Engineering topics

Hui, Man-To

Publications and source records attributed to Hui, Man-To.

9 records

DOE OSTI2019

Distant Comet C/2017 K2 and the Cohesion Bottleneck

The distant long-period comet C/2017 K2 (PANSTARRS) has been outside the planetary region of the solar system for ∼3 Myr, negating the possibility that heat retained from the previous perihelion could be responsible for its activity. This inbound comet is also too cold for water ice to sublimate and too cold for amorphous water ice, if present, to crystallize. C/2017 K2 thus presents an ideal target in which to investigate the mechanisms responsible for activity in distant comets. We have used the Hubble Space Telescope to study the comet in the pre-perihelion heliocentric distance range 13.8 ≤ r {sub H} ≤ 15.9 au. In this range, the coma maintains a logarithmic surface brightness gradient m = −1.010 ± 0.004, consistent with mass loss proceeding in steady state. The absence of a radiation pressure swept tail indicates that the effective particle size is large (radius ≳0.1 mm) and the mass-loss rate is ∼200 kg s{sup −1}, remarkable for a comet still beyond the orbit of Saturn. Extrapolation of the photometry indicates that activity began in 2012.1 ± 0.5, at r {sub H} = 25.9 ± 0.9 au, where the isothermal blackbody temperature is only T {sub BB} = 55 K. This large distance and low temperature suggest that cometary activity is driven by the sublimation of a super-volatile ice (e.g., CO), presumably preserved by K2's long-term residence in the Oort cloud. The mass-loss rate can be sustained by CO sublimation from an area ≲2 km{sup 2}, if located near the hot subsolar point on the nucleus. However, while the drag force from sublimated CO is sufficient to lift millimeter-sized particles against the gravity of the cometary nucleus, it is 10{sup 2}–10{sup 3} times too small to eject these particles against interparticle cohesion. Our observations thus require either a new understanding of the physics of interparticle cohesion or the introduction of another mechanism to drive distant cometary mass loss. We suggest thermal fracture and electrostatic supercharging in this context.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

A Comet Active Beyond the Crystallization Zone

We present observations showing inbound long-period comet C/2017 K2 (PANSTARRS) to be active at a record heliocentric distance. Nucleus temperatures are too low (60–70 K) either for water ice to sublimate or for amorphous ice to crystallize, requiring another source for the observed activity. Using the Hubble Space Telescope we find a sharply bounded, circularly symmetric dust coma 10{sup 5} km in radius, with a total scattering cross-section of ∼10{sup 5} km{sup 2}. The coma has a logarithmic surface brightness gradient −1 over much of its surface, indicating sustained, steady-state dust production. A lack of clear evidence for the action of solar radiation pressure suggests that the dust particles are large, with a mean size ≳0.1 mm. Using a coma convolution model, we find a limit to the apparent magnitude of the nucleus V>25.2 (absolute magnitude H>12.9). With assumed geometric albedo p {sub V} = 0.04, the limit to the nucleus circular equivalent radius is <9 km. Prediscovery observations from 2013 show that the comet was also active at 23.7 au heliocentric distance. While neither water ice sublimation nor exothermic crystallization can account for the observed distant activity, the measured properties are consistent with activity driven by sublimating supervolatile ices such as CO{sub 2}, CO, O{sub 2}, and N{sub 2}. Survival of supervolatiles at the nucleus surface is likely a result of the comet’s recent arrival from the frigid Oort Cloud.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

Split Active Asteroid P/2016 J1 (PANSTARRS)

We present a photometric and astrometric study of the split active asteroid P/2016 J1 (PANSTARRS). The two components (hereafter J1-A and J1-B) separated either ∼1500 days (2012 May to June) or 2300 days (2010 April) prior to the current epoch, with a separation speed V {sub sep} = 0.70 ± 0.02 m s{sup −1} for the former scenario and 0.83 ± 0.06 m s{sup −1} for the latter. Keck photometry reveals that the two fragments have similar, Sun-like colors that are comparable to the colors of primitive C- and G-type asteroids. With a nominal comet-like albedo, p{sub R} = 0.04, the effective, dust-contaminated cross sections are estimated to be 2.4 km{sup 2} for J1-A and 0.5 km{sup 2} for J1-B. We estimate that the nucleus radii lie in the range 140 ≲ R {sub N} ≲ 900 m for J1-A and 40 ≲ R {sub N} ≲ 400 m for J1-B. A syndyne–synchrone simulation shows that both components have been active for 3–6 months, by ejecting dust grains at speeds ∼0.5 m s{sup −1} with rates ∼1 kg s{sup −1} for J1-A and 0.1 kg s{sup −1} for J1-B. In its present orbit, the rotational spin-up and devolatilization times of 2016 J1 are very small compared to the age of the solar system, raising the question of why this object still exists. We suggest that ice that was formerly buried within this asteroid became exposed at the surface, perhaps via a small impact, and that sublimation torques then rapidly drove it to breakup. Further disintegration events are anticipated owing to the rotational instability.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

