Targeting a comet to better understand the solar system
If you crash it, they will come. Thousands of people gathered at events across Hawaiʻi on July 3 to witness a spacecraft’s suicide collision with Comet Tempel-1. Countless others followed events on cable TV or stole glances at the evening sky hoping to see signs of an explosion equivalent to five tons of TNT. Comet watchers beyond Earth’s horizon connected to NASA’s public Deep Impact website in such unprecedented numbers that they temporarily brought the site down.
High atop the quickly darkening Mauna Kea summit, UH Mānoa Astronomer Karen Meech and UH NASA Astrobiology Institute postdoctoral researcher Audrey Delsanti stepped outside the IRTF observatory to see if the flash would be visible to the naked eye. Like the other watchers, they were unrewarded, but hardly disappointed.
For Meech, who is anything but a casual observer, satisfaction goes far beyond being a part of scientific history. As a member of NASA’s Deep Impact science team, she experienced the mission both as the culmination of 10 years of careful planning and the beginning of intense scientific scrutiny that promises to dramatically expand our knowledge of comets.
What we do know about comets is that they are "big dirty snowballs."
Generally small, with a nucleus just a half to a few miles in diameter, they are composed of primarily water and other ices as well as organic (i.e., carbon- and hydrogen-based) material and rocky grains. Trillions of them may exist in a disk of objects beyond Neptune called the Kuiper Belt and the giant Oort Cloud believed to surround our solar system.
Some are nudged by the gravitational pull of other comets or planets into orbits that take them through the inner solar system as often as every three years or as rarely as once in hundreds of millennia. Warmed by the sun, they shed a trail of dust and gasses as much as millions of miles long.
It is this tail that makes some comets visible and, well, pretty.
As a graduate student in astronomy, Meech found her desire to spend her time observing something attractive to look at coincided with the opportunity to work with a new comet expert on the Massachusetts Institute of Technology faculty. Highlights from her own remarkable career at UH’s Institute for Astronomy include 1989 observations that proved the unusual object dubbed Chiron was indeed a comet, 1991 documentation of an outburst from Comet Halley long after it moved away from the sun and discovery that Halley loses about a meter’s depth of material in every pass.
Tempel-1 wasn’t Meech’s first choice as an impact target.
The mission to crash an impactor into a comet while a flyby craft looked on initially considered Phaethon. The asteroid had been well studied, and Meech and others who suspect it is a dead comet wondered if an impact could restart cometary activity by releasing trapped gasses.
"We don’t know what happens to comets," says Meech. "Do they become extinct when the core has no more ice or just turn off because a thick dusty layer insulates the core from heat?"
Phaethon’s velocity made the mission riskier than a slower target, so scientists conducted hundreds of hours of observation on the understudy—Tempel 1. They studied its shape, shade, rotation speed, pole location and so on.
Building the impactor and flyby craft also required compromises.
Engineers demanded addition of navigational aids for last minute course adjustments. Astronomers insisted on substituting alternate materials for components. The impactor was predominantly made of copper because the aluminum traditionally used for spacecraft would interact chemically as it vaporized on impact, interfering with data collection.
As it turned out, the SUV-sized flyby craft used so little fuel on its trip to Tempel that it could be sent to another comet for a flyby or collision mission if astronomers (and public enthusiasm for Deep Impact) can convince NASA that it is worth the cost of the ground crew and antenna time.
Cost was the reason NASA opted for a kamikaze mission.
Deep Impact cost $333 million, compared to the billions of dollars required to land a working probe. Scientists can learn a surprising amount by studying the plume of debris exploded from the comet by the dishwasher-sized impactor’s 23,000-mile-per-hour crash landing. One likened the mission to a geologists hammer, breaking through the weathered crust of a sample to look at the pristine material within.
That’s important because comets are believed to be the bits leftover when planets formed from the dust and debris surrounding a young Sun 4.5 billion years ago. Preserved in a distant deep freeze, they amount to a historical record of the early solar system.
"We don’t know what’s inside.
"Is it dense or fluffy?" says Meech. How thick and strong are the core and outer layers and how do they interact? What structure does the ice take; is it all H20 or also carbon monoxide and carbon dioxide?
One intriguing question is whether comets could have delivered water to a parched early Earth and, with it, the organic compounds needed to produce life. Comparing markers and concentrations in the chemical make up of Earth and comets could yield clues.
Oh, and then there’s that practical matter of gaining information that may help should a comet or asteroid threaten Earth. IfA’s David Tholen works on the near-Earth asteroid hazard.
"We really don’t know how an asteroid will respond to being pushed.
"Will it move like a brick, or will it deform like a pillow? The answer has important implications for how we deflect an object on a collision course," Tholen says.
Deep Impact is expected to slow Tempel’s velocity by just .0001 millimeter per second and shift it a whisker 10 meters closer to the Sun. That’sgood news for those worried that the collision could send Tempel-1 tumbling Earthward, but sobering to those calculating the force needed to counter a real threat.
