\'I) Pergamon Acta Astronautica Vol. 35, Suppl., pp, 171-179. 1995Copyright @ 1994 Elsevier Science Ltd0094-5765(94)00182.0 Printed in Great Britain. All rights reserved
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SOCCER (SAMPLE OF COMET COMA EARTH RETURN) MISSIONKuninori Uesugi*, Jun'ichiro Kawaguchi* and Peter Tsou**
* Institute of Space and Astronautical Science3-1-1 Yoshinodai, Sagamihara, Kanagawa 229 Japan
** Jet Propulsion Laboratory4800 Oak Grove Drive, Pasadena, CA. 91109-8099 U.S.A.
SOCCER, Sample Of Comet Coma Earth Return, is a joint effort between ISASand NASA, and its major scientific objective is to collect intact dust samples from thecoma of a comet and to bring them back to the Earth. However, in order to realizethe SOCCER mission, many scientific and technical requirements and restrictionsshould be considered. The ISAS/NASA SOCCER Joint Working Group (JWG) hasstudied technical feasibility for the SOCCER mission to several comet candidates inthe 2000 time frame, and the comet Kopff has been chosen as a baseline target withWild-2 as a backup. The encounter speed is 8.8 km/s for Kopff and 8.7 km/s forWild-2 respectively. In these cases, however, required C3 and/or ~V exceed thelaunch capability of ISAS's M-V launch vehicle, and the SOCCER mission shouldbe conducted as a "Geotail-type" international cooperation project using a Delta-IIclass launch vehicle . In this paper, the results of feasibility study, which include atypical mission scenario and a draft design of the spacecraft system, are presented.
The intact capture and return of cometary coma material, both dust and volatiles, has anadvantage over orbiter and rendezvous missions in that the captured material can be made availableto all complex, sophisticated laboratories here on Earth. Samples, if properly stored andpreserved, can also be examined by analytical techniques. Lander or rendezvous type of samplereturn missions require rather complex spacecraft, intricate operations, and costly propulsionsystems. By contrast, it is possible to take a highly simplified approach for sample capture andreturn in the case of a comet. This can be accomplished by an Earth free-return trajectory to thecomet, in which passive collectors intercept dust and volatiles from the cometary coma. Bringingsamples back from within the zone of parent molecules of a known comet will provide valuableinformation on the comets, will serve as a rosetta stone for the analytical studies of interplanetarydust particles for the last two decades, and will provide much needed samples for the analysiscommunity.
The Halley sample Earth Return (HER) Mission was the first comet coma sample returnmission studied by the Jet Propulsion Laboratory (JPL) in 1981. The Sample Of Comet ComaEarth Return (SOCCER) was initiated with this proposal team associated with NationalAeronautics and Space Administration (NASA) and the Institute of Space and AstronauticalScience (lSAS) in Japan. This effort makes use of the unique strengths and capabilities of bothcountries in realizing this important quest for the return of samples from a comet, and in sharingthe cost and the science findings. SOCCER has two primary science objectives: nucleus imaging,and dust and volatiles sample return and it can achieve first class science by being the first missionto bring back samples from a known comet, and by obtaining high-quality close-up images of acomet nucleus.
172 Low-cost planetary missions
COMET COMA SAMPLE RETURN BACKGROUND
Interest in cometary missions began in the early 1970s. The first mission was the HalleyFlyby with a Probe and Tempel Rendezvous (HFPTR) mission1 proposed by JPL. Thedevelopment of cometary flyby sample return technology was triggered by the JPL's Halley EarthReturn (HER) Mission". Due to the 70 km/s encounter speed with Halley's comet, atomizedsample return rather than intact sample return was considered for HER. The first comet comasample return mission with intact capture' was jointly proposed by JPL and Goddard Space FlightCenter (GSFC) as a NASA mission for the Planetary Observer Program and proposed direct re-entry via a Discoverer Capsule. Mission costs from GSFC and JPL were compared", Using thespare Giotto spacecraft, JPL proposed Giotto US jointly with the European Space Agency (ESA).Based upon the Giotto II concept, an ESA Comet Atmosphere Encounter and Sample Return(CAESAR)6 was studied in 1986. Since 1987, low-cost flyby sample return missions to comets,SOCCER7, have been jointly studied in U.S. and Japan .
