# Could a cloud of dense gas, such as xenon, deflect a hazardous asteroid?

Impactors or explosive missiles have the advantage of not having to match velocity with an Earth-bound asteroid, so they are cheaper and quicker to launch (or so I understand), and detection need not be as early. But they run the risk of fragmenting the asteroid and making the problem harder to deal with.

So why not send a missile that releases a cloud of dense gas, such as xenon or even one of those hexaflouride gases, into the path of the incoming asteroid? The cloud would not be very dense, but it would be huge, and the relative velocities could be enormous. The cloud would not slow the asteroid with kinetic impact, but with aerodynamic drag. I suppose the tricky part would be timing the release so that the gas does not dissipate too much. Could it deflect the asteroid appreciably?

The scenario I envision is this: we spot an incoming asteroid. We launch a rocket on a trajectory to pass closely in front of the asteroid. When the rocket reaches that point, it releases a cloud of gas so that the asteroid then has to pass through it. This could be done several times in sequence.

I do not envision a cloud surrounding the Earth (we already have one) or hanging in space for days or weeks. I envision a high-speed probe aimed to intersect an incoming asteroid's path, then exploding perhaps a millisecond before it crosses the asteroid's path. In that millisecond, the cloud of gas would expand to some tremendous size, perhaps kilometers wide, that the asteroid must pass through. Several probes could create a series of expanding clouds, one after the other. While the gas cloud will expand rapidly, reducing its density, a proper calculation would take that into account, by integrating over time. Concepts from fluid dynamics such as drag or shock obviously apply, because they apply to mediums as rarified as the solar wind.

So the question really is, how much aerodynamic drag could a cloud of gas expanding into vacuum exert on a largish asteroid? Could it significantly alter the asteroid's course or speed?

(At least two other people, one the famous astronomer Eugene Shoemaker, have suggested similar ideas, according to Wikipedia.)

(Another question asks whether a cloud of gravel could deflect an asteroid with kinetic impact without breaking it apart. This is a different suggestion. Other questions about the behavior of liquids or gas are also different.)

• Nukes are the answer. Cold gas in a cloud doesn't have the energy. Jun 6, 2015 at 12:27
• @Deer Hunter: Thanks! How does is the temperature of the gas relevant? I assume you mean kinetic energy. I posited that the cloud would slow the asteroid with aerodynamic drag, not kinetic impact, i.e. the gas is a fluid, not a set of individual particles like a gravel cloud. The asteroid would create a shock wave, turbulence, etc. Jun 6, 2015 at 16:29
• @KokotheTalkingApe The problem is exactly the fact that the gas is a fluid. A fluid in the absence of pressure behaves like a set of individual particles. Jun 6, 2015 at 17:14
• Jun 6, 2015 at 17:43
• This might help a little: scienceline.ucsb.edu/getkey.php?key=4455 One way to think about gas (or frozen gas) in outer space, is to think about emptying a box of marbles off the top of a high building - the marbles would drift apart, fairly quickly - going from a small volume to quite spread out and that's just random motion. A gas in space there would have high pressure causing it to fly apart, probobly at roughly the speed of sound at the initial pressure. Jun 7, 2015 at 6:08

Gas can act as a brake for moving objects in space, in fact NASA has used the atmosphere of (Planets) as a brake on four different occasions with spacecraft. (edited, correction pointed out in comments).

But it's not very practical to create a cloud of gas in space. Gas requires gravity to maintain it's cohesion. In space, a cloud of gas would become very diffuse and spread out very quickly — even heavy gas.

A cloud of gravel would be better than gas, but neither provides any advantage to a rocket. There's no benefit to leaving a random collection of objects to act as deflection in space for two reasons. One, space is very large, so the amount of material needed would be enormous (and it wouldn't stay in the right place either), and two, a ball of gas or gravel in space would be just as likely to deflect something towards earth as away.

