Project Longshot to Alpha Centauri - part 5

Wednesday, June 18, 2008

Looking toward Sol from Alpha Centauri in Celestia
Looking toward Sol from Alpha Centauri in Celestia

Now done 50 pages out of 74 of Project Longshot, a proposed mission to Alpha Centauri, the closest star system to our own. The last parts of the project paper are admittedly not as much fun, with a lot of numbers which takes more time than the rest, but it should still take no more than two or three days.

3.3 Instrumentation

3.3.1 Instruments

The instruments to be carried on board the probe head are as follows:

IR Imagers
Visual Imagers
UV Telescopes
High-Energy Particle Detectors
Astrometrical Telescopes
Wide-Band Spectrophotometers
Solar Wind Plasma Analyzers
Communications Lasers

Three of each item will be carried on the spacecraft for triple redundancy.

The total weight of the instrumentation package (including everything listed, except the communications lasers) is estimated at not more than 3 metric tons. The estimated weight of the communications lasers is 2 metric tons. The peak power requirement of the instruments is estimated at 300 kW.

The justification for the visual imagers is obvious. Everyone will want to "see" what another star system looks like. In addition to providing important scientific information, a picture will be worth a thousand words or a thousand pages of numbers when it comes to obtaining funding for follow-up missions to other star systems.

IR Imagers and UV Telescopes will provide the first exact data on the characteristics of stars other than our own. Also, one must not forget the possibility that the Alpha Centauri system contains planets. These instruments could also provide data on the radiation and thermal environment of any planets in the system.

The High-Energy Particle Detectors are one of the few types of instruments which will be active during the transit to the objective system. Hard data on the energy level and density of such particles could provide insight into the origin and eventual fate of the universe.

The Astrometrical Telescopes will be the backbone of the mission. By providing the data to accurately determine the distance to the further stars, these instruments will advance the study of stellar characteristics immeasurably. The only limit to this aspect of the mission will be the endurance of the spacecraft. (The section on the objective system contains an explanation of how this will be accomplished.)

The Wide-Band Spectro-photometers will determine the composition of the stars and any planet sized bodies which the system may contain.

The Magnetometers will also be in use during the entire life of the probe. These instruments will provide extensive data on what should prove to be the very interesting magnetic field of a trinary star system. Also, they will provide the first hard data on the galactic magnetic field and how it interacts with the magnetic fields of our own solar system and the Alpha Centauri system.

The Solar Wind Plasma Analyzers will also provide some scientific "firsts". While the composition of the Sun's solar wind is already known, these instruments will accurately determine how far the wind extends into inter-stellar space. This will also be done for the Centauri wind, as well as determining its composition. Also, the close binary pair of Alpha and Beta Centauri should exhibit a very interesting pattern where their solar winds interact.

The communications lasers, in addition to performing their obvious function, will at the same time provide data on extremely long-range laser communication and the extent of spreading losses caused by the interstellar medium.

3.3.2 Instrumentation Configuration

Two possible configurations for the mounting of the instruments were considered. Both consisted of three booms attached to the probe body spaced at 120 degree intervals with each boom supportinga complete instrument package, but one was dynamic and the other static.

The dynamic boom configuration was designed with the intent of retaining the forward particle shield as a structural and operational part of the probe body after orbit is achieved in the target system. The particle shield was to be infused with pipes to provide additional radiating area for waste heat. The shield could also be used to house elements of the probe's central processing units. The major advantages of the dynamic boom configuration are its additional cooling capability and the additional shielding which would be provided in-system for the body of the probe. The major disadvantage is that the mobility of the booms would have to be maintained for the length of the mission. This configuration would require movable mounts at the base of each boom capable of handling the large torques caused by moving the boom. These torques would also pose an additional problem for the probe's attitude control system. Furthermore, the advantage of a larger cooling surface would be offset by the added thermal control complications.

The static boom configuration, (see Fig. 3.3a) was designed to discard the front particle shield upon approach to the target system. The instrument booms will be firmly attached to the probe body and the only movable parts will be the individual instrument mountings. The major advantages of the static boom configuration are the lower number of parts expected to move after the long interstellar transit, and the lower final mass of the probe in-system. The major disadvantage of this design is the need for pyrotechnics to eject the shield after transit. All things considered, the static boom design was adopted, mostly due to its greater reliability.

Fig. 3.3a

3.4 Communications System Design

The major challenges for the communications system of the interstellar probe both occur when the probe enters the target system at a range of 4.3 light-years, or 4.109 x 10^16 meters. This is the maximum transmission range; a fairly high data rate must be maintained, since all probe instrumentation is returning data. The only type of communications system capable of the necessary directivity and data rate is a high-power laser using pulse code modulation (PCM).

Low background noise from the target system is necessary for a low power level, so a laser wavelength of 0.532 microns was chosen. Radiation of this wavelength is almost totally absorbed by the outer atmospheres of K and G type stars, leaving a hole in the absorption spectrum (no transmitted radiation). Laser radiation of this wavelength can be produced by a frequency-doubled diode-pumped YAg laser with an optical attachment to provide a large initial aperture.

The transmitter aperture is 2 meters in diameter with receiving mirrors of 24 meters diameter. The spreading angle is 1.22*lambda divided by the aperture diameter, or 3.25 x 10^-7 radians (0.67 arcseconds). At 4.34 light-years, the spreading results in a footprint radius of 13.4 million kilometers, 8.9% of an Astronomical Unit (AU). Both the pointing accuracy of the laser mount and the attitude determination capability of the probe must be within 0.067 arcseconds, so very low error laser mounts and star trackers will be used.

A total input power of 250 kilowatts is needed for each laser that is transmitting. With an assumption of a 20% lasing efficiency, the transmitted power is 50 kilowatts. If the power is distributed isotropically over an area of 5.64 x 10^20 square meters (the area subtended by the laser bean when it reached Earth), the power density is 8.87 x 10^-17 watts per square meter, or 222 photons per square meter per second. For a 12 meter radius receiving mirror (area of 452.4 square meters), the received power level is 4.01 x 10^-14 watts, or 100,000 photons per second. Using the assumption that a digital pulse 'on' level is 100 photons, the receiver sees 1000 pulses per second. A data rate of 1000 bits per second is low. Note that this rate is the minimum because the transmitter would be at maximum range. If extremely reliable lasers are used, each transmitter can operate at a slightly different wavelength, so the data rate would be up to six times greater depending upon the number of lasers used.

The communications system would use six 250 kilowatt lasers. Three would be placed on the outside of the fuel tanks with the star trackers for communications during the acceleration phase. Three more lasers would be attached to the probe head for communications during the deceleration and in-system phases of the mission. The receiving mirrors would be in geosynchronous orbit about the earth in a constellation of several mirrors with a central node serving as a relay station to TDRSS and the ground.


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