8 Most Interesting Asteroids and Comets

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Want to learn about some interesting asteroids and comets that could help unlock the mysteries of our Solar System? There are so many of them that spacecraft have yet to explore all of them. But we have compiled a list of the top 8 most attractive objects for further exploration. Let’s find out in this article.

Table of Contents

  1. 29P/Schwassmann–Wachmann 1
  2. 3200 Phaethon
  3. 73P/Schwassmann–Wachmann 3
  4. 288P/2006 VW139
  5. 6478 Gault
  6. 101955 Bennu
  7. 162173 Ryugu
  8. 2017 YE5

1. 29P/Schwassmann–Wachmann 1

Let’s start with the most interesting one, comet 29P. As we all know, icy comets have elliptical orbits. If they have circular orbits, the volatiles could evaporate too fast when they’re close to the Sun at all times. If they are farther away, the temperatures may be cold enough that the comet remains inactive forever. Comet 29P, however, is different. It has a nearly circular orbit and yet is an active comet. In fact, it doesn’t eject material when it’s closer to the Sun — it seems to do so at random intervals.

Nobody knows for sure why it behaves so weirdly, although there are a few hypotheses. In fact, it may not be a comet at all; the material ejections could be caused by volcanic eruptions from deep inside the object. However, there still needs to be more information to confirm any theories about that, making it a compelling scientific target to collect data.

Comet 29P, as viewed from the Spitzer Space Telescope
Credit: NASA/JPL/Caltech/Ames Research Center/University of Arizona

2. 3200 Phaethon

This asteroid is weird. The way it looks already makes it extraordinary. Most asteroids are gray or red, but Phaethon is blue. Moreover, the asteroid is ejecting material like a comet, yet its chemical composition does not look like a comet. In fact, Phaethon is the parent body of the Geminids, one of the major meteor showers often caused by comets. The reason for this phenomenon remains elusive until recently, and it points to a comet with no volatiles.

The highly elliptical orbit of Phaethon takes it near the Sun, with its perihelion being just over 0.14 astronomical units, less than half the distance between Mercury and the Sun. It heats the surface of the asteroid to hundreds of degrees Celsius, so hot that the sodium inside the asteroid vaporizes and fizzes out from the asteroid. The force of the outgassing then ejects material into space, causing the Geminids meteor shower and the asteroid’s comet-like appearance.

Although one of the biggest mysteries about the asteroid is solved, it is still a promising scientific target. It will be visited by the DESTINY+ spacecraft by JAXA, launching in 2024.

3200 Phaethon, as seen from the Arecibo Observatory
Note that the bluish hue of the asteroid is not shown because radar images are monochrome.
Credit: Arecibo Observatory/NASA/NSF

3. 73P/Schwassmann–Wachmann 3

Believe it or not, this comet is disintegrating. It was still one comet when it was discovered, but it suddenly started breaking up in 1995, initially in three pieces. Then, when it returned to the inner Solar System in 2006, the fragments broke apart dramatically, specifically into 30 traceable pieces. Since then, the comet is now in at least 74 pieces, as tracked by JPL’s Solar System Dynamics department.

As these comets all recently broke up from a single parent body, 73P, exploring them can help reveal hidden secrets of a comet’s interior and subsurface composition. They can also help scientists learn about how comets fracture under stress from solar radiation and tidal forces. This makes it an excellent target, but since it may turn into an invisible cloud of materials soon, its scientific value may not last long.

Fragments of comet 73P, as seen from the Hubble Space Telescope
Credit: NASA, ESA, H. Weaver (APL/JHU), M. Mutchler and Z. Levay (STScI)

4. 288P/2006 VW139

This object encompasses two rare properties at once. Firstly, it is an equal-mass binary, composed of two objects of approximately the same size. These objects are essentially orbiting each other around their common barycenter. Secondly, it’s an active asteroid, showcasing properties of both asteroids and comets. The system is even a numbered asteroid (300163) and a numbered comet (288P) at the same time! That’s a status that only a few objects have. Why did it develop such a configuration?

One such theory is a sunlight-induced rotational spinup. When a celestial object like an asteroid orbits the Sun, the sunlight exerts pressure on the object and changes its rotation through the YORP effect. In 2006 VW139, the pressure was in the same direction as the rotation, speeding it up and increasing the centrifugal forces inside the comet. This eventually causes the comet to disintegrate and break into two, at a much lower speed than in the case of 73P or other similar events. So instead of ejecting from each other, they are captured in orbit around each other, causing the binary asteroid/comet configuration.

