Aftermath of an Asteroid Collision

Back in the days when the Solar System seemed a simpler place, asteroids were thought to be chunks of rocks whose features could be explained by impacts with other such objects. Comets were altogether different, laden with icy material that erupted when heated by the Sun. It was a straightforward picture, at a time when the system had nine planets, icy ‘dwarf planets’ were not yet in vogue, and distinctions between orbiting objects were clearly drawn. Today we work with a more complicated scenario, one in which some objects once thought of as asteroids develop comet-like features that can last for months.

Thus the interest in the asteroid called (596) Scheila, which late last year developed plumes after brightening unexpectedly in December. Orbiting the Sun every five years, Scheila has a diameter of about 110 kilometers, and evidence from both the Hubble Space Telescope and the Swift satellite now indicates that the unusual activity here was caused by a collision with a smaller asteroid. This is useful stuff, because the first asteroid collision identified — in 2009 involving asteroid P/2010 A2 — showed no collision fragments, whereas Scheila’s activity was caught quickly after the collision, when evidence for what happened was readily available.

Image: The Hubble Space Telescope imaged (596) Scheila on Dec. 27, 2010, when the asteroid was about 350 million kilometers away. Scheila is overexposed in this image to reveal the faint dust features. The asteroid is surrounded by a C-shaped cloud of particles and displays a linear dust tail in this visible-light picture acquired by Hubble’s Wide Field Camera 3. Because Hubble tracked the asteroid during the exposure, the star images are trailed. Credit: NASA/ESA/D. Jewitt (UCLA).

The evidence came in the form of images showing the asteroid with two dust plumes, signs of dust particles being pushed away from the object by sunlight. Archival images show the outburst began between November 11 and December 3 of 2010, and it was on December 11 that the Catalina Sky Survey, which focuses primarily on near-Earth objects, discovered that Scheila had brightened to twice the norm and had begun to display a comet-like aura.

But a follow-up spectrum taken with the Swift satellite’s Ultraviolet/Optical Telescope showed none of the emissions commonly associated with comets, and there was no indication of gases that would flag suddenly exposed ice. The current thinking is that a small asteroid struck Scheila at an angle of less than 30 degrees, leaving a 300-meter crater. It was an impact with a wallop, says Michael Kelley (University of Maryland), a co-author of the paper on this work:

“The dust cloud around Scheila could be 10,000 times as massive as the one ejected from comet 9P/Tempel 1 during NASA’s UMD-led Deep Impact mission. Collisions allow us to peek inside comets and asteroids. Ejecta kicked up by Deep Impact contained lots of ice, and the absence of ice in Scheila’s interior shows that it’s entirely unlike comets.”

The impactor would have been traveling about 17,000 kilometers per hour, striking with the force of at least a 100-kiloton nuclear bomb. It is thought that most of the dust from the collision fell back to the asteroid, but the material that escaped formed plumes that, as seen in the Hubble images taken two weeks after the event, had quickly begun to dissipate. A scant two months after the outburst, there was no sign of the plumes at all. It’s likely that collisions of this kind happen frequently in the main belt, offering a cautionary tale for those keeping an eye on asteroids closer to home.

The paper is Bodewits et al., “Collisional Excavation of Asteroid (596) Scheila,” Astrophysical Journal Letters Vol. 733, No. 1 (2011), p. 733 (abstract/preprint). A short video is available from Goddard Space Flight Center.


Tiny Spacecraft Point to Future Sails

Spacecraft no more than an inch square will fly aboard the next (and last) Shuttle flight to the International Space Station. The work of Mason Peck (Cornell University), the micro-satellites weigh in at less than one ten-millionth of the mass of the original Sputnik, yet can accomodate all the systems we associate with a spacecraft — power, propulsion, communications — on a single microchip. We’ve looked at Peck’s work in previous Centauri Dreams essays (see this one on ‘smart dust’), but it’s great to see some of his concepts put into an actual mission plan for testing in Earth orbit.

What Peck has in mind with the spacecraft he calls ‘Sprite’ is ultimately to create a satellite with different flight dynamics from other spacecraft. Sure, we can miniaturize our electronics and create satellites with small form factors — CubeSats come to mind — but Peck’s craft call up a different analogy:

“Their small size allows them to travel like space dust,” said Peck. “Blown by solar winds, they can ‘sail’ to distant locations without fuel. We’re actually trying to create a new capability and build it from the ground up. We want to learn what’s the bare minimum we can design for communication from space.”

