SOLAR SAIL APPLICATION TO COMET NUCLEUS SAMPLE RETURN Dr. Travis S. Taylor, Tryshanda T. Moton*, Don Robinson, R. Charles Anding Teledyne Brown Engineering, Inc. 300 Sparkman Drive Cummings Research Park Huntsville, AL 35805 Dr. Gregory L. Matloff BangsMatloff Aerospace Consulting Co. 4 17 Greene Avenue Brooklyn, NY 1 12 16 Dr. Gregory Garbe, Edward Montgomery Solar Sail Propulsion Project MSFC, TD 20 Huntsville, AL 35812 Abstract Many comets have perihelions at distances within I .O Astronomical Unit (AU) from the sun. These comets typically are inclined out of the ecliptic. We propose that a solar sail spacecraft could be used to increase the inclination of the orbit to match that of these 1.0 AU comets. The solar sail spacecraft would match the orbit velocity for a short period of time, which would be long enough for a container to be injected into the comet’s nucleus. The container would be extended from a long durable tether so that the solar sail would not be required to enter into the potentially degrading environment of the comet’s atmosphere. Once the container has been filled with sample material, the tether is retracted. The solar sail would then lower its inclination and fly back to Earth for the sample return. In this paper, we describe the selection of cometary targets, the mission design, and the solar sailcraft design suitable for sail-comet rendezvous as well as possible rendezvous scenarios. Selection of Cometary Targets However a dedicated Comet Halley Suitable for Solar Sail Rendezvous rendezvous mission was never flown by the United States. The concept of solar sail rendezvous missions has been studied for quite some Exploration of the mission-enabling time. During the 1970s, a design team at properties of solar sailing to chase and the Jet Propulsion Laboratory (JPL) rendezvous with comets continues as a discovered a trajectory that could allow a small, although important focus of solar sail to rendezvous with Halley’s research interests. New comets approach Comet at its perihelion in the mid-1980s the Sun from the outer regions of the [ 11. A rendezvous with Halley’s Comet solar system, beyond the orbit of Pluto. for that passage was looked at One of these comets shows up every few extensively by NASA scientists. years. These comets are thought to *AIAA Member, propulsion systems engineer 1 American Institute of Aeronautics and Astronautics I . typically have made only a few passes many short-period comets with by the Sun and many are making their perihelion less than or equal to 1.0 AU, first approach to the Sun. These comets which have been observed to have two are thought to have remained essentially or more apparitions [3]. The design unchanged from the early formation of effort for the Halley Rendezvous the solar system. As a result, comets and mission suggested a sail design that the matter that compose them are of very could operate at 0.25 AU or closer from high interest for scientific research the Sun for at least I year without interests [2]. appreciable damage to the solar sail spacecraft. Technology development It is very likely that a solar sail efforts on materials indicate that sailcraft spacecraft could be used to rendezvous capable of sustained operation inside 0.2 with many comets that have perihelions AU might be possible in the near future close to Earth’s orbit. Table I shows [21. Comet -Orbital -Period Perihelion Eccentricity Inclination Aphelion (Years) (AU) ---- ---- (AU) 2P Encke 3.28 0.33 0.850 11.9 4.09 107P Wilson- .oo Harrington 4.29 1 0.623 2.8 4.29 .oo 26P Grigg-Skjellerup 5.10 1 0.664 21.1 4.93 96P Machholz 5.24 0.13 0.958 60.1 5.9 1 45P Honda-Mrkos- Pajdusakeva 5.30 0.54 0.822 4.2 5.54 73P Schwassman- Wachmann 3 5.35 0.94 0.694 11.4 5.18 5D Brorsen 5.46 0.59 0.810 29.4 5.6 1 103P Hartley 2 6.26 0.95 0.7 19 9.3 5.84 3D Biela 6.62 0.86 0.756 12.5 6.19 72P Benning- Fujikawa 9.0 1 0.78 0.820 8.7 7.88 .oo 8P Tuttle 13.50 1 0.824 54.7 10.30 27P Cromrnelin 27.40 0.74 0.919 29.1 17.40 55P Tempel-Tuttle 32.90 0.98 0.904 162.7 19.60 23P Brorsen-Metcalf 70.50 0.48 0.972 19.3 33.70 12 IP Pons-Brooks 70.90 0.77 0.955 74.2 33.50 1 P Halley 76.00 0.59 0.967 162.2 35.30 109P Swift-Tuttle 135.00 0.96 0.964 113.4 5 1.70 35P Herschel-Rigollet 155.00 0.75 0.974 64.2 56.90 Table 1. Comets with Perihelion Close to Earth’s Orbit Table 1 shows that there are at least 18 (these may have been influenced by comets that approach Earth’s orbit. Nine Jupiter, although an analysis of this is of them have aphelions of 4.09-6.19 AU