Sunao Hasegawa

Simulation of space weathering of planet-forming materials: Nanosecond pulse laser irradiation and proton implantation on olivine and pyroxene samples

For the purpose of simulating the surface alteration process called “space weathering”, experiments of pulse laser irradiation, proton implantation, and laser irradiation to proton implanted samples were performed and reflectance spectra of altered materials were measured. To simulate the impact heating by micrometeorite bombardments, we made a new apparatus using a pulse laser whose pulse duration is 6–8 nanoseconds, comparable with a timescale of micrometeorite impacts. We find that the degree of space weathering, i.e., change of reflectance spectrum should depend on mineral composition. Laser irradiation onto olivine produces the largest reduction of albedo and the highest reddening of reflectance spectrum. In general, variation of olivine spectra is much larger than that of pyroxenes. Depths of absorption bands do not change in the scaled spectra. The olivine spectrum after the laser irradiation can match spectra of some olivine asteroids within a subtype of S-type asteroids. Comparison of Vesta spectrum with altered pyroxene spectra suggests that Vesta surface would be relatively older than olivine asteroids. We also investigate the influence of solar wind proton and pyroxene FeO content. The proton implantation causes small changes in olivine and enstatite spectra. Implanted protons do not influence spectral change by the laser irradiation: the laser irradiation and the proton implantation do not produce multiplicative but additive changes on the reflectance spectra. FeO content of pyroxenes does not relate to the degree of reflectance change.

Interpretation of (596) Scheila's Triple Dust Tails

Strange-looking dust cloud around asteroid (596) Scheila was discovered on 2010 December 11.44-11.47. Unlike normal cometary tails, it consisted of three tails and faded within two months. We constructed a model to reproduce the morphology of the dust cloud based on the laboratory measurement of high-velocity impacts and the dust dynamics. As a result, we succeeded in reproducing the peculiar dust cloud by an impact-driven ejecta plume consisting of an impact cone and downrange plume. Assuming an impact angle of 45 Degree-Sign , our model suggests that a decameter-sized asteroid collided with (596) Scheila from the direction of ({alpha}{sub im}, {delta}{sub im}) = (60 Degree-Sign , -40 Degree-Sign ) in J2000 coordinates on 2010 December 3. The maximum ejection velocity of the dust particles exceeded 100 m s{sup -1}. Our results suggest that the surface of (596) Scheila consists of materials with low tensile strength.

Discovery of the Dust Trail of the Stardust Comet Sample Return Mission Target: 81P/Wild 2

We have succeeded in detecting a dust trail along the orbit of the short-period comet 81P/Wild 2, the target of the Stardust mission, which will fly by 81P/Wild 2 in 2004 January. This is the first discovery in optical wavelengths for which an IR counterpart (such as from IRAS) has not been reported. The detected trail extends from the nucleus to a point 23 (0.14 AU) back along the orbit. Its width is 9 (1.4 × 104 km) near the nucleus. Based on a comparison between models and the observed properties of the dust trail, it is likely that the trail is composed of dust particles with a diameter of ~1 mm (corresponding to 0.5 mg in mass). During the flyby phase of the Stardust spacecraft, it is likely that the spacecraft will experience impacts of such large dust particles along the comets orbit as well as from smaller grains in the cometary coma. With an impact velocity of trail particles of 6.1 km s-1, the impact fluence of 0.5 mg trail particles onto the Stardust spacecraft will be ~0.80 day-1 inside the dust trail. However, the Whipple bumper shields should prevent physical disruption of the spacecraft by impacting particles of up to 1 cm in size. Thus, unless the 1 mm trail dust impacts cause severe attitude control problems for the spacecraft, they may not be critically hazardous to the mission but instead may provide a unique opportunity for the first in situ flux measurement of a comet dust trail, using the Whipple bumper shield dust flux monitors.

The first detection of water absorption on a D type asteroid

[1]xa0The D type asteroids are among the darkest objects known in our solar system. Here, we present infrared spectra of one of the main-belt D type asteroids, 773 Irmintraud. In contrast to previous observations of D type asteroids, we found a gap of reflectance around 3 μm in wavelength. The 3 μm gap is one of the spectral signatures of OH or H2O as water ice or in hydrous minerals, which had formed in the processes of aqueous alteration in the early solar system. We suggest that D type asteroids, which are all the while considered as unaltered primitive material, could be aqueously altered. Our data do support the presence of water on a compositionally primitive D type asteroid.

