<p>The <a href="https://en.wikipedia.org/wiki/Chelyabinsk_meteor" rel="noreferrer">Chelyabinsk meteor</a> exploded over Chelyabinsk Oblast just shy of a decade ago. It was estimated after the fact to have been about 20 meters in diameter. No organization saw it before it entered the atmosphere. While detection techniques have improved slightly since then, asking for detecting everything larger than 10 meters across is still asking for far too much.</p>
<p>From <a href="https://www.nasa.gov/planetarydefense/faq" rel="noreferrer">this article</a>, which is slightly dated (2019),</p>
<blockquote>
<p>However, given the current incompleteness of the NEO catalogue, an unpredicted impact – such as the Chelyabinsk event – could occur at any time.</p>
</blockquote>
<p>NASA's goal is to eventually find every near Earth object that is 140 meters across or larger. NASA has not yet reached this goal; new discoveries are made on a weekly basis. Ten meters is asking for far too much.</p>

CC BY-SA 4.0
2022-11-08T11:18:27.237Z
Hi! Great answer. One question: What does "near Earth" refer to in the last paragraph? Is it a specific distance?
Stef
CC BY-SA 4.0
2022-11-08T11:29:49.093Z
@Stef A [Near Earth Object](https://en.wikipedia.org/wiki/Near-Earth_object) (NEO) is any small solar system body whose perihelion distance is 1.3 AU or less.
David Hammen
CC BY-SA 4.0
2022-11-08T15:02:57.640Z
@user253751 The vast majority of those long-period comets have a perihelion distance that is far, far greater than 1.3 astronomical units (AU). That said, a large dead comet that sneaks in unbeknownst from sunward is one of the scariest end of life from the skies scenarios. All mitigation techniques thought of to date would require decades, maybe even centuries, of advance notice to push aside a very large object.
David Hammen
CC BY-SA 4.0
2022-11-08T18:25:45.857Z
And in any case, the objects are tracked based on their natural orbits. An alien spacecraft would just appear in one image and be in totally unexpected place a day later, and we would never determine an orbit for it. It would just be categorized as a spurious dot in a photo.
jpa
CC BY-SA 4.0
2022-11-09T12:25:58.700Z
Add in that the object moves are high speeds - especially if the come from the general direction of the sun, its hard to detect them given the glare
eagle275
CC BY-SA 4.0
2022-11-09T15:58:10.953Z
@DavidHammen Thank goodness anything sunward is spaceward half a year later ;-).
Peter - Reinstate Monica
CC BY-SA 4.0
2022-11-09T16:25:06.593Z
@Peter-ReinstateMonica Just a few days or weeks later, particularly if the comet is a long period comet whose apogee is well beyond Pluto. Comets move very quickly near perihelion. That is why they are scary. If they are undetected while inbound, there is no mitigation time. On the other hand, if such an inbound comet is detected while it is still well outside of Pluto's orbit, there might well be time.
David Hammen
CC BY-SA 4.0
2022-11-08T05:13:43.103Z
2022-11-08T05:31:27.953Z
2022-11-08T05:31:27.953Z
David Hammen
