1. Vol 438|10 November 2005
BRIEF COMMUNICATIONS
Gravitational tractor for towing asteroids
A spacecraft could deflect an Earth-bound asteroid without having to dock to its surface first.
D. DURDA, FIAAA/B612 FOUNDATION
We present a design concept for a spacecraft
that can controllably alter the trajectory of an
Earth-threatening asteroid by using gravity as
a towline. The spacecraft hovers near the aster-
oid, with its thrusters angled outwards so that
the exhaust does not impinge on the surface.
This proposed deflection method is insensitive
to the structure, surface properties and rota-
tion state of the asteroid.
The collision of a small asteroid of about
200 m with the Earth could cause widespread Mass appeal: a spacecraft could use
damage and loss of life1. One way to deflect gravity to tow bodies away from a
an approaching asteroid is to dock a space- collision course with Earth.
craft to the surface and push on it directly2.
The total impulse needed for rendezvous and
deflection is too large for chemical rockets, would reduce the net towing force and stir up is predictable and controllable, as would be
but would be achievable by spacecraft such as unwanted dust and ions). required for a practical deflection scheme.
the 20-tonne nuclear-electric propelled vehi- This scheme is insensitive to the poorly The mean change in velocity required to
cles that were proposed as part of NASA’s understood surface properties, internal struc- deflect an asteroid from an Earth impact tra-
Prometheus programme2. tures and rotation states of asteroids. A space- jectory is about 3.5ǂ10ǁ2/t m sǁ1, where t is
Regardless of the propulsion scheme, a craft needs only to keep its position in the the lead time in years4. So a 20-tonne gravita-
docked asteroid tug needs an attachment direction of towing while the target asteroid tional tractor hovering for one year can deflect
mechanism because the surface gravity is too rotates beneath it. The engines must be a typical asteroid of about 200 m diameter
weak to hold it in place. Asteroids are likely to actively throttled to control the vertical posi- given a lead time of roughly 20 years.
be rough and unconsolidated, making stable tion as the equilibrium hover point is unstable. The thrust and total fuel requirements of
attachment difficult. Furthermore, most aster- The horizontal position is controlled by differ- our mission example would be well within the
oids rotate, so an engine anchored to the sur- ential throttling of engines on opposite sides of capability of proposed 100-kilowatt nuclear-
face thrusts in a constantly changing direction. the spacecraft. The spacecraft can be made sta- electric propulsion systems2, using about
Stopping the asteroid’s rotation, reorienting its ble in attitude by designing it like a pendulum, 4 tonnes of fuel to accomplish the typical
spin axis3, or firing the engine only when it with the heaviest components hanging closest 15 km sǁ1 rendezvous and about 400 kg for the
rotates through a certain direction, adds com- to the asteroid and the engines farther away. actual deflection. For a given spacecraft mass,
plexity and wastes time and propellant. The thrust required to balance the gravita- the fuel required for the deflection scales
Our suggested alternative is to have the tional attraction is given by linearly with the asteroid mass.
spacecraft simply hover above the surface of Deflecting a larger asteroid would require a
the asteroid. The spacecraft tows it without Tcos[sinǁ1(r/d)+Ƞ]ǃGMm/d 2 heavier spacecraft, more time spent hovering,
physical attachment by using gravity as a tow- ⎞r⎞ 3 ⎞ ț ⎞ ⎞ m ⎞ ⎞d ⎞ or more lead time. However, in the special case
line. The thrusters must be canted outboard to ǃ1.7 ⎠ᎏ⎠ ᎏ ⎠
ᎏ
⎠20ǂ103⎠
ᎏ
⎠150 ⎠ in which an asteroid has a close Earth approach,
d ⎠2ǂ103
keep them from blasting the surface (which followed by a later return and impact, the
where G is the gravitational constant; see Fig. 1 change in velocity needed to prevent the impact
Asteroid for definition of other variables. Thus a 20- can be many orders of magnitude smaller if
