Why do comets come back?

[spurius]

Was reading about Hale bopp on wikipedia:

"The comet likely made its last perihelion 4,200 years ago.[29] Its orbit is almost perpendicular to the plane of the ecliptic, which ensures that close approaches to planets are rare. However, in April 1996 the comet passed within 0.77 AU of Jupiter, close enough for its orbit to be affected by the planet's gravity.[29] The comet's orbit was shortened considerably to a period of roughly 2,533 years,[1] and it will next return to the inner Solar System around the year 4385.[4] Its greatest distance from the Sun (aphelion) will be about 370 AU,[1] reduced from about 525 AU"

So how can they work out its aphelion, and what is at its aphelion that swings it back towards the sun? I would think that stuff in the oort cloud isnt moving in sync with such comets and their orbits would be impossible. I know its theoretical, but what do they base these theories on?

[sologuitarist61]

There is nothing at aphelion that swings it back. The comet leaves the sun with any extra momentum that it has picked up swinging around it, it then begins to lose momentum owing to the suns gravity trying to draw it back again, so it continues moving outward until a point is reached where the suns gravity has slowed it enough to start drawing it back in. All orbits are either circular (unlikely) or elliptical (more likely) so it is natural that a comet follows such a path. By tracking its orbit as it swings a round the sun some fairtly simple maths, taking into account the suns gravity, will determine the orbit.

[Stargazer_00]

simple answers are gravity and maths.

[spurius]

There is nothing at aphelion that swings it back. The comet leaves the sun with any extra momentum that it has picked up swinging around it, it then begins to lose momentum owing to the suns gravity trying to draw it back again, so it continues moving outward until a point is reached where the suns gravity has slowed it enough to start drawing it back in. All orbits are either circular (unlikely) or elliptical (more likely) so it is natural that a comet follows such a path. By tracking its orbit as it swings a round the sun some fairtly simple maths, taking into account the suns gravity, will determine the orbit.

Interesting. But what will happen to the space craft heading out of the solar system? Is it possible that the voyagers, new horizions and other such machines could slow down and come back? The aphelion of Sedna is said to be about 900AU so the suns gravitational influence does go on a bit. Based on your post, this would give plenty of time and distance for the sun to slow these tiny objects down and pull them back. If it can do it to a comet which is far bigger and moving far faster.....

[Naemeth]

Interesting. But what will happen to the space craft heading out of the solar system? Is it possible that the voyagers, new horizions and other such machines could slow down and come back? The aphelion of Sedna is said to be about 900AU so the suns gravitational influence does go on a bit. Based on your post, this would give plenty of time and distance for the sun to slow these tiny objects down and pull them back. If it can do it to a comet which is far bigger and moving far faster.....

If they weren't traveling at the speed they are away from the Sun, and just hung there.

To stop something moving, you need to apply a force to it, the Sun's gravity out near Voyager 1 simply isn't enough to pull it from 17km/s to 0km/s.

[Daddystu]

The voyagers gained extra momentum via the slingshot method before hurtling away from (through?) the solar system. I believe at least one of them has reached where the suns influence is thought to end. Where the solar wind contacts interstellar space? Something is slowing them more than if gravity( /relativity?) alone were the sole retarding force apparently.

[sologuitarist61]

As you can see from the other post above, there are many factors that have to be taken into account. With spacecraft they get accelerated at each planet they pass by way of the 'slingshot' mentioned above (Daddtstu) and so gain momentum as they progress further and further away and at those distance the sun's gravity (and the craft speed) mean that the craft can continue outward and as Naemeth says, the sun's gravity out at those distances is smaller than when close in and becomes less noticeable the further away the object is..

[Knight of Clear Skies]

Interesting. But what will happen to the space craft heading out of the solar system? Is it possible that the voyagers, new horizions and other such machines could slow down and come back?

The two Voyager probes, Pioneer 10 and 11, and New Horizons have all reached solar escape velocity. While the Sun's gravity continues to slow them down, it does so at an ever diminishing rate as they move further away. Escape velocity is one of those tricky concepts to get your head around. If Voyager 1 and the Sun were the only two objects in the universe, the former would slow down forever; but it's speed would never reach zero because the rate of deceleration is also continually decreasing.

If it can do it to a comet which is far bigger and moving far faster.....

Not exactly. A comet travels quickly through the inner solar system - because it has been accelerated by the Sun's gravity. At the furthest portion of its orbit, it is moving very slowly with respect to the Sun.

Something is slowing them more than if gravity( /relativity?) alone were the sole retarding force apparently.

Are you thinking of the Pioneer Anomaly? if so, a probable explanation has been found - thermal recoil.

[sologuitarist61]

Are you thinking of the Pioneer Anomaly? if so, a probable explanation has been found - thermal recoil.

Am I the only one thinking that it is remarkable that two different crafts in differing portion of space are both losing speed by radiation pressure, presumably, by the same amount and both in the direction that they are travelling. I know that the Pioneers are similar craft but New Horizon is a different design of space craft, yet they all seem to be losing speed in the same way, which seems strange. Any body got any ideas why this might be?

[Knight of Clear Skies]

I know that the Pioneers are similar craft but New Horizon is a different design of space craft, yet they all seem to be losing speed in the same way, which seems strange. Any body got any ideas why this might be?

The effect has never been seen with the New Horizons probe, just the Pioneers. If there was a something else going on, it's thought that New Horizons wouldn't be suitable for studying it anyway. It's RTG is mounted close to the body of the probe, producing an acceleration far larger than that seen with the Pioneers.

[E621Keith]

The voyagers gained extra momentum via the slingshot method before hurtling away from (through?) the solar system. I believe at least one of them has reached where the suns influence is thought to end. Where the solar wind contacts interstellar space? Something is slowing them more than if gravity( /relativity?) alone were the sole retarding force apparently.

You are thinking about the influence of the sun's solar wind which ends at the Heliopause. Voyager 1 is expected to reach it next year.

The sun's gravity influences stretches out much further. The hypothetical Oort cloud goes out to around 1 light year from the sun. Voyager isn't expected to get out of that for another 14000 to 28000 years.

The classical gravity obeys an inverse square law, so the sun continue to have influence beyond the Oort cloud although it gets weaker an weaker. At some point between us a the nearest star, the influence of the other star's gravitation field will become stronger than the sun, at which point you can say the sun's influence ends (but in reality it's just ceased to be the dominant force).

[sologuitarist61]

The effect has never been seen with the New Horizons probe, just the Pioneers. If there was a something else going on, it's thought that New Horizons wouldn't be suitable for studying it anyway. It's RTG is mounted close to the body of the probe, producing an acceleration far larger than that seen with the Pioneers.

Many thanks for that, I thought I had read somewhere that New Horizon was subject to it as well - getting me facts muddled!

[Knight of Clear Skies]

You're welcome, glad I could clear that one up.