Asteroids and comets exert such a small gravitational force that it is not practical to survey them from orbit. One must instead continuously accelerate using maneuvering rockets to move around the surface. A space exploration craft in three parts connected by lightweight cables can survey asteroids and comets, and deploy landers, without requiring the large thrusters and the continuous depletion of fuel required by a single craft. The spacecraft is deployed by spinning up from a compact configuration using low thrust jets, and then maintain surveying orbit without any major expenditure of energy. The triangular tether arrangement is stable, but care must be taken in changing orbits and with deploying and recovering samples, as can be demonstrated with a simple simulation. Even 100 km long tethers occupy a low payload fraction

Stephens, Richard B.

English

NASA Technical Reports Server

q°
N91-22164
EXPLORATION OF PLANETESIMAL$ fly A TRIPARTITE TVTHERED
SPACECRAFT
l_ichard .13..Stephens
General Atonfics ':i
San Diego, CA 92138
ABSTRACT
Asteroids and comets exert such a small gravitational force that it is not practical to stu'vey
them from orbit. One must instead continuoualy accelerate using maneuvering rockets to
move around the surface. A space exploration craft in three parts connected by lightweight
cables can survey asteroids and comets, and deploy landers, without requiring the large
thrusters and the continuous depletion of fuel r_luired by a single craft.
a) b)
Figure 1: Tripartite tethered spacecraft a) using thrusters and paying out line to spin-up from single
unit to b) spinning configuration up to 10t)km across able to indefinitely survey the surface
of a planetesimal.
The spacecraft is deployed by spinning up from a compact configuration using low thrust
jets, and then maintains surve.__ngorbit without any major ex-,'nditure of energy.
The triangular tether arrangement is stable, but care must be taken in changing orbits
and with deploying and _covering samplers, as can be demonstrated with a simple simula-
tion. Even 100 km long tethers occupy a low payload fraction.
BACKGROUND
An important goal of our space program is m understand t.hec, igins ar.,dev.Aution of
our solar system. The planetesimals -- the comets and asteroids, which litter the system
are some of its oldest, least modified p_,eces,and so it is important to study them and re-
trieve samples.
Though they are tiny relative to the planets there axe still some sizeable pieces (ref. 1);
more than 500 main belt asteroids are known with diameters from 40 up to 600 km. Near
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Earth, most are small objects with diametersup to 10 kin. The iarg_stby far is 433 Eros 1
with dimensions 18x36 kin. There are b,-," --.zito b¢ thovsands of earth applx)achingas-
tezoids w_.thdiat_t,,wsnear_ 1 kin, and n... times thatnumberin the Main Belt. The size
of comets arc less weUknown; their solid nucleus is obscured by evaporating gases and
glowing plasma.. The recent interception and investigation of comet Halley. one of the :
biggest, showed its core to be potato shaped 16 km X 9 kin but its density is only 100 to
300 kglm3 (ref. 2).
The gravitationalattractionof even the largestasteroids is miniscule, and that of the
comets, vdth their low density, is completely insignificant. A 10 km diameter spherical
asteroid(they are actuallyvery irregular),with density 7x103 kg/m3 andmass 3.7x1015 kg
has a gravitationalattraction10-o thatof Earths at its surface. Orbitalvelocity at the sur-
face of such a rock is only 7 rrdsec,and that of a 2 km diameter comet with density 102
kg/m3 is about0.1 m/see.
These numbers are so low that it is impractical to use grav,iy to orbit around such a
body while studying it. Surveyingplanetesimals requiresthe cor_tinuoususe of thrusters.
That energy rtxluir_mcntis a severe problem in spacecraft design, especially for comets
which should be.studiedfor a large fractionof an orbit to observe their structuralanddy-
namicaJevolution asthey approachthe Sun.
PRESENT SURVEY PLANS
Configuration
The Comet Nucleus Sample ReturnMission(rcf. 3) is not yet well defined, but envisions a
space craft of around 1000 kg (600 ?_Gbody, two 150 kg sampler/landers, and a return
capsule). The samplers will return cores from the comet, while their lander will monitor
the surface activity for an extended period. The objective is to return the data and some
unmodified cornet matrix to Earth for study.
Proc_.dure
The exploration strategy after rendezvous is: global characterization from a distance of 200
krn for six days, one day to transfer to '_50 km distance above a specific surface for high
resolution pictures, then transfer to 100 km distance to wait for landing instructions. On a
landing command, move in to ld kin, wait until the proper surface is underneath, and use
:hrusters to maintain 'he spacec,_t in a forced synchronous orbit wi_htkat patch of ground.
