Thursday, November 15, 2018

Target: Earth

Yesterday, 14 November 2018, the discovery of a possible large new impact crater on Earth was publicly announced.  The crater, on the northwest coast of Greenland, was revealed by the seasonal retreat of the ice sheet, partially uncovering a circular feature roughly 20 miles in diameter, presently still filled with ice. 
            The debris from the crater rim exhibits unusually strong traces of platinum, palladium and rhodium, as well as traces of what appear to be grains of terrestrial quartz that has undergone severe mechanical shock, a diagnostic feature of violent impact events that generate high shock pressures.  The article is featured on today’s (15 November), along with text that whets the appetite while flirting with the boundaries of the truth.  We are assured in that article that the shocked quartz grains are the result of the “impact’s force abruptly melting rock”, whereas in reality melting is guaranteed to erase evidence of severe mechanical shock.  We are also unconvinced by the article’s attribution of the impact to a “giant iron meteorite” through the identification of the platinum-group metals in the debris near the crater.  In fact, the large majority of all asteroids and meteorites of asteroidal origin are rich in these elements whether or not they are made of metal, simply because these elements are ubiquitous in chondritic (primitive) meteorites; indeed, they are also present, albeit in somewhat diluted form, in comets.
            The simplest interpretation of the available data, at this early point in the crater’s exploration, is that it was caused by the impact of a near-Earth asteroid or comet with a diameter of about 2.5 to 3 kilometers.  About 1000 Earth-crossing asteroids larger than 1 kilometer in diameter are presently known—and carefully tracked.
            Alternative histories?  The circular shape may be a misleading consequence of some impact-free mode of origin (such as an explosive volcanic eruption), and the platinum-group metals may also be a consequence of a deep-seated volcanic event.  Careful analysis of the proportions of all the platinum-group metals (“PGMs”) and verification of the shocked silica grains should verify or discredit the story of an impact origin of the crater. 
            Let’s keep an eye on the news about this event.  The fact that this news appears on the very day that my science fiction book “A Rending Clash of Worlds: I.  The Astronomers”, a story about the discovery of an Earth-threatening asteroid in 1871, appears as a free offering on is purely coincidental (or reflects a truly high-level conspiracy)!

Monday, November 12, 2018

Rusty Schweickart’s Vision

In the summer of 1968 I received my PhD from the University of California, San Diego, and departed on a cross-country trek with my wife Peg and our two children, Van and Meg, en route to my new job as Assistant Professor of Planetary Sciences and of Chemistry at MIT.
            The race to the Moon was in full swing.  The Soviet Zond 5 spacecraft, an unmanned precursor of a manned lunar-flyby mission, was sent around the Moon in September.  Zond 6 followed in November.  Following the Soviet plan of requiring at least two successful unmanned missions before committing a crew to the same mission profile, Zond 6 was a sobering experience: the spacecraft cabin reentered Earth’s atmosphere at so steep an angle that it would have exposed real cosmonauts to extremely dangerous, and possibly lethal, g-loads during atmospheric entry.  Success with Zond 6 would have permitted a manned Zond mission as early as January 1969.
            At this time, the Apollo program was just getting off the ground; in December 1968 Apollo 8 flew the first manned mission to visit and orbit the Moon, returning the classic pictures of Earth-rise above the lunar horizon.  Thus Apollo 8 achieved a three-man lunar orbit mission at a time when the Soviet Zond program was on the verge of sending two men on a ballistic flyby of the Moon, without orbiting.  The delay of the Soviet lunar program caused by the dangerous reentry of the Zond spacecraft (which was essentially a Soyuz man-rated capsule), combined with the two-month turn-around time for the launch pad at Tyuratam, meant that the first Soviet lunar flyby would be set back for several months.
            In March 1969, with the Soviet lunar program in disarray, Apollo 9 carried the first Lunar Module into a low Earth orbit for a test of lunar landing hardware and procedures.  Aboard that flight, the first to actually occupy and test the Lunar lander in space, was a young red-headed astronaut named Russell L. Schweickart; everyone knew him as “Rusty”. 
            In May, Apollo 10 carried the landing module into lunar orbit for thorough testing of the procedures for a manned lunar landing.  All went well, and the next Apollo mission was given a go-ahead for the first actual manned lunar landing.
            On 13 July 1969, just three days before the well-publicized launch of Apollo 11, the Soviet Union launched a smaller, unmanned probe called Luna 15, an attempt to carry out an automated landing and return a sample of the Moon to Earth.  But Luna 15 impacted the Moon without landing, and the Soviet program was again disappointed.
            Three days later, on 16 July 1969, Apollo 11 blasted off from Cape Canaveral, and the rest is history.
            Some months later the American Institute of Aeronautics and Astronautics, holding a conference in Washington DC, invited a large number of high-school students to a special event at their conference site.  They invited two representatives of the space community with very different expertise and perspectives to address these students.  Those two representatives were Apollo astronaut Rusty Schweickart and MIT planetary scientist John S. Lewis, a notorious asteroid-lover.  We had a great time, and the audience was rewardingly responsive.
            Just today, in a feature article on*, Rusty looked “Back at Apollo 9, and(Forward) to the Next Asteroid Impact”.   He puts a lot of emphasis on the threat posed by impacts of near-Earth asteroids and on their resource potential as a source of water, propellants, energy, metals, etc. for future spacefarers.  It seems that our perspectives are still remarkably parallel after the passage of nearly a half century.  But there have been changes: my brown hair is now gray, and his carrot-top is also strikingly more mature.
            It’s clear; there are asteroids in our future.  Whether the news will be good or bad depends on our decisions and choices.  I, for one, prefer good news.

