This is World Building through a preventative measure.

The Earth has been destroyed a few times in the past and it will happen again in the future, it is just a matter of time. For the first time in history we have the capability to save ourselves if we work together.

What is our current capability when it comes to protecting Earth from a rogue moon, comet or asteroid? I understand after a certain size there is nothing you could do or can we?

Related: https://astronomy.stackexchange.com/questions/32844/how-well-would-the-moon-protect-the-earth-from-an-asteroid

What are the possibilities with current technology if money was no object? Any new realistic answers welcome even if it is your own.


8 Answers 8


First, a little digression on comets: the coma or "hair" (that's the literal Classical Greek translation) of the comet is indeed very, very long, but it's also incredibly rarified (not dense). More importantly, all the material in the coma is being flung away from the comet by the solar wind; it isn't as though the comet will hit us, followed by all the tail material. The nucleus or core of the comet is generally a few tens of kilometers across.

More to the point, what matters isn't so much the volume or composition of the object as its mass. Mass is what will make it hard to deal with. (It's also a large part of what will hurt us on impact. Speed also matters.)

Going off the link Ghedipunk provided, the general strategy is not to blow up the potential impactor but to deflect it. Compared to the size of earth's orbit, earth and any potential impactor are vanishingly tiny, and it wouldn't take much of a course change to make them miss one another. Deflection is easier, relative to the size of the target object, and way less likely to spall off fragments that will hit us anyway.

One way to change course is through an impact, or a series of impacts, from spacecraft. The two ideas NASA seems to be pursuing are kinetic impacts - using the spacecraft itself as a bullet - and nuclear weapons. (As for chemical explosives, my guess is that they don't yield a better bang for the tonnage compared to simply carrying more fuel and accelerating the spacecraft more.) Of these, nukes are quite a bit more powerful, and seem to be the only thing NASA thinks is likely to deflect a >1km diameter asteroid. However they come with all the political considerations you would expect from phrases like "launch nukes into space" and "detonate nukes on incoming asteroids". Also Bruce Willis might get involved and you'd never live it down.

The other method of changing course is to do it gradually. The winning proposal here seems to be the "gravity tractor", which consists in essence of flying a spacecraft very near an asteroid for a prolonged period of time (years to decades) and using its gravity to gently impel the asteroid in a given direction. The advantage is that because no thrust is being directly imparted to the asteroid, its composition and center of mass are irrelevant; this technique will work on so-called "rubble piles" that would fracture from impact or direct thrust. Also, because the magnitude of thrust is so small, the operators have a great deal of fine control over where the asteroid ends up. Finally, there's the benefit that intercepting asteroids and orbiting a thruster around them is something we've done, albeit not for nearly the amount of time required. The disadvantage is the time required, both in terms of detecting it early enough and making sure nothing happens to your spacecraft in that time. (Multiple craft would seem to be prudent, along with a generous margin for error.)

A third option is to laser ablate the impactor. The theory behind this is as follows. When an object is hit by a laser, a small part of its surface absorbs the heat and is converted to plasma, usually. This plasma expands in all directions essentially evenly. The plasma that expands back against the object imparts a force on the object. Therefore, the laser can be used to (very) gradually propel the object in a given direction. The advantage here is that the laser can stay on earth rather than going to the asteroid (space travel is highly inefficient, energetically speaking). The disadvantages are that it's slow, it's never been demonstrated on such a distant target, and of course when all is said and done you have a giant laser emplacement to keep track of.

  • 8
    $\begingroup$ "It's also a large part of what will hurt us on impact. Speed also matters." Just say it! MOMENTUM! $\endgroup$
    – jpmc26
    Commented Oct 3, 2018 at 2:02
  • 1
    $\begingroup$ This does not answer the OP's question. How to prevent a 1,000km diameter object moving at orbital speeds or better with present tech. You've described the tech, but not answered the question. $\endgroup$
    – JBH
    Commented Oct 3, 2018 at 4:59
  • 1
    $\begingroup$ @jpmc26 It’s not momentum either, it’s kinetic energy, which increases faster with speed than momentum does. $\endgroup$
    – Mike Scott
    Commented Oct 8, 2018 at 8:38
  • $\begingroup$ Nukes are by far the most efficient approach. No Bruce Willis, though--blowing the rock up is only an answer when it's a small rock. He's talking about something on the KT-impactor level, blow that up would only make it worse. What you want is to use your nukes as an Orion drive, gently push the rock off course. (You'll probably need to nuke it repeatedly as each nudge needs to be gentle enough to ensure you don't break it up.) $\endgroup$ Commented Jul 23, 2019 at 2:11

Observation and foresight is everything

It all depends on how early you can detect it. This changes drastically the response and the chances of success. Luckily, orbital mechanics is generally well-understood, so if you can detect an object early, you can predict, perhaps even for centuries, if this object is likely to impact the Earth.