φ tonne spacecraft with Ƞǃ20ᑻ hovering one applied before the close approach5. For exam-
r half-radius above the surface (d/rǃ1.5) can ple, the asteroid 99942 Apophis (2004 MN4), a
Spacecraft
d tow an asteroid of 200 m diameter and density 320-m asteroid that will swing by the Earth at a
ρ, M m, T țǃ2ǂ103 kg mǁ3, provided it can maintain a distance of about 30,000 km in 2029, has a small
total thrust T of just over 1 newton. probability (10ǁ4) of returning to strike the
The velocity change imparted to the aster- Earth in 2035 or 2036 (ref. 6). If it is indeed on a
oid per second of hovering (ǵv) is given by return impact trajectory, a deflection of only
Gm ⎞ m ⎞ ⎞150⎞ 2 about 10ǁ6 m sǁ1 a few years before the close
Figure 1 | Towing geometry of a gravitational
ǵvǃ ᎏ ǃ5.9ǂ10ǁ11 ⎠
ᎏ ᎏ approach in 2029 would prevent a later impact
tractor. The asteroid (assumed to be spherical) has d 2
20ǂ103⎠ ⎠ d ⎠
radius r, density ț and mass M. The spacecraft has
(A. Carusi, personal communication). In this
mass m, total thrust T and an exhaust-plume half- So the velocity change imparted to the asteroid case, a 1-tonne gravitational tractor with con-
width Ƞ. It hovers at distance d from the asteroid’s in our example in a single year of hovering is ventional chemical thrusters could accomplish
centre, where its net thrust balances its weight. 1.9ǂ10ǁ3 m sǁ1. Because ǵv is largely inde- this deflection mission as only 0.1 newtons of
The thrusters are tilted outwards to prevent pendent of the asteroid’s detailed structure and thrust would be required for a duration of about
exhaust impinging on the asteroid surface. composition, the effect on the asteroid’s orbit a month. Should such a deflection mission
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©2005 Nature Publishing Group

2. BRIEF COMMUNICATIONS NATURE|Vol 438|10 November 2005
prove necessary, a gravitational tractor offers a 3. Scheeres, D. J. & Schweickart, R. L. The Mechanics of Moving more than half that of a liquid sulphuric acid
viable method of controllably steering asteroid Asteroids (paper 2004–1446, Am. Inst. Aeronaut. catalyst and much higher than can be achieved
Astronaut., 2004).
99942 Apophis away from an Earth impact. 4. Chesley, S. R. & Spahr, T. B. in Mitigation of Hazardous by conventional solid acid catalysts (see sup-
Edward T. Lu, Stanley G. Love Comets and Asteroids (eds Belton, M. J. S. et al.) 22–37 plementary information). There was no loss of
NASA Johnson Space Center, Mail Code CB, (Cambridge Univ. Press, Cambridge, 2004). activity or leaching of –SO3H during the
5. Carusi, A., Valsecchi, G. B., D’Abramo, G. & Boatini, A.
Houston, Texas 77058, USA Icarus 159, 417–422 (2002). process, even for samples subjected to repeated
e-mail: edward.t.lu@nasa.gov 6. JPL Sentry Impact Risk Page reactions at 80–180 ᑻC after having been recov-
http://neo.jpl.nasa.gov/risk ered by simple decantation. The activity is
1. Chapman, C. R. Earth Planet. Sci. Lett. 222, 1–15 (2004).
2. Schweickart, R. L., Lu, E. T., Hut, P. & Chapman, C. R. Competing financial interests: declared none. double that of a carbonized sulphonated naph-
Sci. Am. 289, 54–61 (2003). doi:10.1038/438177a thalene catalyst tested previously5, which
decreased rapidly on recycling at 80 ᑻC.
Carbon catalysts identical to those described
here have also been successfully produced
GREEN CHEMISTRY from carbonized starch and cellulose (results
not shown). Saccharide molecules may there-
Biodiesel made with sugar catalyst fore be generally suitable for preparing these
catalysts, which can be used as a replacement
for liquid sulphuric acid in esterification reac-
The production of diesel from vegetable oil aromatic carbon sheets in a three-dimensional tions. In addition to biodiesel production,
calls for an efficient solid catalyst to make the sp3-bonded structure. Sulphonation of this such environmentally benign alternative cata-
process fully ecologically friendly. Here we material would be expected to generate a stable lysts should find application in a wide range of
describe the preparation of such a catalyst solid with a high density of active sites, enabling other acid-catalysed reactions.