The real trick is to see precisely where something is headed and if it's headed towards the earth, to give it a small push well before it reaches earth.

• Well, I don't imagine a random cloud of gas in space. I imagine a very precisely aimed rocket that will intersect the trajectory of a specific asteroid, and just before reaching the intersection, will release a cloud of gas that the asteroid then has to plow through while the gas has not dissipated entirely. I suppose there could be several such rockets, releasing a line of clouds in the asteroid's path. Again, no need to match velocity with the asteroid, which requires a fueled probe, therefore heavy, therefore expensive and hard to launch. And no chance for fragmenting the asteroid. Jun 6, 2015 at 7:02
• Well, I'm not the smartest guy here by a long shot, but I think it would only work if the cloud was released just prior to contact with the asteroid, like, less than 2 seconds before impact, maybe less than 1 second. At room temperature, air molecules are moving at about 1,000 MPH. It's not hard to see, using that as a baseline, how quickly a cloud in space would disburse. But if it's timed to release just prior to impact with the asteroid, I think it's a solid idea. Jun 6, 2015 at 7:23
• Did Messenger really use Venus's atmosphere for aerobraking? This article says the two Venus flybys were at altitudes of 2992 and 398 kilometers, respectively; this one says the "height" of the atmosphere is 250 km, with an upper limit of 350 km. Jun 7, 2015 at 14:30
• Oops. You're right, I'm wrong. Nasa's done it 4 times, but Messenger didn't do it. Messenger did slow down around Venus (it needed to slow down to get into orbit around Mercury, but it wasn't one of the crafts that Aerobreaked (or is that Aerobroke?). Source: en.wikipedia.org/wiki/Aerobraking and space.com/… I remembered wrong. Jun 7, 2015 at 17:14
• @userLTK it should be Aerobraked. A brake is a device to slow down. The past tense of brake is breaked. "Aerobroke" is what you call an underfunded NASA. Apr 14, 2016 at 13:18

It may be helpful to have a sense of the scales involved to see why gas would be ineffective. A stony asteroid with a roughly spherical shape 50 m across is probably the minimum size that might be worth mounting a mission to deflect (at least arguably, especially once we have greatly expanded into space and such things aren't so hard any more). The estimated mass of such an object is around 170,000 metric tons. Its speed would likely be around 25 km/s.

If the SLS was in operation and we could send a mission to deflect it while it was in Earth's neighborhood (on a pass across our orbit one to a few times before the pass that could result in impact), the maximum payload of gas we could get to it is maybe 10 metric tons1.

If you manage to place that 10 tons of gas right in front of the asteroid in a cloud, it will be immediately dispersing as the asteroid passes through it. It is a collection of particles surrounded by a vacuum, those particles are moving in random directions at high speed as they collide with each other, and they will expand into the vacuum extremely quickly. In this situation drag doesn't apply. The asteroid isn't an object moving through a sea of fluid which exerts pressure on it from all sides, it is larger and far heavier than the cloud of gas it hits, which it disperses with the dynamics of one object hitting a number of tiny objects. The gas cloud might display some behavior analogous to a fluid if it has remained dense enough, but it would be more like the way there are some phenomena like fluids when you run through a pile of leaves. The impact moves the particles out of the way and to the sides, it isn't really like drag.

Consider that all pumps work by creating partial vacuums, which fluids immediately fill. Releasing a gas into a near perfect vacuum is like surrounding it with a perfect pump, sucking it outwards in all directions at once.

The payload size deliverable will of course increase with time, but it would never be more effective than other options.

