A U.S. spacecraft is completing a five-and-a-half month voyage to a distant comet and could create celestial fireworks when it crashes into it Monday, America's Independence Day. Scientists want to know what a comet's nucleus is made of and believe the best way to find out is to blast a hole in one.
The target is the comet Tempel 1, 431 million kilometers away. As NASA puts it, meeting it head on at such a great distance is like threading a needle.
Tempel 1 is typical of the comets that emerge from the so-called Kuiper belt of objects orbiting beyond the planet Neptune and swing around the sun.
NASA scientist Tom Morgan describes a comet as a big dirty snowball in space, a glowing collection of ice, rocks, dust, and gases whose interior composition is intriguing.
"What we're interested in is the source of the dust and gas that forms the coma, which is a small core of ices, organic compounds, and dust called the nucleus," Mr. Morgan explained. "These objects are of interest to us because they are literally the leftover building blocks of our solar system."
Knowing the makeup of a comet would provide information about the chemistry and physics of the solar system as it formed 4.5 billion years ago.
The United States and Europe have a fleet of spacecraft going to various comets to understand them better. Just last year, for example, the U.S. Stardust satellite collected samples as it passed through dust ejected by comet Wild 2 and will be the first to return such matter to Earth for study. The European Rosetta craft is enroute to deposit instruments on the surface of comet Wirtanen in 2014.
The U.S. Deep Impact mission is unprecedented among space projects. The head of NASA's Solar System Division, Andrew Dantzler, says the spacecraft will send a 372-kilogram copper missile to penetrate a comet's surface.
"Deep Impact's rendezvous in space with Tempel 1 will be the first time a spacecraft has ever touched the surface of a comet," Mr. Dantzler said. "In fact, the impactor will penetrate the comet's surface, blowing material away from the surface of the comet and revealing the mysteries of the interior of the nucleus."
It all takes place on Monday when the Deep Impact spacecraft releases the projectile while it is flying in the comet's path. The spacecraft is then to execute a deflecting maneuver to get out of the way shortly before the comet smashes into the projectile at a relative speed of 37,000 kilometers per hour.
Before its demise, the missile is to transmit the sharpest pictures ever of a comet. Cameras on the mothership are to return views of the excavation from above and photograph the structure beneath the surface. Deep Impact project manager Rick Grammier says the three orbiting U.S. space telescopes and many ground observatories worldwide will also be trained on the event, which could be bright enough to be seen.
"So we expect to provide some great fireworks for all our observatories," he said.
Mission scientists do not know how spectacular the space collision will be. If the comet is mostly a pile of rocks held together loosely by ice, the resulting crater could be shallow and wide, with a lot of material thrown out. If it is mostly hard ice, then the hole would be deeper and narrower with less material ejected. Or the impactor could be swallowed without a significant hole if the material is soft and fluffy. The mission's chief investigator, Michael A'Hearn of the University of Maryland, says the crater could be anywhere from centimeters to dozens of meters deep.
"The problem is there are no data on the interior," he explained. "That's what we hope to solve with Deep Impact. A related question is that the comets we know eventually stop outgassing. Is this because they have exhausted all the gas in the interior, or is it because the surface layer has developed some sort of a crust that prevents the ice inside from evaporating and escaping as gas?"
Whatever Deep Impact gleans from its encounter with comet Tempel 1, Mr. A'Hearn says the crash will not push it into a collision course with Earth.
"The impact will, of course, will make a change in the orbit of the comet, but the change will be so small as to be undetectable," he said.
