The past few decades has seen the discovery of hundreds of thousands of small bodies within our Solar System, a process greatly expanded by the introduction of automated sky surveys. However, almost all of these bodies orbit further from the Sun than the Earth, or at least spend part of their orbital cycle outside the Earth's orbit; asteroids which orbit closer to the Sun than the Earth have proven extremely hard to detect, as such bodies are masked by the glare of the Sun. Detecting small bodies with orbits similar to that of Venus requires making observations close to the horizon, where telescopes are looking through the greatest possible airmass, and facing the greatest challenge from ground-based light pollution. Furthermore, these observations must be made close to dawn or dusk, when the observation window is of a very short duration. This is not an insignificant problem, as objects within this area are usually Near Earth Objects (asteroids which either come close to the Earth's orbit, or have the potential to do so), so it is important to know if they are in stable or unstable orbits.
To date only 25 objects have been found which orbit entirely within the orbit of the Earth, these being known as the Atira Asteroids (sometimes the Apophe Asteroids). In contrast, many thousands of asteroids are known which have orbits which cross that of the Earth, known as Aten Asteroids if their semi-major access (average distance from the Sun) is lower than that of the Earth, or Apollo Asteroids if it is larger.
Asteroids are known to occasionally move from population-to-population, as well as sometimes breaking apart, due to tidal stresses, excess rotational speed, or heating from the Sun, and a better understanding of asteroids closer to the Sun than the Earth is likely to enable us to better understand these processes.
Near Earth Objects have orbits which are influenced by that of the Earth, but objects orbiting entirely inside of the Earth's orbit have the potential to be more greatly influenced by the orbit of Venus. Five asteroids are currently known which have orbital periods close to that of Venus, (322756) 2001 CK32, (524522) 2002 VE68, 2012 XE133, 2013 ND15, and 2015 WZ12, and seven have semi-major accesses close to that of Venus, 2020 BT2, 2020 CL1, 2020 QU5, 2021 XA1, 2021 XO3, 2022 BL5, and 2022 CD. All of these asteroids have orbits which cross that of the Earth, and which are therefore considered to be unstable on million-year timescales. All known Earth co-orbital asteroids also have unstable orbits. There is, however, a potential for asteroids to exist in stable orbits resonant with those of Earth or Venus, although no such asteroids have been discovered to date. Venus lacks any known satellites, which suggests that it's Hill sphere (the zone around a planet within which a body could be captured as a satellite) is unstable, due to its proximity to the Sun and other planets. At the same time, objects smaller than 1 km in diameter close to the orbit of Venus, while likely to exist, are beneath the observation limits of most surveys that have looked at this area.
The
HELIOS and
STEREO space missions have both detected a narrow ring of dust following Venus's orbit, which is likely to have originated from a family of asteroids co-orbital with the planet. Models of Solar System evolution have suggested that 8% of a population of small bodies co-orbital with Venus during the early stages of Solar System formation would be likely to remain till today, making it possible that Venus does retain a population of primordial co-orbital asteroids, likely to have a nature quite unlike that of other Near Earth Asteroids or higher eccentricity Venus-co-orbitals. No potential member of such a population has never been observed, but their existence cannot be ruled out due to the paucity of observations in this region.
A significant portion of the near-Venus sky has recently been surveyed using small telescopes, with the most extensive survey of this region having been carried out using the 48-inch (1.2 m) telescope of the
Zwicky Transient Facility. This survey discovered the first known asteroid that orbits entirely inside of the orbit of Venus, (594913) ’Ayló’chaxnim 2020 AV2, thought to have a diameter of about 1.5 km, with the telescope used probably not able to detect objects much smaller.
Shepherd et al. carried out an initial survey in September 2019, which covered about 45 square degrees of sky, followed by a longer survey between June 2021 and January 2022, bringing the total sky surveyed to 624 square degrees. Survey time was limited to about 10 minutes each twilight, just after nautical twilight ends in the evening and just before nautical twilight begins in the morning, when the Sun is usually between 15 and 12 degrees below the horizon. Surveys were carried out close to the horizon, subject to the 23° limit of the telescope.
These conditions generally excluded the observation of asteroids exterior to the Earth's orbit, which would be much further away (on the other side of the Solar System, as the telescope is looking inwards) and therefore dimmer and harder to detect through the high airmass and light interference of the near-horizon near-twilight sky. There will will also be a significant difference in apparent motion of bodies inside and outside the orbit of the Earth. Bodies interior to the Earth generally have an apparent motion in excess of 100 arcseconds per hour, rising to 150 arcseconds per hour in bodies close to the orbit of Venus, while bodies in the Main Asteroid Belt or beyond will have apparent motions of 80 arcseconds per hour or lower.
Shepherd et al.'s study discovered three new asteroids, one of which is thought to be less than a kilometre in diameter, as well as making additional sightings of several previously observed Near Earth Objects, from the Atira, Aten, Amor, and Apollo asteroid populations, adding to our knowledge of those bodies.
The first asteroid described is 2021 PH27, discovered on 13 August 2021 by Scott Shepherd during a collaboration between the Dark Energy Camera twilight asteroid survey and the Local Volume Complete Cluster Survey, and re-observed on 14 August 2021 by the Dark Energy Camera and the Baade-Magellan telescope. The object was relatively bright, with a Magnitude of 19.2, enabling it to be observed by smaller telescopes, and on the third night of observations, 15 August 2021, as well as again being detected by the Dark Energy Camera and the Baade-Magellan telescope, it was observed by two of the 1 m telescopes of the Planetary Defence Office, in Chile and South Africa. This provided sufficient information about the object to search for it in previously recorded images, with it being found in an image taken by the Dark Energy Camera on 16 July 2017. The object was observed again in March 2022.
2021 PH27 has the smallest semi-major axis (average distance from the Sun) of any asteroid ever discovered, and the second smallest semi-major axis of any known Solar System body, behind the planet Mercury. As a consequence of this it also has the lowest known orbital period of any asteroid discovered to date, at 113 days. It is an Atira Family Asteroid, i.e. one which always remains closer to the Sun than the Earth, with a highly eccentric orbit which crosses the orbital paths of both Venus and Mercury, bringing it to a distance of 0.133 AU from the Sun (i.e. 13.3% of the distance at which the Earth orbits the Sun, or slightly a third of the distance at which the planet Mercury orbits the Sun).
The orbit of the newly discovered Atira asteroid 2021 PH27, which has the smallest semimajor axis of any known asteroid. Though it orbits the Sun faster than Venus as its semi-major axis is less than that of Venus, 2021 PH27 has an aphelion exterior to Venus. It also has a perihelion interior to Mercury’s orbit, causing 2021 PH27 to experience the largest general relativistic effects from the Sun’s gravity of any object known in the solar system. Shepherd et al. (2022).
A computer model used by Shepherd et al. suggests that the orbit of 2021 PH27 is unstable on a million-year timescale. The asteroid will pass through the Hill Sphere of Venus (zone within which a body might be captured as a satellite) within the next 950-1050 years, although it is unlikely to be captured, and may already be in a Kozai–Lidov oscillation with the planet (a pattern in which two bodies regularly interact, causing a regular flip between two orbital paths with different eccentricities and inclinations; where one of the bodies is an asteroid and the other a planet this would have a negligible impact on the planet, with all the change being to the orbit of the asteroid).
At its closest point to the Sun, the surface of 2021 PH27 is likely to reach about 500°C, which will presumably have caused considerable thermal processing on the surface, as well as repeated stress to the interior of the asteroid from heating and cooling. It is possible that the asteroid is 'active' (shedding material like a comet), in which case it is likely to cause meteor showers on Venus.
The brightness of 2021 PH27 (magnitude 19.2 at discovery), combined with its low aphelion distance (furthest distance from the Sun) of 0.72 AU, which is just outside the orbit of Venus, combined with its high inclination (32°), suggest that it would be more-or-less impossible for regular Near Earth Object surveys to have spotted.
The most plausible explanation for the origin of 2021 PH27 is that derives from the Main Asteroid Belt, and was knocked from its former orbit by an encounter with Jupiter's gravity field, being captured onto its current orbit by an encounter with Venus. However, there are other possible origins for the asteroid, such as having come from a small population of ancient bodies with orbits resonant with that of Venus, or even a (theoretical) population of Vulcanoid Asteroids orbiting entirely inside the orbit of Mercury.
The second asteroid discussed is 2021 LJ4, which was observed by the Dark Energy Camera on 6 June 2021, and then again on 8 June 2021. Confirming observations were made by the Canada-France-Hawaii 3.6 m telescope on Maunakea on 9 and 11 June 2021, along with further observations by the Dark Energy Camera.
2021 LJ4 is a more typical Atira Family Asteroid, with an aphelion distance of 0.93 AU, just inside the orbit of the Earth, and an orbital period of 202 days. It's orbit crosses those of both Venus and Mercury, and based on it's proximity to Earth and magnitude at its time of discovery (21.4) it is thought to be between 300 and 400 m in diameter.
The orbit of 2021 LJ4, and it's position on 6 June 2021. JPL Small Body Database.
The third asteroid discussed is 2022 AP7, first observed on 13 January 2022 by the Dark Energy Camera, then resighted on 16, 18, 21 and 23 January 2022. A few weeks later the asteroid was spotted by both the Las Cumbres telescopes and the University of Hawaii 88 inch telescope, and subsequently located in images captured in 2017 by the Near Earth Object searches of the
Pan-STARRS telescope and
Catalina Sky Survey.
This enables a fairly accurate reconstruction of the orbit of 2022 AP7, which is calculated to be an Apollo Group Asteroid with a perihelion distance of 0.83 AU (between the orbits of Venus and Earth) and an aphelion distance of 5.0 AU (close to the orbit of Jupiter), making it an Earth-orbit crossing Near Earth Object.
The calculated orbit of 2022 AP7 and its position on 13 January 2022. JPL Small Body Database.
2022 AP7 is thought to have an Earth minimum orbit intersection distance of 0.0475 AU (7 106 000 km or less than twice the distance between the Earth and the Moon), making it a Potentially Hazardous Asteroid. It only had a magnitude of 20.8 at the time when it was detected, but was at that time 1.9 AU from the Earth and about 1.4 AU from the Sun, which even assuming a moderate albido (reflectiveness) implies an object 1.0-2.3 km in diameter, which would make 2022 AP7 the largest Potentially Hazardous Asteroid discovered since 2014.
This is quite surprising; 2022 AP7 is clearly a large object, and regularly comes close to the orbit of the Earth (it has an orbital period of 5 years), something which would ought in theory to have been been detected by earlier Near Earth Object surveys, which have concentrated on the zone outside the Earth's orbit where 2022 AP7 spends much of its time. There are two possible explanations for this; either the asteroid is on an orbit which keeps it away from the Earth, or it is darker than has been calculated, bur undergoes some form of cometary effect at perihelion, causing it to brighten at this point. Shepherd et al.'s orbital calculations suggest that the former is the case, with 2022 AP7 never coming near the Earth on its current orbital trajectory.
The principle objective of Shepherd et al.'s study was to look for Venus co-orbital asteroids, or asteroids with stable Venus-resonant orbits, neither of which were detected. However, the study did detect 4 Atira Family Asteroids (about 15% of the total known Atira population). This enables some constraints to be put on the size of the populations of Atira Asteroids (asteroids that orbit entirely inside the orbit of Earth) and Vatira Asteroids (asteroids that orbit entirely inside the orbit of Venus).
Previous models of the Inner Solar System have suggested that there may be a population of asteroids with stable orbits resonant and co-orbital with Venus, which have remained there since the origin of the Solar System. If these asteroids do in fact exist, then Shepherd et al.'s survey should have detected between 21 and 23 of all such bodies larger than 1 km in diameter (dependent on their albedo). Their failure to discover any such asteroids, in common with that of all previous surveys, suggests that if these asteroids exist at all, then they are very few in number, with probably no more than four objects larger than 1 km in existence. Smaller objects would be less well constrained, although objects greater than 500 m in diameter with high albedos would appear to be equally rare.
