Asteroids: An Introduction

The term “asteroid” was coined by German-born astronomer William Herschel in 1802 following his discovery of Ceres, the first “star-like” object. (From the Greek, asteroeides: aster “star” + -eidos “form” or “essence.”) In the inner solar system, the majority of asteroids are rocks. Some are dirty ice balls (which may appear as comets), and a few are essentially solid metal. Viewed with a telescope, asteroids appear as points of light, like stars. But they move across a stationary stellar backdrop, and their brightness is only the reflection of starlight (from our sun), bouncing off their gray-black surfaces.

Near-Earth asteroids (NEAs), also called near-Earth objects (NEOs), orbit the sun, like all asteroids, but their paths bring them “close” to Earth (within about 45 million kilometers). The first NEA was discovered in 1898, and was called Eros, because it seemed attracted to Earth. Eros (about 35 × 10 × 10 kilometers) was also the first asteroid to be orbited by a robotic spacecraft (NEAR Shoemaker), in 2000.

Today, we know the location and orbits of about 10,000 NEAs. Nearly 1,000 of these are one kilometer in diameter, or larger, and are classified as “civilization killers,” because of the devastation that would result if they were to hit Earth. We’ve discovered most of these giants (more than 90%), but by no means all. Eighteen were discovered in 2012, and two more in the first quarter of 2013.

The biggest challenge now is finding the smaller ones: down to 140 meters, and down to 40 meters. On impact, these could destroy part of a continent, or a city. There are something like a million 40-meter NEAs. We’ve found less than 1%.

Near-Earth Objects: Finding Them Before They Find Us (Princeton, 2012), by Donald Yeomans, the director of NASA’s Near-Earth Object Program Office, is an excellent book-length introduction to NEAs.

The vast majority of asteroids are found in the main belt, between Mars and Jupiter. The main belt constantly replenishes the near-Earth asteroid population.

Potentially Hazardous Asteroids (PHAs or PHOs) are defined as asteroids that are 100 to 150 meters in diameter, or larger, and located in orbits that match the Earth’s orbit even more closely than NEAs. Much smaller asteroids can cause significant damage. The explosion over Chelyabinsk in mid-February 2013 was caused by an asteroid some 17 to 20 meters in diameter.

If an asteroid enters the Earth’s atmosphere it will burn, resulting in a light show that can be explosive. This light show is called a “meteor.” Any rocks that survive atmospheric entry, and are then found on Earth, are called “meteorites.” More than 50,000 meteorites have been discovered.

On 15 February 2013, at about 09:20 local time, a small asteroid (17 to 20 meters in diameter) entered Earth’s atmosphere just east of the Ural Mountains, traveling at about 18 km/second (40,000 miles/hour). The energy released by the air-burst was equivalent to an explosion of more than 440 kilotons of TNT. It was more powerful than 30 “Little Boy” atomic bombs. (Thankfully, asteroids are not radioactive; their impacts do not produce nuclear explosion.) The asteroid that produced the Chelyabinsk explosion was not observed before impact, partly because it came at the Earth from out of the sun. It could have been observed fairly easily, when it was on the opposite side of its orbit, out beyond Mars, if a dedicated search program had been established earlier.

On the same day, an expected, and somewhat larger asteroid (~30 meters), designated “2012 DA” (meaning that it was discovered in 2012), just missed Earth, after passing within 30,000 km of Earth’s surface, closer than our planet’s ring of geosynchronous communication satellites (about 35,000 km above the equator). The approach was so close, Earth’s gravity altered the asteroid’s orbit.

This graphic offers a sense of just how big these NEAs are.

On 30 June 1908, a little after 07:00, a slightly larger asteroid (~40 meters) entered the atmosphere and exploded above Siberia. That air burst is estimated to have been equivalent to around five megatons of TNT. The blast devastated over 2,000 square kilometers of forest, flattening millions of trees. Current estimates suggest that there are a million NEAs that are this size or larger, with expected Earth impacts every few hundred years, or so. We’ve discovered less than one percent of them.

About 50,000 years ago, a metallic asteroid (~50 meters), which was tough enough to pass through the atmosphere without exploding, slammed into the Arizona desert. It left a crater about 1,200 meters in diameter and 170 meters deep — large enough to swallow downtown San Francisco.

This one still pisses off the dinosaurs. The responsible asteroid was ~10 km in diameter (or possibly smaller), and it left a crater that’s 180 km in diameter (or possibly 300 km). Its impact triggered massive earthquakes, volcanic eruptions, and tsunamis. Earth was plunged into an “impact winter” for years, with planet engulfing clouds of ash and dust blocking photosynthesis. This cataclysm is the most likely cause of the Cretaceous–Paleogene extinction event, 65 million years ago, in which more than 75% of all animal and plant species died out. It was, however, good news for our “tree-climbing, furry-tailed, insect-eating” ancestors.

The Sentinel project would put a telescope into an orbit around the sun that is interior to Earth’s orbit, so that it can observe asteroids by “looking out,” away from the sun. From that vantage point, NEA’s can be detected when they are maximally lit by reflected sunlight. The Sentinel telescope will be located in a “Venus-like” orbit, looking out, and scanning the path of Earth’s orbit every 225 days or so (the length of a Venusian orbit).

