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STEREO: Into the Lagrangian Points

I love it when we find uses for instruments that they were never intended for. In deep space terms, we can go back to Voyager 2, which carried a plasma wave instrument that was designed to measure the charged particles inside the magnetic fields of the gas giant planets it would pass. Voyager 2 was able to tell us much about dust impacts on a fast-moving spacecraft when it was realized that the plasma wave instrument would be able to sense the plasma created by vaporized particles. In other words, the instrument became a de facto dust detection device.

Now I see that the two STEREO spacecraft may be pressed into service to study what’s lurking in the L4 and L5 Lagrangian points, each 150 million kilometers from Earth, with L4 60 degrees in front of our planet and L5 60 degrees behind. Balancing the gravitational field of the Sun with that of Earth, the Lagrange points are interesting places, possibly a junkyard of debris from the early Solar System. It’s known that such points appear around other planets, as witness the thousand or so asteroids that make up the Trojans at Jupiter’s L4 and L5 points.


Image: Relative positions of both STEREO spacecraft, also showing the SOHO and ACE spacecraft. Credit: NASA/Solar TErrestrial RElations Observatory team.

Launched in 2006, the STEREO spacecraft were designed to monitor solar storm activity, one of them moving ahead of the Earth in its orbit, the other behind. But as they gradually move through L4 and L5, their heliospheric imagers can be put to work looking for asteroids. New Scientist has just run a story on the prospect, going on to speculate on an audacious idea put forth by Richard Gott (Princeton), who believes a whole population of objects may be found in these intriguing areas.

Addendum: The statement above turns out to be in error. The concept actually originated with Edward Belbruno at Princeton, and resulted in a paper for which he was lead author. That paper is Belbruno, “Where Did The Moon Come From?” Astronomical Journal, Vol. 129, No. 3 (March 2005), pp. 1724-1745 (available online). Dr. Gott was co-author on this work.

Gott wonders whether a large object of the sort believed to have struck the Earth some four billion years ago, thus creating the Moon, may have grown up close by, gradually nudged out of one of the Lagrangian points into collision with our planet. It’s worth considering, since as the New Scientist article points out, an incoming Mars-sized object from elsewhere in the Solar System would have struck the Earth with so much energy that it would have destroyed it. That and the makeup of oxygen isotopes found on the Moon — the same as on Earth — hints that the impactor must have formed in nearby space. From the article:

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 this strengthens the case for the Earth-impactor to have formed there.

Back to that dust issue — the STEREO vehicles have been seeing a wide variety in dust impacts, from just a few up to several thousand per day. What will STEREO’s heliospheric imagers find at the L4 and L5 points? Earth-based telescopes have yet to find asteroids there, so the current thinking is that any that do lurk in the regions are small, less than a kilometer across. But if Gott is right, even a few lucky finds could be fodder for a future mission, one that would give us new insight into the early history of the Solar System.

Comments on this entry are closed.

  • andy February 19, 2009, 16:08

    Talking of Lagrangian points, what’s the current opinion on the Kordylewski clouds which have been claimed to exist in the Moon’s L4 and L5 points?

  • Administrator February 19, 2009, 16:17

    I’ve never heard of Kordylewski clouds. Do you have any background information?

  • devicerandom February 19, 2009, 20:03

    It seems Wikipedia is aware of them:

  • Administrator February 19, 2009, 21:22

    Yes, I read the Wikipedia entry. First time I ever heard about this!

  • Adam February 20, 2009, 1:59

    Hi All

    First I ever heard of them – except in fiction! Larry Niven mentions them as a tale within a tale in his Known Space story “Flatlander” (I think) which has Beowulf Schaeffer describing an early mission to the asteroids that flew into Lagrange dust-clouds and was almost destroyed. I didn’t know where Niven got the idea from, until now!

    Might provide a mechanism for the Long Delay Echoes (LDEs) that are detected with a 3 second delay by radio transmitters. Ron Bracewell’s suggestion of an alien probe in the L4/5 points was inspired by the LDEs, and astronomer Anthony Lawton investigated the LDEs intensively in the 1970s finding that they occurred when the trailing L point was in the sky, sometimes with a delay of many hours. He speculated that a cloud of plasma trapped the signals before releasing them – a neat trick for a cloud of plasma to be so choosy about storage timing. I guess a dusty plasma, charged by solar UV and the solar wind, could do the job, but an alien probe is more fun.

  • igrnemo February 20, 2009, 3:01

    I would like to propose that instead of “Lagrangian Points” rename them to “Lagrangian Fields”. I think this term describes the attributes of them. What is the opinion?

  • ljk February 20, 2009, 13:30

    Why would an ETI probe just sit in the L points all these decades
    after sending back a few of our transmissions? Is it waiting for a
    response? For us to grow up enough to accept the possibility that
    we have visitors in the Sol system?