from common, inexpensive sugars. This a high-performance catalyst to be prepared Masakazu Toda*, Atsushi Takagaki*,
high-performance catalyst, which consists of cheaply from naturally occurring molecules. Mai Okamura*, Junko N. Kondo*,
stable sulphonated amorphous carbon, is The scheme we use to sulphonate incom- Shigenobu Hayashi†, Kazunari Domen‡,
recyclable and its activity markedly exceeds pletely carbonized saccharides is shown in Michikazu Hara*
that of other solid acid catalysts tested for Fig. 1. First, D-glucose and sucrose are incom- *Chemical Resources Laboratory, Tokyo Institute
‘biodiesel’ production. pletely carbonized at low temperature to of Technology, Yokohama 226-8503, Japan
The esterification of higher fatty acids by induce pyrolysis and the formation of small e-mail: mhara@res.titech.ac.jp
liquid acid catalysts such as sulphuric acid polycyclic aromatic carbon rings; sulphonite †Research Institute of Instrumentation Frontier,
(H2SO4) is a process commonly used for groups (–SO3H) are then introduced by sul- National Institute of Advanced Industrial Science
biodiesel production, but it involves high con- phuric acid (see supplementary information). and Technology, Tsukuba, Ibaraki 305-8565, Japan
sumption of energy and the separation of the Structural analysis6–8 indicates that the pre- ‡Department of Chemical System Engineering,
catalysts from the homogeneous reaction mix- pared samples consist of sheets of amorphous School of Engineering, University of Tokyo,
tures is costly and chemically wasteful. Re- carbon bearing hydroxyl and carboxyl (–OH Bunkyo-ku, Tokyo 113-8656, Japan
cyclable solid acids, such as Nafion1–4, make and –COOH) groups, as well as high densities
better catalysts, although they are also expen- of –SO3H groups. 1. Okuhara, T. Chem. Rev. 102, 3641–3666 (2002).
2. Clark, J. H. Acc. Chem. Res. 35, 791–797 (2002).
sive and their activity is less than that of liquid This black powder is insoluble in water, 3. Misono, M. C. R. Acad. Sci. IIc: Chimie 3, 471–475 (2000).
acids1. Sulphonated naphthalene carbonized methanol, benzene, hexane, N,N-dimethylfor- 4. Smith, K., El-Hiti, G. A., Jayne, A. J. & Butters, M.
at 200–250 ᑻC is a solid acid catalyst that has mamide and oleic acid, even at boiling tem- Org. Biomol. Chem. 1, 1560–1564 (2003).
5. Hara, M. et al. Angew. Chem. Int. Edn 43, 2955–2958 (2004).
been used successfully for ethyl acetate forma- peratures. It can be moulded into hard pellets 6. Tsubouchi, N., Xu, K. & Ohtsuka, Y. Energy Fuels 17,
tion5; however, it is a soft material and its or thin flexible films by heating with 0.5–5.0% 1119–1125 (2003).
aromatic molecules are leached out during by weight of binding polymer; the two forms 7. Silverstein, R. M., Bassler, G. C. & Morrill, T. C. in
liquid-phase reactions above 100 ᑻC or when have comparable stability and catalytic perfor- Spectrometric Identification of Organic Compounds 5th edn
218–232 (Wiley, Indianapolis, 1991).
higher fatty acids are used as surfactants, so its mance. The thin films act as electrically insu- 8. Zhang, X. & Solomon, D. H. Chem. Mater. 11, 384–391
catalytic activity is rapidly lost. lating proton conductors whose properties (1999).
We have devised a strategy to overcome these (0.09 siemens cmǁ1 at 50 ᑻC and 100% humid-
Supplementary information accompanies this
problems by sulphonating incompletely car- ity) are comparable to that of Nafion communication on Nature’s website.
bonized natural organic material to prepare a (0.1 siemens cmǁ1 at 80 ᑻC). Competing financial interests: declared none.
more robust solid catalyst. Incomplete car- High-grade biodiesel is produced by esteri- doi:10.1038/438178a
bonization of natural products such as sugar, fication of the vegetable-oil constituents oleic
starch or cellulose results in a rigid carbon acid and stearic acid. The activity of our solid BRIEF COMMUNICATIONS ARISING online
material that is composed of small polycyclic sulphonated carbon catalyst in this reaction is ➧ www.nature.com/bca see Nature contents.
CH2HO H2O
O
OH D-glucose OH SO3H
O
O O
O
OH OH Pyrolysis O O
O
Sulphonation
O
OH > 300° C O 150° C
CH2HO O
O
O O
O
O
CH HO O
O O 2 O
O
OH HO
O
O
O
O COOH
O Sucrose O
OH O
O
OH OH
Figure 1 | Preparation from sucrose and D-glucose of a solid catalyst suitable for biological diesel production. Pyrolysis of the sugars causes their incomplete
carbonization (middle; outlined in blue) and formation into polycyclic aromatic carbon sheets; sulphuric acid (concentrated or fuming) is used to
sulphonate the aromatic rings to produce the catalyst. For details of methods, see supplementary information.
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