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About Asteroids


The Chelyabinsk bolide over the Urals, 2013. Image courtesy of artist Don Davis.

Want to learn more about the history and science behind these celestial objects? Below you will find valuable information on what is known about asteroids – brought to you from our team of notable scientists. We have also included a list of helpful resources on the web for further reading. And remember, you don’t need a degree in astrophysics to care about protecting Earth from threatening asteroids.

For billions of years before Hollywood made the blockbuster movie Deep Impact, Near-Earth Objects (NEOs) have bombarded the Earth. According to the popular “giant impact theory,” our young Earth (at about 50 million years old) was hit by another planet, the size of Mars. Since then, Earth has witnessed the impact of NEOs of varying sizes and frequency. While astronomers and astrobiologists are divided on the details, it is generally believed that water, rock, and hydrocarbons brought to the Earth by NEOs played a significant role in the origins of life.

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Cosmic Shooting Gallery

We are literally living in a cosmic shooting gallery. In 1980, Luis and Walter Alvarez proposed the seminal occurrence that caused the end of the Cretaceous period was the result of a large asteroid hitting Earth. After much debate within the scientific community the clinching event that brought acceptance to this claim was the 1990 discovery of the Chicxulub crater, located near Mexico’s Yucatan Peninsula. It is now generally accepted that the asteroid, about 12 km in diameter, created the global disaster which wiped out the dinosaurs, in addition to 75% of the planet’s species.

And Earth isn’t the only planet in the neighborhood to have documented cosmic collisions. In 1994, four years after the Chicxulub crater discovery, the world’s telescopes witnessed the Shoemaker-Levy comet* colliding with the planet Jupiter. As a result of these discoveries, more scientific thought has been dedicated to considering the historic and future implications NEOs have on our planet.

Just a Matter of Time

What these impact events provide is convincing evidence that being hit by another “big one” is simply a matter of time. How large is a “big one?” What is the average frequency of impact? Why haven’t we seen one in our lifetimes? Questions like these form much of the foundational work the scientific community has been exploring for the past 15 years. One key data point is the 1908 Tunguska event, when a stony asteroid, about 40 meters in diameter entered the atmosphere over Siberia and exploded over a remote and unpopulated region. Recent work has determined that the explosion was equivalent to a 10 megaton bomb. Had the impact occurred in a more populated region of the planet, say New York or London, millions of people would have been killed without any warning.

In recent years the scientific community has dedicated a significant amount of resources to detecting NEOs. The Spaceguard Survey was established and congressional hearings were held in 1993,1998 and 2005. Most of the detection and discovery of these cosmic objects is performed by several survey systems in the United States, as well as the work of dedicated professional and amateur astronomers alike. The Minor Planet Center serves as the primary clearinghouse for all NEO discovery and tracking. These efforts have permitted NASA to have now reached their goal of detection of 90% of the NEOs larger than 1 kilometer in diameter. Unfortunately, these ground-based surveys have greater difficulty in detecting smaller NEOs and have only observed about 1% of the Tunguska-sized ones.

Making Our Own History

While the effort in detecting and tracking NEOs has grown significantly in recent years, the bottom line is we have not yet done a thorough inventory of the NEO population and have not examined the orbits of  the vast majority of the objects that could impact the Earth in the future. Such a survey is difficult to conduct from Earth-based observatories because of sky brightness, weather and other complications. The Sentinel mission is being built by the B612 Foundation to take advantage of the power of an infrared space telescope to find over 90% of the objects such as the one that caused the Tunguska event. The orbits of these objects will be accurately determined so that we can identify whether any of them might present an impact threat in the next 100 years, and give us enough time to prepare and implement missions to deflect such a NEO and prevent disaster.

*The co-discoverer of this comet is Carolyn Shoemaker, who we proudly count as one of our strategic advisors.

Pictured above: 2013 DA14 and Chelyabinsk Meteor relative size. Art by Michael Carroll.


What is the difference between Near Earth Objects and Near Earth Asteroids? Where do comets fit in? Terminology can be confusing.

In general, we prefer to use the term NEA (which stands for Near Earth Asteroid). We think it gives a better understanding of what we’ll be looking for with Sentinel. Our mission is concerned with objects that are orbiting within the inner solar system.

Near Earth Objects (NEOs) is a more general term that includes both the NEAs orbiting within the inner solar system and the comets whose orbits reach into the inner solar system but may then range out further.

NEAs orbit the Sun in the vicinity of our planet and can offer astronomers the possibility of taking an inventory of these inner system neighbors.  Once we discover and start to track a NEA, we can project its orbit ahead many years and can tell whether or not it will hit us (which is really important!). Don’t worry… if we build Sentinel it is quite likely that we will first detect a NEO well ahead of any impact.

Once we detect a NEA, we may find that it will continue to miss us for the next 100 or more orbits around the Sun.  This “early warning” system will generally provide us with decades of advance notice prior to any impact.  (The rare case of discovering an NEA just prior to impact, however, cannot be ruled out—but it’s very very unlikely. In such a case, even a few days warning will permit evacuation of the impact area.)

