I was saddened to learn of the recent death of James Early, author of a key paper on interstellar sail missions and a frequent attendee at IRG events (or TVIW, as the organization was known when I first met him). Jim passed away on April 28 in Saint George, UT at the age of 80, a well-liked figure in the interstellar community and a fine scientist. I wish I had known him better. I ran into him for the first time in a slightly awkward way, which Jim, ever the gentleman, quickly made light of. What happened was this. In 2012 I was researching damage that an interstellar sail mission might experience in the boost phase of its journey. Somewhere I had seen what I recall as a color image in a magazine (OMNI?) showing a battered, torn sail docked in what looked to be a repair facility at the end of an interstellar crossing. It raised the obvious question: If we did get a sail up to, say, 5% of the speed of light, wouldn’t even the tiniest particles along the way create significant damage to the structure? The image was telling and to this day I haven’t found its source. I think of the image as ‘lightsail on arrival,’ and if this triggers a memory with anyone, please let me know. Anyway, although our paths crossed at the first 100 Year Starship symposium in Orlando in 2011, I didn’t know Jim’s work and didn’t realize he had analyzed the sail damage question extensively. When I wrote about the matter on Centauri Dreams a year later, he popped up in the comments:

I presented a very low mass solution to the dust problem at the 100 Year Starship Symposium in a talk titled “Dust Grain Damage to Interstellar Vehicles and Lightsails”. An earlier published paper contains most of the important physics: Early, J.T., and London, R.A., “Dust Grain Damage to Interstellar Laser-Pushed Lightsail”, Journal of Spacecraft and Rockets, July-Aug. 2000, Vol. 37, No. 4, pp. 526-531.

I was caught by surprise by the reference. How did I miss it? Researching my 2005 Centauri Dreams book, I had been through the literature backwards and forwards, and JSR was one of the journals I combed for deep space papers. Later, at a TVIW meeting in Oak Ridge, we talked, had dinner and Jim kidded me about my research methods. As I saw it, his paper was a major contribution, and I should have known about it. Yesterday I asked Andrew Higgins (McGill University) about the paper and he had this to say in an email:

Jim Early’s paper (written with Richard London in 1999) on dust grain impacts addressed one of the bogeys of interstellar flight: The dust grain impact problem when traveling at relativistic speeds. Their analysis showed—counterintuitively—that the damage caused by a dust grain on an interstellar lightsail actually decreases as the sail exceeds a few percent of the speed of light. While the grain turns into an expanding fireball of plasma as it passes through the sail, the amount of thermal radiation deposited on the sail decreases as the fireball is receding more quickly from the sail. This was a welcome result suggesting sails might survive the interstellar transit, and their study remains the seminal work on dust grain interactions with thin structures at relativistic speeds.

Image: Dinner after the first day’s last plenary session in Oak Ridge in 2014. That’s Jim Benford at far left, then James Early, Sandy Montgomery and Michael Lynch. The family has set up a website honoring Jim and offering photos and an obituary. He got his bachelor’s degree in Aeronautics at MIT, following it with a master’s degree in mechanical engineering at Caltech, and a PhD in aeronautics and physics at Stanford University. He was involved with development activities for the Delta launch vehicle while obtaining his bachelor’s degree by working at NASA Goddard Space Flight Center in the summers and then at McDonnell-Douglas after finishing his master’s degree. He joined Lockheed and Hughes aircraft for a time before finally ending up at the Lawrence Livermore National Laboratory working on laser physics until he retired.

Sail in Flight

So let’s look at Jim’s paper on sails, a subject he continued to work on for the next two decades. Although Robert Forward came up with sail ideas that pushed as high as 30 percent of the speed of light (and in the case of Starwisp, even higher), Jim and his co-author Richard London chose 0.1 c for cruise velocity in their paper, which provides technical challenges aplenty but at least diminishes the enormous energy costs of still faster missions, and certainly mitigates the problem of damage from dust and gas along the way. Depending on the methods used, the sail as analyzed in this paper may take a tenth of a light year to get up to cruise velocity. It’s worth mentioning that the sail does not have to remain deployed during cruise itself, but deceleration at the target star, depending on the methods used, may demand redeployment. Breakthrough Starshot envisions stowing the sail in cruise after its sudden acceleration to 20 percent of c. Early and London use beryllium sails as their reference point, these being the best characterized design at this stage of sail study, and assume a sail 20 nm thick. In terms of the interstellar medium the sail will encounter, the authors say this:

Local interstellar dust properties can be estimated from dust impact rates on spacecraft in the outer solar system and by dust interaction with starlight. The mean particle masses seen by the Galileo and Ulysses spacecraft were 2×10^-12 and 1×10^-12g, respectively. A 10^-12g dust grain has a diameter of approximately 1 µm. The median grain size is smaller because the mean is dominated by larger grains. The Ulysses saw a mass density of 7.5×10^-27g cm^-3. A sail accelerating over a distance of 0.1 light years would encounter 700 dust grains/cm² at this density. The surface of any vehicle that traveled 10 light years would encounter 700 dust grains/mm². If a significant fraction of the particle energy is deposited in the impacted surface in either case, the result would be catastrophic.

