• Will the Impactor knock Tempel 1 out of its orbit, change its orbit or cause any debris to hit the earth?
• Why Comet Tempel 1?
• How big is the comet?
• What do we expect to find inside the comet?
• Is comet Tempel 1 going to hit the earth?
• If the comet was split by the impactor, would any debris head towards earth?
• Is there any other comet that is going to hit the earth?
• Where is the comet in its orbit at the time of collision with the impactor?
• How do we know that any results from Tempel 1 will apply to all comets?
• How will you know how deep a crater is formed?
• What is the current best estimate of the mass of Comet 9P/Tempel 1?

Q: Will the Impactor knock Tempel 1 out of its orbit, change its orbit or cause any debris to hit the earth?

     No, the Impactor will not knock Tempel 1 out of it's orbit. However, the impact will cause a slight change in the comet's orbit, but that change is so small that it won't be noticed. A good analogy is to imagine a small pebble hitting an 18-wheeler truck.

     Over time, the changes we introduce might become noticeable except for the fact that this comet also has close approaches to the planet Jupiter which exerts gravitational forces that change Tempel 1's orbit more than the impact. Dr. Don Yeomans has written a detailed history about Tempel 1's orbit.

     Finally, any material knocked off of Tempel 1 will continue along the comet's orbit. Tempel 1 has an orbit that is larger than Earth's so the two orbits do not cross. So debris from Tempel 1 will not hit Earth.

Q: Why Comet Tempel 1?

     Tempel 1 is the comet that best satisfies a variety of constraints for a launch within the 2-year window for which the team was allowed to propose. Constraints require the comet to be large enough to sustain a crater, to have a low rotation rate (so the newly formed crater doesn't rotate out of view before the flyby spacecraft can get pictures!), to have a trajectory that allows natural intercept at high speed and an approach from the sunward side by the spacecraft, and to be observable from Earth at the time of impact. If the launch were to take place at a different time, a different target would have been chosen.

Q: How big is the comet?

     Tempel 1 has an approximate radius of 3km (1.9mi) The crater that will be formed will be about 91m wide (the size of a football stadium) and about 30m (100feet) deep.

Q: What do we expect to find inside the comet?

     This goes back to the purpose of the mission - Why are we going to a comet and excavating a crater? We believe that the surface of a comet nucleus has evolved since the beginning of the solar system. We expect to find fresh, unprocessed material beneath the surface that represents the starting composition of the outer solar system 4.5 billion years ago. One of the species that we hope to find is CO₂ ice, commonly called dry ice. On Earth dry ice sublimates into a gas, in the cold outer regions of the solar system, it persists as a solid. If we see it, it will tell us how cold the outer parts of the solar system were during formation. We will measure the spectral signatures of the surface of the comet and compare it to the interior. By analyzing the differences, we expect to understand the evolution process of comets due to exposure to the sun and the space environment.

     We don't know what else we will see, but we sure are curious and can't wait to find out!

Q: Is comet Tempel 1 going to hit the earth?
Q: If the impactor split the comet, would any debris head towards earth?
Q: Is there any other comet that is going to hit the earth?

     The short answer is "no."

     The long answer is that within the next few hundred years we can be absolutely confident that Tempel 1 will not hit Earth, whether or not Deep Impact goes to Tempel 1. On the time scale of a few million years, on the other hand, we know that the probability of an impact is very low but not zero, again independent of whether or not the Deep Impact spacecraft impacts the comet. Don Yeomans has written a short description of Tempel 1's orbital history and the effect the impactor will have on the orbit.

     Why? Because it is not possible to predict the orbits of all small bodies for more than a few hundred years into the future. We can calculate the orbits of all the bodies and their mutual perturbations sufficiently well to predict accurately where they will be for hundreds of years. We can calculate the orbits for thousands of years, well enough to know whether or not there will be a close approach to any planet that could change the orbit enough to make it uncertain.

