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do Gravity Holes harbor planetary assassins?
Lagrangian Points cancel out Gravity
by Stuart Clark
New Scientist / Feb. 18, 2009
They are the places that Gravity forgot. Vast regions of space millions of kilometers across in which
celestial forces conspire to cancel out Gravity and so trap anything that falls into them. They sit in the
Earth's orbit -- one marching ahead of our planet, the other trailing along behind.
Astronomers call them Lagrangian points. Or 'L4' and 'L5, for short. The best way to think of
them, though, is as "celestial flypaper".
In the 4.5 billion years since the formation of the Solar System, everything from dust clouds to
asteroids and hidden planets may have accumulated there. Some have even speculated that alien
spacecraft are watching us from the Lagrangian points, looking for signs of intelligence.
Putting little green men to one side for the moment, even the presence of plain old space rocks
would be enough to keep most people happy. "I think you certainly might find a whole population of
objects at 'L4' and 'L5'," says astrophysicist Richard Gott of Princeton University.
After nearly a century of speculation, we are on the verge of finding out what they are hiding onceand-for -all. Later this year, 2 spacecraft that spend their lives studying the Sun will begin their slow
journeys through 'L4' and 'L5'.
Space scientists plan to use instruments on board NASA's STEREO probes 'A' and 'B' to search for
celestial objects becalmed at the Lagrangian points. What they find could hugely enhance our view of
how the Solar System formed; tell us more about the colossal impact that formed the Moon; and warn us
if another major collision is on the cards.
The Lagrangian points were first discovered in 1772 by the mathematician Joseph-Louis Lagrange.
He calculated that the Earth's gravitational field neutralizes the gravitational pull of the Sun at 5 regions
in space -- making them the only places near our planet where an object is truly weightless.
Of the 5 Lagrangian points, 'L4' and 'L5' are the most intriguing. They are the only ones that are
stable. While a satellite parked at 'L1' or 'L2' will wander off after a few months unless it is nudged back
into place, any object at 'L4' or 'L5' will stay put due to a complex web of forces. Lying 150 million
kilometers away along the line of Earth's orbit, 'L4' circles the Sun 60 degrees in front of our planet
while 'L5' lies at the same angle behind (see diagram).
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Evidence for such gravitational potholes appears around other planets, too. In 1906, Max Wolf
discovered an asteroid outside of the main belt between Mars and Jupiter and recognized that it was
sitting at Jupiter's 'L4' point. Wolf named it "Achilles" and so began the tradition of naming these
asteroids after characters from the Trojan wars.
The realization that Achilles would be trapped in its place and forced to orbit with Jupiter -- never
getting much closer or further away -- started a flurry of telescopic searches for more examples. There
are now more than 1,000 asteroids known to reside at each of Jupiter's 'L4' and 'L5' points.
Searches for "Trojan" asteroids around other planets have met with mixed results. Saturn seemingly
has none and only in the last decade have Trojans been found at Neptune. Naturally, astronomers have
often wondered about asteroids at Earth's 'L4' and 'L5' points.
Fly-through Zones
The trouble is that our 'L4' and 'L5' points are not easy to see from the ground. They appear to lie
close to the Sun. So by the time night falls, the trailing 'L5' region is low in the sky and setting fast. On
the other side of the sky, the preceding 'L4' point rises in darkness but the dawn is hot on its heels.
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That didn't prevent Paul Weigert at the University of Western Ontario in Canada and his colleagues
from conducting a number of searches in the 1990s with the Canada-France-Hawaii telescope on Mauna
Kea, Hawaii. It was a tough job because 'L4' and 'L5' appear wider in the sky than the full Moon. So a
large number of observations would be needed to search them thoroughly. Alas, Weigert and colleagues
came up empty-handed as their search wasn't detailed enough.
More recently, automated asteroid searches (such as the Lincoln near-Earth Asteroid Research
Project) have begun to creep closer to the Lagrangian points in their nightly robotic scans of the sky.