From this position, the _pacecraft !aunches the sampler/lander and _kes high resolution
pictures. After recovery of the sampler,,the spacec=aftretreats to 100 km to await decision
on a second landing site.
Limitations
The constant maneuvering described above is made.necessary by the dust and gas hazards
near an active comet nucleus. "To provide the additienal thrust required for the 'forced-
synchronous' orbit, it will almost cert_nly be necessaxy to provide an auxiliary chemical
propulsion system on the spacecraft." This propulsion system does not yet exist, and is
one of the critical technologies required to enable CNSR (ref. 4).
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1991012826-342
TETHERED ORBIT SURVEY
Configuration
A radical approach to this maneuvering problem is to divide th_ space_;raft into three parts,
tie them together with thin filx'J,_, and spin them. Once this tripartite spacecraft is spinnin_
fast enough for stability, no further energy is required to maint&in their orbit. It can be ma-
neuvered over the target comet and n._oMtor it from all sides for extended periods with no
further expenditure of maneuve:ing enea-g'y.
These three parts need not be the same mass or have ',.hesame instruments; a configu-
ration consisting of ¢o module types is
A _ overall control, Earth communications, retta'n pod, maneuvering thrasteL atti_
tude jets, spin-up thruster
B --- local control, sampler_ander, photographic exiuipmem, local communications
equipment, attitude jets, spin-up thruster
i B'_ local control, sampler/lander, photographic equipment, local corranunications
' equipment, attitude jets, spin-up thruster
While the spacecraft is together, the attitude jets would combine to give 6 orthogonal pairsr
of orienting jets. When separate, the pair of jets on each unit would orient the spin-up
thruster on each unit as required for spin-up, spin-down, or ,'x-ansversemaneuvering.
The units would be connec{ed with high strength fibers. Studies of tethered probes for
low earth orbit or space shuttle experiments have suggested a variety of materials. Kevlar
29 have been suggested by a number of researchers beth bare (re,*'.5), and metal coated for
protection and communication purposes (ref. 6). A polyethylene (Spectra 1000) was pre-
ferred by one for its temperature stability(ref. 7). 100 kg of Spectra 1000 -'.,ould be suffi-
cient to provide three 100 km tethers with 850 N breaking strength. That is orders of
magnitude greater than needed for this system. Orbiting )htr with a tether length of 100 km
(an altitude of 57.7 km) requires an acceleration < 0.02 N, or .002 of Earth's gravity. If
each part of the spacecraft weighs 500 kg that only requires a tension of 0.5 N in the 100
km fibers. The fibers have a strength of 3x109 N/m 2 = 30 Nl(0.1mm) 2. This 0.1 mm dia
fiber has a safety margin of 100 X! Three 100 km ler_gths weigh only 3 kg (polyethylene).
A thicker fiber may be desirable to limit stretchiness. These fibers have a tensile modulus
of 1.7x10 ll N/m 2 = 1.Tx103 N/(0.1mm) 2, so a 0.5 N force stretches the fiber by .03%, or
30 m in 100 km.
Procedure
This tripartite spacecraft would be launched aod proceed to tt,e rendezvous as a single unit.
On rendezvous with the comet, the parts would spin up to a low speed and then undock so
that the centrifugal force would help control the attitude of each unit. The. att!ude jets on
each unit would align the thrusters in the plane of rotation and they would then spin-up the
system further while the reeling mechanisms paid out line until they reached the desired
configuration -- 100 km separation rotating once per hour, for instance, To limit bounc-
ing, the thrusters and reels would combine to maintain a constant tension on the lengthen-
ing tether. The reels don't need any significant power for this maneuver. Once the desired
cendition is reached, the attitude jets reorient the thrusters so they can maneuver the spin-
ning system around the comet. They can then study the comet indefinitely without using
thruster power.
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If a closer look is de,tired, the thrusters are aligned in the pla_e of rotation again, but
opposite to the rotation direction, and the thrusters are fired to spi,,.-down while the reels
take in tether; again, ti_c two actions combined to maintain a constant tension on the tether. ._
In thi s r.aea _.k. _.1 ..... .-=
I i
t If a sample is required, one of the sampler/landers is launched from as close to the sur-
face as possible. It is not necessary to remain in forced synchronous orbit because one of
the three nodes will zlways be visible to and in communication with the lander. The system
will orbit asymmetrically because of the weight loss (about 150 kg out of 500 kg); the
lighter module farther from the center of rotation. On recovery of the sampler, the weights
and hence the orbiting configuration will change again.
When the mission is over, the thrusters are aligned to spin-down and the tether is