NASA's interest in comets has been reflected in four major JPL efforts beginning with theHFPTR, Halley Intercept Mission (HIM), HER, and finally the recently canceled Mariner Mark IIComet Rendezvous and Asteroid Flyby (CRAF) missions. While ISAS has demonstrated itsunique ability to develop low-cost spacecraft for a variety of missions (SAKIGAKE & SUISEI toHalley, HITEN to the Moon, GEOTAIL to the distant geomagnetic tail). The dominant video andcamera industries in Japan give ISAS a strong basis for developing an imaging camera system.Since 1981, JPL has developed the enabling technology for flyby sample return - intact capture ofhypervelocity particles", and validated techniques for the capture of volatilesl". NASA has anoperational Shuttle capable of retrieving objects in low Earth orbits, and its Deep Space Network(DSN) can track deep space signals around the clock. JPL has mounted sophisticated missionsthroughout the solar system. ISAS has its own Deep Space Center at Usuda, Nagano, Japan(UDSC) equipped with a 64 m antenna, and both agencies have demonstrated their capabilities toconduct aerobraking (HITEN by ISAS and Magellan by NASA). Consequently, both NASA andISAS have common interests in exploring comets and unique capabilities, and SOCCER has beenconceived as a joint mission taking advantage of each agency's strengths. In a tight fundingenvironment, the total cost of a comet flyby sample return mission can be jointly shared, makingthe acceptance of such a mission easier for both countries.
SOCCER Joint Workim: Group
The first meeting of this current ISAS/NASA proposal team was held in Japan-! in 1987.The team was formally installed as a joint study group12in 1988, subsequently promoted to a jointworking group, and met again at ISASI3. The second working group meeting was held jointlywith the Japan - U.S. Workshop on Missions to Near-Earth Objects!" in 1991, while the thirdworking group's meeting 15 was held in May of 1992.
The working division of responsibility between NASA and ISAS follows designatedcapabilities. The imaging system and radio science will be provided by ISAS, while NASA willprovide the major part of sample collection system. The spacecraft and a part of sample collectorwill be designed and built by ISAS, and launched by an ISAS M-V, 3 stage solid propellant rocketor a NASA medium expendable launch vehicle (MELV) depending on required C3 (escape energyfrom Earth) for the selected comet to be aimed at. The return samples will be retrieved by theShuttle. Operations will be led by ISAS, supported by NASA in critical operation phases such aslaunch, encounter with the comet, Earth insertion, aerobraking, and retrieval.
Low-cost planetary missions
SOCCER MISSION DESCRIPTION
Possible comet candidates for SOCCER in the 2000 time frame include Churyumov-Gerasimenko (2002), Wirtanen (2002), Finlay (2002), Du Toit - Hartley (2003), Wild-2 (2003)and Kopff (2002), and important considerations in designing a trajectory for SOCCER are a lowencounter speed to enhance intact capture of dust samples, the required escape energy from Earth(C3) , the required velocity change for Earth return, and the need for a dependable trajectory of thetargeted comet. Based upon these four considerations, the 2002 perihelion encounter with CometFinlay (2000 launch) is the best candidate. Taking into account the launch schedule of ISASmissions, however, it seems difficult to launch the spacecraft to Finlay in 2000. Then, the 2002encounter with Comet Kopff (2001 launch) has been selected as the baseline comet. CometKopff, known since 1906 and observed at 13 apparitions, has a well determined and relativelystable orbit with perihelion just near the Mars orbit (1.572 AU) and a revolution period of 6.43years. As known once selected as the target for the CRAP mission, Kopff has been relatively wellobserved and recognized one of the comets most dusty and of high gas production rates. A plot ofthis baseline trajectory to Kopff is shown in Figure 1. The encounter speed for this trajectory is8.8 km/s, which is within the range of speeds for which successful intact capture has beendemonstrated (i.e., up to speeds of 13 km/s),
AV6-18-2003 ~.;:;,472m1SeC~~. SU EARTH
--- ------- -------,.~~~------------ -TRAJECTORY LAUNCH 11.11.2001
RELATIVE TO FIXED RETURN11-11-2004SUN-EARTH LINE ~15.5 krrr1sec2
KOPFF FLYBY11-21-20028.8 kmlsec
PHASE ANGLE 100
Fig. 1 Baseline Trajectory to Comet Kopff
Ground and space telescopes would begin the tracking phase to acquire Comet Kopffbefore launch and continue after launch to refine Kopffs orbit. After payload integration andtests, SOCCER will be launched by a MELV on November 11, 200I at Kennedy Space Center.
