Venus and Delta-V

[WH40krules]

As I know it, the solar system formed, and all the planets were going in a prograde orbit. Then, sometime later, a meteoriod or another object crashed into Venus and put it in a retrograde orbit. I was wondering how such a thing would happen. Venus's current average orbital speed is about 35km/s (http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html). So, if Venus was in a similar prograde orbit, then it would require 70km/s of delta-v to change Venus's orbit, I think. If I am wrong, please correct me. What I really want to know is how big/ how fast a object would have to go to deliver that delta-v at once.

On a similar note, I was also wondering the delta-v required to blast off a piece of mars and put it into a trajectory to intercept earth and land, like several meteorites have done.

Thanks everyone in advanced!

[phoenix_ca]

...what? Venus is going the same direction as all the other planets. (Edit: and by that I mean orbit)

And when the solar system first formed there were likely lots of objects going in many directions and on various planes around the sun. The current plane and orbital direction of the planets (and asteroids) just happened to be the preferred direction once they all were done (okay, not really) smashing into each other.

Edited May 24, 2014 by phoenix_ca

[Space_Kraken]

Venus rotates retrograde, it orbits prograde...

[xcorps]

https://what-if.xkcd.com/20/

[christok]

Venus (and Mercury) must rotate very slowly due to solar tides, irrespective of the direction. It is then necessary only to account for a much smaller change in angular momentum.

I wish I remember where I read this, but apparently some simulations have shown that multi-body interactions with the other planets may be the cause, rather than an impact. Perhaps someone around here knows the details. There are also some other ideas, such as the manner in which the planet formed by accretion or interactions between tides and solar atmospheric heating.

Short answer: We're not really certain why Venus rotates the way it does.

Since you got confused, lets work out the angular momentum of Venus.

I'll pretend it's a uniform, rigid sphere. The moment of inertia, I, of such about the centre is 2/5 * mass * radius^2, giving us I = 7.133e37 kg m^2

The sidereal period of rotation is 243 days, giving an angular velocity of 2.99e-7 radians per second.

The angular momentum is the moment of inertia times the angular velocity. Which in this case is 2.13e31 joule-seconds if I didn't make a mistake.

As for your other question:

The delta-v required for a Hohmann (i.e. minimum-energy) transfer from Mars to Earth is 5.594 km/s. Actual values can be somewhat lower if the meteorite spent a long time drifting and was sent towards Earth's orbit by multi-body interactions, or they can be higher. It should definitely be at least 5.021 km/s, which is the surface escape velocity of Mars.

I'm ignoring atmospheric drag during ascent. The original meteor may or may not have punched enough of a hole in the atmosphere for this to be realistic.

[WH40krules]

Ahh, my bad. I guess I got rotating and orbiting mixed up. Thanks for clearing it up. Sorry!

christok: That is a lot of delta-v, and must be really rare for a object to impact Mars at the right time for a Hohmann transfer to Earth.

[christok]

must be really rare for a object to impact Mars at the right time for a Hohmann transfer to Earth.

It is not necessary for the impact to occur at that time. Once in a (near) Hohmann transfer orbit, it can keep going round and round until it encounters either Mars or Earth again.