An interstellar object exploded over Earth in 2014, declassified government data reveal

A fireball that blazed through the skies over Papua New Guinea in 2014 was actually a fast-moving object from another star system, according to a recent memo(opens in new tab) released by the U.S. Space Command (USSC).

The object, a small meteorite measuring just 1.5 feet (0.45 meter) across, slammed into Earth's atmosphere on Jan. 8, 2014, after traveling through space at more than 130,000 mph (210,000 km/h) — a speed that far exceeds the average velocity of meteors that orbit within the solar system, according to a 2019 study of the object published in the preprint database arXiv.

2019 study argued that the wee meteor's speed, along with the trajectory of its orbit, proved with 99% certainty that the object had originated far beyond our solar system — possibly "from the deep interior of a planetary system or a star in the thick disk of the Milky Way galaxy...

USSC scientists have officially confirmed the team's findings. In a memo dated March 1 and shared on Twitter on April 6, Lt. Gen. John E. Shaw, deputy commander of the USSC, wrote that the 2019 analysis of the fireball was "sufficiently accurate to confirm an interstellar trajectory."

This confirmation retroactively makes the 2014 meteor the first interstellar object ever detected in our solar system, the memo added. The object's detection predates the discovery of 'Oumuamua — a now-infamous, cigar-shaped object that is also moving far too fast to have originated in our solar system — by three years, according to the USSC memo. (Unlike the 2014 meteor, 'Oumuamua was detected far from Earth and is already speeding out of the solar system, according to NASA(opens in new tab).)

"Did anybody see this rock, or take a picture of it, before it blew up? If not, how do we know it was only 1.5 feet across?"

It looks like they went through the following steps:

Compute an approximate mass based on the impact energy and the impact speed.
Take that approximate mass, and compute the volume using an educated guess for the density of the object (volume = mass / density).
Solve for the radius R: assuming the object was approximately spherical, its volume V can be written as 4π R^3 / 3, so R is the cube root of 3V / 4π.

Here's the relevant quote from the ArXiv preprint, Section 3.2 Size Distribution:

"Given the impact speed of the meteor, ∼ 44.8 km s⁻¹, and the total impact energy, 4.6 × 10¹⁸ ergs, the meteor mass was approximately 4.6 × 10⁵ g. Assuming bulk density values of 1.7 g/cm³ and 0.9 g/cm³ for Type II and Type IIIa objects respectively, we obtain a radius, R, of 0.4m - 0.5m for a spherical geometry (Ceplecha 1988; Palotai et al. 2018)."

This appears to be just an approximation to give an idea of the size, but it's probably about right. I don't think they give an error estimate.

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