Not that the three Sprite chips scheduled for launch tomorrow (April 29) are going to be making any such journeys. They will be mounted on the Materials International Space Station Experiment (MISSE-8) pallet, which will in turn be attached to the ISS. The idea is to expose the chips to space conditions to see how their systems hold up. The MISSE-8 panel will be returned to Earth after a few years, but while they are in space, the three prototypes, built by Cornell students under Peck’s direction, can be tracked individually from their transmissions.

Image: Three prototypes of the chip satellites, named ‘Sprite,’ will be mounted on the International Space Station and are designed to blow in the solar wind and collect data. Credit: Mason Peck / Cornell University.

To see what Peck has in mind for future applications, consider the behavior of dust in our Solar System. We know that for particles at these scales, solar pressure and electrostatic forces are as significant as gravity in producing unusual orbits. Some dust actually leaves the Solar System on an interstellar trajectory, while other particles achieve stable orbits around the Sun, and some particles simply fall to the surface of planets. Peck has been talking for years now about putting the orbital dynamics of extremely small bodies (up to 100 µm in size) to work on tiny spacecraft like Sprite. Such a self-sustaining spacecraft should be able to take advantage of not only photon momentum but the solar wind, unconstrained by the need to carry onboard fuel.

For now, the Sprites being sent to the ISS will have a narrower target, to collect information about the solar wind’s chemistry, radiation and particle impacts on the chips. But they are demonstrations of our ability to pack power, attitude control, communications and more onto a microchip capable of traveling in a non-Keplerian orbit. And what they point to is a different take on the solar sail, one in which it is miniaturized and integrated with onboard spacecraft systems. Such a tiny sail is capable of things that a more conventional design is not. Consider what happens in the presence of a magnetic field, as outlined on this Cornell website:

By artificially charging a spacecraft that is orbiting a planetary magnetic field, we can achieve Lorentz Augmented Orbits (LAO). Here, a spacecraft’s rotating magnetic field transfers energy and momentum to and from a planet via the Lorentz force. LAO offers opportunities that solar-pressure propulsion does not because it requires a magnetic field to operate. This interaction enables energy change maneuvers at outer planets, notably Jupiter with its dense magnetic field, where solar-pressure is too weak to be of much benefit. We show that high charge-to-mass ratios are significantly easier to achieve and maintain at reduced length scales.

And when it comes to reaching the surface of another world, such spacecraft come into their own:

Exogenic dust gently lands on the surface of the Earth while its larger meteorite cousins rapidly ablate in the upper atmosphere. At extremely small length scales, the surface area of the dust can efficiently radiate away the heat generated by aerodynamic friction, even at entry velocities. We seek to use similar geometries and scales to design a passive entry vehicle capable of safely gliding or fluttering down to the surface of neighboring planets.

Peck even considers interstellar possibilities for future generations of Sprites, in which a chain of the tiny spacecraft move away from the Solar System and report data back to Earth through communications relays between each subsequent craft. The tests aboard the ISS are the first chance for the Sprite concept to prove its survivability in space, and Centauri Dreams congratulates Cornell students Zac Manchester, Ryan Zhou and doctoral candidate Justin Atchison on completing the prototypes that will fly aboard Endeavour. Success there could lead to a whole new way of thinking about propellantless propulsion and micro-scale spacecraft.

Among the papers on this work, Atchison and Peck, “A Millimeter-Scale Lorentz-Propelled Spacecraft,” AIAA AIAA Guidance, Navigation and Control Conference and Exhibit (2007) is the place to begin. It’s available online. A Cornell University news release is also available.


Trouble at Hat Creek

What is ‘space situational awareness,’ and what does it have to do with SETI? The answer begins with the collision of a Russian Cosmos 2251 satellite with one of the 66 communications satellites that comprise the Iridium satellite constellation, a worldwide voice and data system. The collision, which occurred on February 10, 2009 produced hundreds of pieces of debris. The Air Force Space Command needs ways of tracking such debris, which poses a threat in the increasingly crowded skies above our planet.

