EXPLORING COMETS, THE SCIENCE OBJECTIVES OF THE DEEP IMPACT MISSION - PART 2
(1) What are the basic properties of the comet's nucleus?
This section deals with discovering the basic physical properties of Tempel 1. We will: explore the terrain, to view the cometscape as it were; and look for evidence of blocks of material being thrown from the crater.
The last look from a doomed spacecraft.
The day before impact, the impactor spacecraft detaches from the flyby spacecraft and continues on to its encounter with Tempel 1. The Impactor Targeting Sensor, ITS, sends images of the target area to the flyby spacecraft. The flyby spacecraft does not use the images (or other data from the impactor), but relays them back to Earth.
As the impactor nears the impact point, the area seen by the ITS will shrink. At first it includes both coma and nucleus. Later only the nucleus is imaged and finally just the impact area.
The ITS will continue to record and send images until it is disabled by striking dust particles in the comet's coma or by reaching the surface. Researchers hope that the final images will cover an area about 30x30 meters (98x98 feet). To put this dimension into context, a regulation NBA basketball court is 94 feet by 50 feet.
A wild and torturous terrain.
Today, we have surface images of only three comets: Halley, Borrelly and Wild 2 (pronounced vilt 2). What the ITS will see on the way down to Tempel 1 might be anticipated from the images of Wild 2 captured by the Stardust spacecraft's navigation camera. According to the Stardust project team, the rugged cometscape sports cliffs 100 meters high, boulders as big as barns, craters a kilometer (0.62 mile) in diameter and several jets of gas blowing particles of dust out of the interior of the comet. See their images at stardust.jpl.nasa.gov/photo/cool.html.
Penetrating the crust and vaporizing.
Scientists who study comets think comets have an outer crust from one to ten meters thick. (A meter is about a yard.) The impactor itself is a hemisphere one meter in diameter. As it tears into the crust at over 10 kilometers per second, it compresses the cometary material at its rounded front end. As the comet-stuff compresses, a shock wave forms. A shock wave is a very thin region of very high pressure that pulses out in the shape of a spherical shell. It moves quickly, faster even than sound would move in the same material.
The shock wave expands from the point of contact backward through the impactor and forward through the comet. In the rearward portion the impactor, where the shock wave has yet to go, material is undisturbed. When the shock wave arrives at the rear of the impactor, the pressure shoots up. When the shock wave passes, the pressure drops precipitously and that release of pressure causes the impactor's material to vaporize!
By the time the shock wave and the reduced pressure region have reached the end of the impactor, less than a thousandth of second has gone by. The actual excavation of the crater, which follows, will take three or four minutes.
Meanwhile back on the flyby spacecraft.After the flyby spacecraft and the impactor separate, the flyby spacecraft and the comet are still on a collision course. But well before the comet overtakes it, the flyby spacecraft maneuvers off the collision course and the comet charges on by. The HRI and MRI aboard the flyby spacecraft are observing the comet before impact and continue to monitor crater formation after impact. To see a good animation of this, click here. Or, for a still picture, click here.
Shape and size of the crater.
The impactor will excavate a bowl shaped crater as wide as a football stadium and up to 15 stories deep. The crater is small relative to the six-kilometer long comet. It's comparable to the size of the crater made by a "bb" on the windshield of a VW bug.
The time it takes for the crater to form is of great importance. That's because it is related to the density of the comet, the comet's ability to resist being pulled apart (tensile strength) and the force of the comet's gravity. Crater formation time is expected to be about 200 seconds.
A translucent cone.
During crater formation, rocks and vaporized ice, called ejecta, hurtle from the developing crater. To the instruments onboard the flyby spacecraft, this will look like the upper portion of an ice cream cone that projects above the comet's surface and whose pointed end is pushed down below the comet's surface. It's called the ejecta cone or the ejecta curtain. Since it's mostly gas and dust, it will be translucent.
Whether the ejecta cone is more or less pointy depends on the strength of the comet's gravity and on how hard it is to pull apart the material of which the comet is made i.e. its tensile strength. Either the strength of gravity or the strength of the material can be the dominant factor in determining how pointy the cone is.
Computations show that if the strength of material dominates then the cone will be pointier than if gravity dominates.
To get an idea of how pointiness is measured, imagine sticking a pencil straight down through the middle of the cone until it touches the point of the cone. Then draw a straight line from anywhere on the cone to the point of the cone (where the pencil ends). Now see the angle that the line makes with pencil. If the strength of material dominates the force of gravity that angle will be about thirty degrees. If gravity dominates that angle will be about forty-five to fifty degrees.
The cone separates from the crater, maybe.
If the material of the comet is strong enough, the part of the cone (of gas and dust) that projects from the rim of the crater will be seen to detach from the comet and float outward toward the coma.
Experiments on Earth.
In order to know in advance what the cone might look like from the flyby spacecraft, scientists have fired small projectiles into targets of various materials. Materials that have been used include light volcanic rocks such as basalt and pumice, plaster of Paris and ice.
For the results of an experiment in which a copper ball was fired into finely ground pumice, click here. The web page is courtesy of NASA Ames Vertical Gun Range, NASA Ames Research Center and science team member Peter H. Schultz, Brown University.
Blocks thrown from the crater.
Images taken by instruments on previously flown spacecraft show blocks of material scattered about on the surface of asteroids and comet Wild 2. It is believed that these are byproducts of impact cratering. If Deep Impact can produce similar flying boulders and track them, can scientists determine the mass of the comet?
To Be Continued...