In a discussion below this answer to Can we use something like RADAR to detect asteroids? I'm complaining that the answer is essentially wrong;

Yes, radar is one of the useful tools for detecting and observing asteroids.

The answer goes on to talk about how radar is used to make measurements, and even that "Yes" link says:

Discovered by astronomers using optical telescopes...

I'd like to ask if radar has ever been used to detect previously unknown asteroids.

Question: Has there ever been an instance of asteroid discovery by radar; seen first by radar rather than being observed after optical discovery?

I find the other answer much more sound:

...Can RADAR be used to systematically detect NEOs that escape telescopes' detection? The answer is: YES, in principle, but it would be very very costly. RADARs have to rely on very directive transmitters. It can illuminate only a small portion of the sky at each time.

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    $\begingroup$ There is discussion in NgPH's answer, although it is a little "cheaty" $\endgroup$ Jul 16, 2021 at 13:30

2 Answers 2


Partial answer to get things moving.

While so far I haven't found an asteroid proper that was first observed by radar, there are at least several examples of objects that were first discovered optically but that were subsequently determined to be binary asteroids or have minor-planet moons of their own.

However, checking the binary asteroids "87 Sylvia, 107 Camilla, 45 Eugenia, 121 Hermione, 130 Elektra, 22 Kalliope, 283 Emma, 379 Huenna and 243 Ida (in order of decreasing primary size)" from that Wikipedia article shows that their multi-body statuses were all determined by optical telescopes.

Minor-planet moon discoveries do happen via radar:

It now becomes difficult to say (for me at least) if these are asteroid discoveries by radar proper, or instances of learning more about a previously discovered asteroid by using radar to detect asteroid moons.

From Wikipedia:

Radar observations during the 2017 flyby have shown that Florence has two moons. The inner of the two moons is estimated to have a diameter of 180 to 240 meters, the outer moon is between 300 and 360 meters across. Each moon is somewhat elongated, and both are tidally locked to the main body. They probably formed as loose material spun away from the main body as its rotation accelerated due to the YORP effect.

A team of NASA's Jet Propulsion Laboratory (JPL) (Pasadena, California) scientists led by Marina Brozovic and Lance Benner, made the discovery using radar imaging at NASA's Goldstone Solar System Radar on June 12 and June 14, 2009. They showed that the near-Earth asteroid 1994 CC is a triple system, which encountered Earth within 2.52 million kilometers on June 10, 2009. This relatively close approach to Earth made the discovery possible, as before the approach, scientists knew very little about this asteroid. In fact, 1994 CC is only the second triple system known in the near-Earth objects population.

In 2008, scientists using the planetary radar at Arecibo Observatory discovered that the object is orbited by two satellites, when the triple asteroid made a close approach to Earth of 0.066 AU (nearly 10 million kilometers). The largest body (preliminarily called Alpha) is spheroid in shape, with principal axes of 2.8±0.1 km, 2.7±0.1 km, and 2.9±0.3 km, with an effective diameter of 2.5±0.3 km and a density of 1.1±0.2 g/cm3. The satellites, named Beta and Gamma, are several times smaller in size. Beta is 0.77±0.12 km in diameter and Gamma 0.43±0.14 km

Asteroid 3122 Florence with its moons.

above: Asteroid 3122 Florence with its moons. Source, also Earthsky.org Asteroid Florence Found To Have 2 Moons. below: Radar imaging of 1994 CC's moons at NASA's Goldstone at different times. Static. Source, also NASA: Triple Asteroid System Triples Observers' Interest

enter image description here

From What is the physical geometry of this apparent “eclipse” of a tiny moon of Asteroid Florence? in Astronomy SE:

below: "A radar image shows asteroid 3122 Florence and tiny echoes from its two moons. Here is an animation that shows them more clearly. The direction of the radar illumination (and thus the direction toward Earth) is at the top." From here. NASA / Jet Propulsion Laboratory. This is a small subset of the frames contained in the original 36 MB GIF, and the size has been decreased by a factor of 2 in order to fit in SE's 2 MB limit.

A radar image shows asteroid 3122 Florence and tiny echoes from its two moons.


This question is sharper than a previous one, to which it is linked by an answer, considered as incorrect by @uhoh. The formulation "Has there been used to discover?" is more challenging than a "can it be used for detection?". The latter can be answered by a few engineering facts on capability, with caveats added for feasibility (to water down enthusiastic misinterpretations). The former requires in priniciple an exhaustive survey of the discoveries so far. Here is the full database, in case you are brave enough to take up the challenge.

