Our Solar System: The Leftovers

By Tom Farr   |   March 25, 2021
Enhanced-color composite image of Kuiper Belt object Arrokoth

We all learned in school that there are eight planets (well, nine if you’re as old as I am), but our solar system is messier than that. There are millions of leftover rocks called asteroids; bits of ice and rock that come and go called comets; and objects out there beyond Neptune called, in dry scientific fashion “trans-Neptunian objects.” These latter are arranged into a nearby grouping called the Kuiper Belt (Pluto is a member) and a hypothesized wispy halo of material at the edge of the solar system called the Oort Cloud. 

The asteroids (also called minor planets) hang out mainly between Jupiter and Mars and are likely debris that Jupiter’s massive gravity never allowed to coalesce into a full-sized planet. Ceres is the largest, at about 590 miles across. It and Vesta, the second largest at about 325 miles, were each orbited by NASA’s Dawn spacecraft. Dawn used its ion drive to fly to Vesta and orbit it from 2011 to 2012 and then break orbit and fly to Ceres, arriving in 2015. The mission ended in 2018; Dawn continues to orbit Ceres, silently. Several other asteroids have been visited by spacecraft and a few have even landed on those small bodies. The smaller asteroids lack enough gravity to pull them into spheres, so often take strange shapes like Eros, visited by NASA’s NEAR-Shoemaker in 1998, which looks like a big banana. A few, like Ida and Dactyl, visited by Galileo on its way to Jupiter, are binary objects. One of the key interests in the study of these objects is to determine what they’re made of. It turns out that they range the gamut from dust-bunnies like Itokawa to huge chunks of metal like Psyche, the target of an upcoming NASA mission. The metallic asteroids have sparked a discussion on “Mining the Sky” in the future. A couple of spacecraft have brought back samples of asteroids: NASA’s Osiris-Rex is on its way back to Earth with samples of asteroid Bennu it collected in October of last year and Japan’s Hayabusa 2 landed in the Australian desert in December with a sample of asteroid Ryugu. NASA will be sending up a couple of new missions later this year to study even more asteroids: Lucy will be sent to asteroids co-orbiting with Jupiter’s while NEA-Scout will be a tiny CubeSat powered by a solar sail to visit a near-Earth asteroid called 1991VG. 

As long as asteroids remain in the main asteroid belt, they’re not of immediate concern for us on Earth. However, as the dinosaurs found out about 65 million years ago, sometimes asteroids find their way to Earth. Near Earth Objects (NEO) include Potentially Hazardous Asteroids (PHA), and NASA’s Jet Propulsion Laboratory in Pasadena is the center for monitoring those dangerous objects. At their website (https://cneos.jpl.nasa.gov), you can see which ones will be coming close in the near future. JPL also operates the NEOWISE spacecraft which is scanning the skies for asteroids. Locally, Las Cumbres Observatory, headquartered in Goleta, operates over a dozen telescopes around the world that also keep an eye out for NEOs. Some objects, like 2020SW, a 15-30 foot wide asteroid, came so close on September 24 last year that it passed inside the orbits of the geosynchronous communications satellites! And of course, space rocks do occasionally hit Earth as meteorites. The smaller ones can escape early detection, like the one that hit Russia in 2013 that was about 65 feet across. Exploding in the air above Chelyabinsk, it caused quite a shock wave. Spectacular images were captured on many Russian dash cameras and can be found on YouTube. You can even learn some good Russian swear words! Astronomers are keeping a close watch on Apophis, which passed by Earth on March 6 and will come close again in 2029. Each time it passes Earth, its orbit is perturbed a little, changing the chances of a collision. At about a quarter mile across, Apophis would be devastating. Because of the huge danger these NEOs pose, NASA and the European Space Agency (ESA) are teaming up to test asteroid deflection techniques. The Double Asteroid Redirection Test (DART) will launch from Vandenberg AFB in July of this year and set on an impact trajectory to an asteroid pair. A tiny CubeSat will be dropped off prior to impact to observe the results and Earth-based telescopes will watch to see if the asteroid’s orbit changes. Five years later, ESA’s Hera spacecraft will visit the same asteroids and determine how well DART performed. 

Comets and More

Comets are rarer than asteroids, but more spectacular when they come into the inner solar system and the sun’s rays boil off their ices and push the dust and vapors away into a bright tail. Comets come in two varieties: Short-period and long-period. Short-period comets return on a regular basis like Halley’s Comet, which last came around in 1986 and will return in 2061. Its orbit takes it out beyond Neptune. Long-period comets come in from a lot further away, probably the distant Oort Cloud. Some never return. Comet NEOWISE, discovered by the aforementioned spacecraft, is a bright long-period comet that lit up our skies last year. Because comets represent some of the oldest, most pristine material in the solar system, they’ve received a lot of attention. Halley’s Comet was visited by spacecraft from the Soviet Union, European Space Agency (ESA), and Japan in 1986 and ESA’s Rosetta orbited Comet Churyumov-Gerasimenko in 2014. It even dropped a lander named Philae onto the comet which relayed detailed information about its composition. Rosetta and Philae are still traveling with the comet. Using a bit more of a brute-force approach, NASA’s Deep Impact mission launched an 820-pound copper ‘smart bullet’ that struck comet Tempel 1 on July 4, 2005. The resulting fireworks were observed by the mother ship, yielding new information about the composition and mechanical strength of the comet. Other missions included NASA’s Deep Space 1, which used an ion drive to visit asteroid Braille and comet Borrelly; and Stardust, which collected particles from comet Wild 2 and returned them to Earth in 2006. It turns out many comet and asteroid missions were multi-purpose to get more bang for the buck. 

Much less is known about objects beyond Neptune because they’re so far away. But several Kuiper Belt objects (KBO) have been discovered in the last couple of decades leading the way to ‘downgrade’ Pluto from planet to KBO. The New Horizons spacecraft from Johns Hopkins University flew by Pluto in 2015 and then was re-directed to pass by Arrokoth four years later. Thus, we have some detailed information about those two bodies, as well as Neptune’s moon Triton, which is thought to be a captured KBO. Pluto in particular was found to be more active than previously thought for an object so small and so far from the sun. Frozen nitrogen glaciers were found flowing from mountainous terrain. Other, more enigmatic features are still being argued over with the New Horizons data. Pluto also has five moons: Charon, which was discovered with Earth-based telescopes, and four new ones discovered by New Horizons. 

The known population of KBO numbers in the thousands, but new ones are still being discovered – it’s estimated that there may be millions of them out there. One of the largest, Eris, is larger than Pluto and even has a moon, Dysnomia. The Caltech team that first discovered it tried the name Xena after TV’s warrior princess, but later the international Astronomical Union insisted on a less pop-cultural name. Careful monitoring of the orbits of the known KBO has indicated that there may be a much more massive ‘Planet Nine’ within the KBO, but so far searches have failed to turn it up. Meanwhile, New Horizons’ journey continues as it threads its way through the Kuiper Belt towards the edge of the solar system. 


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