RESURRECTION OF (3200) PHAETHON IN 2016

We present a study of the active asteroid (3200) Phaethon in the 2016 apparition using the Solar and Terrestrial Relations Observatory ( STEREO ) spacecraft and compare the results with data from the previous two perihelia in 2009 and 2012. Once again, Phaethon brightened by ∼2 mag soon after its perihelion passage, contradicting expectations from the phase function of a macroscopic monolithic body. Subsequently, a short antisolar tail of ∼0.°1 in length was formed within ∼1 day and quickly disappeared. No trail was seen. Our syndyne-synchrone analysis indicates that the tail was comprised of submicron to micron particles and can be approximated by a synchrone coinciding with the outburst. We estimate that the outburst has released a mass of ∼10{sup 4}–10{sup 5} kg, comparable to the two mass ejections in 2009 and 2012, and that the average mass-loss rate is ∼0.1–1 kg s{sup −1}. The forward-scattering effect hinted at low level activity of Phaethon prior to the outburst, which increased the effective cross section by merely ≲1 km{sup 2}. Without the forward-scattering enhancement, detecting such activity at side-scattering phase angles is very difficult. The forward-scattering effect also reinforces the idea that the ejected dust grains rather than gas emissions were responsible for the activity of Phaethon. Despite Phaethon's reactivation, it is highly unlikely that the Geminid meteoroid stream can be sustained by similar perihelion mass-loss events.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

Fragmentation kinematics in comet 332P/Ikeya-Murakami

We present initial time-resolved observations of the split comet 332P/Ikeya–Murakami taken using the Hubble Space Telescope. Our images reveal a dust-bathed cluster of fragments receding from their parent nucleus at projected speeds in the range 0.06–3.5 m s{sup −1} from which we estimate ejection times from 2015 October to December. The number of fragments with effective radii ≳20 m follows a differential power law with index γ = −3.6 ± 0.6, while smaller fragments are less abundant than expected from an extrapolation of this power law. We argue that, in addition to losses due to observational selection, torques from anisotropic outgassing are capable of destroying the small fragments by driving them quickly to rotational instability. Specifically, the spin-up times of fragments ≲20 m in radius are shorter than the time elapsed since ejection from the parent nucleus. The effective radius of the parent nucleus is r{sub e} ⩽ 275 m (geometric albedo 0.04 assumed). This is about seven times smaller than previous estimates and results in a nucleus mass at least 300 times smaller than previously thought. The mass in solid pieces, 2×10{sup 9} kg, is about 4% of the mass of the parent nucleus. As a result of its small size, the parent nucleus also has a short spin-up time. Brightness variations in time-resolved nucleus photometry are consistent with rotational instability playing a role in the release of fragments.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

Gone in a blaze of glory: The demise of comet C/2015 D1 (SOHO)

We present studies of C/2015 D1 (SOHO), the first sunskirting comet ever seen from ground stations over the past half century. The Solar and Heliospheric Observatory (SOHO) witnessed its peculiar light curve with a huge dip followed by a flare-up around perihelion: the dip was likely caused by sublimation of olivines, directly evidenced by a coincident temporary disappearance of the tail. The flare-up likely reflects a disintegration event, which we suggest was triggered by intense thermal stress established within the nucleus interior. Photometric data reveal an increasingly dusty coma, indicative of volatile depletion. A catastrophic mass-loss rate of ∼10{sup 5} kg s{sup −1} around perihelion was seen. Ground-based Xingming Observatory spotted the post-perihelion debris cloud. Our morphological simulations of post-perihelion images find newly released dust grains of size a ≳ 10 μm in radius; however, a temporal increase in a{sub min} was also witnessed, possibly owing to swift dispersions of smaller grains swept away by radiation forces without replenishment. Together with the fading profile of the light curve, a power-law dust size distribution with index γ = 3.2 ± 0.1 is derived. We detected no active remaining cometary nuclei over ∼0.1 km in radius in post-perihelion images acquired at Lowell Observatory. Applying a radial nongravitational parameter, A{sub 1}=(1.209±0.118)×10{sup −6} AU day{sup −2}, from an isothermal water–ice sublimation model to the SOHO astrometry significantly reduces residuals and sinusoidal trends in the orbit determination. The nucleus mass ∼10{sup 8}–10{sup 9} kg and the radius ∼50–150 m (bulk density ρ{sub d} = 0.4 g cm{sup −3} assumed) before the disintegration are deduced from the photometric data; consistent results were determined from the nongravitational effects.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

