How the idea for the Deep Impact mission developed

Alan Delamere
Staff Consultant and Engineer
Ball Aerospace & Technologies Corp.
B.Eng. in electrical engineering 1995, University of Liverpool

Several projects and proposals laid the groundwork for Alan Delamere and his science colleagues to arrive at the precise idea for the Deep Impact mission. Beginning in 1978, Delamere and Mike Belton, then at the National Optical Observatory in Tucson, collaborated on a project involving Comet Halley.

"We got Halley data and investigated it and found the comet was far blacker than we had imagined, blacker than coal. So we asked ourselves: how could this happen?" Delamere said. "There were many comet models, such as the dirty snowball concept, and we became increasingly curious as to just how this black layer accumulated. We suspect that as the comet's ice dissipates, dust is left which becomes a loose, outer crust insulating the trapped, inner comet.

"In 1996 Belton and I decided to submit a proposal to explore the exterior of a dead comet, Phaethon, using an impactor. At this point Mike A'Hearn, now Principal Investigator for Deep Impact, joined our team. We proposed to the NASA Discovery Program using a large mass hitting the comet at about 38 km/sec. We found a way to hit the comet and observe the crater development. However, the proposal wasn't funded because the science panel was not convinced that Phaethon was a comet and they did not believe we could hit it.

"But the idea evolved and in 1998 Mike A'Hearn took over the team leadership and proposed to hit an active comet, Tempel 1. We added a fine guidance system to the impactor. This approach convinced reviewers we could hit the comet.

"I'm looking forward to finding out a lot about the nature of inner cometary material as well as the crust. Deep Impact is letting us gather a great deal of information about these little-known bodies.

"The biggest challenge of the mission is making sure we have a very stable flyby spacecraft that is able to track the event as the impactor reaches the comet. We'll have 800 seconds or so to gather high fidelity images and data. All this makes the flyby spacecraft critical because it will be traveling through a hazardous area filled with cometary material. Moreover, it needs to safely pass through the orbital plane of the comet where there are cometary fragments that could destroy the spacecraft. We've planned to cross the orbital plane as late and as far away as possible while still maintaining a distance that will allow us to get high resolution images."