How Big of an Object Orbiting Closer to the Sun Than Earth Could Be Overlooked?

[ohwilleke]

TL;DR Summary

Aside from Mercury and Venus, what is the largest compact object closer to the Sun Than Earth that astronomers could still have failed to see?

Science involves not just knowing what you know, but accurately estimating how sure you are of what you know and having a decent understanding of what you don't know.

There are two known true planets closer to the Sun than Earth (Mercury and Venus, neither of which has a significant moon), both of which have been known since antiquity and can be seen with a sharp naked eye.

There are five true planets further from the Sun (on average) than Earth. There might be a Planet 9 too, but that would also have an average location further from the Sun than Earth.

All of the known or proposed dwarf planets are also further from the Sun than Earth. The illustration below is (by xkcd) is suggestive of the idea that we're probably not missing much in the way of compact solar system objects with solid surfaces.

There are certainly some rocky objects closer to the Sun than Earth, however.

That's bad, because our current astronomy instruments are very bad at seeing such an object on a collision course with Earth from the direction of the Sun but much better at seeing objects further out from the Sun than Earth.

To plan for the risk of a compact object closer to the Sun than Earth getting on a collision course with Earth, for which we may have little time to react once it is detected, it would be very helpful to know what the current observational bounds are on the largest possible compact object we could be missing that is closer to the Sun than Earth.

Is it 2000km in diameter? 1000km? 500km? 100km?

Background

Dwarf planets defined

To be a planet at all, a body must be approximately round due to gravitational effects which imposes a floor diameter of approximately 650 km to 800 km (with close cases resolved by inspection of whether it is round or not), in addition to not being a moon, or a star (the least massive of which is a brown dwarf with a minimum diameter of about 600,000 km). Thus, dwarf planets have diameters (by definition) from about 650 km to 4879 km, and in practice from 650 km to perhaps 2600 km (assuming that there might be a few Plutons in distant solar orbit a bit bigger than Pluto that are not yet discovered).

An inner dwarf planet and three near misses

One of them, Ceres, and three near asteroids that aren't quite round enough to qualify as planets are in the main asteroid belt and were discovered in the 19th century.

* Ceres 963x891 km
* Vestas 578x560x458 km,
* Pallas 570x525x500 km, and
* Hygiea 500x400x350 km

Ceres makes up about a third of the matter in the main asteroid belt between Mars and Jupiter. These four bodies combined make up about half of the matter in the main asteroid belt. About a third of all asteroids by number are part of one of 20 to 30 clusters of asteriods known as asteroid families.

Ceres was discovered in 1801 and was initially considered a planet. Pallas was the second asteroid discovered and was discovered in 1802 and was considered a planet until the discovery of more asteroids caused Ceres and Pallas to be downgraded to asteroids in 1845. Ceres was upgraded to dwarf planet status again in 2006.

Plutons

The remaining dwarf planet candidates (except Eris, 2007 OR10 and Sedna which are further out from the Sun) are Kuiper Belt Objects with orbits around the sun of 200 years or more (aka Plutons), the largest of which is Pluto, which was first discovered in 1930 before any of the rest. A classical Kuiper Belt object that aren't quite large enough to be classified as dwarf planets are sometimes called a cubewano (after the type specimen 1992QB1) or, depending upon its orbit, a plutino.

In addition to Ceres, the four other officially recognized dwarf planets, also known as Plutons, are (in order of largest dimension of size):

*Pluto 2370 +/- 10 km diameter (as of July 2015).
*Eris (aka Xena) 2326 +/- 12 km diameter (Eris is more massive than Pluto)
*Haumea (aka 2003 EL61 aka Santa) 1920 x 1540 x 990 km.
*Makemake (aka 2005 FY9 aka Easter Bunny) 1434 x 1422 +/- 14 km.

Eris is the most distant from the sun of these, about 60% further out than Pluto.

Another dozen strong dwarf planet candidates in order of size include:

* Sedna 1830-2320 km
* 2007 OR10 1280 +/- 210 km (the largest unnamed object in the solar system)
* Quaoar 1110 +/- 5 km
* Orcus 917 +/- 25 km
* Salacia 854 +/- 45 km
* 2002 MS4 934 +/- 47 km (the second largest unnamed object in the solar system)
* 2002 AW197 626-850 km (best fit 734 km)
* 2003 AZ84 661-789 (best fit 727 km)
* Varda 630-786 km (best fit 705 km)
* Varuna 574-812 km (best fit 658 km)
* Ixion 430-910 km (best fit 650 km)
* Chaos 470-740 km (best fit 600 km)

Beyond these there are probably about a dozen additional solar system bodies without names that are serious dwarf planet candidates.

