astronomy

Damocloid

Damocloid is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Damocloid rather than just read about it. In short: Damocloids are a class of minor planets such as 5335 Damocles (which is also the prototype and namesake of the group) and 1996 PW that have Halley-type or long-period highly eccentric orbits typical of periodic comets such as Halley's Comet, but without showing a cometary coma or tail. David C.

Key takeaways

Damocloid belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Damocloid to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Damocloid from memory before moving on to harder problems.

Reference excerpt

Damocloids are a class of minor planets such as 5335 Damocles (which is also the prototype and namesake of the group) and 1996 PW that have Halley-type or long-period highly eccentric orbits typical of periodic comets such as Halley's Comet, but without showing a cometary coma or tail. David C. Jewitt defines a damocloid as an object with a Jupiter Tisserand invariant (TJ) of 2 or less, while Akimasa Nakamura defines this group with the following orbital elements:

q < 5.2 AU, a > 8.0 AU, and e > 0.75, or alternatively, i > 90° However, this definition that does not focus on Jupiter excludes objects such as (127546) 2002 XU93, 2003 WG166, and (661238) 2004 DA62. Using the Tisserand's parameter with respect to Jupiter of 2 or less, there are currently 220 damocloid candidates as of January 2022. Of these objects, 189 have orbital observation arcs greater than 30 days providing reasonably decent orbits. Their average radius is eight kilometers assuming an albedo of 0.04. The albedos of four damocloids have been measured, and they are among the darkest objects known in the Solar System. Damocloids are reddish in color, but not as red as many Kuiper-belt objects or centaurs. Other damocloids include: (709487) 2013 BL76, (668643) 2012 DR30, (528219) 2008 KV42, (65407) 2002 RP120, and 20461 Dioretsa. Retrograde objects such as Halley's Comet and damocloid 343158 Marsyas can have relative velocities to Earth of 81 km/s (290,000 km/h).

Origin Damocloids are thought to be nuclei of Halley-type comets that have lost all their volatile materials due to outgassing and become dormant. Such comets probably originate from the Oort cloud. This hypothesis is strengthened by the fact that a number of objects thought to be damocloids (and assigned minor-planet provisional designations) subsequently showed a coma and were confirmed to be comets: C/2001 OG108 (LONEOS), C/2002 CE10 (LINEAR), C/2002 VQ94 (LINEAR), C/2004 HV60 (Spacewatch) and possibly others. Another strong indication of cometary origin is the fact that some damocloids have retrograde orbits—they have an inclination beyond 90 degrees up to 180 degrees.

List

As of August 2021 this list from the JPL SBDB contains 20 numbered and 268 unnumbered bodies that meet Akimasa Nakamura's criteria for being classified as a damocloid, that is, either a retrograde orbit, or the following orbital elements: q < 5.2 AU, a > 8.0 AU, and e > 0.75 (also see § top). Tisserand's parameter with respect to Jupiter (TJupiter) is also given. Akimasa Nakamura's criteria and a TJupiter of less than two are largely equivalent as only a few listed bodies do not meet the defined TJupiter threshold. Most damocloids are also listed on MPC's list of other unusual minor planets. The orbital data is sourced from JPL-numbered and -unnumbered element files. The list includes A/-designated objects (introduced in 2017) that were mistakenly identified as a comet, but are actually minor planets. However it excludes hyperbolic bodies such as A/2019 G4 as well as unbounded, interstellar objects such as 1I/ʻOumuamua and 2I/Borisov.

See also Extinct comet List of centaurs (small Solar System bodies)

Notes

References

External links A first look at the Damocloids, David Jewitt, January 2005 Damocloid family, David J. Darling

Worked examples

Example 1 — a first encounter with Damocloid

Start with the simplest possible case. Write down what Damocloid claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Damocloid before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Damocloid ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Damocloid

In research: Damocloid appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Damocloid in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Damocloid is common in secondary-school and first-year university syllabi. It links to neighbouring topics Damocloids, Distant minor planet groups and families, so understanding it makes those chapters shorter.
In everyday life: Look for Damocloid outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Damocloid in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Damocloid means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Damocloid out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Damocloid in simple terms?

Why does Damocloid matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Damocloid?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Damocloid.