    If it is monitoring us simply for study, why respond at all? Wouldn’t
    that disrupt the native life’s natural behaviors and ruin the study?

    One thing is for certain: If the satellites do find something unusual
    in the L points, they will be written off as “just” some planetoids by
    all but the fringe element, who will dilute and ruin any real chance
    to study them further.

  • andy February 20, 2009, 16:21

    I would like to propose that instead of “Lagrangian Points” rename them to “Lagrangian Fields”. I think this term describes the attributes of them. What is the opinion?

    No. Lagrangian points are where the balance of gravitational and centrifugal forces (this problem is easiest to set up in the corotating reference frame – it’s a noninertial frame hence we can talk about centrifugal force) result in a massless test particle remaining stationary with respect to the system of the two massive bodies. The Lagrangian points are the only solutions to this problem – not the regions around the points.

    In the case that the ratio of the primary mass to the secondary mass exceeds 24.96, the L4 and L5 points are stable (i.e. for sufficiently small displacements the particle will experience a restoring force and thus oscillate about the Lagrangian point), but L1, L2 and L3 are unstable, as are L4 and L5 when the mass ratio condition is not met. Talking of these points as “fields” is misleading.

  • Adam February 20, 2009, 17:26

    Hi Larry

    Both Lawton and Freitas did telescopic studies of the Earth-Moon L4/5 points and didn’t find anything down to a few metres in size. If a Bracewell probe was there, then it has moved on, perhaps out of prudence now it knows we can physically intercept it. Duncan Lunan published a translation in 1973 of the LDEs received in the 1920s which indicated the probe had something to do with the constellation of Bootes, and after Lawton’s work Lunan retracted the claim – but 20 years later was of the opinion there might have been something to Bootes being a reference constellation for the probe’s navigation. Maybe. Curious galactic alignments appear around various old paleo-observatories which Lunan thinks might indicate advanced knowledge of the Galaxy, but he’s stretching a long chain of inferences to extract an ETI contact from that.

    I have Lunan’s 1973 book “Man and the Stars” and a book that Lawton contributed heavily to, “CETI”, so I’m in the rare position of being able to look at both sides of the discussion about LDEs. Lunan’s book is still very pertinent to Contact studies and represents the combined brain-powr of the Scottish astronautical society, ASTRA. It’s surprisingly not as dated as you might expect. Its follow-up, “Man and the Planets” (1983) is still full of fresh ideas even after 25 years. The Lawton book is full of 1970s Space Shuttle/O’Neill Colony optimism, refreshingly untainted by the disappointing reality of the Reagan years and beyond.

    Back to the question: perhaps the probe has moved? Or cloaked itself? Perhaps it controls the dust and uses it for obscuring vision and radar? of course it may not exist, but that’s no fun at all!

  • ljk February 20, 2009, 22:35

    And of course we keep assuming these probes are big enough
    to see from a long distance, even after knowing about nanotech
    for decades now.

    If an ETI wanted to examine us without being detected, it would
    not take much – and flying around in giant ships all over the
    place is not it.

  • Thomas February 21, 2009, 17:47

    Not necessarily much of a contribution in this comment, but I wish to say “Thank-you”, for not over-sensationalizing this news story like everyone else is trying to do.

    i.e. “Killer asteroids stalking Earth in gravitational sink-holes.” or something like that.

    Centauri-Dreams seems to have always been very realistic in its news headings, and I respect that very much.

  • Administrator February 21, 2009, 18:57

    Very kind of you, Thomas. I appreciate the comment!

  • slang February 22, 2009, 6:19

    Gott thinks that any objects still in L4 and L5 may be leftovers from the formation of that impacting body.

    If found, might they not equally well be leftovers from the collision itself? Could a sample return mission distinguish between those two options?

  • ljk April 11, 2009, 8:58

    NASA Science News for April 9, 2009

    NASA’s twin STEREO probes are entering a mysterious region of space to look for remains of an ancient planet which might have orbited the Sun not far from Earth. If they find anything, it could solve a major puzzle–the origin of the Moon.



  • jonathan jay August 7, 2009, 15:51

    What if – after STEREO probes A & B have confirmed there is not an equipment-damaging cloud of solar-system dust-bunny-debris gathered around the L4&L5 nodes – a twin set of interferometrically linked observation platforms (Langrangian Space Binoculars, if you will) were placed there and began delivering high-resolution images of extremely bright objects? Wouldn’t that be a highly informing new observation capabilty that would deliver new data presently un-obtainable? Just think what kind of angular resolution could be obtained with a baseline of 1.7 AU! What is the smallest observable wavelength these machines could presently be engineered deliver? I sure wish some group would take this on as a project, and get twin craft into position – i have dreamed about this now for 20 years.