The Case for a Probe

Back in May I [looked][1] at Jean Schneider’s thoughts on what we might do if we discovered a planet in the habitable zone of a nearby star. In an article for _Astrobiology_ called “The Far Future of Exoplanet Direct Characterization,” Schneider (Paris Observatory) reviewed technologies for getting a direct image of an Earth-like planet and went on to discuss how hard it would be to get actual instrumentation into another solar system. His thoughts resonate given recent findings about Gliese 581g (although the [latest data][2] from the HARPS spectrograph evidently show no sign of the planet, a startling development as we investigate this intriguing system).

Whether or not Gl 581g exists and is where we think it is, Schneider’s pessimism about getting an actual payload into another solar system has attracted the attention of Ian Crawford (University of London), who is quick to point out that astronomical remote-sensing, especially for biological follow-up studies of initial biomarker detections, will be inadequate. As we have done with nearby objects like Mars, we will eventually need to send instruments into these systems to study everything from basic biochemistry to evolutionary history there.

**Slower Speeds, Improved Technologies**

But are such missions possible? Schneider and colleagues assumed 0.3_c_ as a typical velocity for interstellar missions and went on to discuss the huge difficulties in accelerating a payload to such values. But Crawford is skeptical. He sees 0.3_c_ as an overestimate, and reminds us that the better developed proposals in the literature tend to focus on values around 0.1_c_ — as he notes, “0.3_c_ will be an order of magnitude more difficult owing [to] the scaling of kinetic energy with the square of the velocity.” Schneider’s 0.3_c_ is, in Crawford’s view, an arbitrary over-estimate of the speed required.

The original Daedalus study is the most detailed engineering assessment yet available, a fusion-based craft that would require fully 50,000 tons of nuclear fuel and attain 12 percent of lightspeed. Daedalus, in other words, is well beyond our capabilities. But Crawford’s point is to remind us that we’re learning more as we go, and that premature pessimism may overlook useful technological advances. On fusion, then:

> …technical advances in a number of fields have occurred which may make fusion-powered vehicles of the Daedalus-type more practical as long-term solutions to the problem of interstellar travel. These include developments in miniaturization and nanotechnology, which would ensure that a much less massive payload would be required than was then assumed, and developments in inertial confinement fusion research for power generation on Earth. Indeed, the National Ignition Facility recently commissioned at the Lawrence Livermore National Laboratory in California ( is, albeit unintentionally, building up technical competencies directly relevant to the development of fusion-based space propulsion systems. For these reasons, there is a strong case for a reassessment of the Daedalus concept in light of updated scientific and technical knowledge, and at least one such study is currently underway…

**Surviving Interstellar Dust**

That study, of course, is [Project Icarus][3], in which Crawford is an active player. But any vehicle, whether fusion based or using more exotic concepts like antimatter or laser-pushed lightsails, runs into the interstellar dust problem. Drop the speed from 0.3_c_ to 0.1_c_ and the issue is partially mitigated. Dust was a showstopper for Schneider, but Crawford notes that assuming an interstellar dust density of 6.2×10-24 kg m-3, erosion at 0.1_c_ would erode on the order of 5 kg m-2 of shielding material over a six light year flight. So that while we are certainly adding to the mass of the probe with shielding material like beryllium, we have not ruled out the mission.

One of the problems with this discussion is that we know so little about the upper bounds of the size distribution of interstellar dust particles. Crawford sifts through the literature on the subject, discussing the effect of impacts of 100-?m grains, which could be as high as two impacts per square meter over the course of a six light year flight. Collecting the needed data on the interstellar medium will be a priority before we can seriously think about launching such a probe.

And if a spacecraft were large enough, Crawford adds, various methods for sensing potential danger could be employed, using radar, for example, to detect incoming grains and laser or electromagnetic methods to destroy or deflect them before impact. The Daedalus study examined the idea of using a fine cloud of small dust particles ejected from the vehicle that would destroy any incoming large grains before they reached the primary vehicle. The latter approach was conceived for a probe entering a denser interplanetary environment at destination, but there is no reason such measures couldn’t be deployed throughout interstellar cruise.

**An Astrobiologically Driven Probe**

In his conviction that interstellar flight is difficult but not impossible, Crawford echoes Robert Forward:

> Journeys to the nearer stars with travel-times of decades (necessitating velocities of the order of ten percent of the speed of light) will be a considerable technological (as well as economic and political) undertaking. The magnitude of the difficulties should not be underestimated, but neither should they be exaggerated. There is a large technical literature…, which demonstrates that rapid interstellar space travel is not physically impossible and is a legitimate technological goal for the centuries ahead. Ultimately, the development of this capability may be the only way to follow-up any detections of biosignatures that may be made in the atmospheres of Earth-like planets orbiting nearby stars in the coming decades…

A primary driver for interstellar flight is likely to be astrobiology. As we develop a mature space exploration infrastructure in our own Solar System to explore life’s possibilities from Mars to Enceladus to the Kuiper Belt, we will also be creating the necessary technologies that will take us further out. Astronomical observations can only tell us so much, leaving us with the need to get an orbiter or a lander to places like Titan or Europa, and giving us the longer term model of direct probes returning data from astrobiologically interesting planets around other stars.

The paper is Crawford, “A Comment on ‘The Far Future of Exoplanet Direct Characterization’ — the Case for Interstellar Space Probes.” Accepted by _Astrobiology_ ([preprint][4]). Jean Schneider has responded to Crawford’s comments and I’ll look at what he has to say tomorrow.


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