Microparticle Acceleration by a Van de Graaff Accelerator and Application to Space and Material Sciences

Abstract A microparticle (dust) ion source has been installed in the 3.75xa0MV Van de Graaff electrostatic accelerator and a new beam line for microparticle experiments has been built at the HIT facility of Research Center for Nuclear Science and Technology, the University of Tokyo. Microparticle acceleration has been successful in obtaining expected velocities of 1–20xa0km/s or more for micron- or submicron-sized particles. Development of in situ dust detectors on board satellites and spacecraft in the expected mass and velocity range of micrometeoroids and investigation of hypervelocity impact phenomena by using time-of-flight mass spectrometry, impact flash measurement and scanning electron microscope observation for metals, polymers and semiconductors bombarded by micron-sized particles have been started.

Response of Piezoelectric Lead–Zirconate–Titanate to Hypervelocity Silver Particles

A lead–zirconate–titanate (PZT) element was studied by bombarding silver particles in the mass range from 1 to 100 pg, and the velocity from 2 to 6 km/s. Output signals were uniquely identified on impact and characterized by Fourier analysis. It was found that incident energies above 100 nJ were uniquely determined by a single PZT element. We discussed its potential as a real-time detector for space dust and debris.

SEARCH FOR THE COMET ACTIVITY OF 107P/(4015) WILSON-HARRINGTON DURING THE 2009/2010 APPARITION

We present the optical observations of the Near Earth Object 107P/(4015) Wilson–Harrington during the 2009/ 2010 apparition taken in search of low-level comet activity. Our photometric and spectroscopic data were collected 28–86 days after the perihelion passage on 2009 October 22 in a wide range of solar phase angles of 39 ◦ –68 ◦ .A disk-integrated phase function was constructed, giving a geometric albedo of 0.055 ± 0.012, phase integral of q = 0.34, and Bond albedo of AB = 0.019. The photometric property shows a profile similar to low albedo asteroids and comet nuclei. No emission lines were found in our spectrum, giving a flat reflectance similar to low albedo asteroids. Although we could not find any evidence for cometary activity in our photometric and spectroscopic data, we found an upper limit of 0.001% on the fractional active area. We derived the upper limit of the optical depth of the dust trail and tail, 7 × 10 −10 . We conclude that 107P/(4015) Wilson–Harrington was completely dormant or inactive in the 2009/2010 return.

Pole orientation and triaxial ellipsoid shape of (25143) 1998 SF36, a target asteroid of the MUSES-C ∗ mission

The near-earth asteroid (25143) 1998 SF36 is a target body for the Japanese sample-return MUSES-C mission. We present here its pole orientation and triaxial ellipsoid shape, using light curve data obtained with three telescopes at Kiso, Mitaka, and Pic du Midi. The solution obtained for the pole orientation has ecliptic latitude λ = 320 ± 30° and ecliptic longitude β = —75 ± 12°. The estimated triaxial ellipsoid shape is a/b= 2.1 and b/c= 1.7 assuming m= 0.03, which is the coefficient of the empirical relation between the light curve amplitude and the phase angle for S-type asteroids (Zappalà et al., 1990). We also found m to be related to asteroid surface roughness, using a light curve simulator.

Microparticle acceleration for hypervelocity experiments by A 3.75MV van de Graaff accelerator and a 100KV electrostatic accelerator in Japan

Abstract In-situ dust detectors have been calibrated by dust electrostatic accelerators that can accelerate projectiles to expected mass and velocity ranges of space debris and micrometeoroids. Unfortunately, In Japan, there was no such a facility dedicated to space science research until our research group was established a few years ago. Therefore, we have developed two high voltage accelerators. One is a modified 3.75MV Van de Graaff accelerator operated by High Fluence Irradiation Facility, Research Center for Nuclear Science and Technology, the University of Tokyo (HIT), and the other is a 100kV accelerator dedicated to dust experiment at the Institute of Space and Astronautical Science (ISAS). The particle velocity using the HIT Van de Graaff accelerator is higher than those reported in other accelerator facilities under the same particle mass conditions and encompasses the desired velocity range of micro-meteoroid. Time-Of-Flight dust mass spectrometer and Hybrid dust detector which are under development in Japan have been investigated using HIT dust accelerators. We have also constructed a 100kV electrostatic accelerator designed for easier handling and lower cost operation which is dedicated to dust acceleration, because the HIT Van de Graaff accelerator is being used for ion beam experiments mainly.

Design of electrostatic accelerators for the development of microparticle detectors in Japan

Abstract Micrometeoroid detection in space usually involves high-velocity impact phenomena. In-situ dust detectors should be calibrated by a microparticle accelerator with a mass and velocity range comparable to micrometeoroids. A pilot model of an accelerator was constructed some years ago and fundamental research for an advanced facility has been performed. We are developing two new accelerators. A 3.75MV Van de Graaff electrostatic accelerator at The Research Center for Nuclear Science and Technology, University of Tokyo, has been modified to accelerate microparticles, and acceleration testing is now being carried out. The expected velocity range is 1–20 km/s for micron or sub-micron particles. Another small accelerator will be installed for easier handling, more frequent use, and lower-cost operation.