     The same answer applies to any debris. The Deep Impact impactor will just scratch the surface of the comet making a relatively small crater compared to the size of the comet. Even if the comet were to be extremely fragile and break apart, the pieces also stay in orbits very similar to the orbit of the parent comet. Danger to the earth is from asteroids and comets whose orbits cross the earth's orbit. Tempel 1 never crosses the earth's orbit. Currently, the orbit of Tempel 1 is at least 0.5 AU (about 46 million miles) from Earth's orbit at their closest points.

Q: Where is the comet, in relation to its orbit around the Sun, at the time of collision? Has it passed around the Sun on its way from the "back" of the solar system?

     The comet will be near its perihelion point (closest point to the Sun) at the time of impact. Its range from the Sun will be 1.506 AU (225 million km), and its range from the Earth will be 0.894 AU (134 million km).

Q: How do we know that any results from Tempel 1 will apply to all comets?

     The short answer is, "we don't". This mission will give scientists a great deal more information on which to base models and predictions about comets, but there is no guarantee that anything learned here is universal to all comets. Further such missions would be helpful to increase the knowledge base, but are unlikely due to time and cost considerations.

Q: How will you know how deep a crater is formed?

     The crater will be in view of the flyby spacecraft for a time after the impact. From images obtained of the crater by the flyby, the depth of the crater should be able to be determined by analysis of the shadow along one side or by counting pixels in the image and accounting for the slope of the crater. Either method will produce an estimated depth, not an extremely precise measurement.

Q: What is the current best estimate of the mass of Comet 9P/Tempel 1?

The simple answer:

     We don't know the mass of comet Tempel 1 at this time. We don't even know its size very accurately yet. We can estimate its mass by assuming a density and multiplying the density by the volume to compute a mass. M=D x V.

     Our best estimate is anywhere from 0.1 - 2.5 x 10¹⁴ kg. When you think that a kilogram is 2.2 pounds, you can see that Tempel 1 is pretty massive on the scale of something human. On a planetary scale, consider the Earth's mass, which is about 6.0 x 10²⁴ kg or asteroid Ceres with a mass of 9.445 x 10²⁰ kg. Compared to them, the mass of the comet is almost nothing, well not quite.

     We should be able to constrain this value better after we analyze images of the impact and have a more accurate shape model of the comet. The goal of the Deep Impact mission is to determine the composition and structure of the interior of the comet. We will arrive at a more accurate value of the mass in the process.

A more involved answer:

     Our current best estimate of the mass of Comet 9P/Tempel 1 is based on the current knowledge of the dimensions of the nucleus and the assumed density of the nucleus.

     The nucleus of Comet 9P/Tempel 1 is elongated and irregular, so the closest example we currently have is the nucleus of Comet Borrelly, but even more elongated. The mean radius of Tempel 1 is 3.25 +/- 0.2 km, and the axial ratio (a/b) is 3.2 +/- 0.4, giving an approximate prolate ellipsoidal shape with semi-major axes a=7.2 +/- 0.9 and b=c=2.2 +/- 0.3 (thus major axes of 14.4 x 4.4 x 4.4 km). The volume is then about 146 cubic kilometers, but could be as low as 90 or as high as 210 cubic kilometers.

     Even more poorly known is the density. From studies of Comet Borrelly, we have a result of 0.49 grams per cubic centimeter, with a range of 0.29 to 0.83 (Farnham, Icarus 160, 398, 2002), but from further refinement through studies of Wild 2, Tony Farnham would now expect that the most probable value is more in the range of 0.3 to 0.4.

     The mass is then calculated by multiplying the volume by the density, and considering the uncertainties. Thus we think we know the mass of the nucleus of Comet Tempel 1 to within about a factor of 3, or a range of a factor of 9.

     We expect to be able to determine the dimensions of the nucleus of Comet Tempel 1 with much higher accuracy once we get close, and we will be tracking ejecta from the impact to try to get the mass of Comet Tempel 1, but that will be much more difficult. We do not expect that the orbit of Comet Tempel 1 will be changed sufficiently by the impact to be measurable, so we don't expect to be able to get the mass of Comet Tempel 1 by that method.