Inhabitable Planets Too Close Together?

Cryo sleep really isn't needed. With a 1 gee acceleration one can accelerate for a year or so and then coast. The idea that one needs to be sleeping forgets the effects of time slowing down when going that fast. The faster you go the less time it effects you. On board. Cryo sleep might be needed if you are on either end. Planet side, but not on the ship.

Going so fast with such an effect upon you makes highest speed computers essential to do anything what appears to be real time. Time literally flies at high speed around, slow you.

JohnRoth wrote:
The take-home here is that the so-called "habitable zone" could be a lot smaller than we think, which has implications for what proportion of systems have a habitable planet.

Yes but current thought is more in the other direction, that the zone may actually be a lot wider.

To get back to an earlier subtopic in this topic, Ars Technica just posted a review of an article on why Earth has plate tectonics while Venus doesn't. There's a fair amount of speculation that plate tectonics are a prerequisite for life as we know it.

But that wouldn´t be an issue for colonisation, as this is just something believed to potentially be a big part of what can make NEW life happen.

Lord Skimper wrote:Cryo sleep really isn't needed. With a 1 gee acceleration one can accelerate for a year or so and then coast. The idea that one needs to be sleeping forgets the effects of time slowing down when going that fast. The faster you go the less time it effects you. On board. Cryo sleep might be needed if you are on either end. Planet side, but not on the ship.

Going so fast with such an effect upon you makes highest speed computers essential to do anything what appears to be real time. Time literally flies at high speed around, slow you.

Time dilation is only really useful at very close to light speed, where the curve grows rapidly to infinity. Even at 90% light, you get less than 10% time dilation, if memory serves. And we are nowhere even close to even a noticeable fraction of that speed.

Lord Skimper wrote:Cryo sleep really isn't needed. With a 1 gee acceleration one can accelerate for a year or so and then coast. The idea that one needs to be sleeping forgets the effects of time slowing down when going that fast. The faster you go the less time it effects you. On board. Cryo sleep might be needed if you are on either end. Planet side, but not on the ship.

Going so fast with such an effect upon you makes highest speed computers essential to do anything what appears to be real time. Time literally flies at high speed around, slow you.

In the following discussion, the term "proper X" means X in as perceived by the passengers on the ship.

Suppose a ship has a proper acceleration of 1 gee for a proper time of 1 year. To an inertial observer, the ship accelerates for 1.19 years, travels 0.56 light-years, and reaches a velocity of 0.77c. This is only a gamma of 1.58--in other words, the time compression is only 0.63. A 500 light-year trip to Manticore would take over 649 years for an inertial observer, and 411 years of proper time.

You need a lot more than just 1 year of acceleration to make interstellar travel of hundreds of light-years practical without cryosleep or taking multiple generations of travel time.

Suppose you could accelerate continuously for a long time. To go 500 light-years, you could have a proper acceleration of 1 gee for a bit over 6 years proper time, then decelerate for the same amount of time. Total trip time would be 12.11 years in proper time, or 501.9 years for an inertial observer. The maximum velocity would be 0.99999254c.

If you had total conversion of matter into kinetic energy with 100% efficiency, you would need 516 kilograms of fuel for every 1 kilogram of ship mass. And that ship mass includes the colonists, their equipment, everything they need to survive for 12 years aboard a ship, the ship frame, the massive shield to protect from relativistic radiation, and the storage unit for all that fuel.

If instead you are using hydrogen fusion (0.008 mass conversion) and 100% efficiency turning that into kinetic energy, you would need 64,500 kilograms of fuel for every kilogram of ship mass!

And it would still take over 12 years from as seen by the passengers!

It is simply not practical to have sublight travel for hundreds of light-years without multi-generation designs or some form of hibernation.

Very well said.

I grew up on stories such as Anderson's Tau Zero and Niven's A World Out of Time. The premiss of a Bussard ramship allowing one to accelerate at one gee almost indefinitely seems so plausible. However; the engineering limitations involved are very daunting. We are stillstruggling to develop a fusion reactor that break-even using D-T fuel which has a fusion reaction cross section orders of magnitude higher than Proton-Proton fusion. We will be elated if we can build a 1,000 ton machine that produces 1,000 Megawatts or 1eex6 Watts perton.. For a one gee fusion rocket we need a power density of about 1eex14 Watts per ton.

SWM wrote:

Lord Skimper wrote:Cryo sleep really isn't needed. With a 1 gee acceleration one can accelerate for a year or so and then coast. The idea that one needs to be sleeping forgets the effects of time slowing down when going that fast. The faster you go the less time it effects you. On board. Cryo sleep might be needed if you are on either end. Planet side, but not on the ship.

Going so fast with such an effect upon you makes highest speed computers essential to do anything what appears to be real time. Time literally flies at high speed around, slow you.

In the following discussion, the term "proper X" means X in as perceived by the passengers on the ship.

Suppose a ship has a proper acceleration of 1 gee for a proper time of 1 year. To an inertial observer, the ship accelerates for 1.19 years, travels 0.56 light-years, and reaches a velocity of 0.77c. This is only a gamma of 1.58--in other words, the time compression is only 0.63. A 500 light-year trip to Manticore would take over 649 years for an inertial observer, and 411 years of proper time.

You need a lot more than just 1 year of acceleration to make interstellar travel of hundreds of light-years practical without cryosleep or taking multiple generations of travel time.

