Pulsars for Deep Space Navigation

Getting an interstellar probe to its target involves navigation of a high order. ‘Marker’ stars — stars that are both bright and distant enough to have relatively fixed positions for the duration of the journey — often show up in the scientific literature. Thus Rigel and Antares, both of which are far larger than the Sun, are attractive markers. Rigel (Beta Orionis) is some 800 light years distant, while Antares (Alpha Scorpii) is over 500 light years away. Are there other, better kinds of markers?

Perhaps so, according to the European Space agency. ESA, through its Ariadna initiative, is homing in on using pulsars for navigation. Ariadna operates under ESA’s Advanced Concepts Team to study new space technologies through linkages with the European academic community; it’s a way to strengthen the agency’s ties with independent researchers. As in the US, creating such connections is tricky business, but Ariadna is already doing interesting work, as its new study on pulsars suggests.

To be precise, the study, called “Feasibility study for a spacecraft navigation system relying on pulsar timing information,” relies on so-called ‘millisecond pulsars,’ those that spin at high rates of more than 40 revolutions per second. The beauty of millisecond pulsars is that they are old and quite regular in their rotation, so much so that their pulses can be used as extraordinarily accurate timing mechanisms. The fastest known millisecond pulsar is PSR 1937+211, which completes 642 revolutions every second.

Image: An artist’s impression of the millisecond pulsar PSR J1740-5340 (spinning at 274 times per second) and its companion star. The absorption of matter from an elderly red giant seems to have revitalized the spin rate of the pulsar, while its companion is being emptied of matter and turned into a white dwarf. Credit: European Space Agency.

We need to use marker objects because Earth-based navigation gets problematic for deep space probes. For one thing, when the Sun moves between Earth and the probe, no communication is possible. For another, navigation from Earth is feasible only along the line of sight to the craft. The kind of autonomous navigation systems that could avoid these issues would require a precisely understood natural reference frame, one that can be accomplished by millisecond pulsars. Autonomous systems developed to use such markers will be what we use on our first robotic probes to other star systems.

The trick, as the Ariadna report makes clear, will be to balance spacecraft mass and power constraints against the navigation hardware that will be developed. Pulsar signals may have an exquisite sense of timing, but they’re also very faint compared to conventional radio tracking signals. The study discusses suitable pulsar sources and derives the minimal hardware configurations that can be used to create such a positioning system. A possible catch: using several pulsar sources with different antennae seems necessary for the best accuracy, but this requires heavy hardware.

J. Sala, A. Urruela, X. Villares, et al., “Feasibility study for a spacecraft navigation system relying on pulsar timing information” has been available on the Net but seems to have been taken down. Until it’s back online, a mission analysis study of pulsar navigation (with link to the online file) can be found here. A backgrounder on Ariadna from ESA’s Advanced Concepts Team is also available. We’ll have much more to say about ACT and other advanced work there in coming days.

Enormous Outburst Seen by Deep Impact

All the observing time on comet Tempel 1 as the Deep Impact spacecraft approaches is really paying off. Scrutinized by ground and space-based telescopes, the comet was seen to emit a small outburst of materials on June 14. Now Deep Impact has seen a much more massive ejection of ice and other particles that occurred on June 22. Although six times larger than the earlier one, the new outburst dissipated quickly, within about half a day.

Intriguingly, the spectrometer aboard Deep Impact showed that the amount of water vapor in the coma doubled during this event, while the amount of other gases, including carbon dioxide, increased even more. From a University of Maryland news release, quoting Michael A’Hearn, who leads the Deep Impact mission:

“Outbursts such as this may be a very common phenomenon on many comets, but they are rarely observed in sufficient detail to understand them because it is normally so difficult to obtain enough time on telescopes to discover such phenomena,” said A’Hearn. “We likely would have missed this exciting event, except that we are now getting almost continuous coverage of the comet with the spacecraft’s imaging and spectroscopy instruments.”

Tempel 1 Outburst

And the wonderfully named Jessica Sunshine, who works on the project for Science Applications International Corporation, noted that seeing significant activity twice in one week suggests that outbursts are common as comets heat up during their approach toward the inner Solar System. At this point, Tempel 1 is near perihelion, the point in its orbit at which it is closest to the Sun.

Image: Raw images taken by the medium resolution imager on the Deep Impact spacecraft. The images were acquired between June 22 and June 24, 2005. A brightening by a factor of about 5 and a rapid decay to baseline brightness were observed on June 22. As the comet moves through space, background stars pass in and out of the field of view. Cosmic rays hitting the spacecraft’s detector give an appearance of flickering. This is an artifact of space cameras that can be removed. Credit: NASA/JPL-Caltech/UMD.

