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Collision Course: Deep Impact
Lucy McFadden and Peter Schultz

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Comet comraderie

On July 3, a spacecraft will separate from its mother ship on a collision course with a comet some 133 million kilometers (83 million miles) away from Earth. The next day, July 4, the 370-kilogram-spacecraft (816 pounds) is scheduled to ram into Tempel 1, giving off what’s sure to be some spectacular celestial fireworks.

Until the planned collision of a spacecraft into comet Tempel 1 on July 4, scientists can only speculate what the moment of impact and the formation of the crater will look like, as rendered in this artist’s interpretation. The aim of this in-space experiment is to learn more about comets and their origins. Image by Pat Rawlings, courtesy of NASA/JPL/UMD.


Called Deep Impact, this project will shed light on some fundamental scientific questions about comets, including what they are made of and how they formed. Comets are some of the oldest and coldest components left over from the outer edges of the solar nebula, from which the planets formed more than 4.5 billion years ago. When heated by the sun upon entering the inner solar system, the ice, dust and organic material that compose comets are driven from their centers into the vacuum of space. A shroud of gas and dust forms the “coma,” a tenuous atmosphere surrounding the comet, which trails off into a tail driven by solar radiation.

The phenomenon of a comet is a truly awesome sight in the night sky. But there is much we do not know about them — ranging from what the nucleus is made of, to whether or not there are craters on a comet’s surface, to what holds together the interior.

From the ground, we can only study the byproducts of the nucleus that are found in the coma and tail. Thus, NASA designed Deep Impact to look inside a comet and see what the icy, solid components are made of and how they are put together. With this experiment, we will deepen our knowledge of the history of the solar system.

Cratered past

The Deep Impact mission reproduces a phenomenon that has affected planetary surfaces throughout the history of the solar system. Just take a look at the moon on a clear night: The dark areas, or maria, are large impact basins that have been filled in with basalt. The bright highland areas are pock-marked with craters from just after the moon formed, a period in planetary history called the Late Heavy Bombardment.

As we look at other surfaces in the solar system, we find craters everywhere. Just after the planets formed, they were sweeping up debris, all of which formed craters on impact.

Impacts act as natural shovels that “dig up” materials from depth and deposit them as “ejecta” near and around the crater. In general, the deepest materials have the lowest velocities and are deposited closest to the crater, whereas the shallowest materials are ejected at the highest velocities and go the farthest.

For more than 40 years, concept studies had looked seriously at the idea of slamming into a planetary body. The use of high-velocity impacts as a geologic tool can be traced to the Apollo program during the 1960s.

Geologists had already used craters on Earth (such as Meteor Crater in Arizona) to study how materials from depth are deposited on the surface, and Apollo scientists wanted to look below the surface of the moon. In the 1960s, the Ranger probes took video images before crashing into the moon, and some scientists watched unsuccessfully with telescopes for the probe’s collision. In the early 1990s, laboratory experiments at the NASA Ames Vertical Gun Range, with the Jet Propulsion Laboratory (JPL) and Brown University, were under way to determine what might be learned from creating an impact on the moon.

Shortly after the Giotto spacecraft flew past Comet Halley in 1986, scientists Alan Delamere and Mike Belton proposed that creating an impact with a comet would excavate material from the comet and permit a look inside to study the differences between surface and interior. Their 1996 proposal was initially rejected due to technical risks. The technical team reviewing proposals was concerned that gas and dust from the comet would divert the spacecraft from its collision course.

In 1998, the team of scientists and engineers (now led by Mike A’Hearn) from universities around the country, as well as in Germany, partnering with Ball Aerospace and Technologies Corp. in Boulder, Colo., and with JPL, proposed the project again and was funded. The team designed an impactor spacecraft with navigation capabilities designed to target the comet and override its forces of gas and dust. Thus, Deep Impact was born in 2000.

Digging out a comet

The Deep Impact mission will create an impact to excavate materials from the greatest depth of comet Tempel 1. Rather than sampling these materials, imagers aboard the impactor spacecraft and the mother ship will capture the first glimpse of the impact, see the crater grow and examine the final crater. At the same time, an infrared spectrometer will record the changes in the gas and dust as they leave the crater, form ejecta around it and contribute to the coma. Major telescopes on Earth and in space will be observing the comet before, during and after the impact, across the range of the electromagnetic spectrum.

