ISON Captured by SUN? Not necessarily

Sunday Nov. 24, 2013 : (Question to JPL) Dear Alan (Chamberlin) at JPL :  Is this statement accurate?  If not how would you reword it?

Big day for ISON yesterday. I noticed today the eccentricity dropped below “1.00”. Does this mean NASA is now calculating it is no longer on a hyperbolic orbit heading out of the solar system, but is now on an elliptical path? Today e is now .9999977109551715 with newly calculated orbit time of 400,864 years – ie Big News – Has ISON has been captured by the SUN and is now in a very long elliptical orbit. I suspect this is a historic moment when we have just seen an exo-COMET captured by our sun.

Monday Nov 25, 2013 : (Response from JPL) Hi Bill, The current orbit (51) for Comet ISON is represented as heliocentric osculating orbital elements at epoch 2013-Nov-04.  The eccentricity listed on our web-site (0.9999977109551715) has a formal uncertainty of 9.8317e-08 (i.e., on the order of 1e-7 1-sigma).  Note that this puts the eccentricity not very far away from 1.0. 

One important issue is that these elements are with respect to the Sun — not with respect to the solar system barycenter.  It is quite common to see comets with e slightly less than 1.0 with respect to the sun and e >= 1.0 with respect to the solar system barycenter.  The other important issue is to understand that these are osculating elements which effectively means they change with respect to the epoch used (i.e., they are representative of a conic orbit which just touches the actual orbit at that epoch and are handy for quick-and-dirty 2-body propagation).

If we look at the heliocentric osculating elements (specifically e) at other epochs, we see that e is greater than 1.0 except for epochs close to perihelion (i.e., the point at which the comet is closest to the sun), 2013-Nov-28.  See output below from Horizons. 

It’s important to be careful about saying too much based on osculating elements and it’s just as important to understand what they are.  I can only recommend you find some reference materials on the subject if you’re interested as I’m unable to give you a substantial primer.  It’s also important to take into account uncertainties in any object’s orbit.  

In any case, it’s not correct to say that we’re witnessing the capture of comet ISON. 

I hope this helps. With kind regards, 



Dr. Alan B. Chamberlin, Jet Propulsion Laboratory, Pasadena, CA 


Post Script

I believe Dr Chamberlain is helping us understand that JPL has currently “51” different sets of orbit parameters for ISON, taken over time . Each set are different – ie the orbit  subtly changes as the comet comes into the inner solar system and gets affected by the inner planets (esp Jupiter).  He points out “we see that “e” is greater than 1.0 except for epochs close to perihelion”. He even notes that although “e” is “< 1 in epoch 51, the “uncertainty parameter ” associated with “e” is sufficiently high to “put the eccentricity not very far away from 1.0″. In effect he is warning me that the orbit is still pretty much parabolic and we cannot draw the conclusion it has changed from “hyperbolic” to ” elliptical” (or that ISON has been captured). Within days, if it fragments and there are large clumps left, these may well become “elliptical” and be CAPTURED by the SUN.  BUT ISON has not been captured by the SUN YET.  i.e. it’s trajectory is still the same : it is heading around the sun and off through the Oort Cloud to another star.