The first drop in the asteroid-moving bucket

 kw: science, orbital mechanics, asteroids, near-earth asteroids, dart mission

Didymos is a small asteroid, about 3/4 km in diameter, or less than half a mile. Not quite mountain-sized, more like a big hill. It has a satellite, named Dimorphos, 170m in diameter, the size of a five-story office building. Unlike an office building, which is 90+% air, it is a big rock. It weighs about five million tons.

This orbiting pair have the great good fortune to be in an orbit that is "reachable" by spacecraft. Not quite Earth-crossing, but much closer than most. As an orbiting pair, these rocks are also much easier to study dynamically. In particular, the orbit of Dimorphos about its common center of gravity with Didymos can be well-characterized, and very small changes in its orbit can be detected. Changes such as the one that will soon occur.

In a couple of weeks, a NASA spacecraft will be smashed into Dimorphos at high speed, and the collision will be watched by a companion spacecraft (the little one shown at center left in this NASA publicity image); and also from ground-based telescopes, symbolized by the icons at lower left.

How sensitive a "probe" is this little satellite? The key is its orbital velocity and period, which can be measured with great precision.

The collision, on September 26, 2022, will be made on the leading side, so it will slow Dimorphos down a tiny bit, moving it into an orbit just a bit closer to Didymos. The key parameters are these:

• Mass of Dimorphos: 4.8 billion kg, equal to 4.8 million metric tons or 5.3 million short tons.
• Mass of the DART impactor: about 500 kg on impact. It starts out weighing 610 kg, and burns propellant to ramp up to speed.
• Impact relative velocity: 6.6 km/sec, equal to 4.1 mi/sec or almost 15,000 mph. For comparison, the orbital velocity of Earth around the Sun is about 67,000 mph or 29.9 km/sec.

The minimum change in the velocity of Dimorphos is calculated by assuming a perfectly inelastic collision. It comes to 0.6875 mm/sec. 

NASA writes that they expect an orbital velocity change of 0.4 mm/sec; perhaps they are assuming lots of kinetic energy will be lost to vaporizing the DART impactor and about a desk-sized chunk of Dimorphos. NASA expects this small slowing of the moonlet to move it into a slightly smaller orbit, such that it will speed up a bit and each orbit will take about 10 minutes less than before. The orbital period is presently just under 12 hours.

It occurs to me that NASA's figure of 0.4 mm/sec must represent the expected increase in orbital velocity resulting from the shrinking of the orbit. Perhaps the instantaneous "bump" will be of the order of 0.7 mm/sec of slowing, but in the resulting, smaller orbit, Dimorphos will have an average velocity as NASA has calculated (their math whizzes are certainly much more savvy than I!).

That's as far as my amateurish figurations can take me. I'll stay tuned to the NASA webcast on the 26th! This is step 1 in figuring out what it takes to shift the orbit of any asteroid that is found to be threatening Earth.

A plea for ensuring human survival in a dangerous universe

kw: book reviews, nonfiction, polemics, space policy, NEAs, NEO's, near-earth objects, near-earth asteroids, cosmic risk, terrorism, superweapons

For at least 2,000 years, the moon and other heavenly bodies were considered perfect, the Earth (and Hell beneath) being the sole demesne of imperfection, sin, and suffering. Aristotle was probably not the first to promote such a view, but was certainly the most influential. Following his lead, the Catholic church built its theology on an Earth set but one step above Hell and Purgatory, with the Heavens (plural concentric abodes) above; the Earth being a place of testing to determine whether one became heavenly and arose, or hellish and sank.

Just under 400 years ago, Galileo first looked at the moon through a small telescope. He saw not the perfect sphere he'd imagined, but a ruined, fully cratered body. However, as late as the 1960s, a strong contingent of astronomers and othes contended that the craters were volcanic, taking a cue from Jefferson, who'd said, "I'd rather believe a Yankee professor would lie than believe stones fall from the sky". The underlying unease was at least partly religious.

Once we saw Mars and Mercury up close, with spacecraft, however, it could not be denied that something had banged them and the Moon about quite thoroughly. The current theory describes an early Earth having its entire outer portion (at least a few hundred kilometers deep) melted by impacts, more than once; even the moon is now seen as originally impact debris, ejected when the Earth was hit by something the size of Mars—this must have happened early enough for it to get quite beat up after such an origin. But the same theory puts an end to this "Hadean Era" at about 3.9 billion years ago.