But at this stage, no Lagrangian asteroids have been identified. "The field has languished because we
are all waiting for somebody to see something," says Weigert.
NASA's STEREO spacecraft could change everything even though they were never designed to look
for asteroids. Launched in 2006, one of the twin STEREO probes was placed ahead of Earth with the
other behind. Tracing Earth's orbit, STEREO 'A' gradually outpaces the Earth while its sister ship
STEREO 'B' trails ever further behind.
From these 2 vantage points, the spacecraft monitor the region of space directly between the Earth
and the Sun, looking for solar storms that can wreak havoc with electrical equipment on satellites and on
Earth.
'L4' and 'L5' are particularly good vantage points from which to warn the Earth of incoming solar
storms. "We talked at one stage about actually stopping the STEREO spacecraft when they got
there,because you get about 2-or-3 days warning of a coming storm," says Michael Kaiser of the
Goddard Space Flight Center in Greenbelt, Maryland and STEREO's project scientist.
Ultimately, the STEREO team discovered that it would take too much fuel to stop their spacecraft at
'L4' and 'L5'. So they settled for a leisurely fly through instead, though they are still travelling too fast to
get stuck. "These are big regions of space," says Kaiser. "It's going to take STEREO months to travel
through them."
That's when it struck Richard Harrison (of the Rutherford Appleton Laboratory in Oxfordshire, UK
and a member of the STEREO team,) that the probes' cameras might be put to another use. He began to
investigate the possibilities and realized that a pair of instruments known as "heliospheric imagers"
could be used to search for asteroids. "They were not designed to do this work," says Harrison, who is
the imagers' principal investigator. "It's an added bonus."
Even so, asteroid hunters face a painstaking task because a Lagrangian asteroid will appear as little
more than a dot moving against a background of thousands of stars. Thankfully, there is already a force
of volunteers who scan the STEREO images via the internet for signs of near-Earth asteroids.
In addition to these efforts, Harrison is hoping to find some professional manpower for the 'L4' and
'L5' crossings. "The close-up investigation of 'L4' and 'L5' is completely new. That makes it something
we should be driving," he says. In these cash-strapped times, however, that might be more easily said
than done.
If investigators do find an asteroid in their sights, it will be worth their effort. "Wouldn't it be
spectacular if we actually backed past an asteroid? Saw it come creeping into view around the camera,"
says Harrison.
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They will be able to watch it and tell how it rotates from the variation in light it reflects from the
Sun. "We will be able to measure the distribution of any asteroids and dust in the Lagrangian points,"
says Harrison.
Armed with that information, we may be able to answer one of the most perplexing mysteries of the
Solar System. Namely, why Earth has such a large Moon.
Most astronomers believe that the moon formed from the debris generated when a Mars-sized object
struck the Earth a glancing blow about 4 billion years ago. Their problem is in understanding where the
object came from.
Computer models show that incoming objects from elsewhere in the Solar System would tend to
strike the Earth with too much energy. Instead of creating the Moon, they obliterate the Earth. So the
impactor must have originated close by, the theory goes, where it could not accelerate too much before
hitting.
Planetary Leftovers
Another clue is that the Moon contains the same abundance of oxygen isotopes as the Earth -hinting that whatever hit us must also have had the same isotope abundance. When astronomers look
out into the Solar System (to Mars, for example), the isotope abundances are different. So this, too,
hints that the impactor formed close by. But where?
What is puzzling is how an object could grow so close to the Earth and reach the size of Mars before
a collision took place. Their mutual gravity should have pulled them together long before. Unless, says
Gott, it formed at a Lagrangian point. "An object could sit at one of these stable points and just grow,"
he says.
Once it grew sufficiently large, gravitational interactions with other objects such as Venus could
nudge it out of the Lagrangian point and onto a collision course with Earth.