<p>Each year, several small NEOs pass Earth closer than the distance of the Moon. <a href="https://minorplanetcenter.net//iau/Closest.html" rel="noreferrer">Here is the list</a> of NEOs that has been detected recently but still, many of them passes through undetected. The main problem with estimating the number of NEOs is that the probability of detecting one is influenced by many reasons. Some of them include:</p>
<ul>
<li>size</li>
</ul>
<p>Most of the NEOs are very small to the point where it is very hard to detect even with advanced technology</p>
<ul>
<li>characteristics of its orbit</li>
</ul>
<p>The orbit of NEO can deviate or perturbate from Moon's, Earth's or any other planets' gravity leading to its detection very hard</p>
<ul>
<li>reflectivity of its surface</li>
</ul>
<p>Smaller NEO reflect less light and hence can't be detected easily.</p>
<ul>
<li>Position of Earth wrt Sun</li>
</ul>
<p>It is much harder to spot objects on the day-side of Earth. The day sky near the Sun is much brighter than the night sky, and there is therefore much better contrast in the night sky. The night-side searcher is also looking at the sunlit side of the asteroids, while in the daytime sky a searcher looks towards the sun and sees the unlit backside of the object. In addition, opposition surge makes asteroids even brighter when the Earth is close to the axis of sunlight. This is equivalent to the comparison of a Full moon at night to a New Moon in daytime, and the light of the Sun-lit asteroids has been called &quot;full asteroid&quot; similar to a &quot;full moon&quot;.</p>
<p>Various astronomical program was initiated dedicating to detect and discover NEOs. In 1998, the United States Congress gave NASA a mandate to detect 90% of near-earth asteroids over 1 km diameter by 2008. Other program and surveys have regulated their own mandating this criteria detection and as a result, estimated total number of NEOs larger than 1 km in diameter rose from about 20% in 1998 to 65% in 2004, 80% in 2006, and 93% in 2011. As of November 2021, 891 NEOs larger than 1 km have been discovered, or 97% of an estimated total of about 920.</p>
<p><a href="https://i.sstatic.net/vc8WJ.png" rel="noreferrer"><img src="https://i.sstatic.net/vc8WJ.png" alt="enter image description here" /></a></p>
<p>In 2005, the original USA Spaceguard mandate was extended which calls for NASA to detect 90% of NEOs with diameters of 140 m  or greater, by 2020. As of January 2020, it is estimated that less than half of these have been found, but objects of this size hit the earth only about once in 2000 years. In January 2016, NASA announced the creation of the Planetary Defense Coordination Office (PDCO) to track NEOs larger than about 30–50 m in diameter and coordinate an effective threat response and mitigation effort.</p>
<p><a href="https://i.sstatic.net/CQAJa.png" rel="noreferrer"><img src="https://i.sstatic.net/CQAJa.png" alt="enter image description here" /></a></p>
<p>Picture Source: <a href="https://en.wikipedia.org/wiki/Near-Earth_object" rel="noreferrer">Near-Earth object - Wikipedia</a></p>

