Nov 08 2021

Hypervelocity Dust Impacts

Space is an incredibly hostile environment, and we are learning more about the challenges of living and traveling in space the more we study it. Apart from the obvious near vacuum and near absolute zero temperatures, space is full of harmful radiation. We live comfortably beneath a blanket of protective atmosphere and a magnetic shield, but in space we are exposed.

Traveling through space adds another element – not only would radiation be passing through us, the faster our ship is traveling the more stuff we would be plowing through. Space is not empty, it is full of gas and dust. In our own solar system, most of the dust is confined to the plane of the ecliptic, in what’s called the zodiacal cloud. But of course, if we are traveling from one planet to another, that would be the plane we are traveling in. At interplanetary velocities, assuming we want to get to our destination quickly (which we do, to minimize exposure to all that radiation) our craft would be plowing through the zodiacal cloud.

We now have some measurements from The Parker Solar Probe regarding the effects of impacts with dust at high velocity. The Parker probe is the fastest human object at 180 kilometers per second. It is also the closest probe ever to the Sun and the one able to operate at the highest temperature. To accomplish this it must keep its heat shield oriented toward the sun. Meanwhile it is encountering thousands of dust particles, tiny grains between 2 and 20 microns in diameter (less than that standard measure of all things tiny, the width of a human hair). We now have data from the probe about the effect of these impacts. Dust grains are striking the probe at hypervelocity, greater than 10,800 km per hour. When they hit they are instantly heated and vaporized, along with a small portion of the surface of the probe. The resulting cloud of debris is also hot enough to become ionized, turning into a plasma. Smaller grains are entirely vaporized in less than a thousandth of a second. Larger grains also give off a cloud of debris that expands away from the craft.

The authors report that the effect of this is:

Some of the impactors encountered by Parker Solar Probe are relatively large, resulting in plasma plumes dense enough to (i) refract natural plasma waves away from the spacecraft, (ii) produce transient magnetic signatures, (iii) and drive plasma waves during plume expansion.  Further, some impacts liberate clouds of macroscopic spacecraft material which can result in electrostatic disturbances near the spacecraft that can linger for up to a minute, which is ~10,000 times longer than the transient plasma plume.

The electrostatic disturbances are enough to temporarily blind or distort instruments on the probe. This could, for example, make it difficult for the probe to keep its heat shield properly oriented. This has implications for future missions as well, as this phenomenon can negatively impact delicate instruments or cause spurious data.

At this point we can only extrapolate to the potential effects of hypervelocity impacts with dust particles on longer distance space travel, whether purely robotic or crewed. The farther we want to go, the more time the craft or probe will spend in space and at higher velocity. We have already sent probes to the edge of our solar system – the New Horizons probe took 9.5 years from launch to reach Pluto. Of course, scientists would love to decrease this time, and if we ever want to send astronauts to the outer planets we will have to decrease the travel time. We can also consider travel time to nearby solar systems. While this is a long discussion unto itself, it is absolutely plausible to send spacecraft to nearby stars, within 10-20 light years or so without requiring new physics (such as faster than light travel). These trips will take years to decades, of course, and also require speeds at about 20% the speed of light (a plausible speed for certain technologies, such as light sails).

Without getting into the broader discussion of interstellar space travel technology, if we can pull off such a feat we will have to contend with the interstellar medium. The density of material will be much less, but the velocity of such craft will be much greater. We now have some data on what the likely effects of impacts with interstellar dust would be. If frequent enough they might blind the ship’s instruments. This reminds me of all those Star Trek episodes and movies whenever they had to risk venturing into a ion storm or dust cloud, their sensors would dissolve into static. This might actually be plausible.

Perhaps worse would be the slow erosion of the outer hull. As each dust particle impacts the hull, in not only gets vaporized itself but it vaporizes a tiny portion of the hull itself. This may not matter for a solar probe on a several year mission. But what would happen to a hull traveling through space at 20% the speed of light for 20 years or more? We could always just add extra shielding, but that means more weight, which would prolong any such trip and might be counterproductive.

All this is why trying to make long distance space travel practical using existing technology (not stuff we have but stuff we can build without introducing entirely new physics, material or technology) is highly problematic. It might be like trying to get to the Moon with the steam technology of the 19th century. That’s why, for this century at least, getting to the Moon and Mars is enough (with crewed missions, that is). These are close enough to get there and back within a survivable time frame at velocities we can accomplish, and using shielding we already have. Anything else is not practical for crewed missions. (We could get to Venus also, but there is no particular reason to send people there rather than just robots.) It’s also hard to say when our underlying technology will improve enough to start looking beyond Mars for crewed missions. We need new methods of propulsion (nuclear, fusion, light sails), we need better physical shielding (perhaps some kind of meta-material), and perhaps magnetic shielding as well. A practical way to produce artificial gravity would be nice also.

This is likely to take a century, and perhaps longer, to get all the requisite technologies together. Of course, disruptive technology breakthroughs are always possible, and impossible to predict. Some lab could develop a kind of nano-meta-material in the next decade that would serve as perfect light shielding, capable of blocking even cosmic rays. Then suddenly, the equation is very different.

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