Engineering topics

Weaver, Harold

Publications and source records attributed to Weaver, Harold.

14 records

DOE OSTI2021

Cometary Activity Begins at Kuiper Belt Distances: Evidence from C/2017 K2

We study the development of activity in the incoming long-period comet C/2017 K2 over the heliocentric distance range 9 ≲ r {sub H} ≲ 16 au. The comet continues to be characterized by a coma of submillimeter-sized and larger particles ejected at low velocity. In a fixed co-moving volume around the nucleus we find that the scattering cross section of the coma, C, is related to the heliocentric distance by a power law, C∝r{sub H}{sup -s}, with heliocentric index s = 1.14 ± 0.05. This dependence is significantly weaker than the r {sub H} {sup -2} variation of the insolation as a result of two effects. These are, first, the heliocentric dependence of the dust velocity and, second, a lag effect due to very slow-moving particles ejected long before the observations were taken. A Monte Carlo model of the photometry shows that dust production beginning at r {sub H} ~ 35 au is needed to match the measured heliocentric index, with only a slight dependence on the particle size distribution. Mass-loss rates in dust at 10 au are of order 10{sup 3} kg s{sup -1}, while loss rates in gas may be much smaller, depending on the unknown dust to gas ratio. Consequently, the ratio of the nongravitational acceleration to the local solar gravity, α', may, depending on the nucleus size, attain values of ~10{sup -7} ≲ α' ≲ 10{sup -5}, comparable to values found in short-period comets at much smaller distances. Nongravitational acceleration in C/2017 K2 and similarly distant comets, while presently unmeasured, may limit the accuracy with which we can infer the properties of the Oort cloud from the orbits of long-period comets.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

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 OSTI2019

Active Asteroid P/2017 S5 (ATLAS)

Observations of active asteroid P/2017 S5 when near perihelion reveal the ejection of large (10{sup 2}–10{sup 4} μm) particles at 0.2–2 m s{sup −1} speeds, with estimated mass-loss rates of a few kg s{sup −1}. The protracted nature of the mass loss (continuous over ∼150 days) is compatible with a sublimation origin, meaning that this object is likely an ice-bearing main-belt comet. Equilibrium sublimation of exposed water ice covering as little as 0.1 km{sup 2} can match the data. Observations a year after perihelion show the object in an inactive state from which we deduce a nucleus effective radius 450{sub −60}{sup +100} m (albedo 0.06 ± 0.02 assumed). The gravitational escape speed from a body of this size is just ∼0.3 m s{sup −1}, comparable to the inferred ejection speed of the dust. Time-series photometry provides tentative evidence for rapid rotation (lightcurve period 1.4 hr) that may also play a role in the loss of mass and which, if real, is a likely consequence of spin-up by sublimation torques. P/2017 S5 shares both physical and orbital similarities with the split active asteroid pair P/2016 J1-A and J1-B, and all three objects are likely members of the ∼7 Myr old, collisionally produced, Theobalda family.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2018

Evolution of H2O Production in Comet C/2012 S1 (ISON) as Inferred from Forbidden Oxygen and OH Emission

We present H2O production rates for comet C/2012 S1 (ISON) derived from observations of [O I] and OH emission during its inbound leg, covering a heliocentric distance range of 1.8–0.44 AU. Our production rates are in agreement with previous measurements using a variety of instruments and techniques and with data from the various observatories greatly differing in their projected fields of view. The consistent results across all data suggest the absence of an extended source of H2O production, for example sublimation of icy grains in the coma, or a source with spatial extent confined to the dimensions of the smallest projected field of view (in this case < 1000 km). We find that ISON had an active area of around 10 sq km for heliocentric distances Rh > 1.2 AU, which then decreased to about half this value from Rh = 1.2–0.9 AU. This was followed by a rapid increase in active area at about Rh = 0.6 AU, corresponding to the first of three major outbursts ISON experienced inside of 1 AU. The combination of a detected outburst in the light curve and rapid increase in active area likely indicates a major nucleus fragmentation event. The 5–10 sq km active area observed outside of Rh = 0.6 AU is consistent with a 50–100% active fraction for the nucleus, larger than typically observed for cometary nuclei. Although the absolute value of the active area is somewhat dependent on the thermal model employed, the changes in active area observed are consistent among models. The conclusion of a 50–100+% active fraction is robust for realistic thermal models of the nucleus. However the possibility of a contribution of a spatially unresolved distribution of icy grains cannot be discounted. As our [OI]-derived H2O production rates are consistent with values derived using other methods, we conclude that the contribution of O2 photodissociation to the observed [O I] emission is at most 5–10% that of the contribution of H2O for ISON. This is consistent with the expected contribution of O2 photodissociation if O2/H2O ∼ 4%, meaning [O I] emission can still be utilized as a reliable proxy for H2O production in comets as long as O2/H2O ≲ 4%, similar to the abundance measured by the ROSINA instrument on Rosetta at comet 67P/Churyumov–Gerasimenko.