"It would have the same oxygen isotopes as Earth because it formed in the same region of the Solar
System," says Gott. Also, being in essentially the same orbit as Earth, the 2 planets would not be
travelling at vastly different velocities when they collided (New Scientist, 14 August 2004, p 26).
Gott thinks that any objects still in 'L4' and 'L5' may be leftovers from the formation of that
impacting body. "Let's say that you find a number of objects there. In that case, they would be great
targets for a sample-return mission to see if they had the same oxygen isotope abundances as Earth,"
says Gott. If they do, Gott believes that this strengthens the case for the Earth-impactor to have formed
there.
In preparation for the crafts' arrivals at 'L4' and 'L5', Harrison is discussing with colleagues how best
to maneuver the 2 probes for the optimal views. Strange as it sounds, the twin spacecraft do not look
where they are going. Instead, they fly facing backwards with several of their electronic eyes pointed
close to our planet on the lookout for incoming solar storms. For the best views of the Lagrangian
points, the spacecraft would have to be flipped over so that the heliospherical imagers point forwards
while the other instruments remain trained on the Sun.
The journey through 'L4' and 'L5' is potentially fraught with danger. Clouds of dust are thought to
be trapped at these Lagrangian points. If the STEREO team is unlucky, a collision with a badly-placed
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dust particle could be devastating! "If one hits us inside the camera, we are no more," says Chris Davis
who is part of the heliospheric imager team from Rutherford Appleton Laboratory.
The risk may be reduced by flipping the cameras back towards Earth as the spacecraft pass through
the most dangerous spots. The rest of the STEREO team is confident that they will survive. Ever since
launch, the craft have been taking hits from dust that happens to lie along Earth's orbit.
"It varies a lot from a few to a few thousand per day," says Chris St Cyr of the Goddard Space Flight
Center, who heads the investigation to understand these dust events.
No one knows how many asteroids the STEREO probes will see. Weigert and colleagues have
performed a number of computer simulations that showed how asteroids can be nudged from a
Lagrangian point due to Venus's gravity. It can happen on a timescale of a million years or so.
However, the same simulations showed that this works both ways with asteroids being nudged into
the Lagrangian points by Venus as well. These results -- and the failure of telescopes to find a
Lagrangian asteroid to date -- have made Weigert cautious about the number and size of the asteroids he
expects STEREO to find.
"I think there may be a few asteroids. But not hundreds. And I'm thinking that they are less than a
kilometer across. In the main asteroid belt, a typical asteroid is 100 kilometers across."
Such doubts do not concern Harrison. "Some think we will see something. Others think we won't,"
he says. "But if we let this opportunity pass us by without even looking, we will regret it."
Could another planet be stalking the Earth?
Most astronomers accept that the Moon was formed when a Mars-sized planet crashed into Earth
during its infancy. Some research suggests that this kamikaze planet formed at either the 'L4' or 'L5'
Lagrangian point where gravity from the Earth and sSun cancel each other out. So could some other
threat be lurking there today?
"Absolutely not," says Paul Weigert at the University of Western Ontario, Canada. There are not
enough dust particles or pebbles in the Solar System.
It was a different story 4.5 billion years ago when the Solar System was swathed in its birth clouds.
The planets were forming from this mix of dust and gas; and 'L4' and 'L5' would have been prime places
for matter to accumulate into larger bodies. "These days, there simply isn't enough material around to
build anything that large," says Weigert.
OK, so nothing planet-sized. But Richard Gott of Princeton University reckons that there may still
be threatening asteroids.
"If we see something big in there, it would be like a ticking time bomb," says Gott. That is because
gravitational nudges from the other planets -- particularly Venus -- could pull the asteroid just far
enough so that it escapes the Lagrangian point. Once outside this trap, it could easily fall into a collision
course with Earth.
"If we see a big asteroid there, it might be worth taking it out pre-emptively," says Gott. "And by
that, I mean blowing it to pieces!"
