astronomy

Ilmarë

Ilmarë 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 Ilmarë rather than just read about it. In short: Ilmarë, formal designation (174567) Varda I, is the only known moon of the large Kuiper belt object 174567 Varda. It was discovered by Keith Noll et al. in 2009, at a separation of about 0.12 arcsec, using discovery images taken by the Hubble Space Telescope on 26 April 2009, and reported in 2011.

Key takeaways

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

Reference excerpt

Ilmarë, formal designation (174567) Varda I, is the only known moon of the large Kuiper belt object 174567 Varda. It was discovered by Keith Noll et al. in 2009, at a separation of about 0.12 arcsec, using discovery images taken by the Hubble Space Telescope on 26 April 2009, and reported in 2011. At approximately 403 km in diameter, it is the fourth-largest known moon of a trans-Neptunian object, after Pluto I Charon, Eris I Dysnomia, and Orcus I Vanth. Under the assumption of a lower density of 0.7 g/cm3, which is thought to be more typical that of trans-Neptunian objects in the ~400 km size range, the satellite-to-primary mass ratio would be approximately 0.11:1.

Observational history

Discovery Ilmarë was discovered by a team of astronomers led by William M. Grundy using imaging data captured by the Hubble Space Telescope on 26 April 2009. The discovery team also included S. D. Benecchi, M. W. Buie, Keith S. Noll, S. B. Porter, H. G. Roe, J. A. Stansberry, and Chad Trujillo. The moon was first spotted at an angular separation of about 0.12 arcseconds from Varda. Although the old discovery photos were taken in April 2009, the moon's existence was not officially announced until October 2011 at the EPSC-DPS Joint Meeting

Further observations

Astronomers continued to observe Ilmarë after its discovery to map its orbital path around Varda. Along with regular tracking by the Pan-STARRS 1 survey between 2010 and 2015, the moon was targeted by specific observation campaigns. On 10 September 2018, astronomers used a non-geocentric occultation observation to collect precise physical characteristics and orbital characteristics of the system. Automated sky surveys, including ATLAS and the Zwicky Transient Facility, continued to track the moon through 2024 to refine its orbital period and distance from Varda.

Nomenclature Names for Varda and its moon were announced on 16 January 2014. Ilmarë (Quenya: [ˈilmarɛ]) is a chief of the Maiar and handmaiden to Varda, the queen of the Valar, creator of the stars, and principal goddess of the elves in J. R. R. Tolkien's fictional mythology.

Orbit

Orbital characteristics

Ilmarë orbits Varda at a distance of 4815±29 km, with a low eccentricity of 0.016±0.004. It has an orbital period of approximately 5.750824±0.000016 d. Its orbit is inclined by 77.4±1.9° with respect to the ecliptic. In a 2021 trans-Neptunian binary study, the Varda system was grouped among massive binaries characterized by close relative separations. The report noted that these tightly bound systems display distinct formation mechanisms from smaller, widely separated equal-mass binary pairs.

Tidal evolution Observations from a 2018 stellar occultation show that Varda's longest equatorial axis points almost directly toward its moon Ilmarë. Varda's elongated axis has a position angle of 67°±8°, while Ilmarë is located at a position angle of 60.8°±0.7°. If Varda were not tidally locked, the chance of this very close alignment happening is only around 2%. Furthermore, the large size ratio between Varda and its moon Ilmarë strongly suggests that tidal synchronization should happen really quickly, especially at the small semi-major axis of the binary, similar to the Salacia-Actaea system. This alignment matches the minimum energy state expected for a tidally evolved binary system.

Physical characteristics

Older estimates for Ilmarë's physical characteristics If Ilmarë and Varda have the same albedo, Ilmarë would be 163+19−17 km in radius, or approximately 8.4% the volume of Varda. If the two bodies also have the same density, Ilmarë would then have approximately 8.4% the system mass of (2.664±0.064)×1020 kg. If, however, the albedo of Varda is 50% greater than that of Ilmarë, Ilmarë would have a radius of 191+22−21 km and the bulk density of the system would be 1.31+0.52−0.36 g/cm3. If Ilmarë has a 50%-greater albedo, then its radius would be 137+16−15 km and the bulk density would be 1.18+0.47−0.33 g/cm3. Because the absolute magnitudes of the two bodies at different wavelengths are similar, it is not likely that their albedos differ by much, so Ilmarë is likely to be in this size range.

Newer estimates for Ilmarë's physical characteristics

Size and albedo However, more accurate measurements in 2025 that was done by Kiss et al., measured a low albedo of 0.068±0.011 for Ilmarë, which is a bit lower than the albedo of Varda (0.099±0.002), yielding a larger diameter of 403±40 km, which is about half that of its primary. Assuming that the following size estimates are correct, Ilmarë is about the fourth-biggest known moon of a trans-Neptunian object, after Charon (1212±1 km), Dysnomia (615+60−50 km), and Vanth (442.5±10.2 km).

Mass and density Under the assumption that both bodies have equal densities (1.15 g/cm3), the resulting satellite-to-primary mass ratio is 0.16:1, closely matching that of the Orcus–Vanth system. Even under the assumption of a lower density of 0.7 g/cm3, which is thought to be more typical that of trans-Neptunian objects in the ~400 km size range, the mass ratio remains high at 0.11:1. This places Varda–Ilmarë among the systems with the largest known mass ratios, comparable to that of Pluto–Charon.

Surface and spectrum The surface of Ilmare's color is dark and moderately reddish, with a low geometric albedo of 0.068±0.011, which is a bit lower than that of Varda, and has a measured B−V color index of 0.857±0.061 and a V−I color index of 1.266±0.052, which is considered a moderately red color.

See also Actaea – a moon of another large trans-Neptunian object, 120347 Salacia, that is similar in size to Ilmarë Hiʻiaka, a similarly sized moon orbiting the dwarf planet Haumea

Notes

References

Illustrations

Ilmarë: Varda and Ilmarë imaged on 31 August 2010 by the Hubble Space Telescope. — Varda and Ilmarë imaged on 31 August 2010 by the Hubble Space Telescope.

Worked examples

Example 1 — a first encounter with Ilmarë

Start with the simplest possible case. Write down what Ilmarë 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 Ilmarë 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 Ilmarë 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 Ilmarë

In research: Ilmarë 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 Ilmarë 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: Ilmarë is common in secondary-school and first-year university syllabi. It links to neighbouring topics 174567 Varda, Astronomical objects discovered in 2009, Trans-Neptunian satellites, so understanding it makes those chapters shorter.
In everyday life: Look for Ilmarë 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 Ilmarë in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Ilmarë 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 Ilmarë out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Ilmarë in simple terms?

Why does Ilmarë 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 Ilmarë?

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 Ilmarë.