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A sentence from the abstract of The population of natural Earth satellites states:

At any given time there should be at least one NES of 1-meter diameter orbiting the Earth. The average temporarily-captured orbiter (TCO; an object that makes at least one revolution around the Earth in a co-rotating coordinate system) completes (2.88 ± 0.82) rev around the Earth during a capture event that lasts (286± 18) d.

As far as I know, the only known, well documented temporarily captured Earth satellite is 2006 RH120, which was in within 1 million km of the Earth for about 1 year. It has an estimated size of 2 to 3 meters, a rotation period (about it's axis) of about 3 minutes (presumably determined by photometry). According to Wikipedia:

2006 RH120 was discovered on 14 September 2006 by the 27-inch (690 mm) Schmidt camera of the Catalina Sky Survey in Arizona.

Using data from the JPL Horizons database, it seems 2006 RH120 was only about 850,000 km from Earth at the time it was discovered.

Deep sky surveys are extremely prolific at detecting small solar system bodies. There was also a recent answer addressing the possibility that the Gaia spacecraft's could detect trojan asteroids associated with Earth's orbit, linking to the paper Detection of inner Solar System Trojan Asteroids by Gaia.

The question asked specifically about TCO's being detected by Gaia, but my question is more general: How would a small TCO (temporarily captured orbiter) or other natural Earth satellite most likely be detected?

There are Earth-based sky surveys, space based visible and near-visible, and possibly even infrared/thermal detections possible, even Earth-based radar, and possibly others.

Are there any searches specifically targeting TCOs?

enter image description here

above: a simple plot of the orbit of 2006 RH120 from JPL Horizions transformed to an Earth-centered Earth-Sun-synodic frame - red line with dots every 10 days, for the period when it was within a +/- 1.6 million km cube centered on Earth in this frame. The green thick band around Earth is the moons orbit during this 448 day period.

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  • $\begingroup$ We likely only see these if we made a serious effort to look for them - so the answer would be "by whatever means we chose to look for them". Satellites at the various Lagrangian points would be strong contenders if you could find somebody willing to spend money on that. Otherwise a systematic phtographic survey around the ecliptic. Such a thing would probably be a good idea if we wished to avoid extinction in the medium term - see theguardian.com/commentisfree/2009/jul/21/… for instance $\endgroup$
    – adrianmcmenamin
    Commented Sep 18, 2016 at 19:29
  • $\begingroup$ @adrianmcmenamin Thanks! Sky surveys may not originally have been primarily focused on finding tiny asteroids, and SOHO was definitely not planned as a prolific comet-finder, and yet... Anyway in this case there is vis/NIR, and there is thermal IR, and there is radar, and there may be others techniques as well. I'm looking for a scientific view of which is most likely considering both existence of instruments, and their capabilities. In this case the TCOs with a useful frequency of capture will be of order 1 meter, not so much a threat & slow enough to be studied or brought back to Earth. $\endgroup$
    – uhoh
    Commented Sep 18, 2016 at 23:19
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    $\begingroup$ It makes sense to me that a temporary captured orbiter and a near earth asteroid would likely be observed by the same method because the 1-3 meter space-rock a million miles away in a weird earth orbit and the 100 meter space rock 20-100 million miles away bear a strong resemblance to each other. But that's just a guess, that seems logical to me, not an answer. Wikipedia chart on how NEOs of sufficient size have been discovered: en.wikipedia.org/wiki/Catalina_Sky_Survey#/media/… $\endgroup$
    – userLTK
    Commented Sep 19, 2016 at 15:04
  • $\begingroup$ @userLTK It looks like two of those have become fairly dominant, and I like your logic. I can't think of anything "special" about TCOs that could warrant a special observation technique, except that when they are already close, a radar measurement might pick them out (strength goes as $1/r^4$ and there is distance information) but I haven't heard of any radar surveys specific for this. Maybe you could post an answer? $\endgroup$
    – uhoh
    Commented Sep 21, 2016 at 2:47

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Cruithne and J002E3 were detected using ground based astrophotography. J002E3 is thought to be "the S-IVB third stage of the Apollo 12 Saturn V rocket ... It is thought that J002E3 left Earth orbit in June 2003, and that it may return to orbit the Earth in the mid-2040s.1" Ground telescope also found 6Q0B44E which orbits beyond the moon. 2002 AA29 was found via automatic sky survey. Several of these objects are in horseshoe orbits.

Regardless, the discovery of these objects usually comes via conventional astrophotography.

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  • $\begingroup$ Since we now have a new discovery and therefore an actual datapoint, you can wrap this up nicely! :-) $\endgroup$
    – uhoh
    Commented Mar 20, 2020 at 0:52
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Both 2006 RH120 and 2020 CD3 are listed as being Catalina Sky Survey discoveries. CSS is not a particularly large telescope, they’re just a tight program with on-the-ball management.

How would a small TCO (temporarily captured orbiter) or other natural Earth satellite most likely be detected?

By some minimum aperture (needs photons and signal-to-noise), some minimum process for faint object motion recognition (we stopped using the ol’eyeball by now) and tracklet linking (have to identify TCOs as such, not Main Belt objects or high-altitude satellites), and some minimum luck. Asteroid search has to economize, and TCOs don’t move like most asteroids do, so there are likely objects slipping through the search parameters.

In the (near-) future, Vera Rubin Obs will be a prime resource due to its aperture, but its search parameters were a compromise with the dark energy people and are unfavorable for TCOs. My guess is that a TCO will be found when someone links a threshold detection of some ‘lesser’ telescope to an unrecognized detection of VRO.

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    $\begingroup$ Thanks for your answer! Citing a supporting source we can all read for "but its search parameters were a compromise with the dark energy people and are unfavorable for TCOs" would be necessary to make this a good Stack Exchange answer. Except in extreme cases, personal knowledge (e.g. "trust me, I worked on it" is not a good basis with which to support an answer. Why is this true? From what sources can others verify it? $\endgroup$
    – uhoh
    Commented Sep 10, 2022 at 14:04

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