CC BY-SA 4.0
2022-11-09T04:40:46.240Z
2022-11-09T04:40:46.240Z
Nilay Ghosh
<p>I am aware of (but not really knowledgeable about) radar and visible device networks that monitor/track asteroids, satellites, space debris, and monitor the world for rocket launches for defense purposes.  It looks like the asteroid network can track items as small as a few meters in diameter. Same goes for satellite monitoring networks.  It seems that all objects in the vicinity of the Earth might be detected.</p>
<p>With all these detection networks, could an object greater than, say, 10m in diameter leave (or enter) the vicinity of the Earth without being detected by one of these networks?    Assuming, of course, that the object is visible and detectable on radar.<br />
Are there holes in the coverage of these networks?</p>
<p>By &quot;Vicinity of the Earth&quot;, I would refer to mean within orbit of Jupiter to LEO.  Perhaps not a good phrase. Is it likely that there could an undetected object, approx 10m diameter, in this region with all the existing detection networks.</p>

CC BY-SA 4.0
2022-11-08T15:38:52.900Z
Can you clarify what you mean by "the vicinity of the earth"? I think you may be asking about an imaginary alien spacecraft that enters / leaves *orbit* around the earth (think of the ISS), but the answer so far has assumed asteroids-that-intersect-with-the-earth (think of the one that killed the dinosaurs). They are different answers.
Bear
CC BY-SA 4.0
2022-11-09T17:45:24.050Z
Ground radar hasn't detected that many, around 1000 objects in the past 50 years.  Compare that to Catalina Sky Survey and Pan-STARRS telescope each discovering over 1000 objects in 2019 which covers most of the discoveries that year with less than 300 coming from other sources.
rtaft
CC BY-SA 4.0
2022-11-09T20:48:22.783Z
The exact capabilities and operating parameters of the system that tracks space debris are probably classified and I couldn't find them in any case.  It wouldn't have to be active all the time and have 100% coverage for that purpose though.  Once you have an the orbit of an item calculated it won't change unless it provides it's own thrust or is acted on by an outside force so you can be confident you will know where it will be well into the future.  A system for finding and tracking objects coming in a single time at 20km/s from outside the Earth/Moon system would be designed differently.
Jason Goemaat
CC BY-SA 4.0
2022-11-09T22:52:21.913Z
@JasonGoemaat, space-debris tracking is mostly done using ballistic-missile warning radars.  Coverage isn't 100%, but it's pretty good, and it's active 24/7.
Mark
<space><near-earth-object>
Could an object enter or leave the vicinity of the Earth without being detected?
tckosvic
# Could an object enter or leave the vicinity of the Earth without being detected?

(Asked by: username_0 on 2022-11-08)

I am aware of (but not really knowledgeable about) radar and visible device networks that monitor/track asteroids, satellites, space debris, and monitor the world for rocket launches for defense purposes. It looks like the asteroid network can track items as small as a few meters in diameter. Same goes for satellite monitoring networks. It seems that all objects in the vicinity of the Earth might be detected.

With all these detection networks, could an object greater than, say, 10m in diameter leave (or enter) the vicinity of the Earth without being detected by one of these networks? Assuming, of course, that the object is visible and detectable on radar.  
Are there holes in the coverage of these networks?

By "Vicinity of the Earth", I would refer to mean within orbit of Jupiter to LEO. Perhaps not a good phrase. Is it likely that there could an undetected object, approx 10m diameter, in this region with all the existing detection networks.

**Tags:** space, near-earth-object
> **Comments:**
> **username_1**: Can you clarify what you mean by "the vicinity of the earth"? I think you may be asking about an imaginary alien spacecraft that enters / leaves *orbit* around the earth (think of the ISS), but the answer so far has assumed asteroids-that-intersect-with-the-earth (think of the one that killed the dinosaurs). They are different answers.  
> **username_2**: Ground radar hasn't detected that many, around 1000 objects in the past 50 years.  Compare that to Catalina Sky Survey and Pan-STARRS telescope each discovering over 1000 objects in 2019 which covers most of the discoveries that year with less than 300 coming from other sources.  
> **username_3**: The exact capabilities and operating parameters of the system that tracks space debris are probably classified and I couldn't find them in any case.  It wouldn't have to be active all the time and have 100% coverage for that purpose though.  Once you have an the orbit of an item calculated it won't change unless it provides it's own thrust or is acted on by an outside force so you can be confident you will know where it will be well into the future.  A system for finding and tracking objects coming in a single time at 20km/s from outside the Earth/Moon system would be designed differently.  
> **username_4**: @JasonGoemaat, space-debris tracking is mostly done using ballistic-missile warning radars.  Coverage isn't 100%, but it's pretty good, and it's active 24/7.  

---

## 2 answers

### Answer by username_5 on 2022-11-08. Score: 20

The [Chelyabinsk meteor](https://en.wikipedia.org/wiki/Chelyabinsk_meteor) exploded over Chelyabinsk Oblast just shy of a decade ago. It was estimated after the fact to have been about 20 meters in diameter. No organization saw it before it entered the atmosphere. While detection techniques have improved slightly since then, asking for detecting everything larger than 10 meters across is still asking for far too much.

From [this article](https://www.nasa.gov/planetarydefense/faq), which is slightly dated (2019),

> However, given the current incompleteness of the NEO catalogue, an unpredicted impact – such as the Chelyabinsk event – could occur at any time.

NASA's goal is to eventually find every near Earth object that is 140 meters across or larger. NASA has not yet reached this goal; new discoveries are made on a weekly basis. Ten meters is asking for far too much.

