Earth Ready to Strike Back at Cosmos
Around Jet Propulsion Laboratory, the Deep Impact comet mission is known as “Revenge of the Dinosaurs.”
NASA’s plan to blow a Rose Bowl-sized crater in a comet comes 65 million years too late to give the dinos payback for the asteroid that may have wiped out much of life on Earth at the end of the Cretaceous period. But the mocking term captures the spirit of a mission -- set to launch Wednesday from Cape Canaveral in Florida -- that is far different from NASA’s usual tiptoe-through-the-cosmos approach.
Instead of gently approaching alien worlds with retro-rockets or parachutes, NASA has adopted the attitude of a 12-year-old with a new slingshot.
“For millennia, comets and asteroids have been hitting the Earth,” said Donald Yeomans, a comet historian and member of the Deep Impact science team. “This is our chance to hit back.”
Or, as one JPL official put it: “We’re going to put the hurt on the thing.”
Despite the locker room bravado, researchers insist there is serious science behind the $328-million mission. Plans call for the spacecraft to make a six-month journey to the neighborhood of Mars’ orbit, where it is scheduled to intercept a comet known as Tempel 1 on the Fourth of July. The craft will then fire a probe at the comet. The resulting collision will, it is hoped, expose secrets of the solar system that have been hidden away for 4.5 billion years.
“I’m trying to solve the mystery of what conditions were like in the early solar system, how things got made,” said Michael F. A’Hearn, an astronomy professor at the University of Maryland and the principal investigator on the Deep Impact mission.
Besides determining what comets are made of and how they form, the mission could yield important information about the hundreds of near-Earth objects moving in space.
Some researchers believe it’s just a matter of time before Earth is in the crosshairs of another large space rock. “We have no idea how to divert some rogue asteroid or comet unless we know what their physical properties are,” A’Hearn said.
No one is quite sure what to expect when the probe collides with Tempel 1.
Will it smash itself to bits against solid rock? Or is the comet more like a flying ice cube? Some speculate that comets like Tempel 1 are nothing but a collection of space flotsam as loosely packed as a bowl of corn flakes. In that case, the probe might fly right through it.
If the comet is made of tougher stuff, the collision could create one of the great Independence Day fireworks shows of all time.
If the theories are right, the explosion should send tons of ice, rubble and gas rocketing into space, causing the comet to temporarily flare up like a lightning bug.
An Ancient Mystery
Although so little is known about them, comets have long played prominent roles in human affairs as harbingers of doom. Because the ancients thought the shimmery tail resembled a woman’s flowing hair, they called the silver-robed wanderers “daughters of the devil.”
Scientists now know that comets are basically clumps of ice and dust. Though they may appear to the naked eye to have a single tail, each comet actually has two tails. The first, bluish in color, is made up of vaporized ice and dust particles heated by the sun and dragged into a tail by the solar wind. The second, yellow and curved, is made only of dust.
Current thinking is that asteroids are remnants of the material that formed the solar system’s inner planets. Comets are associated with the frozen outer planets.
Nobody is sure what happens when they age. Do they eventually run out of ice? When they do, do they turn into asteroids? Or do they break up into dust particles? Some astronomers have speculated that as many as half of the near-Earth asteroids are dormant or extinct comets.
A dormant comet is thought to still hold stores of ice but to have lost its bright appearance because the ice is sealed up in its nucleus.
The idea of reaching out and touching a comet has been around for decades. But serious planning for a mission did not begin until 1999.
Deep Impact was designed and built through a partnership between the University of Maryland, JPL, Caltech and Ball Aerospace & Technologies Corp. of Boulder, Colo.
Tempel 1 was chosen as NASA’s punching bag because it was a comet astronomers knew a lot about. Orbiting between Mars and Jupiter, it is also relatively close, yet far enough away that if it broke up it would not pose a hazard to life on Earth.
A Challenge Head-On
The challenge for the Deep Impact science team will not be finding and intercepting the comet. Since its discovery in 1867 by astronomer Ernst Tempel, scientists have been able to plot the comet’s 5 1/2 -year orbit to an exacting scale.
The plan is for the main spacecraft to approach the comet and then, 24 hours before impact, release its 820-pound probe. A roundish ball of copper and aluminum, the probe will maneuver itself into the comet’s path using its own navigation and propulsion system. It will try for a collision on a sunlit area of the comet about 3.7 miles across after traveling a final leg of 537,000 miles.
Though astronomers talk about shooting the comet as if they are holding a gun, to an observer in space it would look a lot more like a pedestrian walking into the path of a speeding train. The probe will be traveling 23,000 mph, but the comet will be going 70,000 mph in the opposite direction.
This cosmic head-on is expected to generate a force equal to 4.8 tons of TNT. The probe will be annihilated, creating a 14-story-deep crater.
Might this act of cosmic vandalism, as some have characterized it, destroy the comet?
Unlikely, say scientists. Tempel 1 is as much as 9 miles long and 4 miles across. “It will be the celestial equivalent of a bug hitting the windshield of an 18-wheeler,” Yeomans said.
The Deep Impact mother ship will be about 300 miles away, watching the collision through medium- and high-power telescopes, along with a spectrometer to study the material ejected by the explosion.
The probe carries its own digital camera to snap as many pictures as possible before its destruction.
The fireworks should provoke plenty of oohs and aahs on Earth -- except perhaps among the engineers at Ball Aerospace. “Our guys have worked so hard on something that will vaporize completely,” said spokeswoman Emilia Reed.
Still, that’s a far better outcome than the alternative of missing the comet altogether.
In space, there are no do-overs.
“We’ve got one shot at it,” said A’Hearn, who will be at Cape Canaveral for the launch.
