the Sun, + 1AU.

    3. What is this asteroid's period? You must first know Jupiter's peroid. How does semi-major axis relate to period? (Kepler's 3rd Law).

    4. Same as #3.
  4. Dec 4, 2005 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Good point. If astronomystudent's teacher is thinking like an astronomer (OOM), they probably mean for them to use 1 AU, but it might be good to check.
  5. Dec 5, 2005 #4
    Answers to number 1

    1.) perihelion = 0.24
    aphelion = 23.76
    period= 41.57 years (do i need to convert to days?)
    this is the equation i found for velocities of aphelion/perihelion:
    v(perihelion) = [(1+e)/(1-e)]^1/2
    v(aphelion) = [(1-e)/(1+e)]^1/2
    v(perihelion) = 9.95
    v(aphelion) = .1005
  6. Dec 5, 2005 #5
    Answers To Number 3 &4

    3.) orbital period (jupiter) = 4332.71 days
    convert days ------> years
    4332.71 days = 11.8704384 years
    p^2 = a^3
    (11.8704383 years)^2 = A^3
    A = 5.20 A.U. (correct units)??

    4.) orbital perid (mars) = 686.98 days
    convert days -------> years
    686.98 days = 1.882137 years
    p^2 = a^3
    (1.882137 years)^2 = A^3
    A = 1.52 A.U. (correct units)??

  7. Dec 5, 2005 #6


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Those equations are missing a normalization:

    [tex]v_p=\frac{2\pi a}{P}\sqrt{\frac{1+e}{1-e}}[/tex]
    [tex]v_a=\frac{2\pi a}{P}\sqrt{\frac{1-e}{1+e}}[/tex]
  8. Dec 5, 2005 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Closest distance to what?
  9. Dec 5, 2005 #8
    It Is The "nearest Point To Earth"? I Think? That Is What The Problem Is Looking For... I Can't Solve It Without A Distance.

    Are My Other Answers Correct?
  10. Dec 5, 2005 #9


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Decompose the word. What does "helio" refer to?

    Looks right, but you haven't solved 3 and 4 yet, you just found the known values of the planet semi-major axes.
  11. Dec 5, 2005 #10
    1.) for the velocities using the new equation:
    aphelion = .52009654 A.U. (units)?
    perihelion = 18.04673691 A.U. (units)

    2.) helio refers to: having the sun as the center?

    3,4) what do i do with the resonance? do i multiply by 2 in the first one and then 5 in the second one? i'm confused.
  12. Dec 5, 2005 #11


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    What are the dimensions of velocity? Are they distance, time, distance*time, distance/time? Are A.U. units of distance, time, temperature, etc? If you don't know the answer to these things, they can be found on the web or in your textbook.

    "Perihelion" refers to the orbit's nearest point to the sun.

    What does it mean to be in resonance? For example, what does it mean to be in a 2:1 resonance?
  13. Dec 5, 2005 #12
    1.) velocity = distance/time
    thus A.U. is only a measure of distance so I'm stuck again...

    2.) it that is the nearest point, than that deals with solar measurements. I am stuck again, so I am still trying to find out what "its nearest point to Earth" is...

    3,4) resonance is the circumstance in which two characteristic times are related in some simple way??
  14. Dec 5, 2005 #13


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Show your calculation then.

    Just use 1 AU. As tony said, there isn't enough information to determine it, so you'll have to make an approximation.

    The "characteristic times" are the periods. So then how are the periods of two planets related if they're in a 2:1 resonance?
  15. Dec 5, 2005 #14
    1.) I showed all my calculations.

    3,4) I don't know?!? I've given you all I got and that definiton was from my book.

    2.) I am working on.
  16. Dec 5, 2005 #15


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    This is all you showed me:

    Write out the equations you used and the values of each variable you substituted for and I can help you fix what's confusing you. I'm not going to solve the problem for you.

    Ok, then we'll have to back up. What is a ratio? If the length of an object is in a 2:1 ratio with the length of another object, what does that mean?
  17. Dec 5, 2005 #16
    Answers 3&4:
    this is what i think i do with resonance....
    3.) i multiplied the period of jupiter by 2 and then by 5 for this asteroid.. thus:
    period for asteroid = 118.704384 years
    A = 24.153 A.U.

    4.) i did the same thing just 2:5
    period for asteroid = 18.82137 years
    A = 7.076 A.U.
  18. Dec 5, 2005 #17
    2.) he said that the problems 1 &2 go together. I am supposed to use an answer i get from problem one (distances i am guessing) for the distances in 2. does that make sense? is it possible?
  19. Dec 5, 2005 #18
    1.) Most velocity = distance/time
    the units should be km/s or m/s
    using the equation you gave me for velocity: 2(pi)a/p * square root (1-e)/(1+e) doesn't have those units
    the units for a = A.U.
    the units for p = years
    so that would leave me with A.U/years
    but that doesn't seem right? should i convert to kilometers and seconds?
  20. Dec 6, 2005 #19


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Good, this is right so far. It should give you:


    Then you can just solve for the period of the asteroid. The result you got isn't the solution to this equation however. Your result looks like it's just ten times the period of Jupiter.

    Same mistake here.

    Sounds fine to me.

    AU/years is an acceptable unit, but people are more used to m/s or km/s, so it might be a good idea to convert to that. Good job figuring out the units, by the way. :smile:
  21. Dec 6, 2005 #20
    2.) Use the perhelion as my distance than?

    3,4) Confused as to what you mean by that? What is my mistake?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook