The Smallest Objects in the Solar System
At first glance, our solar system might appear to just consist of the eight known planets, a couple of dwarf planets, and their moons. Trying to comprehend the sheer size of some of the planets and the expansiveness that separate them can be very difficult to do. While these astronomical giants are undoubtedly the most prominent objects in our neighborhood around the sun, they are by no means the only bodies orbiting the sun. The vast spaces in between planets are actually teeming with other astronomical objects, most of which are not as massive as the other solar system titans. In fact, they are minuscule comparatively, and these miniature space objects are the focus of this article.
Our solar system contains many different types of bodies, the most well-known distinctions of these objects being planets, dwarf planets, and natural satellites, or moons. But when categorizing different astronomical objects, there is a catchall category of sorts called small solar system bodies,SSSB, or just small bodies. An SSSB is defined as, “any natural solar system object other than the Sun and the major planets and dwarf planets and their satellites” (Tedesco).” Typically, the term is restricted to objects that can be seen with a telescope while still outside of Earth’s atmosphere, resulting in a lower limit requirement of a few meters in diameter. The majority of the small bodies that populate our solar system are minor planets, asteroids, comets, and fragments of such bodies. All SSSBs are thought to be remnant material from the planet-building process that took place during the formation of the solar system from the solar nebula. These are the astronomical objects that go mostly unnoticed, yet dominate the desolate interplanetary regions of the solar system.
As a side note, the difference between asteroids and comets is somewhat cloudy, even in the scientific community, but some clarification might help for the sake of this article. Comets are composed of minerals, rock, and mostly ice. They travel around the sun in elliptical orbits and can pass through the plane of the solar system at any angle. Comets will often have tails that can extend many tens of millions of miles. On the other hand, asteroids have differing compositions and backgrounds and are normally non-spherical. Nearly all asteroids are found in a region between Mars and Jupiter, aptly named the Main Asteroid Belt, orbiting around the Sun along with other rocky bodies. The main distinction between asteroids and comets is that comets have more volatile and elliptical orbits than those of asteroids.
Small bodies are most concentrated in a couple of different areas, the first being the Main Asteroid Belt as previously mentioned. The next regions are the gravitationally stable points, orLagrangian points, along the orbital paths of Jupiter, Mars, Uranus, Neptune, and Earth. These Lagrangian points are areas where the gravitational pull of two large masses precisely equals the centripetal force required for a small object to move with them, resulting in the smaller object maintaining its position relative to the large bodies. The next area of concentration is the Kuiper Belt, a disk-shaped zone of icy bodies that all orbit the sun at distances beyond Neptune. Lastly, many small solar system bodies are congregated in the Oort cloud, which is a spherical cloud of icy bodies orbiting the sun at distances typically more than 1,000 times that of Pluto.
In 2009, the Hubble Space Telescope discovered the smallest object ever seen in visible light at the time within the Kuiper Belt. The Kuiper Belt Object (KBO) found was a mere 3,200 feet (975 meters) across and was 4.2 billion miles away. That is the equivalent of seeing a flea that is nearly 11,000 miles away from you. The smallest KBO that Hubble had previously observed in reflected light was 30 miles across, 50 times larger than the newly discovered object. This tiny KBO should have been much too dim for Hubble to have detected it, so how was it able to find such a small, faint object?
Hubble has three optical instruments called the Find Guidance Sensors (FGS) that “provide high-precision navigational information to the space observatory’s attitude control systems by looking at select guide stars for point.” According to NASA, the sensors harness the wavelike nature of light to make precise measurements of the location of stars. These instruments are also able to see the effects of small objects passing in front of a star that the telescope is focused on. The light from the background guide star would be briefly diffracted from Hubble’s point of view as the light would have to bend around the small object passing between the telescope and the star. Since the FGS instruments make starlight readings about 40 times a second, the telescope was able to detect a single 0.3-second-long occultation event where the KBO object passed in front of Hubble’s guide star. Very similar technology is being used to discover exoplanets, which are planets that orbit around other stars outside of the solar system. Because light diffracts differently through different gases, scientists can determine what gases are present in a potential exoplanet’s atmosphere by analyzing the light’s behavior as it passes around the observed object.