Where do comets fit in with all of this?

To further making things interesting, comets are often labeled as either short-period comets or long-period comets, depending on whether they complete an orbital period (the time it takes to move around the Sun once) is less than or greater than 200 years.

Most comets share certain key characteristics, in particular: they are comprised of ice and dust, and they display those characteristic—and sometimes beautiful—glowing tails as they approach the inner solar system and start to warm up.

Changing course

Some comets have had their orbits altered by close approaches to the planets such that they become inner solar system dwellers.  When this happens, the comet’s outer layers of ice become rapidly depleted and they no longer have visible tails.  These burned-out comets look and act like NEAs.  (Where comets are made of ice and dust, the NEAs are rocky or metallic objects and were formed in the inner solar system.)

The challenge of deflecting long-period comets is quite different from that of deflecting near Earth asteroids.  Relatively few comets are known compared with asteroids and new ones only announce their presence (by displaying tails) as they approach the orbit of Jupiter on the way in toward the Sun.  Falling toward the Sun from the outer reaches of the solar system, these comets go faster, relative to the Earth, than asteroids. From the point when they become visible, it may only a matter of months before they pass by Earth’s orbit.  Because in theory we’d have less time to react, it would be a major challenge to intercept and deflect a comet as compared with an NEA. The good news is that there are 100 times fewer comets that come by the Earth than NEAs, and the overall risk posed by comets is less than 1% of that posed by NEAs.

An asteroid by any other name…

Now that we’ve hopefully clarified some information for you, we use the definition for NEA to include both “genuine” asteroidal NEAs and burned out “NEA-like” comets.  Generally, if it looks like a NEA, and hangs out with NEAs, we call it a NEA regardless of its birthplace.

Image courtesy of the American Meteor Society.

Q. What are Near Earth Asteroids and why is it important for us to track them?

Our planet orbits the Sun among a swarm of asteroids whose orbits cross Earth’s orbit. These are not the asteroids that make up the asteroid belt between Mars and Jupiter, but rather the Near Earth Asteroids whose orbits take them much closer to the Sun, and who regularly approach the orbit of Earth.  These asteroids are remnants of the formation of our solar system, and range in size from pebbles to many miles across.  More than half million of these Near Earth Asteroids are larger than the asteroid that struck Tunguska in 1908, and about 1,000 times more powerful than the atomic bomb dropped on Hiroshima. That asteroid was only about 40 meter across (less than the length of an Olympic swimming pool), yet destroyed an area roughly the size of the San Francisco Bay area, destroying 80 million trees over 1000 square miles.

NASA’s Spaceguard survey has succeeded in mapping 90 percent of the largest NEAs (larger than 1 km), yet due to the limitations of searching for asteroids using mostly ground based optical telescopes, only about one percent of the asteroids larger than Tunguska have been discovered and tracked to date.

We are essentially flying around the Solar System with our eyes closed.  During its mission life of 6.5 years, Sentinel will be able to discover about half a million asteroids, including more than 90 percent of those large enough to cause a 100 Megaton impact should they strike (i.e. greater than about 140 meters in size).

Q. Do asteroids pose danger to humanity?

An asteroid 140 meters across (one that would fit inside a high school sports stadium) packs an impact energy of about 100 Megatons of TNT, which is about five times larger than all the bombs used in WWII.   The consequences of such an impact would be devastating, especially in today’s highly interconnected world, let alone the direct destruction in the area of impact. The consequences are also are very difficult to state in advance because the effects depend  on the location of the exact impact point (i.e. in the middle of the ocean, in a populated area, or in the middle of the desert).   But, is this an experiment we should be willing to let happen?

The Earth is hit surprisingly often by impacts of this size or larger.  The probability of a 100 Megaton impact somewhere on Earth each and every year is the same as the probability of an individual being killed in an automobile accident each year – about .01%.  Yet most of us wear seatbelts and take other simple precautions when driving.  Why do we not take simple precautions with spaceship Earth?  Sentinel will open up our eyes so we will no longer being flying blind in the universe, playing cosmic roulette.

Q. Why isn’t NASA doing this mission?

NASA has done a great job of mapping the largest Near Earth Asteroids. Through NASA’s Spaceguard program, about 10,000 Near Earth Asteroids have been discovered and tracked during the last 15 years.  However, NASA, along with all government agencies, is under severe budget pressure right now, and there is no budgeted plan to find the remaining half million asteroids larger than the Tunguska asteroid.  If there is an asteroid out there on a collision course with Earth, the B612 Foundation believes we should know about it.  It would be unacceptable for there to be an asteroid impact on Earth even though we have the technology to know about it and prevent it, simply because we didn’t choose to solve this issue.  The B612 Foundation is choosing to solve the problem and NASA will play a critical role in Sentinel by managing communications and tracking through the NASA Deep Space Network.

