Cometary bodies and altering orbits

[Allan F]

In another forum there's a discussion about a comet (actually a rogue planet the size of Venus) with a period of 250 years. The discussion is about how much energy would be needed to circularize the orbit in any measurable way.

The mass - apprx 5x10^24 kg.

For the engines, we use some imaginary mass-less thruster which has a finite energy reserve of around 1x10^17 J (that's around 0.6 kg of antimatter + 0.6 kg of matter annihilation).

Now, if this energy was expended as reverse thrust in close proximity to the sun, how much would it lower the aphelion of the planet? Assuming the closest point to the sun was arund the orbit of Mercury.

Edit: Let's assume the entire energy is spent in a negligble time - not days or months, but hours or minutes.

[ka9q]

Energy is not enough; you have to give the amount of reaction mass to which it will be applied.

If by "mass-less" you mean a photon rocket, then the impulse would be the liberated energy divided by the speed of light. Since the liberated energy is mc², the result is simply mc = 3.6e8 N s. Not much compared to your planetary mass of 5e24 kg.

[Allan F]

No, I got something like 0.0001 m/s total.

[ka9q]

I think that's way too high. Now if you could use some of the mass of the planet as reaction mass, you might do better. OTOH, you'd have to do more work overcoming its gravity.

[smartcooky]

Just a semi-related question for all you planetary scientists out there....

On April 13, 2029, the minor planet Apophis is expected to pass Earth within the orbits of geosynchronous communication satellites, about 19,400 miles above Earth's surface. Given that it is about 325m in diameter, is it at all likely that it will significantly disrupt satellite orbits?

[Echnaton]

So how observable will that be?  Will it be visible at twilight with out optics?  How about bright enough to be seen in daytime.

[Luke Pemberton]

So how observable will that be?  Will it be visible at twilight with out optics?  How about bright enough to be seen in daytime.

I was thinking that I'll still be working if the trend in retirment age continues.

[smartcooky]

So how observable will that be?  Will it be visible at twilight with out optics?  How about bright enough to be seen in daytime.

What I have read says around magnitude 3, and my quick back of the enveloope calculation says about 2.2 arcseconds so it should be easily visible in modest binoculars, and with a good pair you should see an extended object.