# Asteroid Impact/Capture/Encounter??????

FWL_Engineer said:
Adam, great Maths mate, but there are a couple of flaws in it. Firstly, most NEO's have an average density of around 2.8g/cm-³, however the greatest flaw is the in the momentum calculations.

Firstly, actually slowing a body down in the solar system is far easier than you might think, as the Sun's and Earth's gravity will work for you to rob angular momentum from the object, and if the orbital calculations are done precisely enough, then you can get Earth to slow the Apophis's orbital velocity by over 90% alone.

Well, I'm only a bit off: 3.3 to 2.8, not that bad!

With regards to gravitational braking: here is how you would do it:

First you would have to ensure the asteroid ends up IN FRONT of the Earth, and slightly outside the Earth's orbit around the Sun. (luckily, this would get you a prograde orbit!). That's OK.

But then you run into a problem... "gravity assists" merely translate angular momentum. It is not "free energy". You can't use one to get an object into an orbit around another body. Their only use has been with interplanetary probes, you swing it around a planet and shoot it off at a different angle. Therefore saving propellant. But the TOTAL momentum of the asteroid remains the same. You would still need to supply 100% of the impulse required to slow the asteroid to earth-orbit velocities, so loadsa rockets. A stable orbit is just a gravity assist, after all. But satellites don't go shooting off at tangents with all the free energy

I still think an easier task would be to place it in an orbit further out, which would require less of a momentum change. It wouldn't be geostationary, but at least that means we can all see it! Dunno about you but I would be pi**ed off if the Earth got a new moon that I couldn't see without flying thousands of miles!!! On the upside, that link reckons London will be a good place to see the asteroid scoot past in 2029.

Adam, your example is flawed, you have assumed that the process would be the same as that used for interplanetary probes, however, whiclst the process is similar in essence, the methodology of execution is somewhat different.

The way in which it would work is to alter the asteroids orbit by increments so that the asteroid actually evolves toward a co-orbital situation with the target planet, in this case Earth. However this is never actually achieved. By whatever means you choose, the idea is to adapt the orbit so that it's orbital parameters are similar to, but not identical to Earth. After several orbits the asteroid would actually catch Earth up in it's orbit, but although the two would still be orbiting the Sun at normal speeds, relative to each other they could pass as slow as a few km/s-¹, or even less if you want to wait longer.

So long as the close approach point has been accurately calculated to lie within the Earth's Roche limit, but above the atmosphere, gravity will do the rest. The object would naturally be captured or ejected, however with careful tinkering you could ensure the object is captured, and to really put nature to shame, you could ensure it is a gravitationally stable orbit.

Usually asteroids that are captured by planets tend to have decaying orbits if they are within 5 radii of the planets centre. Phobos and Diemos are classic examples, and both will impact the surface of Mars within the next 200,000 years. I don't remeber the exact numbers, but I think Diemos will hit the Roche limit in about 15,000 years, be torn apart and the fragments will impact Mars.

This process has occured on Earth, but it is exceptionally rare due to the fact that Earth and Moon are actually a double planet and the Moon's gravity interferes with the natural capture process.

Oh, incidently, if you wanted the asteroid to orbit further out, you would actually need to reduce the asteroids angular momentum more than for a closer orbit. Remeber the Gravity of the planet drops off quickly as you move away from the surface, as a result the asteroid would need LESS energy in order to escape the pull of Earth, as a result it would need to be slowed more than for a tight low orbit where the gravity of the planet is far stronger.

IIRC, Geostationary Satellites can only sit in a band around the equator and whilst this isn't full up at the moment, it almost certainly will be by 2029.

Also, letting this thing get within 22,000 miles seems a bit close to me. Why not keep it in a further out orbit, but place a satellite East and West of it for monitoring purposes ??

JS, it WILL come within 22,00 miles of the surface by natural processes at this time, so altering it's orbit so it is captured into whatever orbit we choose will not be as hard as it may first seem, simply risky.

Regarding Geosynchronous orbits, it is a common misconception that they only exist around the equator. The key to a geosynchronous orbit is to ensure the path of the satellite results in an orbital period identical to the spot on the surface of the globe directly below the satellite.

