On the night of October 19 2017 the Pan-STARRS telescope, atop Haleakela on the island of Hawaii, operated by Robert Weryk, discovered a fast-moving body sailing by Earth at a distance of only 0.2 AU (20% of Earth’s distance from the Sun). The path, after a few days of tracking, showed that the body, now designated C/2017 U1, was “dropping” through the plane of the Solar System from far above the plane of the ecliptic at a phenomenally high speed.
After a week of tracking it was clear that its speed was higher than any resident of the Solar System could have. The eccentricity of its orbit was clearly about 1.2, which made it a unique body in the history of terrestrial astronomy. Bodies in orbit around the Sun pursue elliptical orbits with varying degrees of eccentricity: a perfectly circular orbit has an eccentricity of 0.000, and most of the planets have e of about 0.05. Mercury and Pluto, at the inner and outer edges of the system, have orbits with e = 0.2056 and 0.2566 respectively. The most eccentric orbits are traditionally those of comets, with e rarely less than 0.9; long-period comets may have e as high as 0.9999. The latter corresponds to a comet that approaches within 1 AU of the Sun at perihelion and coasts out to an aphelion distance of 10,000 AU. A body whose orbit extends out to infinite distance would have an eccentricity of 1.0000. Solar heating at such a distance would be incredibly feeble; at 100,000 AU other nearby stars would provide nearly as much light and heat as the Sun.
For comparison, Pluto is close to 36 AU from the Sun: a body at 10,000 AU would receive almost 1000 times less intense sunlight than Pluto. At that distance, depending on the reflectivity of the surface, the body would have a surface temperature that is close to -272 oC, about 1 degree above absolute zero.
After a few more days of observation the general nature of the orbit had become clear, and telescopic observations had revealed that there was no trace of an atmosphere, tail of gases, ices, or dust. Its appearance was not cometary, but asteroidal. The name was immediately changed to A/2017 U1. These few days of further observation refined the orbit enough to fix the orbital eccentricity as 1.195+/- 0.001. Clearly this body is not on a closed orbit around the Sun: it is an unambiguous visitor from interplanetary space; a citizen of the galaxy, not of the Solar System. But late in October when this had become clear, A/2017 U1 was retreating from us at a speed of a little over 26 km/s.
It would have been fascinating to launch a spacecraft to fly to this body and even fly with it out of the Solar System, but there simply was not enough time to build and launch a spacecraft designed for such a challenging mission into the dark.
Other comets have been observed to depart at speeds close to Solar System escape velocity after being gravitationally “kicked” by Jupiter; in this case, it arrived from far out of the plane of the Solar System (orbital inclination of 122.55+/-0.05 degrees), already traveling well above the Sun’s escape velocity.
The body is roughly 160 meters in diameter; had it encountered Earth in its headlong rush through the Solar System it would have delivered a blow of 10,000 to 20,000 megatons of TNT, comparable to Earth’s global arsenal of nuclear weapons detonated in a single event.
What is it made of? Evidence for ices and gases is lacking; astronomers find the surface to be very red, reminiscent of Kuiper Belt bodies, and compatible with a low albedo.
What is its shape? The light curve of the body (the variation in its brightness as it rotates) would be compatible with it being 4 to 10 times as long as it is wide, an uncomfortably large variation that ranges from “unprecedented” to “fantastic” in comparison with native Solar System bodies. Variations in brightness with rotation can be caused either by variations in albedo (as is the case for the 7-fold albedo variation with phase for Saturn’s satellite Iapetus) or in cross-section area (such as a highly non-spherical body), or by a combination of these factors. Measuring the temperature of the body as it rotates (not available in this case) would permit separating these effects.
How common are these titanic “shots in the dark”? We have less than 100 years of data to draw upon. One event in the last century is a poor basis for making statistical predictions, but it certainly does merit our attention. Global extinction events on Earth from all causes occur at a rate of one per several tens of millions of years. How many of these arrive not only unannounced, but unobserved?