Friday, November 2, 2018

3200 Phaethon-- An Asteroid or not?

 The Near-Earth Asteroid 3200 Phaethon has -- in addition to being the most frequently misspelled asteroid -- a number of odd traits that call attention to it. 

First of all, it belongs to a rare and distinctive spectral type; it's a B type asteroid.  That suggests a tribal association with the big Belt asteroid 2 Pallas, which in turn implies cold and wet.

Second, Phaethon's orbit is more like that of a comet than that of a typical asteroid: on each trip around the Sun, it dives in to a perihelion passage a mere 0.140 AU from the Sun, far inside Mercury's orbit, and then coasts out to an aphelion distance of 4.025 AU, beyond the main asteroid belt, crossing the orbits of Mercury, Venus, Earth and Mars twice each on every orbit around the Sun.  Each circuit, with eight crossings of planetary orbits, takes 523.5 days

The intensity of sunlight on Phaethon's surface ranges from 50 times the intensity of sunlight at Earth's orbital distance when Phaethon is at perihelion to 6% of normal sunlight at aphelion.  The surface of Phaethon is quite dark, with an albedo (reflectivity) of only 0.1066: more than 89% of the incident sunlight is absorbed.  At perihelion, the daytime temperature can reach a peak of over 1000 Kelvin.  Another oddity of Phaethon is that its color is unusually bluish; we can be quite sure that it is not blue ice!

Phaethon's seemingly hazardous existence is greatly prolonged by the fact that its orbit is significantly tilted out of the plane of the ecliptic, with an inclination of 22.25 degrees relative to the ecliptic.  Generally, while passing across the orbits of the terrestrial planets, Phaethon is safely out of the plane in which they orbit.  The closest it can get to Earth in its present orbit, a hair less than 0.02 AU: 0.02 x 150 million kilometers, gives us 3 million kilometers of clearance when it is at its closest; about 8 times as far away as the Moon.

The orbital inclination of Phaethon spares us from the hazard of a collision with Earth (and the other terrestrial planets) over long time periods- but not permanently.  Phaethon's diameter is about 5.8 km; for comparison, Meteor Crater in Arizona, about 1.2 km in diameter, is the product of a (relatively) low-speed impact of a metallic asteroid with a diameter about 100 times smaller than Phaethon (and thus a volume about 1 million times smaller; allowing for its high density, a mass of about one 3-millionth of the mass of Phaethon).  The impact velocity of Phaethon, because of its extremely eccentric orbit, could easily be twice as high as that of the Meteor Crater impactor, delivering about 4 times as much energy per unit mass. Thus an impact of Phaethon with Earth would deliver roughly one million times as much energy as the Meteor Crater impactor.  Picture an impact crater 120 kilometers in diameter that produced a thick debris blanket over 300 kilometers in diameter.  The dust raised by the impact would throw Earth into an Ice Age for many thousands of years.

In the near future, Phaethon will not and cannot strike Earth.  In the long run, however, there is about a 50% probability that it will hit Earth; 40% chance of hitting Venus, and a few percent each for Mars and Mercury.

Getting a sample of the material of Phaethon would surely be interesting and informative, and nature does provide Earth with samples by natural means: dust expelled from its surface forms the Geminid meteor shower, which strikes Earth in the middle of December.  Unfortunately, the entry velocity is quite high, and acquiring samples of the dust is extremely challenging.  Spacecraft missions to retrieve surface samples from Phaethon are rendered impractical (but not completely impossible) by the body’s high orbital velocity.

So what is Phaethon? Where did it come from, and how did it get in its present orbit?

One clear possibility is that it was a short-period comet that has been stored in its present orbit long enough for solar heating to dispel its near-surface volatiles.  I have seen ludicrous claims that the intense heating during perihelion passages had heated the entire asteroid to the point of baking all the volatiles out of it.  The brief heating events during perihelion passage barely scratch the surface, and can have no effect on the composition of the deep interior.  Phaethon spends virtually its entire existence soaking in the frigid environment of the outer fringes of the Asteroid Belt.

Baby, it's cold out there.