Not only does early detection give you more time to respond, it also has the advantage that effort required to move something off a trajectory is much smaller for objects that are a long way away, or take a lot of time to reach us. The slightest nudge would have minimal affect in a short time span, but over a long one would have a much larger effect.


  • If it is large and decades away from hitting Earth: Even the smallest nudge would alter its trajectory over the years. Acting (relatively) quickly, we can send a probe with a nuclear device to land on it (which we can already do), then detonate it to nudge it.

  • If it is only a year away from hitting Earth: Depending on the mass to move, you need to increase the yield of your device commensurately

  • If it is only a few days away: Your chances are becoming minimal you can do anything to prevent the impact, and you may need to settle on survival measures.

In all scenarios the best thing you could do is to detect it as early as possible. This means basically more telescopes calibrated specifically to detect objects that may be on a collision course, and vigilance. The Nasa Planetary Defence Office, and associated network of telescopes around the world, is established exactly for this purpose.

Edit - for clarity, the Nasa PDO structure is: enter image description here As you can see, the vast majority of the functions of the PDO is simply in observation, for the above reasons.

  • $\begingroup$ The explanation is spot on and plausible, but the graph is like the heirchy of the organization, I think the mission/vision of the planetary defence is enough for this.(wait, is this actually true, do we have some planetary defence org?) $\endgroup$
    – Mr.J
    Commented Oct 3, 2018 at 3:14
  • $\begingroup$ This does not answer the OP's question. How to prevent a 1,000km diameter object moving at orbital speeds or better with present tech. $\endgroup$
    – JBH
    Commented Oct 3, 2018 at 4:58
  • $\begingroup$ @JBH Although the OP did not state 1000km, it is a simple formula to nudge a trajectory if you can do it early enough. Mass matters less the further out it is. A nudge could be a small nuclear bomb a century before the object becomes a problem, as is stated in my answer. The time it takes to reach Earth, the effect would have magnified more then a thousand bombs closer to Earth. This is a reasonable approach already identified in the PDO Mitigation arm, and is capable in today's technological environment. This still means observation is the best armament we require at the moment, as answered. $\endgroup$
    – flox
    Commented Oct 3, 2018 at 15:54
  • $\begingroup$ @Mr.J Yes we do - actually I volunteer at the local observatory where I live and they have a telescope there dedicated to the NEO programme. Its purpose is to remotely open the dome and scan the sky for objects (no personnel involved except for maintenance). There are many of these around the world, basically rented telescopes. $\endgroup$
    – flox
    Commented Oct 3, 2018 at 15:59
  • $\begingroup$ @flox, the OP stated 500 million km radius. I made the assumption that he mistyped "million" (500 MKm = 3.34 au), so 500km radius = 1,000 km diameter. Further, you need fuel to get the bomb there, an explosion that can move that much potential energy (however small the change), etc. You didn't explain that, and if you try, you'll find we don't have anywhere near the tech to do what you're suggesting. The further out it is, the less likely we can even get the bomb to it (and the less likely we'd even know about it - today's tech...). $\endgroup$
    – JBH
    Commented Oct 3, 2018 at 17:11

I can't find the original source, which I think is either a NatGeo or PopSci article, but here is a source that gives some (slightly crazy) asteroid prevention methods. Here I will take a look at the craziest ones, which haven't been mentioned yet.

Eat it

We can stop asteroids by eating them with robots. The idea is fairly simple: robots sent to the asteroid can grind up the surface and expel the broken up rocks in the other direction. The ejection of mass will decrease the size of the asteroid, but more importantly, it will alter the asteroid's path. With enough advance warning, the asteroid can be steered into the sun or flung out of the solar system.

Paint it

What do you do to a giant asteroid on a collision course with your planet? Make it festive! By pouring paint on the asteroid, sunlight will reflect off of the surface. This tiny push can actually move the asteroid off its course toward earth, but this method seems a lot slower than the first one. Still, looking in the telescope and seeing an asteroid with "Kick me" painted on it sounds pretty cool.

Ram it

If worst comes to worst, slamming a sacrificial space shuttle into an oncoming asteroid is a feasible idea, even if not as cool as the answer by @Muze. A better option is probably sending out a craft to meet the asteroid and giving it a steady push off course so you don't have to deal with all the deadly fragments that are left over from a sudden impact.


Putting together some of the parts of the other answers, and focusing very specifically on the OP's "current technology" requirements, we actually have a very limited tool-kit to work with.

Detection of deep space objects and threats is the first and foremost problem: until we know it's coming, we can't do anything about it. There are several groups observing for objects that are on Earth crossing orbits, the best first step would be tp place them under a singular organization th ensure continuous monitoring and identify and plug gaps in observational capability, either by deploying more resources or different ones (telescopes capable of observing in different wavelengths, deep space radars and so on). This might actually become one of the missions of the proposed US Space Force.