But why did the binary comet have similar masses? Most binary asteroids consist of one smaller object orbiting a much larger one, but it’s different in the case of 2006 VW139. We don’t know yet, as we are far from having enough data for binary comets. That’s because 2006 VW139 is the only binary comet ever discovered!

Comet 288P, as seen from the Hubble Space Telescope
Credit: NASA, ESA, and J. DePasquale and Z. Levay (STScI)

5. 6478 Gault

This asteroid looks like a comet with a tail, and it doesn’t look like an inactive, rocky body. But why is it still classified as an asteroid, not a comet? Well, the reason for its tails is not outgassing. It is the disintegration of the asteroid itself.

As mentioned above, the pressure from sunlight can induce a rotational spinup caused by the YORP effect. Eventually, the rotation becomes so rapid that chunks of material break free of the asteroid. That creates the beautiful dust tails of the asteroid as it intermittently sheds off some of its mass.

6478 Gault, as seen from the Hubble Space Telescope
Credit: NASA, ESA, K. Meech and J. Kleyna (University of Hawaii), and O. Hainaut (European Southern Observatory)

6. 101955 Bennu

You have probably heard of this asteroid before. It is already extensively explored, and it is the target of OSIRIS-REx, a sample-return mission. But why do we still put it on the list of the most interesting asteroids to explore? That’s because it defied scientists’ expectations — and they wanted to know why.

When OSIRIS-REx descended to the surface to grab a sample, researchers and mission operators back on Earth expected the space probe to sit on the surface for a few seconds while it collected a sample. They expected that, although the asteroid is loosely bound, some cohesion is present in the asteroid’s surface, holding the rubble pile together during the sample collection event.

But instead, when analyzing data that the spacecraft sent back home, scientists were shocked. They found out that the asteroid’s surface was almost entirely cohesionless. There is not much force, other than their weak mutual gravity, holding these loosely-bound materials together. Thus, OSIRIS-REx collapsed into the Bennu as its materials fell around the robotic arm of the spacecraft. Only after the thrusters fired did the spacecraft escape from its sinking state and leave Bennu altogether.

Why did this asteroid behave this way, different from other rubble-pile asteroids? One research hypothesized that it’s because Bennu is too small. According to this study, the low surface gravity of the asteroid reduces the effects of cohesion within the asteroid’s particles, making the asteroid crumble with even small amounts of force. But that’s just one research, and we may find something new by exploring objects of its size!

NASA video about Bennu’s surface

7. 162173 Ryugu

Like Bennu, this asteroid is also explored by spacecraft, but it’s intriguing in a very different way. After analyzing the samples brought back to Earth by Hayabusa2, scientists revealed a bizarre explanation for the asteroid’s origin. According to the theory, it was a comet that formed between the outer planets. It was then perturbed by the planets to put it on an elliptical orbit, taking it near the Sun and flying by the inner planets to circularize its orbit.

But why is it classified as an asteroid instead of a comet? That’s because this object is probably an extinct comet. An extinct comet is an object that has survived many close passes to the Sun, expending all of its volatiles through outgassing. What’s left out of the comet is a rocky object, almost indistinguishable from an asteroid. That could be the journey Ryugu went through before the Hayabusa2 spacecraft explored and sampled it.

8. 2017 YE5

You see something interesting when you look at the asteroid 2017 YE5. It’s not one asteroid but two asteroids orbiting each other. But how could that be unusual? Many asteroids are known to have companions, ranging from a small moon to an object almost the size of the primary body. But this object doesn’t only consist of two asteroids of about the same size. In fact, the amount of radar these two objects reflect is very different. This means that the two objects may not be fragments of the same asteroid, which makes it a very special binary asteroid, that may tell us about how binary asteroids in the Solar System form.

2017 YE5, as viewed from the Arecibo Observatory
Note that there are two asteroids orbiting each other in this image
Credit: Arecibo Observatory/NASA/NSF

Conclusion

In this article, we’ve mentioned 8 most interesting asteroids and comets. Their properties are puzzling scientists or provide a rare opportunity to explore some unanswered questions about how our Solar System formed. It involved difficult decisions to put these objects in the current rankings, so if you have other opinions, feel free to leave them in the comments below. Also, if you want to learn more about these minor planets, please visit the webpages in the references below.

