Interstellar Transportation: How?[Part-I]

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Interstellar travel is seemingly impossible within our life time yet we have various technological tactics which could be implemented to it happening before our eyes. Today I stumbled across an excellent paper which explained the intriguing plans in a concise way.

By Dana G. Andrews

Interstellar travel is difficult, but not impossible. The technology to launch slow Interstellar exploration missions, total delta velocities (ΔVs) of a few hundreds of kilometers per second, has been demonstrated in laboratories. However, slow interstellar probes will probably never be launched because no current organization would ever start a project which has no return for thousands of years; especially if it can wait a few dozens of years for improved technology and get the results quicker. One answer to the famous Fermi paradox is that no civilization ever launches colony ships because the colonists are always waiting for faster transportation!

Therefore, the first criteria for a successful interstellar mission is that it must returnresults within the lifetime of the principal investigator, or the average colonist. This is very difficult, but still possible. To obtain results this quick, the probe must beaccelerated to a significant fraction of the speed of light, with resultant kinetic energies of the order of 4 x10^15 joules per kilogram.Not surprisingly, the second criteria for as successful interstellar mission is cost effective energy generation and an efficient means of converting raw energy into directed momentum. In this paper, severalcandidate propulsion systems theoretically capable of delivering probes to nearby starsystems twenty-five to thirty-five years afterlaunch are defined and sized for prospective missions using both current and near termtechnologies.Rockets have limited ΔV capability because they must carry their entire source of energy and propellant. Therefore, they can’t be a probable candidates for interstellar travel. Now one might propose that why not use antimatter rockets? Well, they can’t because in current, we have no mechanism how to quarantine it?

Light Sails

Laser-driven Lightsails are not rockets since the power source remains behind and no propellants are expended. Therefore, the rocket equation doesn’t apply and extremely high ΔVs are possible if adequate laser power canbe focused on the lightsail for a sufficient acceleration time period. The acceleration,Asc , of   a laser-propelled lightsail spacecraft in meters per second is:

Asc = 2PL / Msc

where PL is the laser power impinging on the sail in watts, Ms is the mass of the spacecraft (sail and payload) in kilograms,and c is the speed of light in meters/ second. In practical units, a perfectly reflecting laser lightsail will experience a force of 6.7 newtons for every gigawatt of incident laser power. Herein lies the problem, since extremely high power levels are required to accelerate even small probes at a few gravities.The late Dr. Robert Forward in hispapers on interstellar lightsail missions postulated a 7,200-gigawatt laser to accelerate his 785 ton unmanned probe and a 75,000,000-gigawatt laser to accelerate his 78,500 ton manned vehicle. To achieve velocities of 0.21 c and 0.5 c, respectively,the laser beam must be focused on the sail for literally years at distances out to a couple of light years. In addition, the laser beamwas to be used to decelerate the payload atthe target star by staging the lightsail and using the outer annular portion as a mirror to reflect and direct most of the laser beam back onto the central portion of the lightsail,which does the decelerating. To enable this optical performance, a one thousand kilometer diameter Fresnel lens would be placed fifteen Astronomical Units (AU)beyond the laser and its position relative to the stabilized laser beam axis maintained to within a meter. If the laser beam axis is not stable over hours relative to the fixed background stars (drift <10-12 radians), or if the lens is not maintained within a fraction of a meter of the laser axis; the beam at the spacecraft will wander across the sail fast enough to destabilize the system. While this scenario is not physically impossible, it appears difficult enough to delay any serious consideration of using the large lens/long focus approach to laser-propelled light sails. The alternative approach is to build really large solar-pumped or electrically powered lasers in the million gigawatt range, where we could accelerate a decent size spacecraft to thirty percent the speed of light within a fraction of a light year using more achievable optics (e.g., a reflector 50 kilometers in diameter). Even though space construction projects of this magnitude must be termed highly speculative, the technology required is well understood and LPL systems utilizing dielectric quarter wave Lightsails could accelerate at twenty to thirty meters per second or more.

Laser Propelled Light Sail

The Magsail current loop carries no current during the laser boost and is just a rotating coil of superconducting cable acting as ballast to balance the thrust forces on the dielectric quarter wave reflector. After coast when the spacecraft approaches the target star system the lightsail is jettisoned and the Magsail is allowed to uncoil to its full diameter (80 km for a 2000 kg probe mission). It is then energized either from a onboard reactor or laser illuminated photovoltaic panels and begins its long deceleration. Example interstellar missions have been simulated using state-of-the-art optics designs and the resulting LPL design  characteristics are shown in Table below.
A constant beam power is chosen such that the spacecraft reaches the desired velocity just at the limit of acceleration with fiftykilometer diameter optics. Even though the high-powered LPL appears to meet all mission requirements, this paper explores alternative propulsion systems with potential for significant reductions in power, size, cost, and complexity.

These data show that interstellar exploration is feasible, even with near term technologies, if the right system is selected and enough resources are available. Therefore, once the technology for low cost access to space is available, the primary risk to any organization embarking on a serious effort to develop interstellar exploration/transportation is affordability, not technical feasibility.. The primary issue with respect to any of these systems actually being built is cost, both development cost and operating cost in the price of energy. As for manned exploration, there is a good idea, ride on asteroids- colonize them and go ready for interstellar mission!!