Nuclear weapons as a solution with a short warning time

When the warning time for an approaching asteroid is only a few months, purely kinetic defense methods quickly reach their physical limits, which is why the use of nuclear explosives remains the only option. In a kinetic deflection, space organizations attempt to divert the celestial body from its orbit solely through the mechanical force of a massive, unbraked impact. As can be read in a study published in the journal Space: Science & Technology, a team from the Chinese Academy of Launch Vehicle Technology in Beijing calculated this emergency and came to the conclusion that this simple ramming approach fails for objects larger than 100 meters in diameter.

The basic problem of planetary defense lies in the available reaction time in relation to the enormous mass of the approaching object. If astronomy discovers a dangerous asteroid extremely late, there simply isn’t enough time to slowly change its trajectory using small, continuously acting forces. Such small forces arise, for example, when a space probe acts as a so-called gravitational tractor and flies next to the asteroid for years, pulling it off its course millimeter by millimeter like an invisible tow rope using only its own minimal mass.

The problem of energy transfer in a vacuum

With the Dart mission, the US space agency NASA has already proven that celestial bodies can be measurably deflected by a kinetic impact. However, with massive rocks and a time window of less than a year, the force of such a probe is not sufficient because the enormous mass of the asteroid simply absorbs the momentum.

A nuclear explosive device theoretically supplies the required energy, but does not create a destructive pressure wave in the vacuum of space as within a planetary atmosphere. If the warhead only detonates on the surface, the majority of the thermal radiation released dissipates unused in empty space, which means that the desired deflection effect is almost completely absent.

Two-stage detonation as an optimal solution

To circumvent this inefficient energy transfer, the study outlines a highly complex two-stage process in which a companion probe first flies to the asteroid and fires a conventional explosive device. This first impact serves the sole purpose of making as deep a crater as possible into the hard rock of the celestial body before the actual nuclear warhead is deployed.

In a second, precisely coordinated step, the nuclear warhead navigates into this previously created depression to detonate deep below the surface. Through this underground explosion, the immense energy is trapped by the rock masses and converted into a directed impulse that massively changes the trajectory.

Simulations show massive advantages

The researchers’ calculations impressively demonstrate the advantage of this strategy using simulated impacts on rocks with a diameter of exactly one kilometer. When detonated at a depth of 30 meters, the asteroid achieves a change in speed of a good 30 centimeters per second, which in orbital mechanics means an enormous distance over years.

In direct comparison, a flat surface impact with the same explosive force of three megatons of TNT equivalent would change the speed of the rock by less than ten centimeters per second. Especially with smaller chunks in the range of around 100 meters, the deep detonation is not only enough to distract, but according to the calculations it leads to the complete elimination of the acute threat.

Extreme demands on future carrier systems

This theoretically superior method poses enormous technological challenges for mission planning as it requires complex orbital maneuvers and extremely heavy payloads. The study authors therefore point out that heavy-duty rockets such as the Starship from the US company SpaceX or the Chinese CZ-9 are absolutely necessary for this type of defense.

In addition, despite all the urgency, the incoming celestial body must be discovered early enough, as preparation for the launch of such a massive liquid fuel rocket necessarily takes several weeks. If this time is no longer available, the only plan B, according to the analysis, is to immediately launch a solid-fuel rocket that is always ready for use.

During the direct ramming maneuver that follows, the nuclear warhead inevitably detonates close to the surface, which significantly reduces the effectiveness of the explosion. With an extremely short warning time, this direct approach represents the only remaining option to possibly avert a global catastrophe.

Ultimately, these calculation models make it clear that, in practice, global planetary defense is still far from being an absolutely reliable solution. As long as theoretical answers to impending impacts require such immense engineering preparations, the massive expansion of powerful telescope networks for early detection remains humanity’s best option.