Jane Luu

• Jewitt, David; Luu, Jane & Kim, Yoonyoung (2026). Investigation of Split Comet 240P/NEAT. Astronomical Journal. ISSN 0004-6256. 172(2). doi: 10.3847/1538-3881/ae83ac. Full text in Research Archive Show summary

  We present time-series observations of the split comet 240P/NEAT near perihelion, obtained using the Nordic Optical Telescope. The brighter component, 240P-A, has an estimated radius in the range 400─600 m, and loses dust at the peak rate 130 kg s^−1. The ejected dust has characteristic size ∼50 μm, is expelled sunward at ∼25 m s^−1, with a total ejected mass in the period of observation ∼1.5 × 10^9 kg. Mass loss from the fainter component, 240P-B, peaks at ∼35 kg s^−1 and the total ejected mass was 2.3 × 10^8 kg. The radius of 240P-B is uncertain, with a best estimate ∼300 m and an absolute lower limit 50 m. 240P-A and 240P-B are currently separating at about 1 m s^−1, a speed that is likely accelerating as a result of differential outgassing forces, and have a separation age of >3 yr. The splitting of 240P is incompatible with the action of tides, impact, and internal pressure build up. 240P fits a developing picture, in which small comets are destroyed by rotational instabilities triggered by outgassing torques, an explanation that can be tested in 240P by future observations.

• Jewitt, David & Luu, Jane (2025). Preperihelion Development of Interstellar Comet 3I/ATLAS. Astrophysical Journal Letters. ISSN 2041-8205. 994. doi: 10.3847/2041-8213/ae1832. Full text in Research Archive Show summary

  We describe preperihelion optical observations of interstellar comet 3I/ATLAS taken during 2025 July–September using the Nordic Optical Telescope. Fixed aperture photometry of the comet is well described by a power-law function of heliocentric distance, r H , with the exponent (index) n  = 3.8 ± 0.3 across the 4.6–1.8 au distance range (phase function 0.04 ± 0.02 mag degree −1 assumed). This indicates that the dust production rates vary in proportion to r H − 1.8 ± 0.3 . An r H − 2 variation is expected of a strongly volatile material, and consistent with independent spectroscopic observations showing that carbon dioxide is the primary driver of activity. The measured heliocentric index is unremarkable in the context of solar system comets, for which n is widely dispersed, and provides no basis on which to describe 3I as either dynamically old (thermally processed) or new (pristine). The morphology of the comet changes from a Sun-facing dust fan in the early 2025 July observations, to one dominated by an antisolar dust tail at later dates. We attribute the delayed emergence of the tail to the large size (effective radius 100 μ m) and slow ejection (5 m s −1 ) of the optically dominant dust particles, and their consequently sluggish response to solar radiation pressure. Small (micron-sized) particles may be present but not in numbers sufficient to dominate the scattering cross section. Their relative depletion possibly reflects interparticle cohesion, which binds small particles more effectively than large ones. A similar preponderance of 100 μ m grains was reported in 2I/Borisov. However, 2I differed from 3I in having a much smaller (asteroid-like) heliocentric index, n  = 1.9 ± 0.1. Dust production rates in 3I are ∼180 kg s −1 at 2 au, compared with ∼70 kg s −1 in 2I/Borisov at the same distance.

• Jewitt, David & Luu, Jane (2025). Multiple Outbursts of Halley-type Comet 12P/Pons-Brooks. Astronomical Journal. ISSN 0004-6256. 169(6). doi: 10.3847/1538-3881/add2ff. Full text in Research Archive Show summary

  We present optical observations of the Halley-type comet 12P/Pons–Brooks (12P) on its approach to perihelion. The comet was active even in the first observations at ∼8 au. Starting at ∼4 au, 12P exhibited an extraordinary series of outbursts, in which the brightness changed by a factor up to 100 and the coma morphology transformed under the action of radiation pressure into a distinctive “horned” appearance. Individual outburst dust masses are several ×109 kg, with kinetic energies ∼1014 J, release times ∼104 s, and effective power ∼1010 W. These properties are most consistent with, although do not definitively establish, an origin by the crystallization of amorphous water ice with the related release of trapped supervolatile gases. This interpretation is supported by the observation that the specific outburst energy and the specific crystallization energy are comparable (both ∼105 J kg−1).