Tholen and postdoctoral colleague Yanga Fernandez provided images and precise measurements that helped accurately predict the time of impact. Others at IfA used Mauna Kea telescopes to monitor the impact—Schelte "Bobby" Bus conducting a six-week study of the composition and temperature of dust and gas from the interior of the comet at NASA’s Infrared Telescope Facility, Klaus Hodapp using a new instrument on the UH 2.2 meter to gauge whether the nucleus is chemically uniform and Richard Wainscoat employing a high-speed imaging camera on the UH .6-meter to understand the physics of impact and evolution of dust flowing from the nucleus.
For the first time in modern astronomy, virtually every professional telescope was trained on the same event.
For 800 precious seconds following the impact, the flyby craft recorded the plume. Space based instruments, including the Spitzer Infrared telescope, Chandra X-ray observatory and Hubble Space Telescope, collected spectroscopic, ultraviolet, microwave and other readings. From IRTF’s control room, Meech coordinated a network of more than 80 ground-based research telescopes.
"We’ll be taking data that we’ve never been able to take before. So to all of us in science, this is an extraordinarily exciting night," exclaimed Keck Observatory Director Frederic Chaffee from Keck headquarters in Waimea, where 200 people watched feeds of live NASA coverage and the view from the telescope.
Meech continues to coordinate images and data sent by research and professional astronomers from Australia to Uzbekistan.
"Getting answers to important fundamental properties like nucleus density and composition will take significant work because we have to carefully understand the spacecraft and ground data and make sure it is well calibrated before we begin the science interpretation," she says. Astronomers scrambled to present sound results at the Asteroids, Comets Meteors meeting in Rio de Janeiro beginning Aug 8, 2005.
Early analysis of the plume suggests a low density surface, probably loose and dusty. "If you hit something soft, the energy lifts material out of the crater," explains Meech. "If you hit cement, some of the energy is expended breaking the surface," resulting in a smaller plume.
Infrared characteristics of the post-impact plume varied considerably from readings of the usual emissions, confirming that scientists were looking at previously unexposed material. Observations indicated the presence of hot water, carbon monoxide and organic features. Development of the crater appeared to be controlled by gravity and the dust ejected was microscopic.
Inquiry will continue, of course. When it comes to comet science, Tempel-1’s new crater just scratches the surface.
UH astronomer earns worldwide acclaim and colleagues add to knowledge of the universe
Astronomer David Jewitt is on a roll this year. He was elected to both the National Academy of Sciences and American Academy of Arts and Sciences and earned international recognition for discovery of 12 new moons orbiting Saturn—all within two weeks’ time.
Jewitt attributes his success to the UH Institute for Astronomy, which he joined in 1988. "The institute is undoubtedly one of the best places in the world to be an observational astronomer," he says. "You can’t beat the fantastic astronomical facilities we have in Hawaiʻi."
A 1992 co-discoverer of the comet-filled Kuiper Belt, Jewitt’s team observed the new satellites orbiting Saturn in detailed surveys using two Mauna Kea telescopes. In other recent space news from UH—
* Many young, Sun-like stars in the Orion Nebula are surrounded by enough orbiting dust and other material to form a planetary system like our own.
Data from Mauna Kea’s newest telescope, the Submillimeter Array, suggest that formation of solar systems is a common and continuing event in the Milky Way galaxy, according to IfA’s Jonathan Williams and colleagues. Read the news release.
* Super-massive black holes a hundred-million times the mass of our Sun appear to have gorged on surrounding material and stopped growing when the fuel ran out billions of years ago.
Meanwhile, smaller black holes continue to grow slowly and steadily, Amy Barger and IfA colleagues reported in the February issue of Astronomical Journal. Read the news release.
* Calcium-aluminum-rich inclusions (CAIs) are widely believed to be the oldest solids formed in the early solar system 4.567 million years ago. If so, how could some of them contain younger crystal aggregates called chondrules?
Alexander Krot of ’s Hawaiʻi Institute of Geophysics and Planetology and colleagues suggest an answer in the April 21 issue of Nature. Electron microscopy and electron and ion microanalysis of samples from the chondrite meteorite Allende suggest that the condrules were added during a reheating and melting of the CAIs about 2 million years after the CAIs first formed. Read more.
* In the cover story of Geology’s (June 2005), Victoria Hamilton and an Arizona colleague report that an ancient region of olivine-rich rocks near one of Mars’ largest volcanoes is four times larger than previously estimated.
Because the mineral olivine can weather rapidly when exposed to water, the findings may help determine where and how much water was once present.
A subsequent Nature paper on the global distribution of rock types on Mars describes the planet as more mineralogically diverse than previously thought. Infrared data suggests Mars is more Earth-like in terms of its igneous rock types even though it lacks plate tectonics, a major process for producing evolved rocks on Earth, Hamilton says. Read the news release.
* Nearly $3 million in grants from NASA and the W. M. Keck Foundation will support creation of a new cosmochemistry laboratory in ’s Hawaiʻi Institute of Geophysics and Planetology.
Newly recruited scientists Gary Huss and Kazuhide Nagashima will join UH meteorite and planetary experts in using an ion microprobe to help interpret remotely-sensed data and analyze meteorite samples, lunar rocks, cosmic grains and comet dust. Read the news release.
* What is believed to be the 10th planet was found by a team that included alumnus Chad Trujillo (PhD ’00 ).
It is a Kuiper Belt object larger than Pluto. Release of the name is pending International Astronomical Union acceptance of the discovery. Read the Honolulu Star-Bulletin article.