beyond the scope of this paper). The 2 American Institute of Aeronautics and Astronautics table also shows that 8 have inclinations ecliptic plane, which makes the comets 0-20 degrees, 4 have inclinations 21- difficult to reach in terms of energy 60.1 degrees, 3 have inclinations 61-100 requirements. Since some of these degrees, 3 have inclinations 101-180 comets are discovered only a few degrees. The average inclination for the months prior to their perihelion passage, class is 47.3 degrees. The average it becomes essential to prepare a perihelion for the class is 0.74 degrees. spacecraft for launch for this type of The average eccentricity for this comet mission within a few months notice. class is 0.844. However, high performance sailcrafts having characteristic accelerations (aJ Comet velocity at various points along greater than 5mm/s2, can reach some of its orbit and sailcraft velocity at various these comets on short notice if the orbital locations can be determined inclination of the comet’s orbit is not too according to G. R. Fowles [4]. From Eq. high and if the comet approaches from (6.49) of this reference, we easily obtain and acceptable direction relative to the an equation relating comet velocity at position of the Earth in its orbit. If such a perihelion V, to circular-orbit velocity at high performance sailcraft were ’ perihelion V, and eccentricity e as available when the new comet approaches, a ready-made instrument package could be placed on the ship. In an ideal scenario, this would be done in a high orbit, such as L4 or L5. However, Modifying Fowle’s Eq. (6.51) to find this might be accomplished by a tether perihelion velocity from perihelion solar deployment from a station that has taken separation R, and aphelion solar a few weeks to gain the necessary escape separation R1 yields energy to place it in this high orbit [2]. Sample Mission Design for Solar Sail Rendezvous, Sample Return (SSRSR) to Comet 107P Wilson-Harrington Knowing the spacecraft velocity at perihelion and the perihelion distance, Rationale for Target Choice we can equate energy at perihelion to This object is chosen for a number of energy at any other solar distance R,, reasons. One, its perihelion is at 1 AU, (assuming no orbital energy change) to which requires less spacecraft orbit calculate orbital velocity at R,,, which is adjustment. Two, it orbits the Sun every 4.29 years, allowing ample mission opportunities. Three, its inclination is 2.8 degrees, which also results in less maneuver requirements (less inclination changing). Also called Minor Planet Comet and Sailcraft Orbits 4015, this comet’s aphelion is 4.29 AU and its orbital eccentricity is 0.623 [3]. As comets approach the Sun, they have Interestingly, Comet 107P was an close to the energy needed to escape alternate target for Deep-Space 1. As an from the solar system. They typically Earth-crosser, it belongs to the class of have substantial inclinations from the 3 American Institute of Aeronautics and Astronautics objects, which sometimes strike the order of 0.1. It is the lightness parameter Earth. As a comet, its surface may that drives the capability to perform the contain biological progenitors (it is even SSRSR mission. It should also be noted speculated that primitive life may be here that all materials suggested for the found there). So a sample-return spacecraft architecture are currently mission to this object may have large available although state-of-the-art. scientific and public support. The Launch Vehicle and Departure from Earth-Space Spacecraft Design A Delta-class expendable booster is the Disc Sail: One possible architecture for recommended launcher for the sailcraft. the solar sail rendezvous-sample return We suggest that the upper stage be (SSRSR) solar sailcraft is a disc sail. powerful enough to supply an Earth- We assume a 50-kg payload, a disc sail escape (hyperbolic excess) velocity of radius of 50 m (which implies a sail area about 3 km/s. This is the same of 7,850 m2). The sail-film areal mass hyperbolic excess required to insert an density is 0.006 kg/m2, and the structural Earth-launched spacecraft into a Mars- mass factor is 0.3. We assume a sail bound Hohmann-transfer ellipse [7]. A reflectivity (REFSail) of 0.9. The more capable upper stage supplying the spacecraft areal mass density is (oSlc) 8 km/s hyperbolic excess required to 0.0 12 kg /m2. insert the spacecraft on a Jupiter-bound trajectory is, as will be discussed below, First, we calculate sailcraft lightness