Eris is the most distant dwarf planet or dwarf planet candidate from the Sun other than Sedna. All of these except Orcus and Sedna are between Pluto and Eris in the Kuiper Belt, in their average distance from the Sun. Orcus is only about 1% closer to the Sun than Pluto. Almost all of the potential dwarf planets are no further from the Sun than Eris.

Sedna is the largest body not yet officially recognized as a dwarf planet, and is also about seven and a half times as far from the Sun as Eris.

Other than Sedna, there are only four known objects significantly further out than Eris in the solar system, and only one is further out than Sedna. The sizes and shapes of these four objects aren't known with great certainty. Current estimates putting them on the borderline of qualification as dwarf planets.

Pluto was discovered in 1930 and its moon, Charon, was discovered in 1978.

A host of new dwarf planets have been discovered in the solar system in the last fifteen years. Varuna was discovered in 2000, Ixion was discovered in 2001, Quaoar was discovered in 2002 (its moon, Weywot was discovered in 2007) as were 2002 MS4 and 2002 AW197, Sedna and 2003 AZ84 were discovered in 2003, Salacia, Orcus and Haumea were discovered in 2004, and Eris and Makemake were discovered in 2005. 2007 OR10, unsurprisingly, was discovered in 2007.

Many of the 21st century discoveries were made by Mike Brown. Another leading astronomer in this field is Gonzalo Tancredi.

Charon 1207 +/- 3 km diameter as of July 2015 is part of the same system of Pluto. Since Pluto is larger than Charon, Charon is currently officially classified by the IAU as a moon of Pluto under the existing definition, which disqualifies it from planet status. There are also four other moons in the Pluto-Charon system: Styx, Nix, Kerberos, and Hydra.

The IAU has proposed to classify Pluto and Charon as a double planet (with both Pluto and Charon having dwarf planet status) because the center of their rotation around each other is not within either planet. This would make it the only double planet system in the solar system. But, one analogy that discourages IAU members from adopting the double planet definition, is that the same definition would conclude that Jupiter is not a satellite of the Sun, but is instead as "double" object with it, since their center of gravity is no within either body due to Jupiter's great distance from the Sun.

Charon is not the only big planet-like moon in the solar system, sometimes called satellite planets:

Nineteen moons are known to be massive enough to have relaxed into a rounded shape under their own gravity, and seven of them are more massive than either Eris or Pluto. They are not physically distinct from the dwarf planets, but are not dwarf planets because they do not directly orbit the Sun. The seven that are more massive than Eris are the Moon, the four Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto), one moon of Saturn (Titan), and one moon of Neptune (Triton). The others are six moons of Saturn (Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus), five moons of Uranus (Miranda, Ariel, Umbriel, Titania, and Oberon), and one moon of Pluto (Charon). The term planemo ("planetary-mass object") covers both dwarf planets and such moons, as well as planets. Alan Stern calls them "satellite planets".

 

[Drakkith]

There are certainly some rocky objects closer to the Sun than Earth, however.

It's not the ones closer to the Sun that we have to worry about, it's the ones that potentially cross Earth's orbit.

That's bad, because our current astronomy instruments are very bad at seeing such an object on a collision course with Earth from the direction of the Sun but much better at seeing objects further out from the Sun than Earth.

Virtually all potential Earth impactors spend a significant portion of the time outside of our orbit. The most dangerous ones spend the vast majority of their time outside of our orbit, as the most dangerous objects tend to be the ones with highly elliptical orbits. The further from circular the object's orbit is, the larger the relative velocity between the Earth and the object tends to be. Either because the object is 'falling' almost straight towards the Sun as opposed to moving tangentially to a mostly circular orbit, or because the object's apoapsis is near the Earth's orbit and it is moving very slowly, allowing the Earth to smack into it at nearly 30 km/s.

For example, by the time Halley's comet is 1 AU out from the Sun it is moving at over 40 km/s, with its velocity vector pointing very far off from Earth's. So an impact would occur at a very high velocity compared to an object that was 'catching up' to Earth where the impact would occur at a much lower relative velocity.

As to how small an object would need to be to evade our sight, I admit I don't know.

 

[Keith_McClary]

There are 26 known Atira asteroids, as small as 35m in diameter.

 

[Keith_McClary]

Our NEA dynamical model and an updated model produced by the University of Helsinki’s Mikael Granvik and colleagues in 2016 independently predict that the Amor, Apollo, Aten, Atira, and Vatira subpopulations contain roughly 39%, 55%, 4%, 1%, and less than 1% of NEAs, respectively, at any given time.
...
Asteroids do not remain in the Atira and Vatira populations for long. Detailed dynamical simulations of Atira asteroids performed by Anderson Ribeiro of the Geraldo Di Biase University Center in Brazil and his colleagues indicate that the very planetary close encounters required to enter those hard-to-reach orbits are responsible for continually scattering the asteroids into and out of the Vatira and Atira populations many times during their lifetimes.5 Those events keep the asteroids from lingering in either of the planet-decoupled populations.