There are countless small body objects that have been discovered in our solar system that are only a couple of meters wide. Here are some of the most prominent examples. Duende is an asteroid with a diameter of about 98 feet (30 m). In February 2013, the asteroid passed the Earth at 17,200 miles (27,700 km) from the Earth’s surface which is only about 4.3 Earth radii. Duende was only discovered by scientists seven days after passing Earth.
2012 TC4 is a microscopic 30 feet (10 m) in diameter and passed closest to Earth at roughly 30,000 miles in 2017. This asteroid was the first known asteroid to be observed passing one Lunar distance (the distance between the Earth and the moon, about 240,000 miles) from Earth twice in a row. Amazingly, astronomers were able to detect the asteroid while it was 37,000,000 miles from Earth making it one of the dimmest asteroid recoveries ever.
2004 BL86 is an asteroid with a 980-foot (300 m) diameter and was discovered in 2004. The asteroid made its closest approach to Earth in 2015, during which scientists discovered a small 200-foot moon orbiting around the asteroid, making it a “binary system.” This discovery was made while the asteroid was 750,000 miles from Earth.
There is a subcategory of small solar system bodies called Potentially Hazardous Asteroids (PHA). These are asteroids that cross the Earth’s orbit at any point and have the potential to collide with our planet, posing a legitimate threat. As scientists continue to observe the sky, they are discovering more and more PHAs, some of which are first discovered just after passing close to Earth. As of June 2020, there are 2,093 known PHAs, making up about 9% of the total near-Earth object (NEO) classification. Over 150 of the known PHAs are larger than 1 km across, yet it is estimated that an asteroid would only have to be about 35 meters in diameter to destroy an entire city or town. Scientists understand that it is critical to locate potentially hazardous objects that threaten the Earth and have taken measures to ensure that such objects do not go unnoticed.
Spaceguard refers to a number of different efforts “to discover, catalogue, and study near-Earth objects, especially those that may impact Earth.” Telescopes are used to repeatedly survey large areas of the sky to find asteroids that could be on a collision course with Earth. Since the early 2000s, the Spaceguard effort has not been successful in detecting multiple PHAs before their impact with Earth. But, on October 7, 2008, an asteroid later named 2008TC3 entered Earth’s atmosphere and exploded about 23 miles above the Nubian Desert in Sudan, spreading upwards of 600 meteorites around the area. The asteroid, an astonishingly small 13 feet across, marked the first successful detection and tracking of an object headed towards Earth. This was a huge milestone for the Spaceguard efforts, but more progress is being made “to greatly increase the number of small impactors that are discovered before atmospheric entry” through projects such as the Asteroid Terrestrial-impact Last Alert System (ATLAS) program.
According to a research paper from NASA’s JPL, the exploration of small solar system bodies yields incredible insight to “the characterization of the early solar system history, the search for planetary habitats, and an improved understanding about the nature of planetary processes.” There is something poetic about minuscule objects floating around in the vastness of space because even the smallest astronomical bodies in our solar system have incredible stories and are filled with information to investigate.
Works Cited:
Dunbar, Brian. “Hubble Finds Smallest Kuiper Belt Object Ever Seen.” NASA, NASA, 16 Dec. 2009, www.nasa.gov/mission_pages/hubble/science/hst_img_kuiper-smallest.html#:~:text=NASA's%20Hubble%20Space%20Telescope%20has,solar%20system%20just%20beyond%20Neptune.
J. C. Castillo-Rogez, M. Pavone, J. A. Hoffman and I. A. D. Nesnas, "Expected science return of spatially-extended in-situ exploration at small Solar system bodies," 2012 IEEE Aerospace Conference, Big Sky, MT, 2012, pp. 1-15, doi: 10.1109/AERO.2012.6187034.
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