Centauri Dreams

With NASA’s EPOXI mission closing on comet Hartley 2 at 12.5 kilometers per second, be aware that live coverage of the close encounter will begin at 1330 UTC (0930 EDT) on Nov. 4 from mission control at the Jet Propulsion Laboratory. NASA TV streaming video will be available, and you should also be able to watch the action on a JPL Ustream channel. Finally, NASA’s Eyes on the Solar System Web tool, a 3-D environment for Solar System exploration, is available for viewing a real-time animation of the cometary flyby on your PC.

EPOXI is a great instance of re-purposing a spacecraft to extract maximum value. This is the Deep Impact vehicle that gave us such a spectacular view of the impactor smash-up on comet Tempel 1 back in the summer of 2005. Under the name EPOXI, the mission has pressed ahead with two separate objectives, the first being EPOCh, or Extrasolar Planet Observation and Characterization, which has involved a number of nearby stars known to have transiting exoplanets. The idea here was to observe the stars to see if other worlds might be detectable around them. More on this analysis and its potential for finding Earth-like worlds in a day or two.

The spacecraft has also given us useful studies of Earth as seen from a distance, information that should provide perspective as we learn to use spectral information to map continents and oceans on distant worlds. But Hartley 2 is part of the other extended mission, known as the Deep Impact Extended Investigation (DIXI). The mission will use the two Deep Impact telescopes with digital imagers that were used for Tempel 1, along with an infrared spectrometer, in making this fifth close encounter of a comet. Closest approach at Hartley 2 should be at about 700 kilometers.

Meanwhile, we’re already getting interesting information, including the fact that Hartley 2 is quite active. The spacecraft caught two jets firing off the comet’s surface over a 16-hour period on October 26, viewing these from a distance of 8 million kilometers. The video below shows the jets in action as seen by the High-Resolution Imager and the Medium Resolution Imager.

The jets are thought to originate from similar latitudes on the comet’s nucleus. Says EPOXI principal investigator Michael A’Hearn (University of Maryland):

“These movies are excellent complements of one another and really provide some excellent detail of how a comet’s jets operate. Observing these jets from EPOXI provides an entirely different viewpoint from what is available for Earth-based observers and will ultimately allow a proper three-dimensional reconstruction of the environment surrounding the nucleus.”

Arecibo’s planetary radar has also been focused on Hartley 2, with observations that began on October 24 and continued through the 29th. The comet came within 17.7 million kilometers of Earth on the 20th, its closest approach since discovery in 1986. The comet in these images appears to EPOXI project manager Tim Larson (JPL) as ‘a cross between a bowling pin and a pickle,’ a vegetable analogy quickly picked up by mission scientist Jon Giorgini (JPL):

“Observing comet Hartley 2 from the Earth with radar was like imaging a 6-inch spinning cucumber from 836 miles away. Even without all the data in, we can still make some basic assertions about Hartley 2. Its nucleus is highly elongated and about 2.2 kilometers long, and it rotates around itself about once every 18 hours. In addition we now know the size, speed and direction of particles being blown off the comet, and we immediately forwarded all this information to the EPOXI team.”

Below is the pickle in situ:

Image: Twelve radar images of the nucleus of comet Hartley 2 were obtained by the Arecibo Observatory’s planetary radar from Oct 25 to 27, 2010. Image Credit: NAIC-Arecibo/Harmon-Nolan.

Hartley 2 is offering up the best extended view of a comet in history as it makes its pass through the inner Solar System, and the creative recycled use of the spacecraft offers savings of up to 90 percent over the cost of a similar mission built from scratch. We’re going to be learning a lot more about Hartley 2 in short order, although expect images of closest approach to be delayed after the encounter as Deep Impact reorients its high gain antenna on Earth, at which point the downloading of cometary closeups can begin in earnest.

It’s staggering how much our view of the Solar System has changed over the past few decades. The system I grew up with seemed a stable place. The planets were in well-defined orbits out to Pluto and, even if it were always possible another might be found, it surely couldn’t pose any great surprise in that great emptiness that was the outer system. But today we routinely track trans-Neptunian objects with diameters over 500 kilometers — about 50 of these have now been found, and some 122 TNOs at least 300 kilometers in diameter. We know about well over a thousand objects in that ring of early system debris called the Kuiper Belt.

It’s an increasingly messy place, this outer Solar System, and it has its own terminology. We have centaurs and plutinos, resonance objects, cubewanos, scattered disk objects (SDOs), Neptune trojans, damocloids, apollos and, perhaps, inner Oort cloud objects.

Nope, this isn’t the Solar System I grew up with, and every new discovery adds to the enchantment. Its burgeoning population of outer objects tells us much about its history, assuming we can make the right deductions from what we see. Orbital trajectories are a kind of history written in motion. The reason that a belt of objects beyond Neptune was first suspected was that Jupiter-family comets have orbital inclinations too low to be consistent with an origin in the Oort Cloud, that spherical cloud of comets thought to stretch a light year or more from the Sun. Advances in CCD technology soon made it possible to track down Kuiper Belt objects, and it’s now believed that 100,000 KBOs with diameters larger than 100 kilometers could exist, and perhaps as many as 800 million objects with diameters larger than five kilometers.