As Gladman, Ngo, and I discovered in our simulations,3 the asteroids typically spend only a couple million years—integrated over their lifetimes—as Atiras and a few hundred thousand years as Vatiras. It is possible for asteroids to enter the Vatira region and to remain there for more than a million years before leaving. However, such long-lived Vatiras are extremely rare. If most of those asteroids don’t remain in the Atira and Vatira populations, where do they go? The frequent planetary close encounters generally push the asteroids outward, over tens of millions of years, back onto Venus- and Earth-crossing orbits. (The asteroids become Atens and Apollos.) Because the vast majority of Atiras and Vatiras do not remain decoupled from Earth and Venus, most eventually collide with one of those planets or with Mercury. Any that remain are frequently pushed onto Sun-grazing orbits, and some are even scattered back out to Mars-crossing orbits and potentially beyond the asteroid belt.
...
To date, 23 known asteroids orbit the Sun in the Atira population. They range in size from 50 m to 5 km. Both the Granvik and Greenstreet models estimate the existence of 10 Atiras3,4 with diameters larger than a kilometer. Beyond the six known Atiras in that size range, few large ones are likely left to be discovered. The number of Atiras with increasingly smaller diameters is much greater, so many more remain to be found.

Asteroids in the inner solar system ​

 

[ohwilleke]

There are 26 known Atira asteroids, as small as 35m in diameter.

This and your follow up extended quotation are excellent and clarify a lot of my questions.

Is there a name for compact objects with elliptical orbits such that they cross Earth's orbit around the Sun, excluding those that are always within it, or are always outside it?

Answering my own question from another Wikipedia link:

Near-Earth asteroids, or NEAs, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross Earth's orbital path are known as Earth-crossers. As of June 2016, 14,464 near-Earth asteroids are known[30] and approximately 900–1,000 have a diameter of over one kilometer.

Apparently Near Earth Objects is what I'm really interested in finding out more about:

If a NEO's orbit crosses the Earth's, and the object is larger than 140 meters (460 ft) across, it is considered a potentially hazardous object (PHO). Most known PHOs and NEOs are asteroids, but a small fraction are comets.

There are over 27,000 known near-Earth asteroids (NEAs) and over a hundred known short-period near-Earth comets (NECs). A number of solar-orbiting meteoroids were large enough to be tracked in space before striking the Earth. It is now widely accepted that collisions in the past have had a significant role in shaping the geological and biological history of the Earth. Asteroids as small as 20 metres (66 ft) in diameter can cause significant damage to the local environment and human populations. Larger asteroids penetrate the atmosphere to the surface of the Earth, producing craters if they impact a continent or tsunamis if they impact the sea. Interest in NEOs has increased since the 1980s because of greater awareness of this potential danger. Asteroid impact avoidance by deflection is possible in principle, and methods of mitigation are being researched.

Two scales, the simple Torino scale and the more complex Palermo scale, rate the risk presented by an identified NEO based on the probability of it impacting the Earth and on how severe the consequences of such an impact would be. Some NEOs have had temporarily positive Torino or Palermo scale ratings after their discovery.

Since 1998, the United States, the European Union, and other nations are scanning the sky for NEOs in an effort called Spaceguard. The initial US Congress mandate to NASA to catalog at least 90% of NEOs that are at least 1 kilometre (3,300 ft) in diameter, sufficient to cause a global catastrophe, was met by 2011. In later years, the survey effort was expanded to include smaller objects which have the potential for large-scale, though not global, damage.

 

[ohwilleke]

It's not the ones closer to the Sun that we have to worry about, it's the ones that potentially cross Earth's orbit.

Virtually all potential Earth impactors spend a significant portion of the time outside of our orbit. The most dangerous ones spend the vast majority of their time outside of our orbit, as the most dangerous objects tend to be the ones with highly elliptical orbits. The further from circular the object's orbit is, the larger the relative velocity between the Earth and the object tends to be. Either because the object is 'falling' almost straight towards the Sun as opposed to moving tangentially to a mostly circular orbit, or because the object's apoapsis is near the Earth's orbit and it is moving very slowly, allowing the Earth to smack into it at nearly 30 km/s.

Interesting point. I hadn't considered that an object in a near circular orbit isn't much of a worry.

I wonder if there is good data out there on the distribution of asteroids and comets and other compact objects in the solar system in terms of circular v. highly elliptical orbits, which can be cleanly parameterized with a single parameter?