Enter the Allen Telescope Array, known primarily as a state-of-the-art center for the SETI effort to identify other intelligent species in the galaxy. The ATA caught the Air Force’s eye as a way of tracking and cataloging man-made objects in orbit. Located in a volcanic valley near the Lassen National Forest in California, the array has proven its worth at this task in early tests, a fact that could inspire a new funding source for the observatory. And as we learned to our dismay through a recent post by astronomer Franck Marchis (UC-Berkeley), such funding may now turn out to be crucial.

Marchis notes that the Allen Telescope Array is in deep financial trouble. The ATA has been managed by the SETI Institute and the University of California at Berkeley, the latter’s observations having been funded both by the state of California and the National Science Foundation. Funding from these sources is now drying up.

So far the ATA has put $50 million from donors like Microsoft co-founder Paul Allen to work building the existing 42 dishes, with plans calling for the construction of 350 radio antennae in total. What the ATA now lacks is the income for its daily operational costs, with the National Science Foundation’s support now whittled down to one-tenth of its previous level, and cuts from the state of California as well. The situation doesn’t seem insurmountable — Jill Tarter believes $5 million is needed over the next two years to keep SETI alive — but budgets at the Air Force are as tight as anywhere.

The result: The Allen Telescope Array is, at least for now, in hibernation, with several members of the Hat Creek observatory staff having already been laid off. The installation is non-functional, although safe, according to a late April letter sent to donors by SETI Institute CEO Tom Pierson. While we wait to see whether Air Force funding can help the ATA acquire a new role (and, let’s assume, remain available for future SETI efforts), the timing of the closure has many researchers shaking their heads. After all, Kepler is out there targeting Earth-like worlds, setting up a target list for the ATA to search for signals. The SETI Institute has been asking for donations for a campaign to study the 2000 best Kepler candidates.

So can we raise the $5 million needed to put the formidable resources of the ATA onto the top Kepler candidates? The SETI Institute’s donation page is here. The beauty of the ATA installation, even in its current truncated form, is that competition for observing time on other telescopes can be formidable, whereas even with some ATA time farmed out for Air Force purposes, the array would still be able to hunt SETI targets through a remarkable 10 billion channels. A single dollar contribution buys a 4 million channel look at a single Kepler candidate, and as Seth Shostak has noted, a mere three cents added to US tax forms would keep the facility operational.

I want to point out that this is not a SETI issue alone. The ATA has a wide range of astronomical goals embedded within the SETI mantle, including classifying 250,000 extragalactic radio sources as active galactic nuclei or starburst galaxies, and looking for transient signals caused by accretion onto black holes and other perhaps unknown phenomena. Add the one million stars slated for SETI examination and the 4×1010 billion stars of the inner Galactic Plane to be surveyed for powerful, non-natural transmitters and you have a priceless observational package.

We’re fortunate that the scientific opportunities offered by the ATA are so broad — a fact that should attract funding — and that the array already covers the main space communications bands, which is why it has been proposed for downlink purposes for contestants in the Google Lunar X Prize contest. All of this, plus the ATA’s obvious utility for the Air Force Space Command, leaves me hoping there is a way to work through the financial shortfall and keep the ATA functional. But private donors need to step up – now – to boost our chances of getting the Kepler candidates examined for SETI signals with the equipment most suited for the job.

For more about the ATA closure and what you can do, read SETI Institute Tom Pierson’s April 22 letter, from which this quote:

We are continuing discussions with the USAF and remain hopeful that this effort will help provide future operating funds. At the same time, we must strive to find other sources of funding to supplement operations costs and, very importantly, to support SETI science observations. We are preparing a coordinated campaign to ask for help, and you will be hearing more from us about this. The bottom line is that it takes approximately $1.5M/year to operate the ATA, and at least an additional $1M/year to cover the cost of our SETI science efforts. Thus, right now, we are trying to raise $5M to cover a two-year search of the Kepler Worlds by Jill Tarter and her team. Assuming funding can be acquired, we plan to spend the next two years listening to the 1,235 exoplanet candidates that the Kepler mission announced in February. This fabulous opportunity represents a fundamental shift to be able to point our instruments at known planetary systems, rather than at stars that might or might not host planets.


