It would need a rough {off top of head) 50° change in course and there us nothing out there that would induce that.
And the required change in direction is only growing larger.
Eventually it will require a nosedive.
And the energy to do that just isn’t there.
Something to keep in mind as well: just because Ison sped up as it approached the sun doesn’t mean Ison will retain that speed.
The same towards the sun force that sped Ison up still exists, so Is one remains will slow down at the same rate going out.
Most people think of gravity assists and assume that applies to comets.
Not exactly.
A gravity assist maneuverconsists of a slight change in orbit direction.
(A good illustration of such would be the “New Horizon” page showing the entry and exit course past Jupiter. Wikipedia actually, for once, has something informative on that! Lol.. Section “Jupiter gravity assist”, graphic to right.)
An orbit like Is one, or earth’s for that matter, don’t get a permaboost because it isn’t a glancing pass by.
Ison orbital inclination is 62.4 degrees. In other words, the orbit would need to be bent some 60 degrees - and 60 degrees in the correct direction - to 'straioghten' it out to the plane of the elliptic so that an intersection with anything orbiting the sun in that plane (which is pretty much all the planets) possible at all. Otherwise it would pass through the plane of the elliptic going 'down' to the sun and pass back through the plane of the elliptic going 'up' away from the sun and the longer it goes 'up' away from the sun, the further away from the plane of the elliptic it gets.
As it gets further from the plane of the elliptic, which it will as it 'climbs' away from the sun, the more radical a course change is required to aim it at Earth or any planet. The one big energy sink that Ison passed was the sun and its input would be + g going 'down' and -g going 'up'. In other words, now that the Is one pebbles are climbing up the gravitational hill away from the sun, energy is being subtracted from the Is one system. There is no additional source of + energy to alter the course.
Unless of course, you really think those Nubirans are going to interfere...