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!
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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. |
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