Page F30: Project Longshot to Alpha Centauri

Time for part 2 of my transcription of Project Longshot, a proposed mission to Alpha Centauri, the closest star system to our own. I'm now done a bit over a quarter of the entire thing.

2.0 MISSION PROFILE

2.1 Objective Star System

The star system chosen as our objective system for this mission is the Alpha Centauri system. There are three main reasons for choosing this system as the destination of the probe: (1) the system's proximity to our solar system; (2) the scientific interest of a trinary system; (3) sending a probe to this star system provides an opportunity to make great advances in the field of astrometry.

One of the main concerns regarding the success of the mission is the ability of the probe to be in good working order when it reaches the objective system. While any interstellar mission will require transit time in excess of a century, the Alpha Centauri system is the closest to our own, and thus provides the best opportunity to deliver a functional probe to another star system.

While it would be a great achievement to do a close-up study of any other star system, the Alpha Centauri system promises to be a particularly interesting objective. Alpha and Beta Centauri, which are type G2V and dK1 stars, respectively, orbit each other with a separation of 11 to 35 AU. The third star of the system, Proxima Centauri, orbits the other pair at a distance of about 1/6 of a light year. Proxima Centauri is a type dM5e red dwarf, which exhibits sudden changes in magnitude. It is also one of the smallest known stellar masses. Proxima's position should provide an opportunity for the probe to pass relatively close by on its way to its final destination, a close orbit around Beta Centauri. There is also the possibility that this star system contains planets. Advances in astronomy before the probe is launched should provide much more information about the system to help plan the probe's exploration.

Perhaps the greatest contribution that the mission will make to the scientific community will be in the field of astrometry. Sending a spacecraft to the Alpha Centauri system provides an opportunity to make parallax measurements with a baseline of 4.34 light years (see Fig 2.1a). This is over 63,000 times longer than the present method, which uses the semi-major axis of the Earth's orbit as a baseline (see Fig 2.1b). At this time, parallax measurements are only accurate to about 20 parsecs from the Sun. The longer baseline would allow accurate measurements of stellar distances of more than 1.2 million parsecs. If the probe lasts long enough, it has the potential to accurately determine the distance to hundreds of trillions of stars. Knowing the distance to a star is vital in determining its properties. Such an accomplishment would keep astronomers busy for quite some time.

Text:

PARALLAX DETERMINATION

EARTH

CENTAURI SYSTEM

THE BASELINE FROM CENTAURI SYSTEM IS LONG.

Text:

PARALLAX DETERMINATION

d(parsecs)=b(AU)/IP(arcsecs)

From Earth the baseline, b, is short.

2.2 Operations

Assembly of the probe will occur in two phases: major components and subsystems will be assembled and tested on the ground, and then sent to LEO for final assembly and integration into the spacecraft. Transfer to orbit will require multiple launches with initial estimates indicating that launches will be carried out in the following manner, (based on current projections):

Fusion engine - one launch on an ALS-class vehicle.

Main structure - one launch on a Shuttle-class vehicle.

Fuel tanks - one launch on an ALS-class vehicle.

Fuel - one launch on a Growth ALS-class vehicle.

Payload - one launch on an ALS-class vehicle.

Upper stages for the initial phase of the mission will be launched as required based upon advances in upper stage technology.

The fusion drive will be launched as a finished unit due to its complexity. It will be equipped with a plug-in type interface for integration with the remainder of the ship. The main structure will be a collapsible space frame, which will be erected from its stored configuration by personnel at the space station. Additional structure will also be orbited in collapsed form for erection at Space Station. The fuel tanks will be built of sheet aluminum with formed end caps. The end caps will be launched in a completed and stacked configuration and the sheeting for the bodies will be rolled into a solid cylinder for launch. Assembly will occur in orbit using advanced adhesives technology. The fuel for the fusion engine will consist of He3 and deuterium. The He3 will probably be manufactured on earth using particle accelerators, and then both components of the fuel will be launched in liquid form to orbit for processing into pelletized reactor fuel. The payload, like the fusion drive, will be launched fully-assembled, and have a simple plug-in interface to the rest of the system.

By the time that this mission will be flown, the experience gained through the assembly of Space Station, and the ongoing orbital operations which it is expected to support, will provide an adequate base of technology for personnel to assemble the probe. During assembly, systems checks will be run as each component is integrated into the spacecraft, to prevent failures which could necessitate disassembly of components assembled in orbit. Following assembly, a comprehensive system check-out will be made prior to ignition of the upper stages.

The fission reactor will be used to provide the initial power required to start the fusion reactor. Once the drive is ignited it will become self-sustaining and provide enough excess power to operate the system in use during the transit. During this phase, fuel will be drawn from symmetrically located pairs of fuel tanks to avoid instability caused by non-homogeneous mass distribution. As each pair of tanks is emptied, they will be discarded. Tank volume has been calculated so that four of the ship's six tanks will have been jettisoned by the time it reaches the turn around point (see Figs 2.2a and 2.2b). This point has been chosen so that reversal of the spacecraft and reignition of the main drive will result in arrival at the target at proper insertion speed. When the turn around point is reached, the drive will be shut down, and the probe rotated 180 degrees with respect to its velocity vector using the attitude control system. Following the turn around maneuver, the fission reactor will again be used to provide power at restart the drive. These activities will occur at the 71.27 year point in the mission (see Figs. 2.2c and 2.2d for graphs and appendix for calculations).

Throughout this phase of the flight, data from experiments on interstellar space will be returned to earth at low data rates. This will serve two purposed, providing scientific data, and ensuring that contact will be maintained with the probe. Since the probe will be fully autonomous, any problems with the communications system, due to degradation of the transmitting equipment or faulty link analysis will have to be corrected by making improvements to the receiving equipment. Maintaining constant contact will allow the required lead time to implement any necessary corrective measures in the receiving equipment.

After deceleration is completed, the two remaining fuel tanks and the fusion drive will be discarded and the instruments deployed. The attitude control system which was used to rotate the entire spacecraft will be used as necessary to place the probe into an elliptical orbit around Beta Centauri and to maintain this orbit. Once in orbit, high data rate collection and transfer will begin based on both a preprogrammed series of studies and a set of prioritized experiments (determined by what is found in the system). This will continue until the spacecraft's nuclear plant ceases to produce enough power to operate the communications lasers, which will constitute the end of the mission.