Archival observations of active asteroid 313P/GIBBS

We present prediscovery observations of active asteroid 313P/(2014 S4) Gibbs taken in 2003 and 2004. Sloan Digital Sky Survey observations from 2003 establish that, after 133P/Elst–Pizarro and 238P/Read, 313P is only the third main-belt active asteroid to show mass loss in different orbits. This repetitive activity is consistent with mass loss driven by the sublimation of ice. A Finson-Probstein model is used to analyze the morphology of the object, showing that the activity was continuous but declining steadily over two months of observation in 2003. Ejected dust particles are dominated by relatively large sizes with radii from ∼25 to ∼600 μm. We estimate from photometry the dust mass loss rate to be ≲0.2 kg s{sup −1}. In contrast, Subaru telescope observations from 2004 show no evidence for mass loss and suggest a bare nucleus. The color, nucleus size, and the mass loss rates of 313P measured in 2003 are consistent with published values obtained from data taken in 2014. However, the effective cross-section was larger than in 2014 by a factor of ∼2, indicating a decline in the activity between orbits. A new orbital solution gives a 5σ upper limit to the transverse non-gravitational parameter A{sub 2}≲2.45×10{sup −8} AU day{sup −2}.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

New active asteroid 313P/Gibbs

We present initial observations of the newly discovered active asteroid 313P/Gibbs (formerly P/2014 S4), taken to characterize its nucleus and comet-like activity. The central object has a radius ∼0.5 km (geometric albedo 0.05 assumed). We find no evidence for secondary nuclei and set (with qualifications) an upper limit to the radii of such objects near 20 m, assuming the same albedo. Both aperture photometry and a morphological analysis of the ejected dust show that mass-loss is continuous at rates ∼0.2–0.4 kg s{sup −1}, inconsistent with an impact origin. Large dust particles, with radii ∼50–100 μm, dominate the optical appearance. At 2.4 AU from the Sun, the surface equilibrium temperatures are too low for thermal or desiccation stresses to be responsible for the ejection of dust. No gas is spectroscopically detected (limiting the gas mass-loss rate to <1.8 kg s{sup −1}). However, the protracted emission of dust seen in our data and the detection of another episode of dust release near perihelion, in archival observations from 2003, are highly suggestive of an origin by the sublimation of ice. Coincidentally, the orbit of 313P/Gibbs is similar to those of several active asteroids independently suspected to be ice sublimators, including P/2012 T1, 238P/Read, and 133P/Elst–Pizarro, suggesting that ice is abundant in the outer asteroid belt.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

Where are the mini Kreutz-family comets?

The Kreutz family of sungrazing comets contains over 2000 known members, many of which are believed to be under ∼100 m sizes (mini comets) and have only been studied at small heliocentric distances (r {sub H}) with space-based SOHO/STEREO spacecraft. To understand the brightening process of mini Kreutz comets, we conducted a survey using CFHT/MegaCam at moderate r {sub H} guided by SOHO/STEREO observations. We identify two comets that should be in our search area but are not detected, indicating that the comets have either followed a steeper brightening rate within the previously reported rapid brightening stage (the brightening burst), or the brightening burst starts earlier than expected. We present a composite analysis of the pre-perihelion light curves of five Kreutz comets that cover to ∼1 AU. We observe significant diversity in the light curves that can be used to grossly classify them into two types: C/Ikeya-Seki and C/SWAN follow the canonical r{sub H}{sup −4} while the others follow r{sub H}{sup −7}. In particular, C/SWAN seems to have undergone an outburst (Δm > 5 mag) or a rapid brightening (n ≳ 11) between r {sub H} = 1.06 AU and 0.52 AU, and shows hints of structural/compositional differences compared to other bright Kreutz comets. We also find evidence that the Kreutz comets as a population lose their mass less efficiently than the dynamically new comet, C/ISON, and are relatively devoid of species that drive C/ISON's activity at large r {sub H}. Concurrent observations of C/STEREO in different wavelengths also suggest that a blueward species such as CN may be the main driver for brightening bursts, instead of sodium as previously thought.

79 ASTRONOMY AND ASTROPHYSICS↗