Earth Versus the Asteroids

Recent news and thoughts on near-Earth asteroid encounters and detection

Earth's History is Written on the Moon

Eight days of Arecibo radar images of binary NEA "2000 DP107." The primary is 800-meters across and the secondary 300-meters. The secondary has a 42-hour orbital period at a distance of 2.6-km from the primary. Like our own Moon, the secondary has a rotation period that matches its orbital period; the secondary thus shows the same "face" to the primary. Jean-Luc Margot, Caltech

The Moon clearly bears all its scars from the past few billion years. Passing asteroids and comets have left their mark on our satellite: craters contain craters that contain craters, down to the smallest dimple in the lunar dust. Next time you peer at the marred faced of our nearest celestial neighbor, consider that the Earth and Moon shared the bombardments that shaped those craters. Yet at first glance, craters are not prominent in images of the Earth.

Weather, volcanism, and continental drift are some of the forces that constantly reshape our planet: water can erode mountains and cause glaciers to gouge the land; blowing sands can fill and mask geologic features; exploding volcanoes can remodel the landscape or poke-up islands in the oceans; drifting continental plates rearrange the land-mass of the whole planet -- uplifting mountains, drying up seas by pinching off inlets, or forcing the subduction and recycling of plates into the mantle. Over time, this constant reworking of our planet's topology has erased the vast majority of impact craters that scarred our planet.

But evidence from the Moon, deposits of telltale isotopic material in geologic strata, and the record of recurring extinctions can not be ignored: on the order of every tens of million years, our planet takes a regular punch from outer space. Modern satellite imagery and detective work has even allowed scientists to identify the eroded remains of 28 large impact craters, of at least 20-km diameter, that must have devastated the ecosphere when they formed.

Asteroid Nuisance

Just two generations ago the increasing number of asteroid discoveries was thought to be a nuisance. Not a nuisance to the Earth, but a nuisance to astronomers that had to calculate orbits and catalogue them.

"Discoveries of new minor planets are not encouraged at the present time, unless they are unusual in some respect."

 -- Sky & Telescope, May 1952. Quoted in the "50 & 25 Years Ago" column of S&T for May 2002

Times have changed since the above, pre-space age, quote: something must have created those 28 craters! Large, wandering asteroids could be the culprits that dug such holes in our planet. Astronomers are keenly searching for new asteroids and calculating their orbits to see if they pose an impact threat to us. Not a month goes by without a report of a new near-Earth asteroid (NEA), detected as it sailed by our planet. The media now regularly transmits news of potentially hazardous asteroids (PHAs) to the public. One such example happened this spring, when radar observations were made to refine the size and orbit estimates of an NEA known as "1950 DA." 

Scientists had such confidence in this radar data that they announced to the world their results: the 1-km wide NEA had a 1-in-300 chance of colliding with Earth on March 16, 2880! Striking with an explosive force of a thousand times that of the largest tested hydrogen bomb, a new 20-km class crater would gouge the planet at the center of a 300-km wide area of devastation. In just two generations, scenarios like this have forced scientists to think seriously on the subject of PHAs. Enhanced detection systems may give future generations even more to ponder.

"That's plenty of time to consider the options -- 35 generations, in fact." 

-- Jon Giorgini, a NASA / Jet Propulsion Laboratory senior engineer, commenting on the threat of 1950 DA to the Earth.

Future Threats from Past Encounters

Remnants of material leftover from planetary formation, the asteroid belt between Mars and Jupiter is the source of NEAs. Gravitational interaction with the planets -- mainly Jupiter -- gradually perturbs the orbit of some asteroids to pass from the belt into the inner solar system and become NEAs. Due to the vastness of Earth's orbit, most NEAs pass harmlessly though our orbital path. Some make numerous harmless passes by the Earth, which has spawned a recent theory that considers the threat of the Earth to the NEA!

Recent evidence suggests that NEAs are composed of a loose conglomeration of material -- cosmic rubble piles. A spinning NEA that barely skirts our planet (by a few thousand miles) can be spun-up by the tug of Earth's gravity. This can cause the NEA to break-up into two lumps that orbit each other: a binary asteroid. But, that's not the last Earth sees of the NEA. Perhaps as a sequel to the binary-formation encounter, the NEAs "strike back" on a later flyby: 3 of the 28 largest craters on Earth are double impacts, presumably from binary object encounters.

Analysis of NEA light curves and radar discovery of 5 binary NEAs bolster a theory that 1-of-6 NEAs larger than 200-meters are binary systems. All the binary NEAs spin faster than their single relatives. With an estimated lifetime of 10-million years, there must be numerous encounters with the Earth or other inner planets to maintain a population of binary NEAs.

Defense Starts with Detection

Representation -- not to scale -- of how the Gaia spacecraft can cover most of Earth's "blind zone" in regards to asteroid detection. The Earth (blue orbit) and Gaia (brown orbit) circle the Sun together in the same time period. The highlighted area, above the green horizontal line, is in the daylight sky of Earth: asteroids there cannot be detected because daylight fills the sky with light. Gaia can detect asteroids in most of this region -- except for the red crosshatched area where its sensors would be blinded or blocked by the Sun. ESA 2002

Ground-based observations have detected the bulk of known asteroids. But as WWI fighter pilots learned, it's impossible to visually spot the enemy when they come diving out from the Sun. In the case of ground instruments, the Sun fills the whole daylight and twilight areas of the sky with sensor-blinding light. A recent example of this was NEA "2002 EM7", which whizzed by Earth in March this year. Once it crossed into the night-facing part of the sky, it was it detected; then it quickly crossed back and disappeared into the daylight sky. Only a space-based survey can detect asteroids "diving" out of the sunlit sky. Help in this department will come from a star-mapping probe, to be launched at the end of the decade.

The European Space Agency's (ESA's) upcoming Gaia spacecraft, with a launch date of 2010, will conduct a 5-year mission to make the most precise map of stars in our part of the galaxy. It will be placed in a special orbit: at a location 1.5-million km further from the Sun than Earth. At this position, known as "L2" (Lagrangian point 2), the spacecraft will be "pulled" along by the Earth, and so the spacecraft will always be opposite the Sun -- in the nighttime sky. This gives Gaia an excellent vantage point to detect NEAs, where ground telescopes can't. A study indicates that Gaia's mapping equipment could generate orbits for NAOs that are 30 times better than any observation from the ground.

More information:

Cornell University; April 11, 2002; Press Release: "Radar reveals five double asteroid systems orbiting each other near Earth, likely formed in close encounters with planet"

ESA.int; April 12, 2002; ESA Science News: "European Space Agency to probe asteroid blind spot"

BBC.co.uk; April 5, 2002; Sci/Tech: "Asteroid could hit Earth in 2880"