It has become a trademark of the people/groups in the doomsday business to
link to different orbital elements of a comet, such as Comet Hale-Bopp or Comet
Lee, and to claim that the different orbit solutions indicate that the comet's
orbit is erratic. Is there any truth to these claims? NO! Why then do
orbital elements change?
How Are Orbits Determined?
When a comet is discovered. astronomers, both amateur and professional, from
around the world, observe it, compute its position in the sky, and transmit
their observations to the Central
Bureau for Astronomical Telegrams (CBAT). CBAT computes orbital elements and
also shares these positions with other interested astronomers.
It generally requires a few days after discovery to have enough data for the preliminary
orbit to be calculated and released by CBAT. This preliminary orbit typically
assumes that the comet is traveling in a parabolic orbit. That is, the comet has
an infinite period (ie, it will never come back). We know this isn't true. All
comets orbit the Sun and travel in ellipses around the Sun. For long period
comets, such as Comet Lee, these ellipses are stretched so that they look long
and narrow. The orbital period of such a comet is generally a thousand years or
more. Because of this, a parabolic orbit (eccentricity or e = 1.0) is usually a
good approximation to the real orbit.
A preliminary orbit is based on relative few observations and covers a tiny
portion of the actual orbit. While the first orbital parameters are in the
correct ballpark, they will change as new observations are made and included in
the orbital solution. However, the changes become smaller as more and more
observations are made and a larger arc of orbit is covered.
As more observations become available, our knowledge of the orbit
improves...the comet's orbit doesn't change.
It should be noted that even if our knowledge of an orbit was absolutely
perfect, the orbital elements themselves, evolve with time. This doesn't mean
that they change radically, but they do change. That is why each set of orbital
elements comes with the epoch, a date for which the elements are valid. However,
even months before and after the epoch, a given set of orbital elements will
work sufficiently well for most applications.
What Affects a Comet's Orbit?
Gravity from the Sun is why the comet is in orbit around the Sun in the first
place. However, gravity from all the planets affect the comet. How much depends
on how close the comet comes to a planet and how big (how much mass and thus,
gravity) the planet has. If a comet comes very close to a planet, particularly a
large planet (e.g., Jupiter), its orbit can be drastically changed. Most comets
typically go through the inner solar system with no major changes to their
It is well-known that comets have vents that produce jets of dust and gas
when exposed to sunlight. These jets act as little thrusters. While the effect
of individual jet at any given moment is extremely small, over time these little
thrusts will modify a comet's orbit slightly. The effects of these non-gravitational
forces show up more in periodic comets that return to the Sun's vicinity
frequently. These non-gravitational forces can be modeled and included in the
None of these effects will make a comet's orbit erratic.
How Do We Know a Comet is in Its Predicted Orbit?
Glad you asked...The orbit of a comet is used to generate an ephemeris. An
ephemeris is simply a listing of positions in the sky as a function of time. It
is what astronomers use to find a comet in the sky. Even most amateur
astronomers can generate an ephemeris (or at least a position on a given date)
given the comet's orbital elements.
If the orbit is wrong (or drastically changed), the comet won't be found
when people try to observe it. That we will hear about!
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