Coma↗

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

Anatomy of an Asteroid Breakup: The Case of P/2013 R3

We present an analysis of new and published data on P/2013 R3, the first asteroid detected while disintegrating. Thirteen discrete components are measured in the interval between UT 2013 October 01 and 2014 February 13. We determine a mean, pair-wise velocity dispersion among these components of Δv = 0.33 ± 0.03 m s{sup −1} and find that their separation times are staggered over an interval of ∼5 months. Dust enveloping the system has, in the first observations, a cross-section of ∼30 km{sup 2} but fades monotonically at a rate consistent with the action of radiation pressure sweeping. The individual components exhibit comet-like morphologies and also fade except where secondary fragmentation is accompanied by the release of additional dust. We find only upper limits to the radii of any embedded solid nuclei, typically ∼100–200 m (geometric albedo 0.05 assumed). Combined, the components of P/2013 R3 would form a single spherical body with a radius of ≲400 m, which is our best estimate of the size of the precursor object. The observations are consistent with rotational disruption of a weak (cohesive strength of ∼50 to 100 N m{sup −2}) parent body, ∼400 m in radius. Estimated radiation (YORP) spin-up times of this parent are ≲1 Myr, shorter than the collisional lifetime. If present, water ice sublimating at as little as 10{sup −3} kg s{sup −1} could generate a torque on the parent body rivaling the YORP torque. Under conservative assumptions about the frequency of similar disruptions, the inferred asteroid debris production rate is ≳10{sup 3} kg s{sup −1}, which is at least 4% of the rate needed to maintain the Zodiacal Cloud.

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 OSTI2016

FAR-ULTRAVIOLET OBSERVATIONS OF COMET C/2012 S1 (ISON) FROM FORTIS

We have used the unique far-UV imaging capability offered by a sounding-rocket-borne instrument to acquire observations of C/2012 S1 (ISON) when its angular separation with respect to the Sun was 26.°3 on 2013 November 20.49. At the time of observation, the comet’s heliocentric distance and velocity relative to the Sun were r {sub h} = 0.43 au and r-dot {sub h} = −62.7 km s{sup −1}. Images dominated by C i λ1657 and H i λ1216 were acquired over a 10{sup 6} × 10{sup 6} km{sup 2} region. The water production rate implied by the Lyα observations is constrained to be Q{sub H{sub 2O}}≈ 8 × 10{sup 29} s{sup −1} while the neutral carbon production rate was Q{sub C} ≈ 4 ×10{sup 28} s{sup −1}. The radial profile of C i was consistent with it being a dissociation product of a parent molecule with a lifetime τ ∼ 5 × 10{sup 4} s, favoring a parent other than CO. We constrain the Q {sub CO} production rate to 5{sub −7.5}{sup +1.5} × 10{sup 28} s{sup −1} with 1σ errors derived from photon statistics. The upper limit on the Q {sub CO}/Q{sub H{sub 2O}} is ≲6%.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

HUBBLE SPACE TELESCOPE OBSERVATIONS OF ACTIVE ASTEROID 324P/La SAGRA

Hubble Space Telescope observations of active asteroid 324P/La Sagra near perihelion show continued mass loss consistent with the sublimation of near-surface ice. Isophotes of the coma measured from a vantage point below the orbital plane are best matched by steady emission of particles having a nominal size of a ∼ 100 μ m. The inferred rate of mass loss, dM{sub d} / dt ∼ 0.2 kg s{sup −1}, can be supplied by sublimation of water ice in thermal equilibrium with sunlight from an area as small as 930 m{sup 2}, corresponding to about 0.2% of the nucleus surface. Observations taken from a vantage point only 0.°6 from the orbital plane of 324P set a limit to the velocity of ejection of dust in the direction perpendicular to the plane, V {sub ⊥} < 1 m s{sup −1}. Short-term photometric variations of the near-nucleus region, if related to rotation of the underlying nucleus, rule-out periods ≤3.8 hr and suggest that rotation probably does not play a central role in driving the observed mass loss. We estimate that, in the previous orbit, 324P lost about 4 × 10{sup 7} kg in dust particles, corresponding to 6 × 10{sup −5} of the mass of a 550 m spherical nucleus of assumed density ρ = 1000 kg m{sup −3}. If continued, mass loss at this rate would limit the lifetime of 324P to ∼1.6 × 10{sup 4} orbits (about 10{sup 5} years). To survive for the 100–400 Myr timescales corresponding to dynamical and collisional stability requires a duty cycle of 2 × 10{sup −4} ≤ f{sub d} ≤ 8 × 10{sup −4}. Unless its time in orbit is overestimated by many orders of magnitude, 324P is revealed as a briefly active member of a vast population of otherwise dormant ice-containing asteroids.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2016