Image: Views of the Kuiper Belt and the Oort Cloud. Credit: Donald K. Yeoman/NASA/JPL.

The Outer System Poker Game

All of which is intriguing in its own right, but sometimes it takes a wild card to drive the story forward. That wild card came in the form of Sedna, discovered in 2003 by Mike Brown (Caltech). Brown has been ruminating over the discovery on his Mike Brown’s Planets site, where he notes the fact that the orbit of every object in the Solar System can be explained, at least in principle, by interactions with the known planets. Every object except Sedna:

Seven years ago, the moment I first calculated the odd orbit of Sedna and realized it never came anywhere close to any of the planets, it instantly became clear that we astronomers had been missing something all along. Either something large once passed through the outer parts of our solar system and is now long gone, or something large still lurks in a distant corner out there and we haven’t found it yet.

The possibilities are fascinating, one being the existence of an unknown planet of approximately Earth’s size at roughly 60 AU. Another possibility: A star that passed close to the Solar System at some point in the remote past, perhaps as close as 500 or 600 AU. In both cases, gravitational interactions would have interfered with what would otherwise have been a routine Kuiper Belt object, kicking it into its present orbit. Brown pegs the chances of a rogue star encounter at around one percent, but in any case, finding the culprit star would be impossible. The Sun has orbited the Milky Way 18 times in our Solar System’s history. “Everything is now so mixed up,” he adds, “that there is no way to know for sure what was where back when.”

The View from a Cluster

The third possibility? A kick from not one passing star but from many relatively nearby stars, a kick dating back to the Sun’s presence in the cluster in which the Sun was born. Brown’s description of the process and the place in which it might have occurred is worth repeating:

In the cluster of stars in which the sun might have been born there would have been thousands or even tens to hundreds of thousands of stars in this same volume, all held together by the gravitational pull of the massive amounts of gas between the still-forming stars. I firmly believe that the view from the inside of one of these clusters must be one of the most awesome sights in the universe, but I suspect no life form has ever seen it, because it is so short-lived that there might not even be time to make solid planets, much less evolve life.

A striking view indeed, and the poets among us can muse on its transience. Brown continues:

For as the still-forming stars finally pull in enough of the gas to become massive enough to ignite their nuclear-fusion-powered cores they quickly blow the remaining gas holding everything together away and then drift off solitary into interstellar space. Today we have no way of ever finding our solar siblings again. And, while we see these processes occurring out in space as other stars are being born, we really have no way to see back 4.5 billion years ago and see this happening as the sun itself formed.

But Sedna may help, because its orbit should be a record of what was going on when the Sun and our Solar System were in their infancy, a key to unlocking a 4.5 billion year old puzzle. The problem is that with only a single object of this kind, we wouldn’t have enough information on which to build the bigger picture, which is why researchers like Brown continue to look for other Sednas. It’s also why numerous other theories have sprung up, including the possibility that Sedna once orbited a different star and is actually an extra-solar dwarf planet. Or (an old favorite) that a brown dwarf somewhere in the Oort Cloud could have given it its nudge.

Of Dust and the Disk

All this reminds me of Mark Kuchner’s work on Kuiper Belt dust. Kuchner (NASA GSFC) has been running supercomputer simulations tracking the interactions of dust grains, and points to the Kuiper Belt as not only the home of countless small objects, but of dust and debris that model, though in a much older and developed way, the debris disks around Vega and Fomalhaut. At stake is how dust travels through the Solar System, affected by the solar wind and pushed by sunlight, not to mention the effects of collisions between icy grains themselves.

Kuchner’s team has been able to create infrared simulations of the Solar System as it might be seen from another star, using models of dust generation that could reflect what the condition of the Kuiper Belt was in a series of time frames going back in steps to 15 million years ago. The simulations show that a broad dusty disk like today’s collapses into a dense ring as we go back in time, producing something similar to the rings we’ve found around other stars. But today’s belt is still active. “[E}ven in the present-day solar system,” says Christopher Stark (Carnegie Institution for Science), “collisions play an important role in the Kuiper Belt’s structure.”

Interestingly for our model of dust in the outer system, Neptune’s gravitational effects push nearby particles into preferred orbits, creating a clear zone near the planet and dust enhancements that precede and follow it around the Sun. Kuchner calls this ‘carving a little gap in the dust.’ Our picture of dust in planetary systems is developing, but it’s worth noting how much work we have to do to anticipate the effects of dust on fast-moving spacecraft as we push past the heliopause and into true interstellar space. And Sedna’s odd orbit reminds us how much awaits discovery in our own systems’ furthest reaches.