The question then becomes, just how much of the particle’s energy will be deposited on the sail? The unknowns are all too obvious, but the paper adds that neither of the Voyagers saw dust grains larger than 1 ?m at distances beyond 50 AU, while a 1999 study on interstellar dust grain distributions found a flat distribution from 10^-14 to 10^-12 g with some grains as large as 10^-11 g. Noting that a 10^-12 dust grain has a diameter of about 1-?m, the authors use a 1-?m diameter grain for their impact calculations. The results are intriguing because they show little damage to the sail. Catastrophe averted:

At the high velocities of interstellar travel, dust grains and atoms of interstellar gas will pass through thin foils with very little loss of energy. There will be negligible damage from collisions between the nuclei of atoms. In the case of dust particles, most of the damage will be due to heating of the electrons in the thin foil. Even this damage will be limited to an area approximately the size of the dust particle due to the extremely fast, one-dimensional ambipolar diffusion explosion of the heated section of the foil. The total fraction of the sail surface damaged by dust collisions will be negligible.

The torn and battered lightsail in its dock, as seen in my remembered illustration, may then be unlikely, depending on cruise speed and, of course, on the local medium it passes through. Sail materials also turn out to offer excellent shielding for the critical payload behind the sail:

Interstellar vehicles require protection from impacts by dust and interstellar gas on the deep structures of the vehicle. The deployment of a thin foil in front of the vehicle provides a low mass, effective system for conversion of dust grains or neutral gas atoms into free electrons and ions. These charged particles can then be easily deflected away from the vehicle with electrostatic shields.

And because the topic has come up in a number of past discussions here, let me add this bit about interstellar gas and its effects on the lightsail:

The mass density of interstellar gas is approximately one hundred times that of interstellar dust particles though this ratio varies significantly in different regions of space. The impact of this gas on interstellar vehicles can cause local material damage and generate more penetrating photon radiation. If this gas is ionized, it can be easily deflected before it strikes the vehicle’s surface. Any neutral atom striking even the thin foil discussed in this paper will pass through the foil and emerge as an ion and free electron. Electrostatic or magnetic shields can then deflect these charged particles away from the vehicle.

Consequences for Sail Design

All of these findings have a bearing on the kind of sail we use. The thin beryllium sail appears effective as a shield for the payload, with a high melting point and, the authors conclude, the ability to be increased in thickness if necessary without increasing the area damaged by dust grains. Ultra-thin foils of tantalum or niobium offer higher temperature possibilities, allowing us to increase the laser power applied to the sail and thus the acceleration. But Early and London believe that the higher atomic mass of these sails would make them more susceptible to damage. Even so, “…the level of damage to thin laser lightsails appears to be quite small; therefore the design of these sails should not be strongly influenced by dust collision concerns.” Dielectric sails would be more problematic, suffering more damage from heated dust grains because of their greater thickness, and the authors argue that these sail materials need to be subjected to a more complete analysis of the blast wave dynamics they will experience. All in all, though, velocities of 0.1 c yield little damage to a thin beryllium sail, and thin shields of similar materials can ionize dust as well as neutral interstellar gas atoms, allowing the ions to be deflected and the interstellar vehicle protected. These are encouraging results, but the size of the problem is daunting, and given the apparent cost of the classically conceived interstellar probe, the prospect of impact damage calls for continued analysis of the medium through which the probe would pass. This is one of the advantages of sending not one large craft but a multitude of smaller ‘chipsat’ style vehicles in the Breakthrough Starshot model. Send enough of these and you can afford to lose a certain percentage along the way. I can only wish I could sit down with Jim Early again to kick around chipsat concepts, but what a fine memorial to know that your paper continues to influence evolving interstellar ideas. The paper is Early, J.T., and London, R.A., “Dust Grain Damage to Interstellar Laser-Pushed Lightsail,” Journal of Spacecraft and Rockets, July-Aug. 2000, Vol. 37, No. 4, pp. 526-531.