• Jewitt, David; Luu, Jane & Li, Jing (2025). Demise of Kreutz Sungrazing Comet C/2024 S1 (ATLAS). Astronomical Journal. ISSN 0004-6256. 169(2). doi: 10.3847/1538-3881/ada3c0. Full text in Research Archive Show summary

  Most Kreutz family sungrazing comets are discovered only days before perihelion, severely limiting observational opportunities to study their physical nature and decay. Kreutz sungrazer C/2024 S1 (ATLAS) was discovered a month before reaching its perihelion distance of 0.008 au, allowing physical observations from both ground- and space-based telescopes. We present observations from 0.9 au to 0.4 au using the Nordic Optical Telescope showing that 1) nucleus disintegration was on-going already at 0.7 au pre-perihelion, 2) the activity varied unpredictably with distance and 3) the nucleus radius was 600 m (red geometric albedo 0.04 assumed). We also use coronagraphic observations from the STEREO-A spacecraft to study C/2024 S1 at heliocentric distances 0.1 au. We find that the coma scattering cross-section peaked near 0.075 au and faded progressively, by a factor 20, towards the last observation at 0.02 au. We interpret the near-perihelion fading as a result of the sublimation of refractory coma grains, beginning at blackbody temperatures 1000 K, consistent with olivine composition. The comet was not detected after perihelion. We consider processes operating to destroy the nucleus when near perihelion, concluding that rotational instability and sublimation losses work together towards this end, even before entry of the comet into the Roche lobe of the Sun.

• Luu, Jane & Jewitt, David (2024). Continuous Fragmentation of Comet 157P/Tritton. Astronomical Journal. ISSN 0004-6256. 169(1). doi: 10.3847/1538-3881/ad8e62. Full text in Research Archive Show summary

  We observed the split comet 157P/Tritton in 2022 October–November and 2024 January with the Nordic Optical Telescope. Our observations show that the splitting continued during the entire observing campaign. Fragmentation was associated with outbursts, consistent with the action of outgassing torques that spun up the nucleus and its fragments to the point of rotational instability. The outburst-fragmentation events can lead to a runaway process where the increasing spin rate, driven by outgassing torques, results in repeated mass loss until the sublimating body completely disintegrates.

• Kim, Yoonyoung; Jewitt, David; Luu, Jane; Li, Jing & Mutchler, Max (2023). Comet 108P/Ciffreo: The Blob. Astronomical Journal. ISSN 0004-6256. 165(4). doi: 10.3847/1538-3881/acba07. Full text in Research Archive Show summary

  Abstract Short-period comet 108P/Ciffreo is known for its peculiar double morphology, in which the nucleus is accompanied by a comoving, detached, diffuse blob. We report new observations of 108P/Ciffreo taken with the Hubble Space Telescope and the Nordic Optical Telescope and use them to determine the cause of this unusual morphology. The separation and the longevity of the blob across several orbits together rule out the possibility of a single, slow-moving secondary object near the primary nucleus. We use a model of coma particle dynamics under the action of solar gravity and radiation pressure to show that the blob is an artifact of the turnaround of particles ejected sunward and repelled by sunlight. Numerical experiments limit the range of directions which can reproduce the morphology and explain why the comoving blob appearance is rare.

• Luu, Jane; Jewitt, David C.; Mutchler, Max; Agarwal, Jessica; Kim, Yoonyoung & Li, Jing [Show all 7 contributors for this article] (2021). Rotational mass shedding from asteroid (6478) Gault. Astrophysical Journal Letters. ISSN 2041-8205. 910(2). doi: 10.3847/2041-8213/abedbc. Full text in Research Archive
• Luu, Jane; Flekkøy, Eirik Grude & Toussaint, Renaud (2020). 'Oumuamua as a Cometary Fractal Aggregate: The Dust Bunny Model. Astrophysical Journal Letters. ISSN 2041-8205. 900(2). doi: 10.3847/2041-8213/abafa7. Full text in Research Archive
• Jewitt, David; Kim, Yoonyoung; Luu, Jane; Rajagopal, Jayadev; Kotulla, Ralf & Ridgway, Susan [Show all 7 contributors for this article] (2019). Episodically Active Asteroid 6478 Gault. Astrophysical Journal Letters. ISSN 2041-8205. 876(2), p. 1–11. doi: 10.3847/2041-8213/ab1be8. Full text in Research Archive Show summary