Once a target is identified, we need to find a way to deflect it. Since the odds are the object will be small, dark and moving very quickly, it may be detected with only a small window to react. in that case, using a nuclear weapon to heat and ablate a portion of the object to create a rocket like thrust to change orbits may be the only feasible defence. If we are thinking ahead, it is possible to build nuclear warheads that can concentrate much of their energy in a narrow direction, much like a convention HEAT warhead can focus the explosive energy of high explosives into a narrow jet. The project was conceived in the 1960's under the name CASABA Howitzer. Prebuilt warheads using these principles will be more efficient in focusing the energy of a nuclear explosion onto the target, so can be smaller and easier for a rocket to carry (or carry multiple warheads, if that is desired).

Finally, we will need a large and powerful rocket to carry the warhead(s) to the target. Currently the best option would be to contact SpaceX for a Falcon Heavy. This is currently the largest and most powerful rocket in service, giving us lots of options for interplanetary orbits for intercept, and also is the only rocket essentially built on an assembly line, so can be ordered and assembled quickly, assuming no one has a rocket already on standby somewhere.

So the order of events would be observe the target and calculate the orbital parameters. Prepare a contract with SpaceX to build and prepare a Falcon Heavy. The USAF or other nuclear force prepares one or more warheads on a missile bus to mate with the Falcon Heavy, and prepared the flight computers.

Once the assembly is put together, it is launched under Space Force control, which ensures the missile bus is guided to the rendezvous point, selects the optimum time and place to fire the warhead and releases it from the bus. After the explosion, observations take place, and if necessary secondary explosions are conducted to ensure the object is pushed far enough off course to miss the Earth. The missile bus will continue on its orbit, and if any warheads are not used on the mission, likely detonated to prevent their recovery sometime in the future.

So for the foreseeable future, this is what an asteroid deflection mission would look like.

Edit to add (just because!): the launch would sound like this.


Hit it with an asteroid while it is still far away from us.

To do this we would have to have a lot of notice, probably decades. But given the size of the rogue moon, that might not be a bad assumption.

We would compare the orbit of the rogue moon around the sun with asteroids that we consider movable and have orbits that we could adjust to intersect with the rogue moon. If we're lucky, and that interaction happens far enough away from the earth, then we could undergo a mission to adjust the asteroid's orbit so that it collides with the moon and thereby avoiding the collision of the moon with the earth.

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    $\begingroup$ This does not answer the OP's question. We do not have the ability to capture and move an asteroid with today's tech. $\endgroup$
    – JBH
    Commented Oct 3, 2018 at 5:00
  • $\begingroup$ @JBH Your comment on a separate answer, including "You've described the tech, but not answered the question.", suggests that you agree with that answerer that we do have that ability today. Also, the question asks basically "What could we do potentially with our current capability and unlimited money," meaning tech that is designed but not tested should count, hence we do have the tech as explained in that other answer. $\endgroup$
    – Loduwijk
    Commented Oct 3, 2018 at 18:22
  • $\begingroup$ @JBH A better rebuttal to this answer might be "Now show us what the odds are that there will be an asteroid with a close enough approach to the target that we would be likely to be able to do this at all." I don't know, but my gut is telling me this might be a great technique but with only a tiny chance the hijacked object will align with its requirements. $\endgroup$
    – Loduwijk
    Commented Oct 3, 2018 at 18:25
  • $\begingroup$ @Aaron, (a) No, I did not assert that I agreed the presented tech in the other answer was available. I was pointing out that the answerer provided an explanation without evaluation (which I expect to prove it isn't viable). (b) Unlimited money isn't unlimited time. Today's tech does not mean tomorrow's tech. We do not have the ability to travel to Mars today (regardless the size of our bank account). Therefore, we don't have the ability to get a solution to an approaching something in a timely manner - regardless our bank account. $\endgroup$
    – JBH
    Commented Oct 3, 2018 at 19:24
  • $\begingroup$ @JBH No, you didn't assert that; you are correct. I did not mean to imply you did. As I said, your comment suggested that (whether you intended for it to or not). Addressing it the way you did made it look like an endorsement of the process. (b) We do not have the ability to go to Mars today, but if we suddenly were faced with the scenario "We have to have a human set foot on Mars within three years, or else!..." we could probably make that happen. We very well might accidentally kill people in the process, but it is within the realm of possibility. $\endgroup$
    – Loduwijk
    Commented Oct 3, 2018 at 19:58

(My answer will assume that we have detected said Earth-destroying object early enough to be able to implement the solutions below.)

With our current technology, we have two possibilities: destroy or deflect.

There are two ways of destroying a spatial object inbound to Earth, depending on it's composition and mass:

  • Kinetic impact: Send a probe on a collision course with the object, calculating the orbital path and angle of impact to maximize the relative speed in order to shatter the object into harmless smithereens. The probe needs to have enough mass to gather enough kinetic energy to achieve this result.