If the orbit results in such an alignment, then the satellite shall remain above the same spot, and it will be Geosynchronous.

The orbits that do require equitorial orbits are Sun synchronous as they orbit earth with the Sun and so they rise and set at different times each day. It is a coimplicated orbit to achieve and maintain, constantly using propellant to maintain position, as a result satellites in these orbits tend to be short lived, usually no more than 900 solar days.

OK, I let this ride the other day when you did it on the do-gooding hippy post, but I have to ask now.....

WHY DO YOU KEEP CALLING ME JS ??

Is it some Trigger, Rodney, Dave Thing ?

JT..

I am frigging blind!! I have always read your name as J Staunton for some insane mental health reason

From now on it will be JT and nothing else..well apart from the Hippy jibes from time to time

Groovy, Man !

Phew, glad you said you need constant firing with non-equatorial geostationary orbits! I was about to start my spiel on

'spose it could be done, the Earth's linear velocity around the sun is about 110,000 kph, so if you could get the asteroid to head off at a tangent to our orbit you could capture it... only a 5,000kph-or-so change in velocity required, as you say it could reduce the energy requirements by 90% or more.

All depends on getting the right bodies in the way at the right time though. Did those articles mention the sidereal period of the asteroid? If it only has another one or two passes to make then it doesn't leave much time to wang it around other planets...

You're right on the faster orbit/further out... also subject to the rigours of beer and tiredness I had started hypothesising without finishing my thought process... If the asteroid WERE to come in at geostationary orbit radius, at such a relative speed, then by Kepler's laws it would enter an ELLIPTICAL orbit where the APOGEE would be much farther out than 36,000km. Of course at the apogee the speed would be much less than 105,000kph, but nonetheless it would be a highly eccentric orbit. And of course it would be waaaaay beyond escape velocity anyway!

Y'know, might be worth starting a competition to see who could find the asteroid easiest to capture, I'm sure there must be a few bodies that are heading near enough other planets that a minor push could send them here, could be a laugh... Perhaps there's an honourary doctorate in there somewhere!

A very large spanner has just occurred to me...

Obviously the asteroid won't be crossing our orbit at right angles, or it would be headed right for the sun.

Now, they say the best time to view it would be about 10pm, and specify a location, London. This suggests that it would be near-enough overhead London at 10pm (common sense dictates the "ideal location" would be the one the object is over)

Now, picturing where 10pm is on Earth relative to the Sun and the Earth's orbit... This would mean the asteroid's path is almost parallel to our orbit at that point of it's trajectory... so you would not be able to gain much (if anything) through a slingshot (it's already pretty much on the ideal trajectory at that point). So, you would have to find almost all of the braking-energy with rockets.

If you have been wondering where i have been i have moved to a safe place this astroid i was telling you about which is about to stike the earth
in september may just skim it i mean grase the earth but will kill millions
the million doller question is where is safe for a small donation i can let you into my secret me and Otto are well safe if you would like to join us
great , just make that small donation. See you soon

I really wish Arthur C Clark was here in this forum to kick all our butt's on this one!
Number crunching and astrological hypotheses you guys are wasted!

Plumb-boy, it's ASTRONOMICAL not Astrological.

One is the study of stellar facts, the other stellar crap!!

FWL_Engineer said:
Plumb-boy, it's ASTRONOMICAL not Astrological.

One is the study of stellar facts, the other stellar rubbish!!

it's a fact that the price of that other Stella rubbish is astronomical.

Slighly off topic.......have any of you seen this?

It was mentioned today on Radio 2 (Steve Wright).

Unfortunately I'm having problems loading it onto my computer at the moment

JulieL said:
It's interesting though...........have you and Freddie had a 'falling out' somewhere along the line?......hence the 'feisty' posts here.

It depends on what you would term a falling out. I would have seen it as debate in the early days where I objected to voilence and anger directed towards specific groups which then began to (or maybe it always did) include anyone who didn't share freddie's views. Since then, ANY comment I make results in accusations of following people round the forum etc etc with a sprinkling of rule breaking abuse. Since there's probably fewer than 20 people IN THE WHOLE WORLD who post on the general chat forum, it's more or less impossible not to follow someone at some time or other.