another option. factor, p using Eq. (4.19) of Ref. 3 s/c Sail Maneuver 1 : Inclination Change ps/c= 0.000787[(1+ REF,,,,) / Our first sail maneuver is inclination (q,J= 0.12 (4) change, using a curve-fit to Fig. 4.23 of McInnes [5] This means that the sail acceleration is 7.2 X IO" g or 7.3 X m /sec2 if the N /AT = (p,./,/0. .05) sail is oriented normal to the sun. exp[- 1.323In(RI<,-) 2.31 (5) Square Sail: A second architecture we where AI / AT is inclination change in considered is the typical square sail. The degrees per week, and R,, is the constant sail is assumed to be 100 m on a side. cranking-orbit radius from the Sun The sail areal density is the same as that during the inclination-change maneuver, of the disc. The support booms for the square sail have a linear density of 50 in Astronomical Units (AU). To alter g/m. The total lightness parameter for inclination by 2.8 degrees to match the the square sail is about 0.12 and comet's inclination takes 12.7 weeks or therefore has similar sail acceleration as 89 days. This maneuver will actually for the disc sail. take a bit longer (about 10%) since the s/c is moving at a higher velocity than Either of the above architectures will Earth's circular velocity after it departs suffice for the SSRSR mission provided Earth-space. that the lightness parameter is on the 4 American Institute of Aeronautics and Astronautics acceleration at 1 AU if it is oriented normal to the Sun. From Forward’s “grey solar sail paper “ [6], tangential acceleration can be written as Figure 1. Inclination cranking versus time It is worth noting here other possible where a = sail absorptance, Rb is sail back (mirror-like) reflectance, R, is sail target missions and mission durations specular reflectance, 8 is the sail-Sun required. Figure 1 shows the inclination aspect angle (0 degrees for sail normal to cranking achieved as a function of Sun), S is the solar flux in watts per weeks. After 100 weeks an inclination square meter, and c is the speed of light. of about 22 degrees could be reached. For a specular sail, the tangential From Table 1 this shows that possibly 8 target comet orbits could be matched in acceleration component will be very low. about two years or less. Estimating Comet Perihelion Velocity But if the sail is back-reflective, we can apply Forward’s Eq. (50) from ref. 6 to From Equation 1 where V, is comet obtain velocity at perihelion and V, is circular velocity at comet’s perihelion, ACC, = ACC,y,cs in Bcos8, (7) substituting Earth’s circular velocity as where ACC,/, is the sailcraft acceleration the circular velocity at comet perihelion, if oriented normal to the Sun. In this and using e = 0.623, we find that the case, an angle of 45 degrees between comet’s perihelion velocity relative to normal to the sail and the Sun results in the Sun is 38 km/s, or about 8 km/s a tangential acceleration of 3.25 X IO4 faster than the Earth at perihelion and 5 m/s2, which is more than enough. km/s faster than the spacecraft in its post-earth-escape solar orbit. Forward suggests that we might obtain a mirror-like sail coating by embossing the Comet Rendezvous Alternative 1: A sail reflective layer with images of Holographic Sail corner-cube reflectors [8]. This might be done holographically [9]. This rendezvous alternative assumes a tangential component of sailcraft Comet Rendezvous Alternative 2: High- acceleration after the inclination- Energy Upper Stage cranking maneuver. Elementary kinematics reveals that the spacecraft This alternative replaces the tangential can match the comet’s perihelion acceleration by sail near perihelion with velocity in 0.5 years if it has a tan 9e ntia1 an upper stage capable of leaving Earth- acceleration of 3.17 X m/s , less space at a hyperbolic-excess velocity of than 1/2 of its solar-radiation-pressure 8 km/s [4]. The sail is only used during 5 American Institute of Aeronautics and Astronautics the pre-rendezvous phase of the mission other impact from the comet’s matter for inclination cranking. Interestingly, that may potentially damage the solar this approach has been used before-for sail. The capsule will then be closed and the launches of Pioneer 10 and 11 and retracted and a sample of the comet’s Voyager 1 and 2 in the 1970’s. If this nucleus matter is returned to scientists option is selected, more time must be for examination back on Earth. devoted for the inclination-cranking maneuver. Considering the reference mission design discussed thus far, the sample Comet Rendezvous Alternative 