In part 1 I described what an interstellar probe might contain, and what technologies we’d need. Here I get more into what the trajectory looks like. The calculations and estimates are all back of the envelope – i.e. they are a very rough order of magnitude estimate.
In designing a multistage trajectory, it is often helpful to start at the destination and work backwards along the trajectory to see what is needed. We’ll assume that the Gliese 581 system is our destination. It is 20.3 light years away and well off the Solar ecliptic plane. At Gliese 581 we’ll want a set of science instruments, about 10,000 kg (22,000 lbs) worth. For comparison, the entire Hubble spacecraft is 11,000 kg. We’ll need really good communications, call it 1000 kg, and a power plant to power both the instruments and radio, several small fission reactors should do well, another 10,000 kg. We’ll need a propulsion system, for that we’ll choose a Bussard Ramjet. The Bussard Ramjet is best thought of as an augmented sail. It is not capable of large velocities relative to the local space medium, but it doesn’t need to carry fuel and we’ll mostly be wandering from orbit to orbit within the star system anyway. We’ll give the engineers 20,000 kg for the ramjet. That gives us a total spacecraft mass of 41,000kg (90,000 lbs).
Working backwards, we need to slow down coming into the Gliese 581 system from our interstellar velocity. Again the Bussard Ramjet is perfect, it is just an augmented sail. Our relative ‘airspeed’ is very high, slowing down with the Bussard Ramjet is straightforward. About one half a light year out from the star, we start up the ramjet collector and by the time we cross into the Gliese 581 solar wind, we’ll be at orbital capture speeds. So we don’t need to add any additional mass or capability for this stage.
Working backwards again, we have a long coast phase. We will need small course corrections, again we already have the necessary equipment. A small (in the noise) amount of stored fusion materials can be run through the Bussard Ramjet’s fusion reactor to make any minor course changes.
Now we come back to the boost phases, and we need to work forward. We’ll start at L4. First phase is earth departure, we need to go from Earth orbit to Solar orbit. Then we need to do plane change to line us up with Gliese 581, then do our solar departure burn. For these phases I calculate that a four stage booster system using nuclear fission fragment engines will be necessary. The gross mass of the vehicle at L4 where it starts will be 13 kilo-tonnes. For reference, this about the same size as 4 Saturn V rockets. Not small. The first stage thrust is fairly small, providing about 60 micro-g of acceleration. And the first stage will be burning for over 36 years! At staging the spacecraft will be nearly 1/2 a light year away (28,000 au) travelling at 2.5% of the speed of light. The first stage is ditched and the second stage starts its burn. After another 36 years the second stage shuts down. The spacecraft is now going about 5% of the speed of light and is 1.35 light years away. At the end of the 38 year third stage burn, the spacecraft is cruising along at 7.5% light speed and a little over 4 light years away. At the end of the 36 year long fourth stage burn, the spacecraft is moving at 10% light speed and is over 7 light years away. At this point the spacecraft coasts for about 124 years, which brings it to within about 1/2 light year of the Gliese 581 system. At this point we kick on the Bussard Ramjet collectors which slows the spacecraft down with a deceleration of 0.1 g. For ten years the spacecraft slows down and enters an elliptical orbit within the Gliese 581 system. Using the ramjet for both power and orbital maneuvering the spacecraft becomes a permanent part of the Gliese 581 system and can systematically find and observe every chunk of rock, snowball, or ball of gas in the system until it wears out.
All told we are looking at 280 years of transit time. At the end of the transit, a scientist will be getting the data she requested as a grad student when she retires! Most importantly, we will have an entirely new view on the universe.
But why only do one?