Category Dreams of Other Worlds

Rethinking Comets

The comet cloud defines the outer edge of the Solar System, ex­tending a thousand times further than the planets from the Sun, and a significant fraction of the distance to the nearest stars. It had always been presumed that these small bodies were frozen rel­ics from the formation epoch, essentially unchanged in 4.5 billion years. The expectation was that comets would be made of dust from a previous generation of stars, or pre-solar grains. That’s why the mission was named Stardust.

Instead, something quite unexpected was found: fire and ice.24 Comets contain ice that formed in the frigid zone beyond Nep­tune, but the bulk of a comet’s mass is rock, and that rocky ma­terial seems to have formed under conditions hot enough to va­porize bricks. The comet particles embedded in Stardust’s aerogel included two ingredients that are found in meteorites, debris from asteroids that formed between the orbits of Mars and Jupiter. One is chondrules, rounded droplets of rock found in many primitive meteorites that melted and quickly cooled as they orbited the Sun. The other is a much rarer mineral called a Calcium Aluminum Inclusion, irregular white particles of very unusual chemical com­position that can only form at very high temperature. The Stardust team puzzled over this discovery, but the implication seemed clear. Matter that formed readily in the inner Solar System was some­how transported to the edge of the young Solar System where the comets formed. Wild 2 does contain grains of pre-solar stardust material, but they’re very rare. Comets are not made of material left over from other stars; they’re made mostly from material that formed close to the Sun. As such, they provide insights into how planets and moons were built 4.5 billion years ago.

Stardust’s primary contribution to planetary science is verifi­cation of the idea that there was extensive radial mixing of ma­terial in the solar nebula just as the Solar System was forming. Also, comets are more physically diverse and variegated than any­one imagined. Each of the comets where we’ve got up close and personal—Halley, Borrelly, Tempel 1, and Wild 2—have different shapes, surface textures and features, and levels of activity. Com­ets may be modest in size compared to planets and moons, but their role in planetary systems is anything but modest. They deliver water and organic building blocks of life to terrestrial planets, and as we know from studies of our geological record, when they hit a planet they can decisively alter the history of ecosystems and even the entire biosphere. It’s appropriate that all human cultures have been fascinated by comets, and viewed them as harbingers of life and death.

To learn more requires an increase in our modest amount of di­rect information. So it was very exciting when Stardust was given a new lease on life after its sample return. In 2007, NASA approved the New Exploration of Tempel 1 (NExT) mission, a return to the site of the Deep Impact mission in 2005.25 Stardust had con­served just enough hydrazine fuel to make a second journey to a comet, and the goal was to observe the impact made by the ear­lier spacecraft and see any changes on Tempel 1 caused by its last close approach to the Sun (Deep Impact’s cameras were blinded by dust released from the impact). In early 2010, a controlled burn ensured Stardust would approach at the optimal speed. The flyby was challenging because the camera had to catch the correct side of the tumbling comet nucleus while coming up on it at 7 miles per second. Appropriately, our first second date with a comet took place on Valentine’s Day, 2011. Stardust showed that the 500-foot wide crater created in 2005 was indistinct and had a mound in the center, indicating that much of the ejected material had fallen straight back down in the weak comet gravity. The results indi­cated that the comet nucleus was fragile and only weakly held together.

All good things must come to an end. For Stardust the end came on March 24, 2011, when mission controllers deliberately ordered a “burn to completion.” They told the spacecraft, which was al­ready running on fumes, to fire its main engines until there was no fuel left. NASA used the final burn to refine fuel consumption models for these kinds of engines, ensuring that Stardust gave use­ful data right up to its last gasp. The next day, Project Manager Tim Larson put the spacecraft into a state called safe mode, turned off the transmitter, and walked away from the console. Mission accomplished.

