The View from Sun-Earth L5

NASA’s new twin STEREO solar observatories launched October 25th, 2006, if successfully deployed in separate orbits ahead and behind Earth in its orbit about the Sun, will give us our first 3D look at Sunspots and the powerful solar flares and the coronal mass ejections that can originate in these areas.

Sunspots are born when strong magnetic fields break through the Sun’s surface. These fields are frequently unstable and can explode. The result is a powerful burst of energy that can exceed that of 10 billion hydrogen bombs exploding in unison.

Our astronauts in low Earth orbits, aboard the shuttle or the International Space Station, are protected by Earth’s magnetic van Allen Belts. Yet on Earth, these events can cause a lot of mischief with our ever more intricate and growing electronic communications and computer systems.

But the real danger will be to people caught on the Sunlit side of the Moon, or in space, in transit, beyond the van Allen Belts’ protective shield. It is vital, if we are going to have a any kind of permanent presence on the Moon and elsewhere in space, that we both better understand these events - that is the goal of the STEREO mission - and that we can predict them with greater accuracy and with more lead time, to allow everyone potentially at risk to reach shelter safely.

Per the NASA release, “STEREO's nearly identical twin, golf cart-sized spacecraft will make observations to help researchers construct the first-ever three-dimensional views of the Sun. The images will show the star's stormy environment and its effects on the inner solar system, vital data for understanding how the Sun creates space weather.

“After about two months, STEREO's orbits will be synchronized to encounter the Moon. The "A" observatory will use the Moon's gravity to redirect it to an orbit "ahead" of Earth. The "B" observatory will encounter the Moon again for a second swingby about one month later to redirect its position "behind" Earth. STEREO is the first NASA mission to use separate lunar swingbys to place two observatories into vastly different orbits around the Sun.”

The initial separation will be less than one degree, as seen from the vantage point of the Sun, but will increase steadily as the craft continue on their two opposing trajectories. The goal is to get a 3 dimensional view of these events. But long term, what we need is something else, advance warning.

The Sun rotates on its axis in “about a month.” As it is not a solid body, it’s rotation is faster at the equator than at the poles, by about a day or so. As a result, through the months, the Sun’s internal magnetic fields get increasingly twisted, until they snap, disappear, and reform with opposite polarity. This is the source of the well known 22-year solar cycle. It is also what creates the periodic high solar storm activity seasons.

We can only see the side of the Sun facing us at the time. If we could see around the advancing limb, we would know of potentially explosive Sun spots before the Sun’s rotation brings them into view. If we had a solar observatory parked in the Sun-Earth L5 position, trailing Earth in its orbit about the Sun by 60 degrees, we could get as much as a week’s early warning.

This L5 point or Lagrange 5 point is one of the dynamically stable points where Earth’s and the Sun’s gravitational influence cancel out. There, 93 million miles or 150 million km trailing Earth in its orbit, a dedicated solar observatory could keep watch through its lifetime, probably on the order of one 22 year solar cycle. So it would be something we would expect to replace regularly, maybe once a decade to increase the likelihood that we have one that is fully operational during all active Sun periods.

Such an observatory could do double duty, serving as a data and communications relay for spacecraft or stations further out in the Solar System (on Mars or Europa, for example) temporarily behind the Sun from Earth’s point of view.

Advance warning as important as it is, will be only part of an all-approaches effort to minimize the problem of solar flares and coronal mass ejections. On the Moon, there needs to be a network of shelters placed so that given the ample lead time afforded by an L5 solar observatory, safety can be reached by all those operating on, or in transit across, the exposed lunar surface.

The problem occurs every eleven years. We do not need “blackout periods” for spacefaring. That would have an enormous dampening effect on solar system exploration, and on efforts to open Mars in particular. While trips to the Moon from Earth take only a few days, and would be covered by the kind of lead time we are talking about, presently, there could be one or two launch windows to Mars every decade that would have to be skipped because of high solar flare risk, unless we can develop low-mass (low fuel penalty) radiation protection systems for spacecraft in transit. But that’s another article.

Ideally, a ring of radiation-hardened solar observatories could circle the Moon closer in, possibly even within the orbit of Mercury, giving us maximum coverage of solar weather systems. In the meantime, we urge design and development of a Sun-Earth L5 observatory, to be launched in time for the onset of human crew missions to the Moon.

The first humans to return to the vicinity of the Moon could come well before that time, as early as two years from now, if someone plunks down the $100 million the Russians are asking for a loop-the-Moon flyby tourist experience. That would have to be scheduled for a quiet Sun period.

For more on the problem, check out this online report:

Peter Kokh