I visited Barringer Crater near Winslow, Arizona once. It is quite impressive; nearly a mile across and 570 feet deep. The mini-asteroid that caused it came in about 50,000 years ago, and was perhaps 150 feet in diameter. This isn't the only recent or "nearly recent" meteorite impact crater. This set of lists in Wikipedia lists 36 well-known craters by size, then 182 (including a few clusters of craters as one item) in a continent-by-continent list.

Half of the craters listed exceed five miles (8 km) in diameter, and the two largest are over 300 miles across. The Chicxulub crater, scar of the impact that destroyed the dinosaurs, is just over 100 miles across; four of the known craters are larger than that, and one of these that is offshore of India is the same age, so maybe there were several dino-killers, perhaps a string of chunks like the broken comet that peppered Jupiter in 1994.

Some statistical points: The craters are clustered in age-size space. Craters older than 10 million years range upwards from about 1.3 km in size, with no visible trend. Younger craters show a size versus age trend, which is probably due to smaller craters being eroded quickly. Interestingly, the third-youngest crater listed is Mahuika crater, offshore of New Zealand, about 600 years old and 20 km in diameter. That's big enough to have multi-year weather effects, which may explain the century-long cooling after about 1420 AD.

This brings us to the major point of The Survival Imperative: Using Space to Protect Earth by William E. Burrows, the reason he wrote the book. Several (about 4?) tons of meteoric dust settles on Earth's surface each day; a few pebble-size or larger meteorites land each month, somewhere; a chunk big enough to crash through a roof does so about every decade (if we count the oceans and uninhabited places, it could be several per year). What do we know about larger, more damaging objects?

Researchers such as Louis Frank state that a number of meter-scale to "house size" comets hit the Earth's atmosphere daily, depositing mainly water. Based on dust-gathering studies, these little comets must have very little solid content.

The crater lists I mention above give us a rough idea, which competent statisticians restate thus: the chance of another Barringer Crater object (50 meter size) coming in is about one per ten thousand years. At kilometer size, it is one per few million years. The dino-killer was probably about six kilometers in size, and arrived 65 million years ago. Two of nearly that size hit about 35 million years ago, and one about 2 km across hit some five million years ago, causing a 50-km (30 mile) crater.

We know how big a blast a particular asteroid will create: it is all physics, once you know its mass and velocity. Size isn't the only criterion, because asteroids that circle the sun in the "normal" direction (prograde orbits) can hit Earth with speeds mostly under 25 km/s, while those going the opposite way (retrograde orbits) can hit at a speed as high as 75 km/s, which means ten to twenty times the impact energy.

Author Burrows wishes to influence policy so that we'll create and maintain a permanent presence in space, with the aim of preserving human life in the face of the next big asteroid to come our way. However, his is no simple nor single goal, as he details throughly. We are at risk from a violent universe, it is true. We are also at risk from our own violence. To put his thesis simply, if human life is made extinct, there is a roughly equal chance that it will occur from our own use of superweapons as from external asteroids and other cosmic causes.

However our potential demise is figured, Burrows and others wish to have a backup strategy. First and foremost, he desires a human presence in multiple off-Earth locations, first the Moon, then one or more large orbiting colonies. This is getting some of our eggs out of the single basket in which they currently reside. A lunar colony would have as its main aim keeping a breeding population alive through any disaster Earth might suffer and keeping a record of all of earth's cultural, scientific, and artistic attainments. This is backing up your hard drive in a big way (and how many of us actually do so?).

With equally great threats from Earth (evil or stupid people with megaton bombs or bioweapons) and from space, there would be two other significant thrusts to a space endeavor, but probably physically located in other venues. One is detection and exposure of superweapon preparations by rogue states (he has a long discussion of North Korea), the other is detection and diversion of NEAs, near-earth asteroids. Pitifully little money is being supplied to the latter endeavor, but one day of the Pentagon's budget per year would make it a much more robust, and effective, enterprise.

Thus the book is a long polemic (positively speaking) toward these aims. Sadly, the writing is too scattered to easily grasp. The author appears to have a hop-toad mind, and the focus jumps accordingly. It is written on too high a level to hold the interest of the average congressman or senator. Also, just as very few of us, and even very few corporations, do a really good job of backing up our computer data, the human race is probably way, way to short-sighted to do much of anything before Rome or San Diego vanishes into the next crater.