reeled in. Collected samples are transferred to the return pod, and then the A module sepa-
rates from the oth,_,'two modules and returns home.
Dynamics
Algorithms for stably paying out tether have been investigated for leashed experiments in
Earth orbit (ref. 8), though only up to lengths of about 20 km, That dynamics problem is
difficult because leashed experiment_ have to come to the end of their tether with zero ve-
locity and acceleration simultaneously, or the experiment slowly bounces _fllthe way back
to the experimenter. It is not so hard for a rotation triangular system. We have set up a
simple simulation program to model its kinematic behavior. We found it very stable if, 1
during spin-up, one lets the fiber pay out at a rate so that the tension in the tethers remains 1about constant. Errors onl:,, cause some jello-like wobbling around the equilibrium trian-gle. One prevents that by turning on and off the thrusters slowly relative to the time re-quired for a wave to travel the length of the tether. That time seems to be a few minutes for
i00 km tethers(ref. 9).
Release and recapture of the sampler/lander modules causes an abrupt change in the
mass of the modules, m=dcould cause instability problems. Our simulation shows the need
to use thrusters in a radial direction to compensate for mass loss and to smooth out the
transition.
Energetics
For stated conditions, need to accelerate 1000 kg to about 300 km/hr. That is 1/2 x 10"_N-
sec per spin-up. If spin-up and spin-down are done maintaining a constant tension in the
tether, the reels would require no net power; they would be electric generators during spin- '_
up and use their stored energy to reel in the tether during spir: '_.awn. Motor, generator,
and battery inefficiencies would cause some net power drain. That power could be pro-
vided by solar cells.
Limitations
Tether malfunction would abon the mission. Impact by a micrometeeroid, breaking a
tether is the most likely cause, but the probability may be hard to estimate; there is only
sparse data on the micrometeoroid density outside the Earth's moon orbit. Making the
tether of a braided tow reduces that problem (ref. 10). There is also substantial ionized gas
-- presumably corrosive, in the vicinity of comet which could weaken the tethers.
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Mechanical problem in the reels would also be fatal. Designs have been studied for and
will be tested in low Earth orbit (ref. 11). .
SUMMARY I
We have sugges:ed a tripartite spacecraft connected by long lightweight tethers as an )alternate approach to sarveying microgravity planetesimals -- _:,mets and asteroids. The Imass of the tethers is an small fraction of the total spacecraft mass. A configuration and
exploration plan ha,,'e been sketched out. This configuration is stable _iuring spin up and
spin down operations, but there may be problem_ stabilizing it during release and recapture
of sampler/lander modules. In addition, there may be a problem insuring that the tethers
are not broken by micrometeoroids.
REFERENCES
1. Planetary Exploration through Year 2000, Part I: A core program, (Govt. Pr,inting
Office, Washington, 1986) p. 106.
2. H. Balsiger, H. Fechtig, and J. Geiss, "A close look at Halley's Comet," Scientific
American, 259 #3, 96 (Sept. 1988).
3. Planetary Exploration through Year 2000, P_,r: H: An augmented program, (Govt.
Plinting Office, Washington, 1986) p. 113.
4. ibM., p. 208.
5. W. Seyboldt, J. Puls, W. Hallman, W. Ley, W. Wienss, "Tether Interests and
Activities within Germany," Tethers in ,Space: Toward Flight, proceedings from the
3M Int. Conf. on Tethers in Space _ Toward Flight, San Francisco, CA, 17-19 May
1989 (Am. Inst. of Aeron. & Astron., Washington, 1989), p. 9.
6. R.F. Orban, "Advances in space tether materials," ibid., p. 333.
7. J.K. Harrison and C.C. Rupp, J.A. Carroll, C.M. Alexander, E.R. Pulliam, "Small
Expendable-Tether Deployer System (SEDS) Development Status," ibid, p. ',.9.
8. J.R. Glaese, "A length rate control law applicable to space station tether deploy-
ment/retrieval," ibid., p. 162; P.K. Lakshmanan, V.J. Modi, A.K. Misra, "Space
Station based tethered payload control strategies and their relative merits," ibid., p.
166.
9. D.D. Tomlin, D.K. Mowery, "Tethered satellite system control system design," ibid,.
p. 143; M. Pasca, M. Pignataro, A. Luongo, "Three dimensional vibratiohs of teth- :1"
ered satellite system," ibid., p. 153.
10. E. Scala, "Tethers in space, and micrometeoroids," ibid., p. 372
11. J.A. Hoffman, "Operational complexities of real tether system.-: in space," ibid., p.
381.M. Greene, D. Gwaltney, G. Stover, D. Kromann, J. Walls, "GetAway Tether
Exwrirr, ent (GATE) for file Tether Dynamics Explorer (TDE) Series,:' ibM., p. 33.
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Exploration of planetesimals by a tripartite tethered spacecraft

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