NASA has doubled its estimate of the number of killer space rocks following orbits closely aligned with Earth’s in a paper accepted for publication in the Astrophysical Journal.

It says there are now believed to be about 4700 potentially hazardous asteroids (PHAs) sharing the space near our planet, which it defines as having diameters at least 100 metres.

The space agency says in a statement:

Observations from NASA’s Wide-field Infrared Survey Explorer (WISE) have led to the best assessment yet of our solar system’s population of potentially hazardous asteroids. These asteroids have orbits that come within eight million kilometres of Earth, and they are big enough to survive passing through Earth’s atmosphere and cause damage on a regional, or greater, scale.

The assessment is the more alarming in that it leaves out near Earth objects including known comets that are also capable of crossing the planet’s path and posing an albeit lesser risk, but a nevertheless a risk with a catastrophic outcome, as their orbits also undergo changes than cannot be adequately calculated in advance if they lose mass as frozen ices within them are released, or become perturbed by their own close encounters with other rogue space rocks or comets.

The forthcoming NASA paper is also likely to revive the cosmic disaster p-rn frenzy that briefly flared in 2004 after the near-Earth asteroid later named Apophis was discovered.

Apophis was not only verballed incorrectly as being named after an Egyptian god of destruction, but its orbit showed that on Black Friday, April 13, 2029, it would pass so close to Earth it could destroy higher orbiting satellites, be visible to the naked eye as a medium bright third magnitude star, and get so bent by our planet’s gravity that it could come back for a full-on collision on Easter Sunday, April 13,  2036.

Apophis is not a welcome orbital dance partner for Earth.

At an estimated diameter of 300 metres, it is up to 100 times more massive than the infamous Tunguska meteorite of 1908, which exploded over central Siberia with the energy output of a city-burning hydrogen bomb.

However, the current estimates are that the chance of an impact in 2036, once put as high as one in 37, are now better estimated at only one in 250,000. Only. The path along which Apophis could impact stretches across Asia and central America, literally across the Panama Canal, and the northern Pacific, and the central Atlantic, all points where it might take out 10 million people on the ground, or generate a massive tsunami.

There are loosely articulated plans among Russian and Chinese space scientists, who are arguably looking for a lifetime heavily funded project, to devise the means to rendezvous with Apophis and nudge it out of is risky orbit into one that may never again threaten Earth.

The NASA paper is likely to strengthen their case, whether for Apophis deflection, or a yet to be identified threat, as the paper’s authors say they estimate that of the likely 4700 PHAs in existence, only 20-30% of them have already been detected and catalogued, and not all of them are under continuous surveillance for orbital changes.

And, in the fine print of observational near-Earth astronomy, Apophis is currently rated as a collision risk in 2053, depending on the still incalculable degree to which the very close encounters of 2029 and 2036 contribute to changes in its future path.

It’s an interesting thought. Excess fossil carbon could be joined by a rogue asteroid as the great big global risk of the 21st century, although that isn’t to downplay mad despots with nukes, or a pandemic.