3: return spacecraft will match velocities Optimized Sailing Application with the 107P/Wilson-Harrington comet. Matching velocities reduces the risk of This approach applies results of an particle impacts since it is likely to only optimized-sail trajectory analysis incur very minor damage and slight reported on pp 139-140 of McInnes [l]. course alterations from low velocity gas In the results of this computer and particle collisions. If the relative simulation, a sail with p = 0.05 departs s/c velocity of spacecraft to dust is high, a Sun-centered circular orbit at I AU to problems could occur similar to the a high-eccentricity solar elliptical orbit. European Space Agency (ESA) Giotto The spacecraft requires 3 orbits of the mission which intercepted Halley’s Sun to increase its semi-major axis to 3 comet in March of 1986 [IO]. Giotto AU, which we require for comet received significant course alteration and rendezvous. Since acceleration scales damage. Fourteen seconds before with increasing p we can do it with s/c, closest approach Giotto was hit by a our sailcraft configuration in 1.5 orbits ‘large’ dust particle which caused the or about 5 years. spacecraft angular momentum vector to shift 0.9 degrees. The primary concern Sample Collection for SSRSR is likely to be the possibility Previous missions describe comet of gas jets erupting from the comet with sample collection by placing solar sail course altering force. However, some of probes into orbits where they pass as the comets we are considering are close to the surface of the comet nucleus somewhat inactive, including 107P/ as the thermal capacity of the probe will Wilson-Harrington according to the allow. The system proposed here utilizes work and assessment by Marsden [ 113, a similar concept of releasing a sample “the observations suggest that the object capturing device such as a capsule is a largely inactive comet that roughly the size of a 12-ounce soda can undergoes occasional outburst.” attached to the sailcraft by a tether (possibly Spectra 1000TM). Once the The spacecraft will utilize cameras and sailcraft is within the orbit of the comet, built in digital signal processing a tether system will be spring launched techniques to analyze the surface for an toward the comet to pick up a sample of optimal landing location. The mission matter from the comets nucleus. The may last weeks, which will allow the sailcraft will be capable of releasing the Comet Mission Earth Team to make the tethered capsule and then altering its final decision of the landing site. If any trajectory to avoid incineration or any of the potential landing sites show gas 6 American Institute of Aeronautics and Astronautics jet activity, landing will be attempted in tool will rotate slowly and be pulled some other area. The soda can sized deeper into the comet nucleus as it lander will separate with the gentle rotates. The counter-rotation will release of a spring-loaded mechanism produce almost zero net torque to help and will begin to move away from the minimize the probability that the comet main body of the sailcraft very slowly. nucleus section being drilled will break A tether reel on the sailcraft will away in an uncontrolled manner. The maintain approximately zero tension as comet nucleus material is collected in a the tether reels out at a speed matching comet nucleus material bag. When the the lander’s speed. The tether will also desired size of the comet material have a fiber optic cable wound around it sample has been obtained the drilling that will enable direct optical portion of the sampling tool will be communication between the sailcraft and ejected as a volume exchange and the the lander’s main computer. The main material bag is sealed. The sampling computer on the sailcraft will use data tool ejection mechanism will also from the lander’s proximity detector to provide an external seal where it was control very small cold gas thrusters for previously housed and provides a proper comet relative velocity additional protection for the material touchdown at a selected comet landing bag. site. The current assumption is that the comet velocity matching will not leave a In order to obtain separation of the significant comet rotational component lander from the comet the lander will relative to the sailcraft and subsequently release the Gecko skin landing pads and the lander. leave them on the comet in much the same way the Lunar Excursion Modules The soda can lander will adhere to the left the landing gear