> **Comments:**
> **username_6**: Hi! Great answer. One question: What does "near Earth" refer to in the last paragraph? Is it a specific distance?  
> **username_5**: @Stef A [Near Earth Object](https://en.wikipedia.org/wiki/Near-Earth_object) (NEO) is any small solar system body whose perihelion distance is 1.3 AU or less.  
> **username_5**: @user253751 The vast majority of those long-period comets have a perihelion distance that is far, far greater than 1.3 astronomical units (AU). That said, a large dead comet that sneaks in unbeknownst from sunward is one of the scariest end of life from the skies scenarios. All mitigation techniques thought of to date would require decades, maybe even centuries, of advance notice to push aside a very large object.  
> **username_7**: And in any case, the objects are tracked based on their natural orbits. An alien spacecraft would just appear in one image and be in totally unexpected place a day later, and we would never determine an orbit for it. It would just be categorized as a spurious dot in a photo.  
> **username_8**: Add in that the object moves are high speeds - especially if the come from the general direction of the sun, its hard to detect them given the glare  
> **username_9**: @DavidHammen Thank goodness anything sunward is spaceward half a year later ;-).  
> **username_5**: @Peter-ReinstateMonica Just a few days or weeks later, particularly if the comet is a long period comet whose apogee is well beyond Pluto. Comets move very quickly near perihelion. That is why they are scary. If they are undetected while inbound, there is no mitigation time. On the other hand, if such an inbound comet is detected while it is still well outside of Pluto's orbit, there might well be time.  

---

### Answer by username_10 on 2022-11-09. Score: 5

Each year, several small NEOs pass Earth closer than the distance of the Moon. [Here is the list](https://minorplanetcenter.net//iau/Closest.html) of NEOs that has been detected recently but still, many of them passes through undetected. The main problem with estimating the number of NEOs is that the probability of detecting one is influenced by many reasons. Some of them include:

* size

Most of the NEOs are very small to the point where it is very hard to detect even with advanced technology

* characteristics of its orbit

The orbit of NEO can deviate or perturbate from Moon's, Earth's or any other planets' gravity leading to its detection very hard

* reflectivity of its surface

Smaller NEO reflect less light and hence can't be detected easily.

* Position of Earth wrt Sun

It is much harder to spot objects on the day-side of Earth. The day sky near the Sun is much brighter than the night sky, and there is therefore much better contrast in the night sky. The night-side searcher is also looking at the sunlit side of the asteroids, while in the daytime sky a searcher looks towards the sun and sees the unlit backside of the object. In addition, opposition surge makes asteroids even brighter when the Earth is close to the axis of sunlight. This is equivalent to the comparison of a Full moon at night to a New Moon in daytime, and the light of the Sun-lit asteroids has been called "full asteroid" similar to a "full moon".

Various astronomical program was initiated dedicating to detect and discover NEOs. In 1998, the United States Congress gave NASA a mandate to detect 90% of near-earth asteroids over 1 km diameter by 2008. Other program and surveys have regulated their own mandating this criteria detection and as a result, estimated total number of NEOs larger than 1 km in diameter rose from about 20% in 1998 to 65% in 2004, 80% in 2006, and 93% in 2011. As of November 2021, 891 NEOs larger than 1 km have been discovered, or 97% of an estimated total of about 920.

[![enter image description here](https://i.sstatic.net/vc8WJ.png)](https://i.sstatic.net/vc8WJ.png)

In 2005, the original USA Spaceguard mandate was extended which calls for NASA to detect 90% of NEOs with diameters of 140 m or greater, by 2020. As of January 2020, it is estimated that less than half of these have been found, but objects of this size hit the earth only about once in 2000 years. In January 2016, NASA announced the creation of the Planetary Defense Coordination Office (PDCO) to track NEOs larger than about 30–50 m in diameter and coordinate an effective threat response and mitigation effort.

[![enter image description here](https://i.sstatic.net/CQAJa.png)](https://i.sstatic.net/CQAJa.png)

Picture Source: [Near-Earth object - Wikipedia](https://en.wikipedia.org/wiki/Near-Earth_object)


---

