Fermi Paradox or 'Where are they?'

[chrispenycate]

Stars are easy; you know where they are, you know where they are going to be for the next ten thousand years or so. Plan your course in advance not to fly through any of them. Or close to; stars have a tendency to have planetary systems, and, even though most of the space is full of vacuum (i.e. empty of worthwhile blobs of matter) it's a higher risk region than most of interstellar space. We know interstellar space contains a statistically insignificant percentage of matter because it isn't diffusing the light from stars the other side of it. But this doesn't mean zero, just less likelihood of hitting a free comet than being struck by lightning in Hendon, and Murphy's arm is long.

We can improve our odds. Once we are up to speed there is no real argument about building a kilometre wide telescope, capable of detecting anything big enough to do damage at several light weeks distance – plenty of space to decide whether to steer round it or laser zap it. Dark matter is indetectable but since at last hearing it can interpenetrate normal matter with no interaction we can ignore it totally. Which leaves us with dust clouds.

Now, we know there isn't much in the way of interstellar dust in the region (same way as we know about comet density) but we really don't need much friction to heat everything up and slow everything down. We'll start with unmanned drones, of course, which will partially map out the region, but we might turn out to need the Bussard as a vacuum cleaner, in which case we'll definitely collect the reaction mass to throw away.

And really, I'm not expecting to hit more than a few kilograms of matter on the entire trip.

[TheEndIsNigh]

You'll be taking plenty of mice with you then Chris.

(Care of Douglas Adams)

[skeptical]

End

You may be right, but on the basis of what science currently knows, it appears unlikely that, after leaving the solar system, you will encounter much. Hydrogen molecules, even at 0.1 c are not an issue. Grains of sand are.

On dark matter, chris is correct. Take note. Large objects such as comets will be so rare as to be essentially not worthy of worrying about. However, running into a grain of sand, or even dust, at 0.1 c would be a disaster.

The biggest risk will be while still inside the solar system. However, the velocities involved will also be way less, and in 500 years, we should be able to plot a clear course through that part of space.

[Xelebes]

Originally Posted by skeptical

Interstellar travel is theoretically possible, with a velocity of 0.1c (10% of light speed) potentially achievable even using techniques within reach today.
Interstellar travel - Wikipedia, the free encyclopedia

Sadly, in spite of Larry Niven's enthusiasm, a Bussard Ramjet will not do it.
I quote from the reference above:

"the Bussard ramjet, a fusion rocket in which a huge scoop would collect the diffuse hydrogen in interstellar space, "burn" it on the fly using a proton-proton fusion reaction, and expel it out of the back. Though later calculations with more accurate estimates suggest that the thrust generated would be less than the drag caused by any conceivable scoop design, the idea is attractive because, as the fuel would be collected en route, the craft could theoretically accelerate to near the speed of light."

In other words, it works better as a brake than a throttle!

Bussard's ramjet uses a parabolic receptacle, but makes no mention of a magnetic receptacle which could serve two purposes: 1) deflect/intake harmful radiation and particles for conversion and 2) reduce drag by having things not rub against each other.

[skeptical]

xelebes

I kind of wondered if a Bussard ramjet field could be used to delelerate an interstellar craft. If you could accelerate to 0.1c using something like a super-iondrive engine, and use a bussard ramscoop to do the decelerating, then you could cut the amount of reaction mass you had to carry quite dramatically??

[TheEndIsNigh]

skeptcal:

You're not by any chance trying to have it both ways are you

Not many hydrogen molecules for me to run into yet enough to slow the ship with the scoop

[Urien]

We had a super-ion drive on our old Datsun when I was a kid. My dad dug it up from the potato patch. We found it next to the wax sealed Roman cylinders.

Simultaneously we proved there was no Fermi paradox and (from the contents of the cylinders) that the Romans reached America.

The potatoes also made good chips.

[skeptical]

To the End
Sure - lots of Hydrogen atoms to run into. However, even at 0.1 c, their mass is so tiny as to make the impact negligible. However, with a wide spanning ramscoop field, there may be useful braking???