Impact Experiments

Research Projects


IMPACT EXPERIMENTS


Introduction

The objective of this research is to study the collisions which are believed to occur between asteroids and other small bodies during the formation and evolution of solar systems. The results from the experiments allow us to develop improved numerical models of evolution small body populations including the asteroid main belt and the Edgeworth-Kuiper Belt, as well as providing comparison to recent radar, lightcurve and optical (via spacecraft) observations of real asteroids. Current collaborations include the Space Mechanics Group (Pisa, Italy), Osservatorio Astronomico di Torino (Italy) and The Planetary Science Institute (Arizona, USA).

This page is divided into the following sections:

  • Catastrophic Disruption (1992)
  • Mega-craters (1996)
  • Ice Targets (1997)
  • Imaging
  • Data Reduction
  • See also my Thesis or look at the abstract.


    Water Ice Targets, NASA AVGR (1997, 1998)

    In order to study the nature of collisions in the Edgeworth-Kuiper Belt (EKB) - at least, based upon our current understanding - we have carried out impacts of ice projectiles into water ice targets of varying porosities. Impact velocities were of a few hundred m/s, representative of those in the EKB. The figure top right shows a 15cm diameter target made from standard ice pellets (also shown; cylinders of 1/4 inch diameter and length). The experimental arrangement was similar to that used in previous AVGR runs (see above) with similar instrumentation; we hope to be able to measure the velocity and roation of the majority of ejected fragments. The lower right image shows the centre of one frame from a similar sequence right after impact (image resolution has been reduced to 50% for this web page).




    Only limited work has so far been completed on these data and experimental film. Please look first at the PSI side of the collaboration; follow this link for a report by Eileen Ryan.

    Various works in progress on this experimental run:
    Projectile Integrity table and examples.
    Ejecta envelope illustrating crater and straight-through projectile exit effects.

    Some (test) MPEG movies are available:
    '9723-0.MPG' 879 KB: Porous (59%) crushed ice, full-on direct hit. Side camera.
    '9725-0.MPG' 1318 KB: Solid 15 cm ice sphere, gentle oblique impact. Side camera.

    There are also some interesting stills here from a polycarbonate projectile impact into a dusty ice target (shot 981004).


    Mega-Craters, NASA AVGR (1996)








    The objective of these experiments is to study the transitional regime between cratering (where 99% or more of the target is left intact) and catastrophic disruption where less than 50% of the target mass remains in the largest remnant (LR). By studying the mass and velocity of ejected fragments from large craters we can provide vital input to models of asteroid family origins and evolution, as well as direct comparisons to hydrocode simulations in this important regime.

    In a progression from the previous campaigns, and in close collaboration with PSI, these experiments were performed using the Ames Vertical Gun Range (AVGR) at the NASA Ames Research Centre, Mountain View, California. Cement targets were again used, but due to the size of the available impact chamber the target mass is of the order of 1 kg (compared to 10 kg in the contact charge experiments). The impactor was an Aluminium sphere of either 1/4 inch or 3/16 inch diameter depending on the velocity required. Velocities were from 1.07 to 5.51 km/s (as measured by a sequence of in-line X-ray photographs taken after the projectile was launched). Instrumentation for these experiments was similar to that used in 1989 and 1992 but now consists of 3 high-speed cine cameras and one high speed video recorder. All systems were operating at 500 frames/sec.

    These images are from impact 961003, a particularly high-energy shot in this run. The impact velocity is 2.43 km/s and the specific energy 1.25 J/g. 21% of the target mass remained in the largest remnant, so it was in fact a catastrophic outcome; too disruptive to be called a mega-crater. (oops!)


    Catastrophic Disruption, Open-air (1989, 1992)

    The technique involves the use of a contact explosive charge to simulate the impact of a projectile. By careful choice of the charge diameter and burial depth it is possible to closely simulate an impact at the detonation velocity of the explosive; in this case 6.1 km/s. This is a typical velocity for collisions between asteroids in the main belt.

    Several experimental programmes have been carried out over the past few years. Most recently were 4 experiments in 1989 and 8 experiments in 1992, all using spherical 21 cm diameter targets of artificial rock (alumina cement, homogenous except for the presence of a harder core in two cases). The target rests upon a steel table and the disruption is filmed using one or two high speed cameras as well as a fast-shuttered CCD video system.

    The images on the right are sequential frames from shot 1992-2. The frame rate is 400 per second - it is very unusual to see the flash of the exploding contact charge like this (frame 2).








    We have carried out an extensive study of these experiments. Data collected cover fragment size, shape, rotation rate distribution, 2D and 3D velocity studies, tumbling fragment studies, rotational bursting. More elaborate analysis has been applied to fragment initial orientation, initial position and effect on ejecta properties, and a study of the possible existence of a radiant point as discussed by Paolicchi et al in a series of papers on the Semi-Empirical Model (e.g. Icarus 77, 177-212, 1989). For more results, see references [6], [8], [9], and [11] to [14] on this page. See also Giblin, 1994, Ph.D thesis


    Imaging

    Although the imaging systems aren't necessarily of much scientific interest, it may be worth documenting them here for the benefit of anyone with similar imaging needs or who wants to carry out their own expeiriments. For that purpose a page of background is provided here.


    Data Reduction

    The high speed cine film is processed, digitised and recorded onto CD-ROM before being analysed using an Acorn RiscPC. A British computer with a small but dedicated user base, the RiscPC is a great programming platform and ideal for this project. Digitisation has been done by either going first to video followed by a video digitiser, or more recently by using a film scanner (Epson FilmScan 200 or Minolta Dimage Dual) to directly scan the 16mm cine film strips. The digital movies are then displayed by using specially written software which allows individual fragments to be followed and studied in detail. By correlating the observed fragment properties (size, orientation in flight, rotation rate, etc.) it is possible to match some fragments between the two views and thereby reconstruct a 3D velocity field (see the imaging page for the viewing geometry, etc.). This system is partially automated but is not very reliable - the images are very messy - and is always followed up by direct visual study of each apparent fragment match. A nice animation (250K) of a set of trajectories from a 3D analysis can be seen on this page

    Primarily for those involved in this project, here is a chart showing current status of the analysis. This chart is automatically generated but not automatically uploaded - it might be slightly our of date but it gives a good idea of where we're at. (Aside: This doesn't seem to display properly in Netscape 4.72. but it works in MSIE 5 and Fresco 1.73).


    This document was constructed by Ian Giblin