Icarus: Fusion and Secondary Technologies

Discovery News now offers fully ten articles on Project Icarus and its background, written by the Icarus team and assembled on the site by Ian O’Neill. I was startled to realize how the list had grown, but it reminds me to point periodically to this collection, because Icarus — the attempt to re-design the Project Daedalus starship study of the 1970s — is a work very much in progress. Icarus is a joint project of the Tau Zero Foundation and the British Interplanetary Society. The latest article on the team’s work is by physicist Andreas Tziolas, who in addition to being a frequent Centauri Dreams contributor is also secondary propulsion lead for the Icarus effort.

It’s no surprise that the biggest issue surrounding an interstellar probe is the propulsion system, which for Icarus means fusion, a method offering as much as a million times better performance than our current chemical rocket technologies, if we can ever figure out just how to harness it. The Icarus team chose fusion deliberately and with full knowledge that alternatives were out there. Fusion, after all, was the propulsion method of choice for the Daedalus designers, and a major part of the Icarus effort has been to take an existing design and look at it in today’s terms. Have a look at Using Fusion to Propel an Interstellar Probe for more on this design choice.

Breakthroughs vs Evolution

We’d all like to see the kind of sudden breakthrough that would in a single stroke put deep space in range, and perhaps one day something like this will happen. But the gap between what we can do with today’s technology and what we’d like to achieve with such a breakthrough is a daunting one, wide enough that it tends to shut down active research. What Icarus is all about is the idea that we need to do the research anyway, that a series of designs can get better over time, and that each time we run the new iteration, we learn more about where we need to go.

Tziolas’ latest contribution to the Icarus story is his take on secondary propulsion systems, technologies that would come into play both as the Icarus craft is built and when it arrives in its destination solar system. And as he notes, what is considered secondary for the Icarus mission is essentially what is primary for all contemporary space missions, a fact that highlights how far we have to go to mount a true interstellar effort. Solar sails, for example, might be the technology to use when a craft like Icarus deploys small probes to study its target system. And sails are one among a host of options that might help in spacecraft construction:

…solar sails may not be a good option for accelerating what would be a very massive Icarus to another star, but offer unique advantages for exploring the target solar system with probes. The advances in ion (NASA’s NEXT and NSTAR) and electromagnetic thrusters (VASIMR) in recent years seem to indicate a path towards some new and exciting propulsion technologies being developed. Some of these systems might be used on the Icarus interstellar spacecraft itself, and others may play supporting roles. For example, the technologies necessary for the spacecraft construction and fuel gathering stages of the mission.

We know where chemical rockets are at their most useful, and that is in scenarios demanding high thrust. Building massive and complex objects in space demands a much improved way of getting to low-Earth orbit, almost certainly a vehicle that can not only make frequent flights, but operate as a single-stage-to-orbit craft. Thus Tziolas’ interest in Skylon, an unpiloted craft from the British firm Reaction Engines Ltd. that is envisioned as operating in a fleet of vehicles equipped with air-breathing jet engines to assist them in reaching orbit in a single stage.

Image: The unpiloted Skylon is one possibility for getting frequent payloads to low-Earth orbit. Credit: Adrian Mann.

SpaceX’s plans for a Falcon Heavy, based on the company’s existing Falcon 9 booster but capable of launching 117,000 pounds into orbit (that’s twice the capacity of the Space Shuttle) point in the same direction. The original Daedalus design was for a craft weighing some 50,000 tons, almost all of which were comprised of the Helium-3 essential for its fusion engines. Before any interstellar craft can get built, we have to have the kind of space-based infrastructure that will allow us to move within the Solar System (Daedalus envisioned mining a gas giant’s atmosphere for fuel — Icarus has not yet made a decision on a fuel source). And the ability to launch heavy loads frequently is the biggest piece missing in the infrastructure puzzle.

Fusing Futuristic Ideas

The effort to design a craft that is well beyond our current capabilities leads us to re-examine and perhaps combine developing technologies. We’ve begun to fly solar sail demonstrator missions, and we have begun to consider whether the solar wind could drive a large magnetic sail, a device already known as a magsail. Beyond that, we have laboratory work suggesting that beamed propulsion using laser or charged particles for acceleration is possible. Tziolas wonders how we might draw from such technologies to assist a future interstellar probe:

Combining these two ideas may lead to an advanced space tug, responsible for pushing spacecraft equipped with magsails into their orbits. Perhaps the Icarus could serve as the beam generator which pushes its planetary explorer probes into place. Alternatively Icarus could use an extremely large magnetic sail to help decelerate, once within reach of the target star’s solar wind or magnetosphere.