There are five asteroids for which repeat observations have been made, which are on orbits with similar periods to Venus, (322756) 2001 CK32, (524522) 2002 VE68, 2012 XE133, 2013 ND15, and 2015 WZ12, as well as a number of asteroids detected since 2020 which appear to have similar properties. All of these are thought to be less than a kilometre in diameter, with orbits unstable on a millions-of-years timescale. Shepherd et al.'s study was predicted to find between 5 and 10% of any population of similar asteroids with diameters in excess of 1 km, but did not detect any, suggesting that larger asteroids with these properties are again very rare.
Shepherd et al. discovered two new Atira Family Asteroids, 2012 LJ4 and 2021 PH27, as well as making additional observations of two previously discovered Atiras, 2019 AQ3 and 2021 BS1. The survey was predicted to detect between 5 and 8% of the population of Atira Asteroids larger than 1 km in diameter, which suggests that the total population of such asteroids is between 50 and 75, of which 25 have been discovered to date. Assuming a total population of Near Earth Asteroids larger than 1 km in diameter of about 1000, then the Atiras would make up about 5% of this population. This was predicted in earlier models of the Near Earth Object population, although more recent models have tended to put the number of Atiras lower, at about 1% of the total population.
This has two possible implications. Firstly, the lower estimates of the population size of the Atira Asteroids is correct, and we have actually discovered the majority of these bodies. Secondly, there are more Atiras than we previously realised, and many remain undetected. This later possibility would be more likely if many of these bodies have either low semi-major axes, low eccentricities, or high inclinations, all of which would result in objects which do not closely approach the Earth. This later explanation fits with the known properties of 2021 PH27, which has a low semi-major axis and a high inclination.
Vatira Asteroids orbit entirely inside the orbit of Venus. To date, only a single such asteroid has been discovered, (594913) ’Ayló’chaxnim 2020 AV2. Shepherd et al.'s model suggests that about 5% of Vatira Asteroids with populations larger than 1 km should have been discovered to date, implying that there are about 20 such asteroids, although extrapolating from a single known asteroid gives quite a large margin of error. Again, a larger population of such asteroids could be present if their orbits tended to keep them out of the observed survey fields, due to low semi-major axes, more circular orbits, or higher inclinations.
The Dark Energy Camera Twilight Survey is the largest and most sensitive survey aimed at objects interior to the Earth's orbit to date. The survey discovered two new Atiras and one new Apollo Asteroid, but no new Vatiras.
One of the newly discovered Atira Asteriods, 2021 PH27 has the lowest known semi-major axis and shortest orbital period of any asteroid discovered to date. This asteroid is calculated to have regular close encounters with Venus, and to be likely to pass through that planet's Hill Sphere within the next 950-1050 years, making it a Potentially Hazardous Asteroid to Venus. 2021 PH7 has an orbit which crosses those of both Venus and Mercury, with a perihelion distance of 0.13 AU. This body will thus also be more subject to the effects of General Relativity than any other known body in the Solar System, resulting to a change in its trajectory of about 53 arcseconds each century.
The newly discovered Apollo Asteroid, 2022 AP7, is the largest Potentially Hazardous Asteroid discovered since 2014, with an estimated diameter of 1.5 km.
Modelling of the Atira Asteroid population suggests that there are probably several more such asteroids larger than 1 km in diameter to be discovered, probably with low semi-major axes and high inclinations, making them hard to detect for most surveys.
Although the Dark Energy Camera Twilight Survey was intended to be more sensitive to smaller objects than previous studies, two of the three bodies it discovered were in excess of 1 km in diameter. This may indicate that smaller asteroids are less stable in the region covered by the survey. This is probably due to the thermal and gravitational effects of the Sun, which could cause smaller bodies to break up more readily. However, this cannot be stated with any confidence at the current time due to the low sample size of detected Atira Asteroids.
See also...
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