Q. What kind of data will the spacecraft mission deliver?

As Sentinel orbits the Sun every 7 months, its field of view (looking away from the Sun) will sweep around the sky.  Sentinel will make repeated observations of the sky in infrared looking for objects that move: asteroids.  Sentinel will transmit the data on the locations of these asteroids back to Earth where the discoveries will be confirmed and the positions of the asteroids will be mathematically pieced together into orbits.  Sentinel will complete its survey of the inner solar system in 6.5 years, allowing asteroid locations to be forecast for more than 100 years into the future

Q. If an asteroid is found on a collision course with Earth, then what? 

With decades of advance notice, which Sentinel will provide, only a tiny change in an asteroid’s trajectory is sufficient to deflect it away from Earth (usually less than 0.02 mph change in velocity).  This makes the future task of protecting Earth relatively straightforward with several promising technologies.  Conversely with little or no notice of an impending impact (less than a few years), we limit our options to few or none – making the Sentinel Map critical to the future of humanity.

Q. How will the data be analyzed and posted?

The spacecraft and telescope will be operated by the Sentinel Operations Center, located at the Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder (CO)

  1. Data will be transmitted through NASA’s Deep Space Network
  2. Data analysis (calculation of orbits and identification of threats) will use the existing NASA data pipeline through the Minor Planet Center, Cambridge, MA, and NASA Jet Propulsion Laboratory, Pasadena, CA

Q. How might the Sentinel Mission aid in asteroid mining or other resource development? Are you in competition with Planetary Resources or how will you work together?

We do not have any official relationship with Planetary Resources or Deep Space Industries, although we do know them and support the work they are trying to do. The most easily accessible asteroids are those that pass close by Earth at a relatively (in cosmic terms) slow velocity, but asteroids can only be mined once they are discovered and tracked.   The Sentinel data set will provide the definitive list of accessible asteroids that is critical in selecting which asteroids to target for either scientific exploration or economic exploitation.  In fact, Sentinel will discover and track more asteroids in the first month of operation than all other telescopes combined throughout history.  After 6.5 years of operation, nearly all known Near Earth Asteroids (more than 98%) will have been discovered by Sentinel.

Q. How will the B612 Foundation raise the needed capital to fund Sentinel?  Why does B612 Foundation think it will be successful?

The B612 Foundation is a nonprofit 501(c) 3 organization that intends to raise the money for the Sentinel Mission in a manner similar to philanthropic capital campaigns for civic projects.  In fact the history of large telescopes is that many of them have been funded privately, so we hope to continue in that tradition.  Since the size of our raise is similar in scale to a medium sized civic capital campaign for a building such as a museum, performing arts center, or academic building, we believe raising the funds for Sentinel is feasible.  An advantage we have over these campaigns is that our target donor audience is global.  We believe our goal of opening up the solar system and protecting humanity is one that will resonate worldwide.  We have garnered the support and advice of a number of individuals experienced with successful philanthropic capital campaigns of similar size or larger, and will continue to build our network.


B612 was established in 2002. In our first ten years as a volunteer organization, our focus was on asteroid deflection research and advocacy. The team developed several deflection concepts, ones that are accepted today as standard techniques to prevent an impact. The following blog posts were issued/posted prior to B612 deciding to build, launch and operate a telescope in June 2012.


Ed Lu speaks at TedX NASA

Decision program on asteroid threat mitigation

Joint study with JPL

National Research Council Report on NEO hazards

NASA Advisory Council Report on Planetary Defense

JPL Evaluation of Gravity Tractor

Congressional hearing on NASA’s NEO Report

NASA’s NEO Report to Congress

Independent Analysis of Alternatives to Divert NEO

NASA NEO Workshop 2006

NEOs: The Katrinas of the Cosmos?

Towing Asteroids with Gravitational Tractor

B612 Foundation Press Statement

Better Collision Insurance

National Space Society, International Space Development Conference

World Federation of Scientists’ Multidisciplinary Core Group on Planetary Emergencies

World Federation of Scientists’ International Seminars on Planetary Emergencies

Testimony of Rusty Schweickart and Ed Lu

The hazard of near-Earth asteroid impacts on earth

AIAA 2004 Planetary Defense Conference; Protecting Earth From Asteroids

The Asteroid Tugboat

Letter to the President

OECD Global Science Forum Workshop on Near Earth Objects

The Gravity Tractor deflection concept

Asteroid Apophis and the B612 exchange with NASA

Conceptual illustration of a gravity tractor. Art by Dan Durda.

Learn More on the Web

Near-Earth Object Program, NASA/JPL

American Meteor Society

NEO Shield

Meteor Crater

Minor Planet Center

Planetary Defense Conference (2013)

Impact Earth! (Game)

Defending Planet Earth – report of the National Academies on asteroid surveys and deflection

Report of the NASA Advisory Council on Planetary Defense

A Brief Note on the Economic History of Space Exploration in America, by Alexander MacDonald

Asteroid Threats: A Call for Global Response, Association of Space Explorers, October 2008

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