Centauri Dreams‘ take: This mission is already producing superb science, with the spectrometer providing a solid analysis of the material these outbursts have ejected even at this distance. The prognosis for a successful encounter on July 4 is promising indeed. The crater that results from the collision of Deep Impact’s 820 pound impactor is expected to span hundreds of feet, with an accompanying ejection of ice, dust and gas that will reveal the primordial materials beneath. All told, we should know much more about the early Solar System after the Deep Impact collision.

And note what A’Hearn says about observing time above. While the improvements to ground-based telescopes have been extraordinary, observing rare phenomena with them is always subject to the intense demand for telescope time. There is no substitute for a spacecraft’s instruments on the scene when it comes to continuous observation.

About the spectrometer: Deep Impact’s instrument measures not visible but infrared light. But the principle is the same: take light that is scattered by materials like the dust and gas of the comet and break it into its spectrum. Doing this, you can determine the composition of the materials being ejected. The ‘flyby’ spacecraft will use medium and high resolution imagers along with the spectrometer to return data from the event.

A movie of the recent cometary outburst can be found here. The University of Maryland conducts the overall Deep Impact mission, with mission operations handled at the Jet Propulsion Laboratory.

Planet Formation Around TW Hydrae

If you want to see planet building happening before your eyes, turn your attention to TW Hydrae. Located 180 light years from the Sun in the constellation Hydra (the water snake), TW Hydrae is ten million years old, a celestial infant, with a mass four-fifths that of the Sun. Now researchers have discovered that the protoplanetary disk surrounding it contains more than enough material to form at least one and probably more Jupiter sized planets.

Artist\'s conception of TW HydraeBehind the new study are David Wilner (Harvard-Smithsonian Center for Astrophysics) and colleagues, whose work was just published in the June 20, 2005, issue of The Astrophysical Journal Letters. The team has shown that a vast swarm of pebbles extending out a billion miles from the star is in the early stages of planet formation. The small objects, according to current models, should grow in size as they continue to collide and eventually form planets. “We’re seeing planet building happening right before our eyes,” said Wilner. “The foundation has been laid and now the building materials are coming together to make a new solar system.”

Image: The star system TW Hydrae, shown here in an artist’s conception, possesses a protoplanetary disk holding vast numbers of pebble-sized rocky chunks. Those pebbles eventually should grow to become full-sized planets. Credit: Bill Saxton (NRAO/AUI/NSF).

To measure the size of the disk, the scientists used the National Science Foundation’s Very Large Array to measure radio emissions from TW Hydrae. The radiation showed a cold, extended dust disk filled with centimeter-sized pebbles. Dusty disks like this one emit radio waves with wavelengths similar to the size of the particles in the disk, an unusually useful fact that in this case was amplified not only by the relative closeness of the system, but also by the current stage of the young star’s evolution, which allows the particle size/wavelength relationship to be readily observable.

But is there already a planet in the TW Hydrae system? A colleague of Wilner’s at the CfA, Nuria Calvet, has created a computer simulation of the disk, demonstrating a gap from the surface of the star out roughly 400 million miles, about the distance between the Sun and the asteroid belt in our own Solar System. Chances are the gap is the result of a giant planet pulling in nearby material, though the planet itself has yet to be observed.

The paper is Wilner, D’Alessio, Calvet, et al., “Toward Planetesimals in the Disk around TW Hydrae: 3.5 Centimeter Dust Emission,” The Astrophysical Journal, 626:L109-L112, (June 20, 2005). An abstract is available here.

Hubble Sees Activity on Comet Tempel-1

Here’s the Deep Impact target, as seen by the Hubble Space Telescope in a dramatic set of images that show a jet of dust blowing away from the comet’s nucleus. At the time the photos were taken — seven hours apart on June 14 — Hubble was 120 million kilometers away; the images come from the space observatory’s Advanced Camera for Surveys’ High Resolution Camera. Tempel 1, it is hoped, will provide an even more spectacular show when Deep Impact reaches it on July 4, releasing an 820 lb. copper impactor that will slam into the comet.

A Jet Leaves Tempel-1The image on the left shows the comet before the new jet formed. In the center of the image, the bright dot is the reflection of light off the comet’s nucleus, which is too small at these distances for Hubble to resolve. The nucleus is thought to be about 14 kilometers wide and 4 kilometers long, about as hard to see, according to an ESA press release, “…as someone trying to spot a potato in Stockholm from Madrid.”