Here’s an interesting bit of news from the New Horizons team. Remember that the spacecraft, having made its pass by the Pluto/Charon system in 2015, will be moving ever deeper into the Kuiper Belt. It’s been the hope of mission planners that a close study of one or more objects there might be possible. Now astronomer Scott Sheppard (Carnegie Institution of Washington) has announced that he has detected the first asteroid in Neptune’s trailing Trojan zone (the planet’s L5 point), an area New Horizons will fly through before arriving at the Pluto/Charon binary.

2008 LC18 is not itself in range for a New Horizons flyby, but mission principal investigator Alan Stern notes its significance in a recent report on the mission’s Web site: ” …its discovery shows that additional and potentially closer Neptune Trojans that New Horizons might be able to study could be discovered in the next three years.” And that gives us an interesting mission extension for New Horizons, to take advantage of the instrumentation we’ve put deep into the outer system. Closest approach to Pluto occurs in 1719 days.

Colors of a KBO

Thus far we’ve been able to image about 1000 objects in the Kuiper Belt, and it’s interesting to note that they come in a range of colors. Work at NASA GSFC (Greenbelt, MD) now offers up a computer model that tells us how the icy bodies acquire their red, blue or white tints. The model is based on incoming radiation and its effects on the different layers of a KBO.

Image: This cutaway model shows a red “shelf” layer of a Kuiper Belt object peeking through the thin, darkened crust above so that the object appears red in telescopes. Credit: NASA/Conceptual Image Lab/Tyler Chase.

Active Processes Deep in the Outer System

I find Kuiper Belt objects utterly fascinating, and not least because of the possible astrobiological interest. Eris, for example, has a bright, icy surface. According to the GSFC model, deeper layers of relatively pure water ice could erupt upwards to form new outer layers on KBOs, accounting for Eris’ brightness. The potential for active processes deep within a KBO is intriguing, and I remember discussing astrobiological possibilities with Joel Poncy at last year’s Aosta conference. Poncy and team (Thales Alenia Space) were investigating a fast orbiter mission to Haumea, which is, like Eris, highly reflective and evidently covered with water ice.

Poncy’s point was that if you’re going to study KBOs in terms of astrobiology, Haumea isn’t the place, because its unusual shape (a flattened ellipsoid) probably derives from a major collision, which would have disrupted the interior processes we want to study. But we now have numerous objects with diameters over 500 kilometers beyond the orbit of Neptune, and we’ll doubtless find hundreds more in coming years. Some smaller KBOs, like 2002 TX300, are possibly the result of the same collision that produced Haumea. 2002 TX300 is highly reflective, its ice covering evidently fresh and thus somehow resurfaced periodically. The case for life inside a KBO is slim, but these intriguing objects may teach us something about life’s early chemistry.

Explaining How Colors Emerge

But back to the GSFC work. The colors of Kuiper Belt Objects seem related to the different sizes and orbits we’ve observed since 1992, when the first KBO, 1992 QB1, was discovered. John Cooper, a physicist at Goddard, takes note of KBO diversity:

“There’s a group called the Cold Classicals that move in relatively circular orbits, and are nearly aligned in the same plane as the orbits of the other planets. These are all consistently reddish. Other objects, which might range from red to blue to white, tend to move in more elliptical or inclined orbits, which suggest they came from a different location within the solar system early in its history. So, it’s possible that the uniformly red Cold Classicals represent a more pristine sample, showing the original composition of the Kuiper Belt with minimal disturbances.”

Cooper’s work tells us that radiation should affect different objects in different ways, depending on their location. The so-called Cold Classicals would have formed in an area where plasma ions from the Sun aren’t intense enough to darken the KBO’s outermost surface. Instead, the plasma ions ‘sandblast’ the topmost layer to expose the layer immediately below, with further erosion being produced by dust grains from collisions in the belt. Given enough time, simple chemical reactions producing organic molecules, a kind of radiation ‘cooking’ process mediated by radiation from interstellar space, can then produce the red tint we see on many KBOs.

Ice and Complex Molecules

White Kuiper Belt objects also fit within Cooper’s layer model, which assumes water ices in a deep mantle layer that can erupt onto the surface to leave bright icy patches. “So these may not be dead icy objects,” says Cooper, “they may be volcanically active over billions of years.” Usefully, New Horizons’ pass through this region may yield better surface observations not just of Pluto and Charon but objects beyond that will help us confirm what materials are present. And its readings of the energy distribution and particle count may confirm the factors needed to make Cooper’s model work.

And about astrobiology in the Kuiper Belt?

“When you take the right mix of materials and radiate them, you can produce the most complex species of molecules,” says Cooper. “In some cases you may be able to produce the components of life — not just organic materials, but biological molecules such as amino acids. We’re not saying that life is produced in the Kuiper Belt, but the basic chemistry may start there, as could also happen in similar Kuiper Belt environments elsewhere in the universe and that is a natural path which could lead toward the chemical evolution of life.”