The Gaia space telescope is in a Lissajous orbit around the Sun-Earth Lagrange point 2. The orbit period is about 180 days and the size of the orbit is 263,000 x 707,000 x 370,000 km. It has been theoretically concluded that there should be a population of temporarily captured orbiters (TCO's) around that Lagrange point. All but a handful of them should be less than a meter in diameter and too small to be observed from Earth. Only one TCO, 2006 RH120, has ever been observed, a 5 meter diameter one.

Since Gaia will be in a somewhat similar orbit as the TCO's and will scan the entire sky several times over the coming years, and an IR telescope is sensitive to asteroids, I wonder if it will be able to detect such TCO's. And if so, what could we find out about them? Gaia will not track any asteroids nearby, if it catches the same TCO more than once it would be by chance, I suppose. Could Gaia learn about their sizes, orbits, surface compositions?


2 Answers 2



Near-Earth objects (NEOs) may be temporarily captured to oribt Earth as discussed in the reference quoted in the question. Such a capture must invariably involve the NEO to pass by the L1 or L2 points, which are the saddle points in the co-rotating binary potential of the Earth-Sun system.

Thus, these objects have just high enough co-rotating energy (Jacobi constant) to enter the region around the Earth via those saddle points. Since they can only escape that region by exiting through the same saddle points, it may take a while before they "find" the exit, i.e. they are temporarily captured.

However, as I read that paper, there is no expectation of NEOs captured on orbits around the L1/L2 points. In fact, there are no stable orbits around those Lagrange points and hence no such captured satellites are expected. Indeed, this is one of the major benefits of putting an artificial satellite there. It also means that, in order to keep Gaia at L2, it will need to correct its orbit from time to time. Otherwise, it will eventually drift off the L2 region. This has the advantage that no space debris from missions like this will accumulate there.

Thus, to answer your question: there are no satellites orbiting near L2 to be discovered. The chance of any asteroid passing by (on its way to or from temporary capture) is rather small or, equivalently, the typical NEO passing there on a regular basis is tiny (1m or so), i.e. hardly detectable. Moreover, any NEO moving near Gaia will have a huge proper motion and be highly unlikely to be seen on more than one scan (and different sightings cannot be related).

Of course, NEOs elsewhere can and will be detected by Gaia, as discussed in another answer, in particular Trojan asteroids (orbiting L4/L5 on stable orbits) of Earth and Mars.

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    $\begingroup$ The chance being small of an asteroid passing is irrelevent to whether such an object could be detected should one actually pass by. Furthermore, being detected on 'only on one scan' is still detection which is the question which was asked. Arguing improbability and inability to correlate differnt glimpses of the same object does not move actual detection of such objects by Gaia from the yes column to the no column. $\endgroup$ Sep 3, 2014 at 1:07
  • $\begingroup$ A small object reflects less light and my be below the significance limit of the CCD. From a single scan one only knows "there was something there in this direction" (which is accurate in one dimension and inaccurate in the other), but god know what. This is not a detection. $\endgroup$
    – Walter
    Sep 3, 2014 at 7:37


Detection of inner Solar System Trojan Asteroids by Gaia (pdf):

The Gaia satellite, planned for launch by the European Space Agency (ESA) in 2013, is the next generation astrometry mission following Hipparcos. While mapping the whole sky, the Gaia space mission is expected to discover thousands of Solar System Objects. The se will include Near-Earth Asteroids and objects at Solar elongations as low as 45 degrees, which are difficult to observe with ground-based telescopes. We present the results of simulations for the detection of Trojan asteroids in the orbits of Earth and Mars by Gaia.

  • $\begingroup$ Yes, but trojan asteroids would be 1,000 times further away than temporarily captured orbiters around the same lagrange point as Gaia. $\endgroup$
    – LocalFluff
    Aug 28, 2014 at 16:28
  • $\begingroup$ That just implies that the ones orbiting the same Lagrange point will appear brighter. It's doubtful anything'll get close enough to show a disk, much less be out of focus. Depending on exposure time, you'd likely see more streaking with the nearby ones, but that'd just tell you that they're nearby, or moving absurdly fast. $\endgroup$ Aug 28, 2014 at 17:10
  • $\begingroup$ Your answer is wrong. The question considered asteroids near Earth's L1 point. Trojans are near L4 and L5. Detectability of the latter doesn't imply that of the former, even if they existed. $\endgroup$
    – Walter
    Sep 3, 2014 at 7:42
  • $\begingroup$ @Walter L2, not L1. Your comment is wrong :-) $\endgroup$
    – uhoh
    Sep 16, 2016 at 16:17

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