The reality of global
warming is receiving growing acceptance.Even the Bush
Administration seems to be modifying its previous hard-line position rejecting
the idea.Projections of the Intergovernmental Panel on
Climate Change indicate an average global temperature rise of between 1.5 and
4.5 degrees C by the year 2100.Climate change simulations
suggest that we may be approaching (or may already have passed) a tipping point
in global warming.Data records show a progressive increase
in average temperature starting about 1920, with the average temperature of the
Earth now about 1 degree C higher than it was in 1920.
There are predictions
that fertile farmland, for example in the Great Plains region of the USA, will
be replaced by new deserts, that temperate zones will become more tropical, that
the ecology of the ocean will be radically altered, that glaciers will melt,
that the level of the ocean may rise by up to 20 feet, perhaps drowning costal
cities around the world.Over the coming decades, the Earth
may become a very different and less pleasant place.
Is there anything that
can be done to avert this global calamity?Several technical
fixes have been suggested.One of them is based on the
cooling effects of volcanic eruptions.The progressive
increase in average temperature over the past few decades shows a pronounced dip
of a few tenths of a degree C spanning a decade that corresponds to the eruption
Pinatubo, a massive eruption that dumped many tons
of sulfur into the upper atmosphere.It has been suggested
that by putting 3 to 5 megatons per year of sulfur into the upper atmosphere on
purpose, we could counteract the effects of global warming.
This “cure”, unlike an eruption event, would have to be done continuously for
many decades.The side-effects of such a remedy, however,
appear to be as bad as the problem it is intended to fix.
Acid rain form the sulfuric acid formed from the sulfur dioxide would become the
standard kind of rainfall, irreversibly altering the ecology of the planet.
Prof. Roger Angel of the
University of Arizona,
a prominent astronomer and creator of some of the world’s largest telescope
mirrors, has proposed an interesting alternative.He would
like to place scatterers at the L1 Lagrange point of the Earth-Sun
system that would remove about 1.8% of the ambient sunlight.
To understand the
proposal, let’s start with what Lagrange points are.The
Italian-French mathematician and mathematical physicist Joseph-Louis Lagrange
(1736 –1813), in attempting to solve the gravitational three body problem,
discovered that in a simplified Solar system in which the Earth orbits the Sun
with no other planets or moons, there are five points of stability.They are now called “Lagrange points” and labeled L1 through L5.If a massive object is at one of these Lagrange points and displaced
slightly in a particular direction, there may be a restoring force that pushes
it back toward the stability point.The “Trojan” Lagrange
points L4 and L5, which are in the Earth’s orbit 600 ahead and behind
the Earth, have such stability in all three directions in space.However, Lagrange points L1, L2, and L3, which lie on the line through
the centers of the Earth and Sun, are stable only in the two directions
perpendicular to the line connecting the two gravitating bodies, but are
unstable to “radial” displacements along that line.
The L1 Lagrange point,
about 1.5 million kilometers above the Earth in the direction of the Sun, could
be called the “sunshine” Lagrange point.It is closer to the
Sun than The Earth, but the “back-pull” of the Earth partially cancelsthe Sun’s gravitational pull, so that it has the same orbital period as
the Earth.It is stable to perturbations perpendicular to
the Earth-Sun axis, but it is unstable to perturbations along that line, so that
some active thrusters are occasionally required to maintain a satellite in this
orbit.The Solar and Heliospheric Observatory (SOHO) and the
Advanced Composition Explorer (ACE) are presently located at L1.
Roger Angel’s sunshield
would be placed just beyond the L1 point.Because the Earth
and Sun have about the same density, the penumbra shadow of blocked sunlight
from an object placed at L1 almost precisely covers the disc of the Earth.Thus, it is the ideal location for an object blocking Earth-bound
sunlight. Angel estimates that a reduction in the intensity of solar radiation
by about 1.8% would full reverse the effects of a doubling of atmospheric CO2.
However, in maintaining
an orbit at L1, the action of light-pressure is a problem.A
square meter of radiation absorbing material (assumed to be 1.06 mm
thick and to have an average density of 2.35 g/cm3) at the orbit of the Earth
and perpendicular to the Earth-Sun axis receives a push from solar radiation of
(1 mN = 10-6
newtons).The gravitational pull of the Sun on the same
square meter of material is 15 mN,
so light pressure would cancel about 1/3 of the gravitational pull, and
maintaining an orbit precisely at L1 would be impossible.
Roger Angel’s solution
to this dilemma is to make several innovations.First, make
the material transmit most of the light that strikes it and to
scatter about 4% of the light at an angle of a few degrees, just large enough to
miss the Earth but not large enough to absorb much momentum.
The remaining light pressure still requires him to put the object in an orbit a
bit closer to the Sun (about 1.8 million kilometers above the Earth) to achieve
a stable L1-type orbit.The downside of intercepting only 4%
of the light is that you need 25 times more area than if you intercepted all of
the light.As we’ll see below, that raises the cost.
What goes into the L1
orbit and how much will it cost?The cheapest solution would
be to place a light-absorbing dust cloud there.However,
light pressure and the radial instability of L1 orbits would rapidly dissipate
such a cloud.Therefore, one must instead use a “cloud” of
autonomous sunshade spacecraft with “station-keeping” capabilities.Angel’s unit sunshade spacecraft design is essentially a navigable sheet
of silicon nitride containing holes with their centers placed 15 mm
apart in a vast hexagonal planar array, so that light passing through the holes
is coherently deflected in an interference pattern by a few degrees.Each unit has a mass of about a ton (1000 kg) and has a shade area of
about 2.4 square kilometers.
The total area that must
be occupied by these sunshades is very large, about 4.7 million square
kilometers.The total mass of the spacecraft needed to cover
this area is estimated to be 20 million tons (2.0×1010 kg).
Angle tries to get the
launch cost down by suggesting the construction of an electromagnetic launcher,
a “space cannon” mounted on a high mountaintop and having a “muzzle velocity” of
12.8 km/s.He describes a 2 km long magnetic coil launch
system using peak magnetic fields of 24 tesla and requiring an energy input of
65 billion joules that is projected to provide such a capability.
Suppose it was decided
that the effects of global warming must be mitigated in a 10 year period using
this method.Angel estimates that with flyer payloads of
1000 kg each, about 20 million launches would be needed to deploy the sunshade
system.He envisions 20 of the electromagnetic launchers,
each costing about $30 billion, launching one flyer every 5 minutes for 10
years.A stretch-out to more decades of launch would require
a smaller number of launchers operating for a longer period.
The total capital cost of the launchers would be about $600 billion and the
electrical energy cost about $150 billion.Added to that,
the production cost of the flyers would be about $1 trillion.These figures do not include the development and operations costs,
estimated to be less than $5 trillion.If the lifetime of
the project is 50 years, than average annual cost would be $100 billion, about
0.2% of the world’s gross domestic product.
Is this sunshade
project, or one like it, likely to become a world priority and to be
implemented?It’s difficult to say.
Concern about global warming is rising in all parts of a planet, but such
concern would have to rise much higher to reach a level at which the megaproject
envisioned by Roger Angel would be seriously undertaken.The
resources of the planet have never been mobilized in a coherent way on such a
massive scale, and it is not easy to visualize the political processes that
might bring this about.
Nevertheless, it’s an
interesting idea, and it certainly has implications for science fiction as well
John Cramer's new book: a non-fiction
work describing his Transactional Interpretation of quantum mechanics, The
Quantum Handshake - Entanglement, Nonlocality, and Transactions,
(Springer, January-2016) is available for purchase online as a printed or eBook