  We present imaging and spectroscopic observations of 6478 Gault, a ~6 km diameter inner main-belt asteroid currently exhibiting strong, comet-like characteristics. Three distinct tails indicate that ultra-slow dust (ejection speed 0.15 ± 0.05 m s−1) was emitted from Gault in separate episodes beginning UT 2018 October 28 ± 5 (Tail A), UT 2018 December 31 ± 5 (Tail B), and UT 2019 February 10 ± 7 (Tail C), with durations of ΔT ~ 10–20 days. With a mean particle radius 200 μm, the estimated masses of the tails are M A ~ 4 × 107 kg, M B ~ 6 × 106 kg, and M C ~ 6 × 105 kg, respectively, and the mass-loss rates from the nucleus are 20–40 kg s−1 for Tail A, 4–6 kg s−1 for Tail B, and ~0.4 kg s−1 for Tail C. In its optical colors Gault is more similar to C-type asteroids than to S-types, even though the latter are numerically dominant in the inner asteroid belt. A spectroscopic upper limit to the production of gas is set at 1 kg s−1. Discrete emission in three protracted episodes effectively rules out an impact origin for the observed activity. Sublimation driven activity is unlikely given the inner-belt orbit and the absence of detectable gas. In any case, sublimation would not easily account for the observed multiple ejections. The closest similarity is between Gault and active asteroid 311P/(2013 P5), an object showing repeated but aperiodic ejections of dust over a 9 month period. While Gault is 10 times larger than 311P/(2013 P5), and the relevant timescale for spin-up by radiation torques is ~100 times longer, its properties are likewise most consistent with episodic emission from a body rotating near breakup.

• Jewitt, David; Kim, Yoonyoung; Luu, Jane & Graykowski, Ariel (2019). The Discus Comet: C/2014 B1 (Schwartz). Astronomical Journal. ISSN 0004-6256. 157(3), p. 1–11. doi: 10.3847/1538-3881/aafe05. Full text in Research Archive Show summary

  Long-period comet C/2014 B1 (Schwartz) exhibits a remarkable optical appearance, like that of a discus or bi-convex lens viewed edgewise. Our measurements in the four years since discovery reveal a unique elongated dust coma whose orientation is stable with respect to the projected antisolar and orbital directions. With no tail and no trail, the limited influence of radiation pressure on the dust coma sets a lower limit to the effective particle size 100 μm, while the photometry reveals a peak coma scattering cross-section 2.7 × 104 km2 (geometric albedo 0.1 assumed). From the rate of brightening of the comet we infer a dust production rate 10 kg s−1 at 10 au heliocentric distance, presumably due to the sublimation of supervolatile ices, and perhaps triggered by the crystallization of amorphous water ice. We consider several models for the origin of the peculiar morphology. The disk-like shape is best explained by equatorial ejection of particles from a nucleus whose spin vector lies near the plane of the sky. In this interpretation, the unique appearance of C/2014 B1 is a result of a near equality between the rotation-assisted nucleus escape speed (~1–10 m s−1 for a 2–20 km scale nucleus) and the particle ejection velocity, combined with a near-equatorial viewing perspective. To date, most other comets have been studied at heliocentric distances less than half that of C/2014 B1, where their nucleus temperatures, gas fluxes, and dust ejection speeds are much higher. The throttling role of nucleus gravity is correspondingly diminished, so that the disk morphology has not before been observed.

• Jewitt, David & Luu, Jane (2019). Disintegrating Inbound Long-period Comet C/2019 J2. Astrophysical Journal Letters. ISSN 2041-8205. 883(2). doi: 10.3847/2041-8213/ab4135. Full text in Research Archive Show summary

  We present observations of the disintegrating long-period comet C/2019 J2 (Palomar), taken to determine the nature of the object and the cause of its demise. The data are consistent with the break-up of a sub-kilometer nucleus into a debris cloud of mass ~109 kg, peaking on UT 2019 May 24 ± 12. This is ~56 days before perihelion and at a heliocentric distance of ~1.9 au. We consider potential mechanisms of disintegration. Tidal disruption is ruled out, because the comet has not passed within the Roche sphere of any planet. Impact disruption is implausible, because the comet orbit is highly inclined (inclination 105fdg1) and disruption occurred far above the ecliptic, where asteroids are rare. The back pressure generated by sublimation (0.02–0.4 N m−2) is orders of magnitude smaller than the reported compressive strength (30–150 N m−2) of cometary material and, therefore, is of no importance. The depletion of volatiles by sublimation occurs too slowly to render the nucleus inactive on the timescale of infall. However, we find that the e-folding timescale for spin-up of the nucleus by the action of sublimation torques is shorter than the infall time, provided that the nucleus radius is rn < 0.4 km. Thus, the disintegration of C/2019 J2 is tentatively interpreted as the rotational disruption of a sub-kilometer nucleus caused by outgassing torques.