1 the Orion MPCV has a dry mass of 21 metric tons, but isn't for missions with the kind of delta V this would take. 10 tons is a total loose guesstimate of what might be left for payload once the delta V capacity had been satisfied

• I don't see why the concept of drag doesn't apply. Even if the gas cloud were smaller than 50 m at impact (which seems impossible), it would still conform to the asteroid and flow around it, creating drag. Also, all fluids are collections of particles, but fluids are dense enough to transmit movement and pressure while conforming to objects (my very rough definition of fluid). The cloud would clearly stay fluid-like even if just for a moment. As I said above, even the solar wind itself is fluid-like, since it carries shock waves. Jun 6, 2015 at 19:48
• Shock waves occur when the Earth's magnetic field meets a coronal mass ejection from the sun, which is billions of tons of charged plasma. The dynamics in that situation are due to the sheer size of the phenomena, the fact that it is ejected at high speed, and the fact that because it is charged it responds to the sun's magnetic field. Even so, the mass is very diffuse by the time it reaches Earth. Jun 6, 2015 at 19:59
• Particles in the solar wind are also charged and thus behave the way they do because of their interaction with magnetic fields from the sun and other bodies. They can also be called a wind because they stream constantly off the sun in all directions and thus there is a constant density of them. Jun 6, 2015 at 20:03
• @brillig, So, my point is that even very rarified gas can act like a fluid, meaning it can exert drag. My question, for the umpteenth time, is HOW MUCH drag? Jun 8, 2015 at 4:29
• @KokotheTalkingApe, going with the most optimistic assumptions possible, the total drag from 10 tons of gas will reduce the velocity of a 170,000-ton asteroid from 25 km/s to 24.997 km/s.
– Mark
Jun 8, 2015 at 6:05

Let's say you catch the hazardous object one orbit before it's going to hit earth. That means it's got about a billion kilometers left to go, and you want to deflect it by say 10,000 kilometers (almost an earth-diameter). That's a (very) roughly speaking ten parts per million adjustment of the momentum.

In that case you'll need it to intercept a cloud of "stuff" with an (also very) roughly speaking mass of ten parts per million of the hazard. Assuming similar densities, that means your stuff - in the rocket - before expansion - will have a size of 1/100,000^0.33 or about 1/50th of the size, or the mass, of the hazzard.

If your hazard is 1km in diameter with a density of 3 g/cm^3 and your liquid Xenon has the same density (it does) then you need a 20 meter diameter ball of liquid xenon to boil, turn into a gas, and then stay put. Of course if it's got much much farther to go, then you can use less mass.

If your gravel ("micro-impactor") cloud, or particulate ("nano-impactor") cloud, or molecule ("pico-impactor") cloud is released in space, then it is in orbit - in an orbit - whether you like it or not - around the sun.

That means if you want it to stay in front of the hazardous object for a long time, it would have to be in the same or similar orbit - with the easter egg that it could be moving in either direction. So you could put it in the "backwards" orbit and maximize the momentum loss.

All of the impactor clouds mentioned above work similarly - whether it's molecular collisions or gravel collisions, the collisions "remove" momentum from the hazardous body loosely speaking. They will all have some sticking fraction - sticking is half as effective as rebounding for momentum transfer.

The term "fluid" refers to an approximation - temporarily forgetting that molecules exist and pretending you have a uniform material. It's and extremely useful approximation, but here we should just stick to the somewhat more realistic view of the gas as individual impactors and momentum transfer.

The difference between the cloud of molecular particles, and the clouds of larger particles is the relative speeds, and effect of collisions. If you think of temperature as kinetic energy per particle, then the lighter the particle, the faster velocity for a given temperature.

However, by the time the gas expands from 1 meter to 10 kilometers in size, it's really ultra high vacuum (UHV). The mean free path is as big as the cloud itself. It is now "cold" after expansion - in the sense that the random motion is low, but the velocities are now "ordered". The molecules are still going fast, but radially expanding.

When a gas cools by expansion in space, the molecules don't slow down and stop. "Cold" just refers to the random part of the motion. The ordered part (expansion) will continue at roughly the same average speed.

Punch line: So if you want to keep your particulates in place (meaning - all in one orbit) longer, then possibly molecules are not the best bet because they will have the fastest expansion rate.