We know how fast and how big the Deep Impact probe is, but we cannot predict exactly what the impact will be like because we do not know the nature of the surface of the comet. It could be smooth or very rough. It could be covered by a layer of organics and silicates (left over from repeated visits to the inner solar system), and that layer could be highly porous or welded together by ice. Below that surface, the comet may be highly porous and weakly bonded or held together like a fragile fairy castle. And we will not know the angle of impact until we watch the images from the probe and see the event unfold from the flyby cameras.

Ultimately, the probe will penetrate deeply enough to excavate volatiles — ices of carbon, hydrogen, oxygen and nitrogen — trapped below the reworked surface. These ices are the ultimate prize because those materials have been there since the time when the solar system formed.

Unknowns concerning the actual impact conditions have led to a series of experiments at the NASA Ames Vertical Gun Range, to prepare ourselves for the different possibilities. These experiments use a two-stage light-gas gun, which was specially developed in the 1960s for the Apollo program. The gun can be positioned at different angles, allowing a wide range of gravity-sensitive targets (for example, sand or water) to be placed in a large chamber. It is called a two-stage gun because it uses gunpowder (the first stage) to compress hydrogen gas to high pressures. When released (the second stage), the hydrogen gas propels the projectile at velocities high enough (6 to 7 kilometers per second) to melt and vaporize the projectile when it hits the target, even if the target is water.

The gun range allows us to measure various crater formations and to watch as they happen, just as will occur with Deep Impact. Two processes affect the size and appearance of the final crater: how the probe transfers all that energy and momentum to the comet, and what stops the crater from growing.

If the target (in the case of Deep Impact, Tempel 1) is much less dense than expected (less than 0.1 grams per cubic centimeter), then there are two possibilities. The first is that the probe will compress the material in front of it with little material coming out: It disappears. The second possibility, however, was discovered in the experimental series: The probe could penetrate very deeply before exploding, which results in ejecta coming out of the crater in a column at high angles. This is similar to a fireball entering the atmosphere and exploding above the ground. In such a case, a smaller crater forms with plenty of ejecta, but the crater collapses into the underground cavity.

Forces stopping the crater from forming will determine the crater size. In one scenario, the crater grows until the weak gravity of the comet stops it; the ejecta do not have enough velocity to get out of the crater. In another scenario, the strength of the comet (after the shock passes through) prevents the crater from growing too large; it grows as large as the shock-disrupted surface allows. And of course, there are combinations: a weak surface layer over a strong substrate or a hard surface layer over a delicate one. The exciting thing is that we are using the Deep Impact spacecraft to find out.

The navigation team at JPL has pointed the spacecraft such that Tempel 1 will crash into the impactor spacecraft at a relative velocity of 10.2 kilometers per second (23,000 miles per hour). At these speeds, the impactor will mostly likely melt and vaporize, with ejecta from the comet first propelled out of the initial cavity. Watching the crater as it forms on Tempel 1 will allow us to understand which prediction for the comet is correct. One key aspect is what the ejecta will look like when they leave the crater. In loose particulates (sand and powder), the ejecta form a sheet that looks like a snow cone (see image, opposite page). This sheet, called the ejecta curtain, marches outward quickly but slows down as the crater finishes forming.

After the crater formation is complete, the curtain speeds up, as it is composed of higher velocity ejecta. If the target is made of material that is strongly held together, then the ejecta will come out in chunks and stringers. But if the target is highly porous, then the laboratory experiments reveal that the ejecta will come out in a narrow plume. Other things to look for are the flash that’s created at the moment of impact and the evolution of the temperature. One thing is for certain: Whatever the outcome, it will be different from our expectations.

En route

The Deep Impact mission is the eighth in the Discovery Program, a series of planetary exploration missions managed by NASA’s Science Mission Directorate. These missions are capped at $360 million each, with a development time to flight of three to four years. Initially scheduled to launch in January 2003, Deep Impact was delayed while computer bugs were worked out and preflight testing was completed. The mission successfully launched last Jan. 12 from Cape Canaveral in Florida.

