Fastest Spacecraft Ever?!?, Which one is it? Options

[ljk4-1]

G. Landis had a similar idea to Meyer's one year to Pluto drive in 1989.


The Landis paper online:

http://www.sff.net/people/geoffrey.landis/...ightsail89.html


The Centauri Dreams article:

http://www.centauri-dreams.org/?p=652

[ugordan]

Also the payload would probably have to have some kind of spaced multiple shielding. At 185 kms-1 even micrometeorites would be deadly.

As Arthur C. Clarke used to point out - when something hits you in space, it doesn't really matter whether it's travelling at 10 or 1000 km/s. Either way, you're toast

185 km/s probably doesn't require additional shielding. You just have to hope that every dust grain that hits you is microscopic. If it's not -- you'll buy the farm anyway.

[djellison]

Well - consider a 1 gramme projectile at 10km/sec - that's 50,000 joules of energy

Consider it moving at 185 km/sec - that's 17112500 joules, or 342 times as much (i.e. 18.5 ^2 )

Doug

[ugordan]

That's true, but consider what are the odds of flying into a one-gram chunk (assuming you're not flying by a comet - in which case 200 km/s doesn't give you a big science op)? Present multi-layer protective blankets are IIRC designed to withstand micron-sized (or whatever) dust impacts. A 1-gram dust grain would punch through the shielding in both cases and would potentially be disastrous to the s/c. IMHO, it would be unfeasible to add shielding mass such that it would specifically protect the s/c against such large and statistically unlikely impacts.

Which reminds me of "whipple-shields" used on Stardust - how large impactors were they designed to hold?

[djellison]

I wasn't suggesting we would encounter 1 gramme particles - I was just using that as a simple figure from which to demonstrate the way the energy ramps up with the square of the velocity.

Doug

[ugordan]

energy ramps up with the square of the mass.

You mean velocity?

[djellison]

Oop- yes

Doug

[Guest_DonPMitchell_*]

I believe it is the case that it takes far more energy to send a probe to the Sun than it does to send a probe out of the solar system. The third cosmic speed is about 17 km/sec. But to cancel the Earth's orbital speed and fall into the Sun would require reaching about 30 km/sec.

[mchan]

If one were not permitted to cheat and use one or more gravity assists.

[Phil Stooke]

No, that would just be the method.

Phil

[djellison]

Yes - it seems unintuitive at first, but the number of flybys and the fuel mass ratio of Messenger show just how difficult Mercury is.

Doug

[edstrick]

"...square of the velocity...."

When we're dealing with speeds of tens of kilometers per second, I'm inclined to think of the kinetic energy in terms of the "square of the ferocity"!

Zip-CRUNCH!

[mchan]

No, that would just be the method.

I had interpreted "energy" in the post as the C3 "energy" for injecting into a solar orbit. The C3 for a direct trajectory to intersect the Sun's surface is greater than, say, a VEEGA trajectory to a Jupiter gravity assist back towards the Sun. I hesitate here because I don't know if it is possible to get the required inflection without going into Jupiter's atmosphere.

[Bob Shaw]

I had interpreted "energy" in the post as the C3 "energy" for injecting into a solar orbit. The C3 for a direct trajectory to intersect the Sun's surface is greater than, say, a VEEGA trajectory to a Jupiter gravity assist back towards the Sun. I hesitate here because I don't know if it is possible to get the required inflection without going into Jupiter's atmosphere.

You can actually get *very* close to the 'surface' of Jupiter and still be out of it's atmosphere, the pressure of which drops of rapidly with altitude (unlike, say, Mars). Of course, if you get it wrong, it's a problem...

Bob Shaw