A sharp question is not necessarily better. In my opinion the base question is "CAN RADAR be a better technique for searching for all NEOs, especially the hard-to-detect ones, yet potentially hazardous for humanity?".


This in turn calls for several interrogations: (a) Why do we need a better early-warning system?; (b) How have discoveries been performed so far and what are the short-comings?; and (c) Which new technologies could be in our road-map (RADAR among them)?

Answering these questions satisfactorily are far beyond my capability (and may need a full book). But here are some indicative clues that I have found:

-WHY DO WE NEED AN EARLY WARNING SYSTEM? It suffices to link these two facts:(a) the 2013 impact; and (b) the goal set by the US congress in the 2018 bill, Section 321. In Section 321, the set goal is to detect and catalog 90% of NEOs of size >140 meters, ONLY by the Year 2033. To put this into perspective, the Chelyabinsk meteor has an estimated diameter of ~20 m, a “too small” size to be contemplated for this goal. And yet, a 500Kg fragment of its meteorites has been found(!). Would it not be important to be able to detect the meteor, even a couple hours before impact?

-HOW DISCOVERIES HAVE BEEN PERFORMED SO FAR? Mostly passively, ground-based detections, in the visibility and infrared spectra. “Passive” means that the illuminator is not under our control (but “free”. Here, the Sun or the object’s radiation itself, if it does radiate strongly enough). The process involves taking “pictures” of a small portion of the sky at different times. Something that is “near-earth” must move in the captured pictures if the timing is set adequately. Basically it is a movement-detection, passive process. The main drawback of this process is that the favorable geometry puts the Earth between the illuminator and the target. Chelyabinsk is an example of an unfavorable geometry.

-WHY NOT “ACTIVELY” OR SPACE-BASED? “Active” means that we control the illuminator (time, direction, intensity, modulation), but the downside is that it is not free. You can play with this tool to have a first idea of the RADAR limitations. “Space-based” means that we put the detector in-orbit, possibly around the Sun.

Note that the US Congress Bill of 2018 mentions space-based technologies, BUT NOT RADARS (sorry to fans of Arecibo and like).


Discovery of objects on known trajectories (such as moons of known asteroids, of multi-body asteroids), YES.

Discovery of an object with unkown trajectory, very likely NO (see above discussions). For NEOs, it is possible to scan the database of discovered objects and have an definitive answer. The OP should know the existence of this database (otherwise the question is open-ended).

  • $\begingroup$ "unstated motivation" My motivation is to get an accurate answer to this question as it is written. Please don't say that I have an ulterior motive or I'll flag your post as "targeting"; your answer should not address me or your imaginations of my motivations. Please stick to the question as asked. Thanks! If you have a problem with the question, flag or post something in meta. $\endgroup$
    – uhoh
    Jul 19, 2021 at 10:40
  • $\begingroup$ A temporary, reversible -1 for not supporting the last paragraph beginning "Discovery of an object with unknown trajectory, NO" which is the only part of the post currently that addresses the question. How does the author know it's "No"? How can future readers judge the veracity of this "No" if it's unsourced? $\endgroup$
    – uhoh
    Jul 19, 2021 at 10:42
  • $\begingroup$ "Here is the full database, in case you are brave enough to take up the challenge." If one isn't willing to do so, nor to find an alternate search method (there may be lists sorted by discovery instrument for example, or one might simply search for "discovery" and "asteroid" and the names of the major deep space radar installations) then it seems one is not prepared to post an an actual answer, in which case what is posted is not an answer and can be flagged for deletion. What is posted here now is an opportunistic blogpost, hijacking my question. $\endgroup$
    – uhoh
    Jul 19, 2021 at 10:58
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    $\begingroup$ @uhoh, I expect that one day you would post such a "nice" result to your own question (unless you have concluded that it is a boring task). I wonder still whether this constitutes a "fact" to SE standard, for a nitpicking person on SE. $\endgroup$
    – Ng Ph
    Jul 19, 2021 at 13:56
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    $\begingroup$ The very small part of this answer that actually attempts to answer the question as written is an unreferenced guess. $\endgroup$ Jul 19, 2021 at 13:56

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