Far-Ultraviolet Observations of Comet C 2012 S1 (ISON) From Fortis

We have used the unique far-UV imaging capability offered by a sounding-rocket-borne instrument to acquire observations of C/2012 S1 (ISON) when its angular separation with respect to the Sun was 26 deg.3 on 2013 November 20.49. At the time of observation, the comet's heliocentric distance and velocity relative to the Sun were r h = 0.43 au and r (sub h) = 62.7 km s)exp. -1). Images dominated by C Iota Lambda 1657 and H Iota Lambda 1216 were acquired over a 10(exp. 6) x 10(exp. 6) km(exp. 2) region. The water production rate implied by the Ly Alpha observations is constrained to be Q H2O is much greater than 8 x 10(exp. 29) s(exp. -1) while the neutral carbon production rate was Q(sub c) is much greater than 4 x 10(exp. 28) s(exp. -1). The radial profile of C I was consistent with it being a dissociation product of a parent molecule with a lifetime T is approx. 5 x 10(exp. 4) s, favoring a parent other than CO. We constrain the Q(sub co) production rate to 5(+1.5)(-7.5), 10(exp. 28) s(exp. -1) with 1 sigma errors derived from photon statistics. The upper limit on the Q (sub CO)/Q(sub H2O) is 6%.

sounding rocket↗

DOE OSTI2016

HUBBLE AND KECK TELESCOPE OBSERVATIONS OF ACTIVE ASTEROID 288P/300163 (2006 VW139)

We present Hubble Space Telescope (HST) and Keck 10 m telescope observations of active asteroid 288P/300163 (2006 VW139) taken to examine ejected dust. The nucleus is a C-type object with absolute magnitude H{sub V} = 17.0 ± 0.1 and estimated diameter ∼2.6 km (for assumed visual geometric albedo p{sub V} = 0.04). Variations in the brightness of the nucleus at the 10%–15% level are significant in both 2011 December and 2012 October but we possess too few data to distinguish variations caused by activity from those caused by rotation. The dust scattering cross-section in 2011 December is ∼40 km{sup 2}, corresponding to a dust mass ∼9 × 10{sup 6} kg (88 μm mean particle radius assumed). The FWHM of the debris sheet varies from ∼100 km near the nucleus to ∼1000 km 30″ (40,000 km) east of it. Dust dynamical models indicate ejection speeds between 0.06 and 0.3 m s{sup −1}, particle sizes between 10 and 300 μm and an inverse square-root relation between particle size and velocity. Overall, the data are most simply explained by prolonged, low velocity ejection of dust, starting in or before 2011 July and continuing until at least 2011 October. These properties are consistent with the sublimation of near-surface ice aided by centrifugal forces. The high spatial resolution of our HST images (52 km pixel{sup −1}) reveals details that remained hidden in previous ground-based observations, such as the extraordinarily small vertical extent of the dust sheet, ejection speeds well below the nucleus escape speed, and the possibility of a binary nucleus.

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 OSTI2015

EPISODIC EJECTION FROM ACTIVE ASTEROID 311P/PANSTARRS

We examine the development of the active asteroid 311P/PANSTARRS (formerly, 2013 P5) in the period from 2013 September to 2014 February using high resolution images from the Hubble Space Telescope. This multi-tailed object is characterized by a single, reddish nucleus of absolute magnitude H ≥ 18.98 ± 0.10, corresponding to an equal-area sphere of radius ≤200 ± 20 m (for assumed geometric albedo 0.29 ± 0.09). We set an upper limit to the radii of possible companion nuclei at ∼10 m. The nucleus ejected debris in nine discrete episodes, spread irregularly over a nine month interval, each time forming a distinct tail. Particles in the tails range from about 10 μm to at least 80 mm in radius, and were ejected at speeds <1 m s{sup –1}. The ratio of the total ejected dust mass to the nucleus mass is ∼3×10{sup –5}, corresponding to a global surface layer ∼2 mm thick, or to a deeper layer covering a smaller fraction of the surface. The observations are incompatible with an origin of the activity by impact or by the sublimation of entrapped ice. This object appears to be shedding its regolith by rotational (presumably YORP-driven) instability. Long-term fading of the photometry (months) is attributed to gradual dissipation of near-nucleus dust. Photometric variations on short timescales (<0.7 hr) are probably caused by fast rotation of the nucleus. However, because of limited time coverage and dilution of the nucleus signal by near-nucleus dust, we have not been able to determine the rotation period.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2014

Geology Before Pluto: Pre-Encounter Considerations

The cameras of New Horizons will provide robust data sets that should be imminently amenable to geological analysis of the Pluto systems landscapes. In this paper, we begin with a brief discussion of the planned observations by the New Horizons cameras that will bear most directly on geological interpretability. Then we broadly review the major geological processes that could potentially operate on the surfaces of Pluto and its major moon Charon. We first survey exogenic processes (i.e. those for which energy for surface modification is supplied externally to the planetary surface): impact cratering, sedimentary processes (including volatile migration), and the work of wind. We conclude with an assessment of the prospects for endogenic activity in the form of tectonics and cryovolcanism.

Pluto↗