174 Low-cost planetary missions
The kick motor will be ignited after the ejection of the second stage motor of the MELV and willinsert SOCCER into the heliocentric trajectory needed to encounter comet Kopff. During the pre-encounter cruise phase the solar cell paddles will be deployed and the spacecraft placed into threeaxis stabilization mode. During cruise, the covers of the sample collectors will be kept closed anda high gain antenna will be kept oriented to Earth. A couple of months before encounter, thecamera will initiate the pre-encounter observation imaging science and optical navigation imagingas well to aid in precise orbit determination. Final targeting from optical navigation update will beperformed one day before the encounter (November 21, 2002). Before the encounter phase,which lasts for approximately one hour, the covers of the sample collection system will be opened,volatiles capture getters will be deposited, solar paddles will be retracted, and the spacecraft will beplaced into spin stabilized mode.
SOCCER's two primary science systems require a very close encounter with the cometnucleus to achieve good close up imaging and to capture cometary samples with minimumprocessing. A target encounter distance of 10 km from the comet nucleus is highly desirable.Detailed study, involving orbit determination simulations (including the effect of center-findingerror) and assessment of the coma environment, is necessary to determine the minimum feasibleencounter distance. During the final approach, nucleus images will be acquired at the maximumrate. Some selected, compressed images will be processed and transmitted in real time. After thesealer getters are deposited and the mission begins the post-encounter observation phase, thesample collector covers will be closed. The solar paddles will be redeployed and observatoryimaging will resume after the spacecraft replaced into 3-axis stabilization mode. The spacecraftwill then commence the trip back to Earth. A deterministic maneuver of 472 mls will be performedon June 18,2003. After having completed two orbits around the Sun in three years, the spacecraftreturns to Earth on November 11, 2004.
o Spin stabilizedo Paddle Retractedo Dust Collector Deployed
Cornel Encoynter Phase
Earth Return Phase & Aerohrakjng Phaseo 3-Axis Stabilizedo Paddle Deployedo Dust Collector Retracted
o 3-Axis Stabilizedo Solar Cell Paddle Deployed
Cry ising Phase
Spacecrart Separationo Spin Down -> Rate Dump
o Spin Stabil izedo Active Nutation Control
Fig. 2 Baseline SOCCER Mission Scenario
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After a final trajectory correction maneuver, an 1.274 krn/s Earth insertion burn isperformed at an altitude of 200 km bringing the SOCCER spacecraft into a highly elliptical orbit.Aerobraking is subsequently used to circularize the orbit at an altitude low enough to allowrendezvous with the Shuttle. Before aerobraking is initiated, a 7 mls maneuver will be executed atan apogee of 50,000 km to reduce perigee to 120 km. Attitude of the spacecraft will bekept in 3-axis stabilized mode during the aerobraking phase, which lasts approximately 7 months. Then, an109 mls orbit circularization bum will insert the spacecraft into the Shuttle orbit. After sightingSOCCER, the shuttle will make final adjustment for an Extra Vehicular Activity to attach theRemote Manipulator System to the grapple. After grapple attachment, the sample collectionsystem will be separated from the spacecraft and placed into a refrigerated container for Earthreturn. On reaching the ground, the entire container will be moved into the laboratory fordeintegration, sample examination, and sample distribution. The baseline mission scenario isillustrated in Fig. 2.
All flight hardware handled by the Shuttle is subject to stringent safety requirements. SinceSOCCER will not be launched by the Shuttle, and may not be designed to meet Shuttle launchrequirements. Furthermore, the retrieval of a spacecraft into the cargo bay of the Shuttle after thecraft has been in deep space for three years would have severe safety, and thus, cost implications.Consequently, it is cost effective for the Shuttle to retrieve the sample collection system only.
SOCCER SCIENCE PAYLOADS
Each part of the SOCCER science payload serves a dual function, science and engineering:the imaging camera will perform optical navigation for comet locating and encounter targeting; thesample collector will serve as a dust shield for the spacecraft during encounter, and, of course, theradio transponders will serve for the spacecraft communication as well as for radio science.
SOCCER ImaLdng System
The closest image of a nucleus was obtained by Giotto, a spin-stabilized spacecraft. Thereis a need for images of much higher resolution. Though the SOCCER spacecraft should be alsospin stabilized during the encounter phase, a charge coupled device (CCO) camera can achievefiner resolution of the dynamics of the comet nucleus surface than ever obtained. The high-resolution imaging of the comet coma and nucleus can be achieved in both the pre- and post-encounter phases, in which the spacecraft is three axis stabilized. The goals of the SOCCERimaging system are:
1. Obtain nucleus surface images at the closest distance with fine resolution,2. Obtain observatory, wide coverage of large-scale dynamic coma phenomena,3. Obtain pre- and post- perihelion images of the comet coma development at regular and frequent
intervals beginning several months before encounter and after encounter.
The SOCCER camera will be an improved version of a solid state camera designed forseveral ongoing ISAS flight programs. These include the PLANET-B mis...