An engineering team at Ball Aerospace and Technologies Corp. stacks the flyby spacecraft atop the impactor spacecraft for NASA’s Deep Impact mission, in which the impactor will slam into comet Tempel 1. Courtesy of NASA/JPL/UMD.

After launch, the instruments were turned on and pointed at familiar bodies, including the moon and Jupiter, as well as star clusters. After a planned period of 35 days in which the instruments were heated to drive off water adsorbed from their time on the launch pad in Florida, the science team and engineers realized that one instrument, the high-resolution imager (HRI), was not focusing as expected. Furthermore, when we heated the telescope more, the focus did not improve. A “tiger” team of instrument specialists was put in place to analyze the problem, and unfortunately determined that the focus of the HRI instrument was not where it was expected.

As a result, some images are a bit blurry, and the science team is investigating image-processing techniques to digitally correct the focus. In spite of this disappointment, however, the project members are optimistic that our primary science objectives can be achieved, especially with the medium-resolution camera and the imager on the impactor both operating as expected.

The team was very happy on April 28, when the cameras first spotted comet Tempel 1 itself. We are currently imaging the comet every day and waiting to get close enough to see through the gas and dust in the coma and see the nucleus itself.

A camera on the impactor will reveal the surface of the comet as it approaches until seconds before impact, with data relayed to the mother ship and then back to Earth. On its flyby, the mother ship will record the impact and its aftermath with cameras and an infrared spectrometer. Meanwhile, back on Earth, telescopes, both ground-based and in orbit, will be observing for evidence of the impact at a distance.

For those with access to a telescope and a view of the constellation Virgo, the comet is visible now in the evening sky in the Northern Hemisphere. The Deep Impact project is asking amateur astronomers with digital, charge-coupled-device cameras for their telescopes to record the brightness of the comet as often as possible to fill in the gaps in coverage by large telescopes (see related story). It is important for the science team to know how the comet is behaving so that we can set the exposure times on our camera.

We are monitoring the comet from the spacecraft on a daily basis and will be observing more often as we approach impact. Yet the role of ground-based observers is important, too, for their ability to continuously monitor as the weather permits.

It’s an exciting time to be exploring the solar system, with missions reaching new places, deeper than ever before. The sense of anticipation is growing within our science team; we feel like we are moving at the speed of the spacecraft as it approaches the comet for its impact this month.

Watch with us!

Comet Comraderie

Throughout the planning of the Deep Impact project, NASA has tried to get as many people as possible excited about slamming an 816-pound spacecraft into the Tempel 1 comet. By studying the size and shape of the hole left behind from the impact, scientists hope to understand more about the mysterious comet body and how it formed. Already, thousands of people are involved in various outreach projects, from lending their names to the mission to checking out the night sky with their telescopes.

The latest public project is sponsored by the Planetary Society, a nonprofit space advocacy group, and invites people around the world to place their bets on how big of a crater Deep Impact will form on Tempel 1’s surface. The contest encourages participants to first learn all they can about the comet and the mission from the society’s Web site. The person with the guess that is closest to the actual crater size will win a laser-engraved plaque made of the same kind of copper material that comprises the impactor spacecraft.

In another outreach project, NASA asked members of the public to send in their names to adorn a disc mounted on the impactor spacecraft. Between May 2003 and January 2004, the “Send Your Name to a Comet” campaign, 560,000 people sent in their names and received personalized certificates, thanking them for their involvement.

In a more hands-on public project, amateur astronomers around the world are observing Tempel 1 and the impact themselves, using small telescopes to beef up the dataset Deep Impact scientists are compiling. For more on the observing project, read this month’s Education & Outreach column. And for links to the other Deep Impact projects, see below.

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McFadden is a Deep Impact science team member and an associate research professor at the University of Maryland’s Department of Astronomy in College Park, Md. Schultz is a professor in the Department of Geological Sciences at Brown University in Providence, R.I., and co-investigator on the Deep Impact science team.

Links:
"Making a Public Impact," Geotimes, July 2005
Deep Impact home page
The Planetary Society's Deep Impact page


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