Astronomy’s Human Genome Project

Michael Perryman has dubbed the Hipparcos mission astronomy’s equivalent of the Human Genome Project.22 Perryman explains that as astronomers more accurately map the location, velocity, and vector of stars in our galaxy we can understand the age and morphology of the Milky Way, how our galaxy has evolved in the past, and what the future holds for our Solar System and the gal­axy. For instance, the Hipparcos mission has contributed to our better understanding of the galaxy’s current structure. We know our galaxy is not a perfect spiral, but is instead a barred spiral that’s warped so that the limbs at one end curve up and at the other bend down (figure 8.3). Another major contribution of Hip – parcos, for astronomers and popular audiences, is that the mission improved the estimates of distances to stars harboring exoplanets. In this way, it has crystallized our sense of the growing number of distant worlds in space. We’ve seen in the earlier chapters on the Solar System that planets and moons are potential abodes for life. As the Human Genome is a project to map the underlying structure of terrestrial life, so Hipparcos is a tool to help astrono­mers map plausible sites for extraterrestrial life. The search for life beyond the Earth is a foundational scientific pursuit, and it has attracted attention from some unlikely quarters.

The Vatican has maintained an observatory over the centuries in order to officially determine dates of the calendar year; the Gre­gorian calendar has been used in the Western world since 1582. However, astronomers of the Vatican Observatory more recently

Astronomy’s Human Genome Project

Direction of Magellanic Clouds

Figure 8.3. Hipparcos measured the positions for hundreds of thousands of stars and so was able to map out the disk of the Milky Way over 500 light-years. This was enough to detect a subtle warp in the disk, exaggerated in this schematic view. The shape of the disk is like a brimmed hat with the brim turned down on one side (ESA/Hipparcos).

have been focusing on other concerns. In November 2009, Pope Benedict XVI called leading astronomers, astrobiologists, and cos – mologists to Vatican City to spend a week presenting recent find­ings regarding exoplanets orbiting nearby stars and to discuss the possibilities of intelligent life in those star systems.

Of the Vatican’s interest in exobiology, science reporter Marc Kaufman noted: “Just as the Copernican revolution forced us to understand that Earth is not the center of the universe, the logic of astrobiologists points in a similarly unsettling direction: to the likelihood that we are not alone, and perhaps that we are not even the most advanced creatures in the universe. This. . . may conflict with the stories we tell about who and what we are.”23 During the five-day meeting scientists addressed subjects such as the origins of life, extremophiles and their habitats, the likelihood of such life thriving on moons in the outer solar system, and whether life’s bio­signatures could be detected on exoplanets.

As yet, exoplanets are mostly gas giants with little chance of life on them, but as the detection limit has reached Earth mass with NASA’s Kepler satellite, research spurs scientists, philosophers, and theologians alike to contemplate the implications for our place in the universe. “The questions of life’s origins and of whether life exists elsewhere in the universe. . . deserve serious consideration,” explained Jose Gabriel Funes, a Jesuit priest who is also the di­rector of the Vatican Observatory. Co-author Chris Impey, who presented a paper at the meeting and co-edited the written pro – ceedings,24 comments: “Both science and religion posit life as a special outcome of a vast and mostly inhospitable universe. There is a rich middle ground for dialog between the practitioners of as – trobiology and those who seek to understand the meaning of our existence in a biological universe.”25 Reporter David Ariel, who also covered the meeting, aptly noted, “The Church of Rome’s views have shifted radically since Italian philosopher Giordano Bruno was burned at the stake as a heretic in 1600 for speculating, among other ideas, that other worlds could be inhabited.”26

For the moment, most of the vast inventory of stars remains out of reach. But several hundred relatively nearby stars are known to have planets, and Hipparcos has been an essential tool in measur­ing their distances. These new and potentially habitable worlds range from a dozen to a few hundred light-years away. Spanning the entire galaxy, one estimate is of 8 billion terrestrial habitable worlds around Sun-l ike stars, each of which has the potential to host life.27 This number is the same order of magnitude of the number of base pairs derived from the Human Genome Project, making literal the analogy of a vast mapping project to parse life in the Milky Way.