At the right, a bright area in the shape of a fan shows the new jet. It extends outward roughly 2,200 kilometers. The reason for such outbursts is poorly understood, but may involve increased heat as the comet approaches the Sun, causing cracks in the surface of the nucleus, which would allow trapped gas and dust to spew out. Another possibility is that the pressure of heated gas beneath the surface has lifted a part of the nucleus, causing it to crumble into small dust particles.

It was the American astronomer Fred Whipple (1906-2004) who proposed that comets are ‘dirty snowballs’ of rock and ice left over from the formation of the Solar System. Whipple viewed the cometary nucleus as a mixture of dark organic material, rocky grains and water ice. Bear in mind that the word ‘organic’ when used in this context means that the compound is based on carbon and hydrogen — it does not imply a biological origin. Remarkably, comets are about 50 percent water by weight. It is possible that the carbon and water now found on Earth was originally delivered here by cometary bombardment from the outer Solar System.

Whipple’s classic 1950 paper is “A Comet Model. I. The Acceleration of Comet Encke,” Astrophysical Journal 111: 375, 1950. After proposing that comet Encke was made of a conglomeration of ices — water, carbon dioxide, carbon monoxide, methane, and perhaps ammonia — Whipple would see forty years pass before observations of comet Halley proved that his ‘dirty snowball’ theory was correct.

Image credit: NASA, ESA, P. Feldman (Johns Hopkins University), and H. Weaver (Applied Physics Lab).

Remembering ‘Gateway to Strangeness’

One of the earliest appearances of solar sails in the American science fiction magazines was Jack Vance’s “Gateway to Strangeness.” Appearing in the August, 1962 issue of Amazing Stories (two years after Cordwainer Smith’s solar sail story, “The Lady Who Sailed the Soul,” ran in Galaxy), the oddly named tale is actually an account of a young crew being put through its training aboard a solar sail-powered spacecraft. Invariably, they run into trouble, and are forced to find a way out of their life-threatening dilemma by the hard-as-nails Henry Belt, a space veteran who just might be on his last mission.

Cover illustration for \'Gateway to Strangeness\'The story later appeared with a title more suited to its content — “Sail 25” — in Vance’s Dust of Far Suns (1964) and in a number of later anthologies, including The Best of Jack Vance (1976). Here’s a snippet, recounting the crew’s work in getting their ship ready for its mission by setting up and securing the huge sail:

“Around the hull swung the gleaming hoop, and now the carrier brought up the sail, a great roll of darkly shining stuff. When unfolded and unrolled, and unfolded many times more it became a tough gleaming film, flimsy as gold leaf. Unfolded to its fullest extent it was a shimmering disk, already rippling and bulging to the light of the sun. The cadets fitted the film to the hoop, stretched it taut as a drum-head, cemented it in place. Now the sail must carefully be held edge on to the sun, or it would quickly move away, under a thrust of about a hundred pounds.

“From the rim braided-iron threads were led to a ring at the back of the parabolic reflector, dwarfing this as the reflector dwarfed the hull, and now the sail was ready to move.

“The carrier brought up a final cargo: water, food, spare parts, a new magazine for the microfilm viewer, mail. Then Henry Belt said, ‘Make sail.'”

Would that solar sail deployment turned out to be this straightforward! I remember reading “Gateway to Strangeness” when it came out, and it’s from my still-preserved 1962 copy of Amazing that the quote comes. So little science fiction had been written about solar sailing in those days. There was the Smith story, a classic, and the 1951 article by Carl Wiley in Astounding that laid down the basics in a venue other SF writers might follow. But soon there would be Arthur Clarke’s “The Wind from the Sun” (1964) and Poul Anderson’s “Sunjammer” from the same year (Clarke’s story, confusingly, was originally called “Sunjammer” as well).

What a pleasure it is to recall how the idea of this breakthrough propulsion method gradually established itself in both fiction and theoretical studies. Just before the Vance story ran, Robert Forward pondered the possibilities of beamed propulsion in the journal Missiles and Rockets, an article that was reprinted in December, 1962 in Galaxy. In the scientific literature, the most recent examination of the technology had been Richard Garwin’s “Solar Sailing: A Practical Method of Propulsion within the Solar System,” which ran in 1958 in the journal Jet Propulsion. The idea has been around for a long time, and it’s helpful to remember in the aftermath of the Cosmos 1 failure that great concepts find their way, even if it takes decades to make them happen.