Sentinel technology is derived from two successful NASA telescopes, the Spitzer telescope (launched in 2003), which utilized a cryogenic cooling system similar to the one that Sentinel will use, and the Kepler telescope (launched in 2009), which provides heritage avionics and structural components. The Sentinel telescope will detect light in the infrared, which allows for more effective asteroid discovery and for more accurate determination of asteroid sizes. The B612 Foundation has signed a no-cost, Space Act Agreement with NASA, which provides for use of NASA’s Deep Space Network (DSN) to transmit data, and for the public release of all data collected, following an optional six-month embargo.

Scheduled for a SpaceX launch in 2018, Sentinel is expected to discover 1,000,000 NEAs during its operational life of 6.5 years, for a total cost of about $500 million. The Sentinel is not a science mission. It is designed to use tested and well known technology, and it is not expected to uncover new scientific knowledge. It is intended to accomplish one goal: Find and map a million near-Earth asteroids.

Operated by the Smithsonian Astrophysical Observatory (SAO), under the auspices of the International Astronomical Union (IAU), the MPC records and publishes all asteroid observations. The IAU website offers a detailed chronology of many NEA milestones.

NASA’s NEO program, run by the folks at the Jet Propulsion Laboratory (JPL), was set up to “coordinate NASA-sponsored efforts to detect, track, and characterize potentially hazardous asteroids and comets that could approach the Earth.”

NASA established the Small Bodies Assessment Group (SBAG) in 2008 to “identify scientific priorities and opportunties for the exploration of asteroids, comets, interplanetary dust, small satellites, and trans-Neptunian objects.”

The “Origins Spectral Interpretation Resource Identification Security Regolith Explorer” (OSIRIS-REx), is NASA’s third New Frontiers program. Planned to launch in 2016, it will visit the near-Earth asteroid 1999 RQ36 (a potential Earth impactor) to gather and return a mineral sample to Earth. OSIRIS-REx follows in the footsteps of the first successful asteroid sample return mission, which was run by Japan’s Aerospace Exploration Agency (JAXA). In 2010, the Hayabusa returned to Earth a pinch (rather than the hoped for handful) of regolith that it had gathered from the surface of Itokawa, a Mars-crossing NEA. OSIRIS-REx is intended to accomplish several science objectives, including the return of up to two kilograms of regolith to Earth in 2023, at a cost of about one billion dollars.

The Committee on the Peaceful Uses of Outer Space (COPUOS), in Vienna, has been studying the technical and societal challenges presented by potentially destructive asteroid impacts for several years.

In response to the 1991 recommendations of UNISPACE III, COPUOS established an “Action Team” on near-Earth objects in 2001 (AT-14), which reports to the COPUOS Scientific and Technical Subcommittee. In December 2012, AT-14 delivered their final report, which noted that “the first step in addressing the risk posed by an NEO [is] to detect its presence and determine its trajectory.” In addition, they were “pleased to learn that the B612 Foundation … was continuing the development of its Sentinel infrared space telescope.”

COPUOS established a Working Group on NEOs in 2007, which regularly contributes an appendix to the annual report of the Scientific and Technical Subcommittee (Annex III). Their presentations to the Subcommittee in February 2013 included a recommendation to create an “International Asteroid Warning Network” (IWAN), which emphasizes the need to “find them as early as possible.”

The Space Situational Awareness program (SSA) of the European Space Agency (ESA) recently initiated an NEO Segment. They plan to launch a new “Small Bodies Data Centre” in May 2013, which will include priority listings of NEAs most in need of new observational data.

The International Academy of Astronautics (IAA), in association with ESA, and others, is holding their third planetary defense conference on 15-19 April 2013, in Flagstaff, Arizona.

A hearing: “Threats from Space: A Review of U.S. Government Efforts to Track and Mitigate Asteroids and Meteors, Part 1” (19 March 2013).

Before the hearing, Congresswoman Donna Edwards (D-MD), ranking minority member of the House Subcommittee on Space, wrote a Washington Post Op-Ed, along with Congressman Rush Holt (D-NJ), former assistant director of Princeton’s Plasma Physics Laboratory, in which they note, “Many countries lack the United States’ sophisticated sensors that can help determine whether a large explosion is nuclear in nature. The damage that could occur if a nation were to misidentify a meteor explosion and launch a counterattack is chilling.” They conclude, “We should make the investments necessary to track near-Earth objects and prepare for disasters of all kinds.”

During the hearing, Dr. John Holdren, director of the White House Office of Science and Technology Policy (OSTP), testified, “The most important single thing we could do to improve our capacity to see any asteroid of potentially damaging size coming, would be [to launch and operate] an orbiting infrared telescope of the sort that the B612 Foundation is working on.”

A hearing: “Assessing the Risks, Impacts, and Solutions for Space Threats” (20 March 2013).

The New York Times reported that the Committee chairman, Senator Bill Nelson (D-FL), as well as the ranking minority member, Senator Ted Cruz (R-TX), “expressed little skepticism about the scientists’ testimony.”

Dr. Edward Lu, CEO of the B612 Foundation testified, “The probability of a 100-megaton asteroid impact somewhere on Earth this century is about 1%. The odds of another [Tunguska-like] five-megaton event this century are much higher, about 30%…. We know the locations and trajectories of the million nearest stars, because our telescopes can look away from the sun. But we do not know the locations of the million nearest asteroids, and yet those things [sometimes] hit the Earth.”

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