Suppose you could accelerate continuously for a long time. To go 500 light-years, you could have a proper acceleration of 1 gee for a bit over 6 years proper time, then decelerate for the same amount of time. Total trip time would be 12.11 years in proper time, or 501.9 years for an inertial observer. The maximum velocity would be 0.99999254c.

If you had total conversion of matter into kinetic energy with 100% efficiency, you would need 516 kilograms of fuel for every 1 kilogram of ship mass. And that ship mass includes the colonists, their equipment, everything they need to survive for 12 years aboard a ship, the ship frame, the massive shield to protect from relativistic radiation, and the storage unit for all that fuel.

If instead you are using hydrogen fusion (0.008 mass conversion) and 100% efficiency turning that into kinetic energy, you would need 64,500 kilograms of fuel for every kilogram of ship mass!

And it would still take over 12 years from as seen by the passengers!

It is simply not practical to have sublight travel for hundreds of light-years without multi-generation designs or some form of hibernation.

namelessfly wrote:Very well said.

I grew up on stories such as Anderson's Tau Zero and Niven's A World Out of Time. The premiss of a Bussard ramship allowing one to accelerate at one gee almost indefinitely seems so plausible. However; the engineering limitations involved are very daunting. We are stillstruggling to develop a fusion reactor that break-even using D-T fuel which has a fusion reaction cross section orders of magnitude higher than Proton-Proton fusion. We will be elated if we can build a 1,000 ton machine that produces 1,000 Megawatts or 1eex6 Watts perton.. For a one gee fusion rocket we need a power density of about 1eex14 Watts per ton.

Great stories. I love Tau Zero and A World Out Of Time. For that matter, I love everything Poul Anderson and Larry Niven have written.

The Bussard Ram Jet is a wonderful inspirational concept. It is unfortunate it doesn't work as well at relativistic speeds as the stories. When using it at high velocity, you must accelerate all that hydrogen you are scooping in up to your own relative velocity before fusing it. If you don't, you get almost no thrust from the fusion because you are already moving away from the fusion products at high velocity. So, either you get less and less thrust as your velocity approaches the expansion velocity of the fusion products, or you get more and more drag as you try to bring interstellar hydrogen up to your own speed. Either way, your efficiency goes way down as you reach relativistic speeds.

But it really does make a great story.

TheMonster wrote:…Lastly, when it comes to the durability of the ship, the logical design for a generation ship is to hollow out an asteroid, leaving a large amount of rock as a shield against impacts along the way...

Not really - that would be a least efficient design. It would take more effort to hollow out the asteroid (unless you were already doing it for mining purposes) than to manufacture the armor for the bow. The rock would also be much softer & less capable than manufactured armor. The manufactured armor can also be of composite type to allow ray shielding to be included in the layers (lower/deeper layers to prevent damage) the moon landings had thin gold foil which sufficed (at their speed) a marginally thicker composite (or electromagnetic field) would be needed for high %c travel. The armor would be less massive, provide more protection & require less energy to push along with the rest of the ship than a hunk of asteroid would. Plus the armor would make for a better looking ship than a hunk of rock. The use of an asteroid for a ship would only be practical if you wanted to hid your “ship” nature as a disguise (which might come in handy if the destination turned out to be already claimed)!

Tenshinai wrote:But that wouldn´t be an issue for colonisation, as this is just something believed to potentially be a big part of what can make NEW life happen.

right - this may actually help colonization - just because a planet hasn’t developed life doesn’t mean it can’t sustain life. If we find lots of planets with the requirements for life, but no life on them, then those are prime candidates for terraforming and colonization.

HB of CJ wrote:…the more we understand that everything must be darn near perfect for even genetically modified people and things to survive, live and multiply there…

I've heard that, but if you look at the Earth, there’s life in the scorching desert and life under the frozen Antarctic. They’ve even found life in the scalding & poisonous underwater active volcanos. Life may surprise you. It really dose adapt to its environment (especially for the environment it develops in!)

MAD-4A wrote:

Tenshinai wrote:But that wouldn´t be an issue for colonisation, as this is just something believed to potentially be a big part of what can make NEW life happen.

right - this may actually help colonization - just because a planet hasn’t developed life doesn’t mean it can’t sustain life. If we find lots of planets with the requirements for life, but no life on them, then those are prime candidates for terraforming and colonization.

To some extent that is true. It eliminates worries about displacing native intelligence or preventing it from appearing in the future. There is no need to worry about the fate of local species. It removes the danger of local diseases, pests, and predators. And you can terraform as much as you like.

However, an empty planet also imposes significant hurdles for colonization. A planet without life will not have oxygen. An oxygen atmosphere is unstable, and cannot last without being replenished. Free oxygen will oxidize numerous materials on a planet, and will disappear on less than geological timescales. So colonizing a planet without any life means a lot of terraforming, to create an oxygen atmosphere. It would take at least several generations to have a breathable atmosphere. It appears that very few planets without life have been colonized in the Honorverse.

A planet without any life means a lot of terraforming, to create an oxygen atmosphere and a biology base where people could live outside of the kind of domed environment of Grayson if they even got that far and didn't just try to set up in oribit somewhere.
Even if the planet had an atmosphear humans could breath witout assistance, you have to worry about what the local biology would do to the humans and whatever they import. So far, human interplanitary colonization has tended to assume that the colonists are going to take all kind of plants and animals to provide things we could safely eat (presuming they didn't get contaminated with local toxins anyway.

It would take at least several generations establish all sorts of Earth based plants, animals, insects etc. More than one planet mentioned in the Honorverse books almost died because of what the local biology did to people or terrafroming species and crops.