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Readers' Comments
1. M-o-m-e-n-t-u-m
by Cynthia / Wed Feb 18, 2008 18:56:23 GMT
> "celestial forces conspire to cancel out gravity and so trap anything that falls into them"
That's rather poorly worded as it implies some sort of energy dissipating gravity well that
ensnares objects which innocently pass by these regions. You need to remove an appropriate
amount of kinetic energy or alter the momentum possessed by an object before it will happily remain
in these regions.
2. Goldmine?
by Chris Simmons / Wed Feb 18, 2009 19:36:32 GMT
Rather than blow it up, wouldn't this be an ideal place to build a space mine? Minimal gravity
well, you could shunt vast amounts of material into Earth orbit at little cost. You could bootstrap the
space economy!
3. M-o-m-e-n-t-u-m, M-o-m-e-n-t-u-m
by adamabyss / Thu Feb 19, 2009 00:12:11 GMT
Well said, Chris. If only the Powers-that-Be would start the process of colonizing and mining
space now for the improvement of all people instead of subjecting us to another thousand years of
petty and meaningless politics, casino-style economics and financial markets, greedy capitalism
(nothing wrong with a fair and overseen free market -- I'm not a communist), and possible selfdestruction in pointless conflicts and wars instead of opening their minds to the amazing
opportunities just beyond the horizon.
4. Goldmine?
by Markus / Wed Feb 18, 2009 22:34:56 GMT
I love this comment, Chris! Mining asteroids makes so much sense. I've loved this concept
since I first heard about it when I was a kid some 40 odd years ago.
There is so much Iridium in space, we could use the 'L4' and 'L5' as minimum maintenance way
stations; stockpiling minerals; and for a location to permanently keep an observatory, space station,
etc.- beam microwaves around. The weaponizing of space means the end of Mankind. Back to the
Stone Age or worse. We need to collectively lighten up with lower vibrational idiocy. Hope and
Faith prevail; Fear and Evil self cancel. History will prove this soon. Clear the air and the space
junk -- there is a way to do it. Courage and Imagination, Hope and Faith. The way we do it now
sure isn't working. Too much pollution!
5. Weapons Everywhere.
by Eric M. Jones / Wed Feb 18, 2009 20:44:23 GMT
I was told by a JPL scientist that whatever rocks there may be in the 'L4' and 'L5' Lagrange
points in the Earth-Moon system were considered as possible weapons. Find some big rock there
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and boost it towards Earth. A whole lot of Bang! for very little money. Whether-or-not there are
rocks there is highly classified.
6. Weapons Everywhere.
by Dann / Thu Feb 19, 2009 01:46:42 GMT
The hard part would be determining exactly where on Earth they impacted. You could wind up
shooting yourself in the foot.
7. Solar Sails versus Blowing It Up
by David Feany / Wed Feb 18, 2009 21:03:38 GMT
Blowing it up would destroy evidence of protoplanetary formation or L4/L5 asteroid formation
that we would probably like to keep around for further investigation.
Maybe instead, we could strategically attach a solar sail to it that would eventually pull it out of
the L4/L5 and away from Earth's orbit. To make sure it has enough force to leave the gravity well,
we could start the motion with a rocket booster or small explosive force on the side closest the Sun.
8. This Is New *scientist*?
by Alan Fletcher / Wed Feb 18, 2009 22:49:00 GMT
Gravity is not "neutralized" at the Lagrange points.
Lagrange Point.
An object is not "weightless" at the
A "Lagrange Point" just a place where the gravitational forces of a large body (Sun) and a
smaller body (Earth) act to keep a third miniscule body (Asteroid or Probe) in the same position
RELATIVE to the smaller body (Earth).
As the Earth revolves around the Sun, the Langrange Point revolves in lock-step.