References

  1. Pultarova, T. (2021, November 2). This mysterious comet’s super-bright outbursts have astronomers puzzled. Retrieved November 25, 2022, from https://www.space.com/comet-29p-strange-outbursts-puzzles-astronomers
  2. Yirka, B. (2021, November 3). Mysterious comet has been having multiple large outbursts – Phys.org. Retrieved November 25, 2022, from https://phys.org/news/2021-11-mysterious-comet-multiple-large-outbursts.html
  3. (2021, August 16). Fizzing Sodium Could Explain Asteroid Phaethon’s Cometlike Activity. Retrieved November 27, 2022, from https://www.jpl.nasa.gov/news/fizzing-sodium-could-explain-asteroid-phaethons-cometlike-activity
  4. Byrd, D. (2021, December 12). 3200 Phaethon: The Geminid’s mysterious parent object. Retrieved November 27, 2022, from https://earthsky.org/astronomy-essentials/rock-comet-3200-phaethon-geminid-meteor-shower/
  5. Starr, M. (2022, October 17). This Unusual Asteroid Keeps Spinning Faster, And We Don’t Know Why. Retrieved November 27, 2022, from https://www.sciencealert.com/this-unusual-asteroid-keeps-spinning-faster-and-we-dont-know-why
  6. Howell, E. (2017, February 15). Comet Breaks in Two — May Be Close to Disintegration – Space.com. Retrieved November 25, 2022, from https://www.space.com/35710-comet-73p-breaks-in-half-photos.html
  7. Yeomans, D. (2006, April 24). Comet 73P/Schwassmann-Wachmann 3 Shows That Breaking Up Is Not So Hard To Do. Retrieved November 25, 2022, from https://cneos.jpl.nasa.gov/news/news150.html
  8. (n.d.). Hubble Provides Spectacular Detail of a Comet’s Breakup. Retrieved November 25, 2022, from https://hubblesite.org/contents/media/images/2006/18/1902-Image.html?news=true
  9. Hille, K. (2017, September 20). Comet or Asteroid? Hubble Discovers that a Unique Object is a Binary. Retrieved November 28, 2022, from https://www.nasa.gov/feature/goddard/2017/hubble-discovers-that-a-unique-object-is-a-binary
  10. Plait, P. (2017, September 24). A binary asteroid that’s actually a binary comet. Retrieved November 28, 2022, from https://www.syfy.com/syfy-wire/a-binary-asteroid-thats-actually-a-binary-comet
  11. Garner, R. (2019, March 28). Hubble Watches Spun-Up Asteroid Coming Apart. Retrieved November 27, 2022, from https://www.nasa.gov/feature/goddard/2019/hubble-watches-spun-up-asteroid-coming-apart
  12. Shekhtman, S. (2022, July 7). NASA Reveals Surface of Asteroid Bennu is Like Plastic Ball Pit. Retrieved December 1, 2022, from https://www.nasa.gov/feature/goddard/2022/surprise-again-asteroid-bennu-reveals-its-surface-is-like-a-plastic-ball-pit
  13. (2022, July 7). SwRI-led study provides new insights about surface, structure of asteroid Bennu. Retrieved December 1, 2022, from https://www.swri.org/press-release/swri-led-study-provides-new-insights-about-surface-structure-asteroid-bennu
  14. Mathewson, S. (2022, April 3). Asteroid Ryugu may be a remnant of an extinct comet. Retrieved December 1, 2022, from https://www.space.com/asteroid-ryugu-remnant-extinct-comet
  15. Lerner, L. (2022, October 20). Rock scooped off speeding asteroid may be comet that lost its tail. Retrieved December 1, 2022, from https://news.uchicago.edu/story/rock-scooped-speeding-asteroid-suggests-it-was-once-comet-lost-its-tail
  16. Greicius, T. (2018, July 11). Observatories Team Up to Reveal Rare Double Asteroid – NASA. Retrieved December 1, 2022, from https://www.nasa.gov/feature/jpl/observatories-team-up-to-reveal-rare-double-asteroid

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