The Japanese space agency JAXA just successfully put a probe into orbit of 1 km wide asteroid moving quite fast through space, sent three rovers to it's surface to explore and photograph it, and the probe will bring back samples from the asteroid in a few years.

  • Explosion: Same as above, except the probe is fitted is a powerful explosive; current plans by space agencies call for an atomic warhead; in order to vaporize most of the object, and ensure that the rest is broken down into small pieces that are harmless for Earth and would burn up in the atmosphere.

There are also two ways to deflect an Earth-destroying object, depending on it's composition and mass:

  • The destructive way: Send a probe to impact or explode against the object, basically pushing it out of the way.

The only drawback to that method is that, at the moment, you cannot predict the new trajectory very accurately, although 'away from Earth' would be good enough for everyone, and there's a risk that some debris still keep going towards Earth, perhaps some that are big enough to do some damages.

  • The non-destructive way: Land a probe, or several, on the object and use some means to push it away.

The simplest way is for the probe to use it's own engines to gently push the object on a new orbit. With constant acceleration over a period of time, a satellite could move even a kilometer's wide object away from Earth.

Another way to do this is to strap a solar sail to the object and let the sun do the work. It would be slower than with the engines of a probe, but the object would be pushed out of the Solar System in the end, never to return.

Those are the solutions we have at the moment. Size wouldn't be a problem, up until a certain point, if money is unlimited. Even an object the size of the Moon, if detected early enough, and it would be detected early, whether by professional astronomers or amateurs, could be nudged away by landing hundreds of probe on the same side and have all the probe nudge it away. Something with similar size than Earth might be a bit too big for us to handle at the moment.


I take issue with some of your terminology in this question; the Earth has never been destroyed, we wouldn't be here if it had, occasionally the established ecosystem gets a good kicking is all. Anything actually capable of destroying the planet has to have about 2.5x1032J of kinetic energy, that's the same as the total solar energy output for about 4 months. Something with that much energy is either too big or going far too fast for us to notice it coming before it hits. Kinetic energy is equal to 1/2mv2 where m is the mass of the object and v is it's velocity:

  • From memory the maximum relative velocity an object from within our solar system has is something like 11ms-1 putting that in we get an object that weights 4.1x1027 tonnes, a little over twice the mass of Sol, and if it's a comet with a specific gravity of 0.6-ish (which is average for observed objects) it will actually be much larger than (up to three times the volume of) the sun.

  • Or if we instead look at an average comet; that weights about 8x1010 tonnes so it would have to be going roughly 2.4 billion ms-1 or 8 times the speed of light to have that amount of energy.

So in short although a much smaller and/or slower object might kill everything bigger than a bacterium on Earth, and be stoppable. Anything that's capable of actually destroying the planet that comes our way we don't stand a chance of stopping.


Do we have enough nuclear material to move the Moon? I propose with enough foresight and time we could not try to send nuclear devices to the whatever that threatens the Earth but we use the moon by changing its orbit just a little by timing a block of the large object. The nukes can either be planted on the Moon in anticipation or guided one after the other until the moon and object coincide.

To increase blast efficiency place each following nuke in the same crater creating a deeper crater focusing each following blast unilaterally.

The unknown Earth killer could be lined up to be a near miss with the Moon and push the Moon a little with the right timing to block.

enter image description hereenter image description here

I would just be worried about the Moons orbit. The entity and moon debris be pushed into the Earth or out of orbit but still would be better then a direct hit. In my question I had some pictures I had added to illustrate and placed them here. Rough moon vs Earth would look like them below. Thanks. enter image description here

enter image description here

On this opposite of Earth everything is under water.

enter image description hereenter image description here

  • $\begingroup$ Then a thousand asteroids would be raining down on earth instead of just one... $\endgroup$
    – John Locke
    Commented Oct 4, 2018 at 1:21
  • $\begingroup$ @JohnLocke most of it will escape orbit and burn up. $\endgroup$
    – Muze
    Commented Oct 5, 2018 at 18:35
  • $\begingroup$ You are moving the moon between the earth and the asteroid as a shield, so the moon will explode and move toward the planet, which is the same direction the rest of the debris will go. See this video $\endgroup$
    – John Locke
    Commented Oct 5, 2018 at 18:41
  • $\begingroup$ @JohnLocke not if the angle of impact is right it all will escape, but if it is wrong its still better than a direct impact. $\endgroup$
    – Muze
    Commented Oct 5, 2018 at 18:49
  • $\begingroup$ Space is tridimensional, you know. Your solution might work if the "destroying object" comes along our orbital plane - but if it comes perpendicularly, I don't see how it would be possible. $\endgroup$ Commented Oct 8, 2018 at 12:06

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