behind. The initial comet by implementing synthetic Gecko separation from the comet is powered by skin coated landing pads. Geckos are stored energy in a spring mechanism. small reptiles whose feet have hundreds The lander computer will send a signal of thousands of hair-like “setae” with to sailcraft indicating a successful hundreds of submicroscopic pads separation and the sailcraft will then reel (“spatulae”) at each seta tip, which in the tether. The maximum tension appear to cling by van der Waals forces expected on the tether will be due to the to almost any surface even while under force necessary to accelerate the conditions of vacuum and particulate approximately 10 kg lander and full contamination. Experiments and sample material bag to the sailcraft analyses have been conducted that velocity. Development of a sample suggest the skin can be synthesized and storage system for the flight back to may deliver adhesion forces of as much Earth must be considered if ice particles as 10 N per 100 mm2 [12]. The lander are to be maintained. A detailed analysis will simply bump into the comet nucleus of the environmental requirements and the pads will adhere to the surface. necessary to maintain the sample should be conducted, but is beyond the scope of The comet nucleus is sampled via a this paper. “sampling tool” which is a counter- rotating drill-like system. The sampling 7 American Institute of Aeronautics and Astronautics ~ References Earth Return 1. C. R. McInnes, Solar Sailing, The sailcraft will fly the sample back to Springer-Praxis, Chichester, UK Earth by undoing the orbit and (1 999), pp.4- 1 I ; pp 139- 140. inclination changes discussed 2. Wright, J. L., Space Sailing, previously. Once the sailcraft Gordon and Breach Science approaches Earthspace it is possible that Publishers, PA, (1992), pp.42-45. it can survive an aerobraking encounter 3. R. P. Binzer, M. S. Hanner, and with the Earth whereas its sail is used as D. 1. Steel, “Solar System Small the aerobrake ballute. Therefore only Bodies,”A. N. Cox ed., Allen’s inclination-cranking and minor Astrophysical Quantities, 4‘h. ed., maneuvers may be required on the Springer-Verlag, NY (2000) Earthbound leg. The aerobraking 4. G. R. Fowles, Analytical maneuver could leave the sail in an Mechanics, Holt, Rinehart, and elliptical orbit around Earth with its Winston, NY (1962). periapsis at a low enough orbit that the 5. N. C. Wickramsinghe, F. Hoyle, Shuttle could capture the sample and S. Al-Mufti, and D. h. Wallis, return it to Earth. It is also possible that “Infrared Signatures of if the periapsis is too high for Shuttle Prebiology-or Biology,” in capture that the soda can lander could be Astronomical and Biochemical launched toward Earth using a stored Origins and the Search for Life energy spring or cold gas thruster. The in the Universe, ed. C. B. tether would be reeled out in order for Cosmovici, S. Bowyer, and D. the Shuttle to capture it. This should be Werthimer, Editrice Compositori, investigated further. Bologna, Italy (1997), pp 61-76. 6. G. L. Matloff, Deep-Space Conclusion Probes, Spri nger-Praxi s, Chichester, UK (2000). We have shown here a concept mission 7. R. R. Bate, D. D. Mueller, and J. architecture for a comet nucleus sample E. White, Fundamentals of return using a solar sail rendezvous. A Astrodynamics, Dover, NY small tethered soda can spacecraft could (1971), pp. 362-365. then be used to capture a sample of the 8. R. L. Forward, “Grey Solar comet’s nucleus and return it to Earth. Sails,” J. Astronaut. Sci., 38, The sailcraft would return the sample to 16 1 - 185 (1990); also published Earthspace whereas the sail is used for as AIAA 89-2343. an aerobrake. The sample could then be 9. G. L. Matloff, G. Vulpetti, C. returned to Earth via the Space Shuttle. Bangs, and R. Haggerty, “The The analysis presented here suggests that Interstellar Probe (ISP) : Pre- a solar sail spacecraft is ideal for the Perihelion Trajectories and comet nucleus sample return. More Application of Holography, detailed studies for the mission concept ”NASA I CR- 2002-21 1730, should be conducted. NASA/MSFC, Huntsville Alabama (2002). 8 American Institute of Aeronautics and Astronautics . .. ’ . 1 10. D.R. Williams, http://nssdc.gsfc.nasa.gov/planeta ry/giotto. html, 2003. 11. B. G. Marsden, http://cometograph y.coni/pcornet s/ 107p.h tml, 2003. 12. Autumn, K., et al. (2000). Adhesive force of a single gecko foot-hair.” Nature, 405, pp. 68 1- 685. 9 American Institute of Aeronautics and Astronautics