Such musings offer a futuristic roadmap of ideas to examine as we ponder propulsion methods that are interstellar in implication and remain within the bounds of known physics. We need to push current technologies hard. Icarus project leader Richard Obousy made the case for fusion on Seth Shostak’s radio show recently, where he noted ‘To use rocket fuel to reach the stars in a time-frame consistent with a human lifetime is completely inconceivable.” It’s a good interview (starting at 41 minutes 35 seconds into the main show), one in which Seth probes Richard for details about the new paradigm Icarus will need to surmount chemical rocket limitations.


Saturn Aurora Offers Clues to Enceladus

Last week we looked at the possibility of using a planet’s aurora as an exoplanet detection tool, speculating that the LOFAR radio telescope in Europe might be able to detect such an emission, and I reminisced about listening for emissions from Jupiter on an old shortwave receiver. Jonathan Nichols’ work at the University of Leicester makes the case for exoplanet detections, and recent news from analysis of Cassini data indicates that planetary aurorae can do more than just flag the presence of a planet. In some cases, they can provide information about that planet’s moons, as in the case of Saturn, where careful analysis may offer us new insights into Enceladus.

For it turns out that the electrical connection between Saturn and Enceladus is rather robust. Moreover, it was a connection that scientists had anticipated, given that in the Jovian system, Io creates a glowing auroral footprint near Jupiter’s north and south poles. Why not expect something similar in the case of Enceladus, even if it’s 240,000 kilometers from Saturn? Even so, the search for an auroral footprint had come up dry until new analysis of Cassini data collected in 2008. Now we have images and audio highlighting the interactions between planet and moon. First the audio:

The eerie sounds accompanying the above video (credit: NASA/JPL) are the result of electrons moving along the magnetic field lines between a glowing region of ultraviolet light on Saturn, and Enceladus. The sound has been amplified to make it audible to the human ear. Look below, where you’ll see that an auroral ‘patch’ appears at the end of the magnetic field line connecting planet and moon, near Saturn’s north pole, where energetic electrons plunge into Saturn’s atmosphere. The region measures 1200 by 400 kilometers, and it’s a faint catch, far less bright than the planet’s polar auroral rings.

Image: The two images shown here were obtained by Cassini’s ultraviolet imaging spectrograph on Aug. 26, 2008, separated by 80 minutes. The footprint moved according to changes in the position of Enceladus. In the image, the colors represent how bright the extreme ultraviolet emissions are. The lowest emission areas (one to two extreme ultraviolet counts per pixel) are in black/blue. The brightest emission areas (500 to 1,000 extreme ultraviolet counts per pixel) are in yellow/white. The footprint appeared at about 65 degrees north latitude. It measured about 1,200 kilometers (750 miles) in the longitude direction and less than 400 kilometers (250 miles) in latitude, covering an area comparable to that of California or Sweden. Credit: NASA/JPL/University of Colorado/Central Arizona College.

The discovery of a beam of energetic protons near Enceladus, aligned with the magnetic field, goes back to 2008, and further work in 2009 detected the hissing sound of the magnetic connection. Where this gets truly ingenious is in the way these data can be used to ferret out information about Enceladus itself. Because the moon is active, with jets that spray water vapor and organic particles into space, the water cloud above its jets produces an ionized plasma cloud as it interacts with the magnetic field around Saturn. The magnetic field lines are disturbed by this interaction in a way that suggests the geyser discharges from Enceladus are variable.

Abigal Rymer (JHU/APL) is one of the lead authors of the paper on this work:

“The new data are adding fuel to the fire of some long-standing debates about this active little moon. Scientists have been wondering whether the venting rate is variable, and these new data suggest that it is.”

Thus we learn more about the complex interactions between a planet and its moons, information that may one day help us untangle the extremely faint auroral signatures we pick up from exoplanetary systems. The paper is Pryor et al., “The auroral footprint of Enceladus on Saturn,” Nature 472 (21 April 2011), pp. 331–333 (abstract).