This said, the reason to worry about the one's closer to the Sun, even if only temporarily due to a highly elliptical orbit, I've learned from other sources, is that an object on the Sun's side from us is harder to observe, which means that there is little or no warning time between seeing it coming and impact, leaving little time to plot an projected impact point, evacuate that area, or take other planetary defense measures. In contrast, many compact objects approaching Earth from the outer solar system can afford us weeks to months to years to a decade of advanced notice.

An object that poses a survivable threat if its impact point can be localized and its high risk of impact can be anticipated long in advance can be much more problematic if there is no reaction time.

For example, by the time Halley's comet is 1 AU out from the Sun it is moving at over 40 km/s, with its velocity vector pointing very far off from Earth's. So an impact would occur at a very high velocity compared to an object that was 'catching up' to Earth where the impact would occur at a much lower relative velocity.

O.K.

As to how small an object would need to be to evade our sight, I admit I don't know.

This is another point I'm working on and interested in both from the perspective of impactors and from the independent basis of just wanting to know what might be undiscovered out there. Apparently, one asteroid with an orbit entirely within that of Earth as small as 35 meters has been seen.

But, I still don't have much of a sense of how complete our catalog of inner solar system compact objects is likely to be, even though it seems like we ought to have some way to reasonably estimate that. I'd imagine that the end result of an analysis like that would look like a heat chart with one axis in a two dimensional figure representing size, and another representing albedo or so other continuous trait relevant to detection likelihood.

 

[DaveC426913]

On a side note:

I wonder how much care Randall put into this illo, particularly in regards to the asteroids:

They say the asteroids, all told, may have a mass as much as a small planetoid - which would jive with Randall's map, but a small planetoid, broken down into untold billions of smaller bodies could have a much larger surface area.

A single planetoid, say 4,000 km³, would have a volume of 33x10⁹ km³ and a surface area of 17x10⁶ km².

But 33 x 10⁹ asteroids - all of volume .1 km³ - would have a surface area of 25x10¹¹ km³ - or about 65 times larger (i.e. as big as the rest of the map).

Randall's data.

 

[DaveC426913]

Apparently Near Earth Objects is what I'm really interested in finding out more about:

More specifically, Earth-crossing asteroids:
https://www.britannica.com/science/Earth-crossing-asteroid
https://en.wikipedia.org/wiki/List_of_Earth-crossing_minor_planets

 

[ohwilleke]

More specifically, Earth-crossing asteroids:
https://www.britannica.com/science/Earth-crossing-asteroid
https://en.wikipedia.org/wiki/List_of_Earth-crossing_minor_planets

Yipes! From the second link: "99942 Apophis will approach Earth so closely in 2029 that it will get under the orbit of the Earth's geostationary satellites."

 

[Drakkith]

Interesting point. I hadn't considered that an object in a near circular orbit isn't much of a worry.

Well, an object just barely catching up to Earth would start to free fall towards us once it entered our sphere of influence. This equates to a delta-v gain of about 11-13 km/s. So at minimum, an object is going to impact us with about that much speed which is obviously still very dangerous. Still, better than a 40-50 km/s impact velocity.

 

[cormsby]

the object's apoapsis is near the Earth's orbit and it is moving very slowly

...so this apoapsis is the object's aphelion, making it an inner-system object?

 

[Drakkith]

...so this apoapsis is the object's aphelion, making it an inner-system object?

Yes, if the aphelion (apoapsis) is near Earth's orbit then it would be an inner-system object.

 

[collinsmark]

Aside from Mercury and Venus, what is the largest compact object closer to the Sun Than Earth [...]

(boldface mine)

At the risk of being nit-picky, I think you're looking for a term like "solid object."

Compact objects typically refer to things like white dwarfs, neutron stars, and black holes. As in really compact compared to matter we are familiar with on an everyday basis.

 

[ohwilleke]

(boldface mine)

At the risk of being nit-picky, I think you're looking for a term like "solid object."

Compact objects typically refer to things like white dwarfs, neutron stars, and black holes. As in really compact compared to matter we are familiar with on an everyday basis.

My intent was to exclude diffuse dust-like matter, without excluding things like comets with gaseous tails that aren't necessary rigid bodies.

 

[Keith_McClary]

https://en.wikipedia.org/wiki/Small_Solar_System_body
https://en.wikipedia.org/wiki/Asteroid#Terminology

 

[ohwilleke]

If there are term that covers everything on the chart?

 

[DaveC426913]

If there are term that covers everything on the chart?

Solar System Bodies? (That's the title of the Wiki page.)

 

[Drakkith]

If there are term that covers everything on the chart?

"Objects close to us, in astronomical terms, that appear on this chart"