• Flekkøy, Eirik Grude; Luu, Jane & Toussaint, Renaud (2019). The interstellar object ’Oumuamua as a fractal dust aggregate. Astrophysical Journal Letters. ISSN 2041-8205. 885(2), p. 1–6. doi: 10.3847/2041-8213/ab4f78. Full text in Research Archive Show summary

  The first known interstellar object 'Oumuamua exhibited a nongravitational acceleration that appeared inconsistent with cometary outgassing, leaving radiation pressure as the most likely force. Barring the alien lightsail hypothesis, an ultra-low density due to a fractal structure might also explain the acceleration of 'Oumuamua by radiation pressure. In this Letter we report a decrease in 'Oumuamua's rotation period based on ground-based observations, and show that this spin-down can be explained by the YORP effect if 'Oumuamua is indeed a fractal body with the ultra-low density of 10−2 kg m−3. We also investigate the mechanical consequences of 'Oumuamua as a fractal body subjected to rotational and tidal forces, and show that a fractal structure can survive these mechanical forces.

• Jewitt, David & Luu, Jane (2019). Initial Characterization of Interstellar Comet 2I/2019 Q4 (Borisov). Astrophysical Journal Letters. ISSN 2041-8205. 886(2). doi: 10.3847/2041-8213/ab530b. Full text in Research Archive Show summary

  We present initial observations of the interstellar body 2I/(2019 Q4) Borisov taken to determine its nature prior to the perihelion in 2019 December. Images from the Nordic Optical Telescope show a prominent, morphologically stable dust coma and tail. The dust cross-section within 15,000 km of the nucleus averages 130 km2 (assuming geometric albedo 0.1) and increases by about 1% per day. If sustained, this rate indicates that the comet has been active for ~100 days prior to the observations. Cometary activity thus started in 2019 June, at which time C/Borisov was at ~4.5 au from the Sun, a typical distance for the onset of water ice sublimation in comets. The dust optical colors, B − V = 0.80 ± 0.05, V − R = 0.47 ± 0.03 and R− I = 0.49 ± 0.05, are identical to those of a sample of (solar system) long-period comets. The colors are similar to those of 1I/(2017 U1) 'Oumuamua, indicating a lack of the ultrared matter that is common in the Kuiper Belt, on both interstellar objects. The effective size of the dust particles is estimated as $\overline{a}$ = 100 μm, based on the length of the dust tail and the 100 day lifetime. With this size, the ejected dust mass is of order 1.3 × 107 kg and the current dust mass loss rate ~2 kg s−1. We set an upper limit to the nucleus radius using photometry at r n ≤3.8 km (again for albedo 0.1) and we use a statistical argument to show that the nucleus must be much smaller, likely a few hundred meters in radius.

• Jewitt, David; Luu, Jane; Rajagopal, Jayadev; Kotulla, Ralf; Ridgway, Susan & Liu, Wilson [Show all 7 contributors for this article] (2017). Interstellar Interloper 1I/2017 U1: Observations from the NOT and WIYN Telescopes. Astrophysical Journal Letters. ISSN 2041-8205. 850(2). doi: 10.3847/2041-8213/aa9b2f. Full text in Research Archive Show summary

  We present observations of the interstellar interloper 1I/2017 U1 ('Oumuamua) taken during its 2017 October flyby of Earth. The optical colors B – V = 0.70 ± 0.06, V – R = 0.45 ± 0.05, overlap those of the D-type Jovian Trojan asteroids and are incompatible with the ultrared objects that are abundant in the Kuiper Belt. With a mean absolute magnitude <i>H _V</i> = 22.95 and assuming a geometric albedo <i>p _V</i> = 0.1, we find an average radius of 55 m. No coma is apparent; we deduce a limit to the dust mass production rate of only ~2 × 10⁻⁴ kg s⁻¹, ruling out the existence of exposed ice covering more than a few m² of the surface. Volatiles in this body, if they exist, must lie beneath an involatile surface mantle ≥0.5 m thick, perhaps a product of prolonged cosmic-ray processing in the interstellar medium. The light curve range is unusually large at ~2.0 ± 0.2 mag. Interpreted as a rotational light curve the body has axis ratio ≥ 6.3_-1.1/+1.3:1 and semi-axes ~230 m × 35 m. A ≥6:1 axis ratio is extreme relative to most small solar system asteroids and suggests that albedo variations may additionally contribute to the variability. The light curve is consistent with a two-peaked period ~8.26 hr, but the period is non-unique as a result of aliasing in the data. Except for its unusually elongated shape, 1I/2017 U1 is a physically unremarkable, sub-kilometer, slightly red, rotating object from another planetary system. The steady-state population of similar, ~100 m scale interstellar objects inside the orbit of Neptune is ~10⁴, each with a residence time of ~10 years.

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