Comets: Killers, Creators, or Cotton Candy?

Author:  Calamia Joseph
Institution:  Physics & Science Writing
Date:  April 2005

Grab your gas masks--here comes Hailey. Though today we don't normally view comets through plastic visors, in 1910, comet-protection proved a profitable business. Yet, this is not the first time we have feared the heavens. Ancient astrologers cited comets as ill omens of death and famine. Now, planet-destroying comets and asteroids inspire terror in popular films such as Deep Impact and Armageddon. Why have humans always feared what scientists dub celestial "small bodies"? On all accounts, the problem seems to be a lack of understanding.

NASA's Discovery Program, created in 1992, aims, as its name implies, to discover the truth behind celestial small bodies. Of the ten planned Discovery missions, six focus on the study of comets and asteroids. The number of these investigations hints at a deep importance in studying these small bodies, an importance beyond applications in Earth protection and perhaps even space science itself.

Dusting off an antique

Imagine the Earth 3.9 billion years ago. Debris, from the solar system's formation, litters the sky. The constant bombardment of our hot, waterless planet makes life impossible. But then the heavy attack ends. Almost immediately, the roots of life, "fragile carbon-based molecules," begin to form. Where did these molecules come from? Was it the burning planet--or, perhaps, was it the attackers themselves?

Many scientists question whether comets or other extraterrestrial debris could have brought life to earth. Previous projects to fly near comets, such as the 1990 Ulysses probe mission, have produced detailed pictures of comets and also provided composition information through spectroscopy, or light analysis. But in January, NASA scientists went beyond all previous investigations–they captured a piece of a comet to take home.

Discovery Program's Stardust mission to comet Wild 2 is the first project to bring materials to Earth from beyond the orbit of the moon. Traveling a total distance (round trip) of 3.2 billion miles, this "robotic lab assistant" collects samples much like a child with a butterfly net.

Artist rendering of Stardust probe. Note sample collector deployed out probe front. Source: NASA/JPL.

Artist rendering of Stardust probe. Note sample collector deployed out probe front. Source: NASA/JPL.

A specially designed, "tennis racket" sample collector protects tiny comet dust particles as the probe dips through Wild 2's coma, a comet-cloud of expelled materials. The secret of the tennis racket probe is Aerogel, the lowest density material in the world, which slows and stops particles, even during Stardust's 13,000 mph comet fly by.

Each sample is extremely small, probably less than a micron (1/100th the width of a human hair), and Don Brownlee, principal investigator of the mission, notes that his team will analyze samples "a single grain at a time."

The "ghost-like" foam, Aerogel, used by the Stardust probe for its strength and insulating properties. Source: NASA/JPL.

The "ghost-like" foam, Aerogel, used by the Stardust probe for its strength and insulating properties. Source: NASA/JPL.

Yet what will Brownlee and his team search for in these tiny grains? One practical application is Earth protection.

"Humans could not currently survive an impact of the size that killed the dinosaurs 65 million years ago," states Brownlee, who notes, that though the chance of such an impact during our lifetime is one in a million, these odds of destruction are much greater than other popular concerns--such as environmental poisons.

Co-investigator in the Stardust mission, Scott Sandford, agrees. Sandford believes the only way to understand the threat of such an impact (and the defense necessary to survive it) is through the investigation of asteroids, comets, and other celestial small bodies.

"Asteroids and comets represent a significant impact hazard for the Earth. An impact by such an object would certainly cause massive destruction, and could potentially result in the extinction of most forms of life on our planet. A key part of understanding and mitigating this hazard is to have a good understanding of what these objects are made of and how they are put together."

But to both of these scientists, Earth protection seems a secondary goal; the material in Wild 2 is much more than a defense tool.

Sandford explains that, in many ways, the samples from Wild 2 are clues to our planet's past. Because Earth is constantly renewed through volcanism, weather, and other natural processes, none of the planet's original material now remains.

"Everything has been altered at least once," explains Stanford. "In contrast, the materials in small bodies (asteroids and comets) are not subjected to all these planetary processes."

In many ways, collecting materials from Wild 2 is like collecting materials from our primordial past. Brownlee notes, "Earth retains no records of its formation and its early history." The oldest rocks on our planet are only about 3.8 billion years old; comparatively, the dust samples from Wild 2 may contain particles from the time of the Solar System's creation, 4.6 billion years ago.

Blasting through--deep impact

Yet, some scientists want a deeper look–-a look provided by Discovery's most recent mission, Deep Impact. Instead of taking samples from the comet's surrounding coma cloud, this July 4th, Deep Impact will blast a "stadium-size" hole in the comet Tempel 1 with a 23,000 mile per hour, head-on collision. A second probe component will then analyze the newly created crater and the comet's exposed composition.

Like Stardust researchers, Joseph Veverka, Deep Impact co-investigator, also cites defense as one reason for the Deep Impact mission. He grins as he explains the project's necessity, "When you are trying to deflect a comet you need to know if its composition is concrete or cotton-candy."

Though Deep Impact will not bring back any samples, Veverka sees benefits in his project beyond previous NASA missions. The Sun literally "cooks" any body that comes close to it, thus the outer surface and expelled coma of close-orbiting comets, such as Tempel 1 are not truly the primordial materials many researchers hope for. Deep Impact cuts deeper than Tempel 1's burnt crust, revealing comet composition and chemistry as it was at the dawn of the Solar System.

Artist rendering of Deep Impact probe. Source: NASA/JPL.

Artist rendering of Deep Impact probe. Source: NASA/JPL.

Veverka feels this research will greatly benefit all planetary studies and stresses that comets are "windows to the past."

"If we want to know more about ourselves, we might start looking at our parents or grandparents," he says.

Many other scientists agree; the desire "to know more about ourselves" is an overarching mission in each of these small body investigations–to understand, without fear, our past.

As Sandford states, "This sort of work helps address the issue of where we fit in the larger scheme of things.'"

References and Suggested Reading

Jet Propulsion Laboratory. Deep Impact Home Page.

Jet Propulsion Laboratory. Stardust Home Page.

National Aeronatics and Space Administration. Deep Impact Launch: January 2005.

National Aeronatics and Space Administration. Stardust Launch: February 1999.

- Joseph Calamia