It Came from Outer Space

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?

Want a Piece of Mars? It’ll cost you a billion dollars—or be free!

Anyone who wants a piece of Mars can imagine sending an automated spacecraft to land on Mars, grab a sample, and blast off for return to Earth.  No problem; just pay a billion dollars or so and you can have it!  Such a deal!  As they tell everyone in the bazaar in Istanbul, “Just for you!”

But asteroid and comet impacts on Mars happen from time to time, blasting fragments of Mars’ crust into orbit around the Sun.  Some of these fragments drift into Earth-crossing orbits; a few enter Earth’s atmosphere and a very lucky few survive entry heating to fall on the ground and be recovered as meteorites.  Most of them will never be recognized as meteorites, especially once their fusion crust weathers off.  A few decades ago we knew of only about two dozen such Martian meteorites, but since then we have reaped the benefits of meteorite searches in two locations where meteorites are so different from the local country rocks that they virtually stand up and say “look at me”!  One of these is the vast sand seas (ergs) of the Sahara Desert; another is the snowy wastes of Antarctica.  Rocks in either place are rare, and the overwhelming majority of them are meteorites.  In turn, the large majority of these Saharan and Antarctic meteorites are fragments of asteroids—but some are pieces of the Moon or Mars, and possibly other bodies as well.

To date some 200 meteorites found on Earth have been traced back to an origin on Mars.  Virtually all are members of three meteorite classes, shergottites, nakhlites, and chassignites, collectively called SNC meteorites.  They do not sample Mars’ surface either widely or democratically; they may indeed represent only three or four places on Mars where recent and unusually violent impacts occurred.  But they are real pieces of Mars, and they are vastly cheaper than a billion dollars!

Neither Fish nor Fowl; Neither Comet nor Asteroid?

The Hubble Space Telescope has presented us with telling images of a peculiar body that seems to share the attributes of an asteroid and a comet.  It has been christened with the double name 300163 2006 VW139/288P, alias P/2006 VW139, acknowledging its asteroid-like and comet-like traits and declining to make a decision between the two categories. 

The first part of the name attests that the body was discovered as an asteroid in October 2006, but the recognition of its strange appearance and its ambiguous identity had to await the Hubble observations a year ago.  The second part of its name identifies it as a body with cometary appearance (a tail) and a “periodic” orbit, meaning that it has a known orbital period.  And what should we call it?  The exceptional clunkiness of the name suggests nicknames such as “2006 VW”, which unfortunately conjures up a misleading (non-duplicitous) image.

A “video” of the body, constructed out of a number of snapshots taken as it rotates, can be found at http://hubblesite.org/videos/news/release/2017-32.  Form your own opinions (and suggest your own nicknames).

So what do we see when we look at that video?  Two bright, roughly equal-sized apparently solid bodies, just a few body diameters apart, trailing a bright “tail” of generally cometary appearance.  The tail appears to be a dust stream rather than a gaseous tail, tempting us to speculate about the chain of events that produced this strange display. 

The “just-so” story has to take into account the duplicity, proximity, and activity of the body.  There are several possible scenarios to consider:

1a and b. Collisional Fission.  An asteroid in the Belt was recently clobbered by a glancing impact sufficiently violent to get it rotating fast enough to fission into two bodies in very close orbit about each other, perhaps even a contact binary.  This happened so recently that the dust sprayed out by the impact has not had time to dissipate.  Sounds improbable, but there are examples galore to whet our imagination.  The Trojan asteroid 624 Hektor was studied in 1980 by a former student of mine, Stu Weidenschilling, and interpreted as a contact binary; a Wikipedia article on “Contact Binary: Asteroid” lists 13 strong candidates for the same distinction.  At least two comets (67P/Churyumov-Gerasimenko and 8P/Tuttle) also appear to have the same structure.  Such structures can arise either by (1a) the glancing collision of two large independently-orbiting bodies or by (1b) rotational fission of a single asteroid that was spun up to the point of splitting apart by a relatively small but sufficiently violent impact event.  Since the encounter velocity of two asteroids in independent orbits around the Sun is typically several kilometers per second, the overwhelming majority of asteroid/asteroid collisions must result in explosive disruption rather than capture into closed orbits.  Low-velocity encounters must be rare; but contact (or near-contact) binaries ARE rare.  The dust-generating activity can then be attributed to the collision event, which must have been fairly recent; otherwise, the dust would have been swept up by the bodies or blown away by solar radiation pressure.  Oh, we also know from independent evidence that 2006 VWs may survive low-velocity collisions. 

2.  It is just a comet whose gas emission is undetectably small, but sufficient to maintain a dust cloud, according to H. H. Hsieh and 41 other coauthors, Discovery of main-belt comet P/2006 VW139 by Pan-STARRS1, Astrophysical Journal Letters, 748 (1). (L15) 1-7 (2012).  They interpret observations by the Pan-STARRS1 survey telescope as showing a striking similarity to the main-belt cometary object 133P/Elst-Pizarro, which has a rather similar orbit.  Both may be outliers of the Themis family of main-belt asteroids.  They argue that the stable maintenance of the dust cloud is indicative of cometary outgassing, most likely of water vapor, but they are also unable to detect any gaseous molecular emissions spectroscopically.

If I had to bet on the strength of present evidence, I’d have to prefer the latter explanation.  The implication of a very high water content in some Belt asteroids is worth remembering—especially if you are a water-based spacefarer.

See also: 

https://www.space.com/38214-spitting-asteroids-hubble-telescope-discovery.html?utm_source=sdc-newsletter&utm_medium=email&utm_campaign=20170922-sdc