See http://math.ucr.edu/home/baez/lagrange.html
9. This Is New *scientist*?
by Valerie / Thu Feb 19, 2009 11:00:11 GMT
The gravitational forces at the Lagrange points balance, so there is no net gravity. The Lagrange
points are the only places in space where you would be truly weightless.
10. Are You So Sure?
by ChrisChaos / Thu Feb 19, 2009 00:52:23 GMT
> "Alas, Weigert and colleagues came up empty-handed as their search wasn't detailed enough."
Sure? Or could it just be that there is nothing to find there.
11. Are 'L4' & 'L5' "Rahu" And "Ketu"
by soumyasrajan / Thu Feb 19, 2009 01:18:59 GMT
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I am not an expert in astrophysics or Indian astrological studies. But I find an intriguing
similarity.
In the Indian astrology since ancient times apart from the planets, they always use 2 more
imaginary planets (which do not exist) called "Rahu" and "Ketu". T hese imaginary planets are also
considered a bit of destroyers.
I wonder whether Rahu and Ketu have anything to do with 'L4' and 'L5'. The positions of Rahu Ketu in Indian astrology are charted well for many years. I t will be interesting to find out whether
those positions have anything to do with positions of 'L4' and 'L5.' Also whether even in ancient
times, people may have means to identify positions of 'L4' and 'L5'?
12. Gravity "Holes" -- what Nonsense!
by Derek Smith / Thu Feb 19, 2009 09:46:02 GMT
The Earth and the Sun are gravity traps. If an object gets caught by one and sinks into its gravity
hole, then on Earth you have to strap a Titan to it to shift it out of the gravity hole.
By contrast, 'L1' and 'L2' can e thought of as gravity hills with relatively flat tops. Stand right on
the top, and all the forces of gravity are balanced out -- NOT "cancelled out". Trouble is that at 'L1'
and 'L2', the hills are always on the move. So to stay in the balance point, you need to move with the
hill.
'L3' and 'L4', however, can be thought of as vast gravity plains. If you happen to be on one with
no velocity relative to it, then gravity is essentially equal in all directions. So the chances of getting
a tug out of there are low. But to have got up there in the first place means that the object would
have had to have been travelling. Then because there was no restraining gravity well, the object
would have simply carried on straight over the almost flat gravity plain and down the other side.
The only way an object is going to stop there is if it carries enough fuel to sump its velocity with
or if it happens to meet an object with equal and opposite momentum. What are the chances of that?
Because there is no friction out there, given long enough, everything is going to slide very slowly
off the plain or be blown off by the solar wind.
So if we do find stuff in an 'L' point, we should look at it very carefully and ask why is it still
there. Does it have a mind of its own??
13. re: Gravity "Holes" -- what Nonsense!
by Freegan / Thu Feb 19, 2009 12:06:02 GMT
Derek Smith correctly defines the nature of the Lagrangian points but forgets that any body will
create its own gravity well by virtue of its mass. Even at a Lagrangian point.
Should 2-or-more bodies coincidentally approach any point in space, they will attract one another.
Even at a Lagrangian point. Hence, there may exist clouds of minor bodies orbiting one another at these
points.
Something I find fascinating in the article is the precise 60 degree lead and lag of two of these points
relative to the position of the Earth.
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In my mind this raises the (admittedly remote) prospect of eventually finding a star system whose
outstanding feature is the existence of six similar planets in the same orbit, each occupying the
coincident Lagrangian points of its neighbors.
Perhaps with a little ingenuity and a lot of technology, in the future we may engineer such a situation
occupying the habitable zone of a star. Perhaps even ours.
14. Mr-breakdown
by RmR / Thu Feb 19, 2009 10:03:27 GMT
For an alien space craft to get "stuck" in 'L4' or 'L5, it would need to sync velocity with Earth
and then strangely break down, losing all power. It's not really a "trap", is it?
If a craft was wandering powerless through space, the probability of having a matched velocity
are next to nothing.
Who writes this stuff? PKD even after his most sleepless nights had a better grasp!
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