Frequently Asked Questions

asteroid Vesta
Fireball Meteor Over-Groningen
comet 67P/Churyumov-Gerasimenko

  

Questions

What is a near-Earth object?    
How often do near-Earth objects come close to Earth?    
Do we know of any asteroids that pose a threat to Earth?    
How are NEOs found and tracked?    
What is a NEO close approach?    
How can we prevent an asteroid from hitting Earth?    
What is planetary defense?    
What does the Planetary Defense Coordination Office (PDCO) do?    
What does the Near Earth Object (NEO) Observations Program do?    
What is an asteroid?    
What is a comet?    
Is it likely that an asteroid will impact Earth in the near future?    
How many near-Earth asteroids have been discovered so far?    
How good are we at finding and tracking NEOs?    
What can be done to improve the NEO detection rate?    
How are NEOs characterized?    
How do NEO scientists assess asteroid impact risks?    
Does Bennu, the asteroid that will be studied by the OSIRIS-ReX spacecraft, pose an impact risk to Earth?    
Is it likely that an asteroid will impact Earth in the near future?    
Would it be possible to shoot down an asteroid that is about to impact Earth?    
Why is it necessary to characterize a NEO that might impact Earth in order to develop a plan for an impact mitigation mission?    
What is the role of planetary radar in NEO observations?    
Can planetary radar be used to search for near-Earth objects?    
What is NASA’s Near-Earth Object Observations Program funding the Arecibo Observatory to do?    
What is NASA’s role in the Large Synoptic Survey Telescope (LSST) project?    
How will LSST contribute to finding, tracking, and characterizing near-Earth asteroids?    
How are human-accessible asteroids identified?    

Answers

What is a near-Earth object?
A near-Earth object (NEO) is an asteroid or comet whose orbit periodically brings it within approximately 121 million miles (195 million kilometers) of the Sun – that’s within about 30 million miles (50 million kilometers), of Earth’s orbit. Like the planets, all asteroids and comets orbit the Sun, although some asteroids also are in orbit around planets or even larger asteroids. Some of the smaller moons of other planets may be captured asteroids. Most asteroids are in what is called “the main belt” between Mars and Jupiter. The vast majority of near-Earth asteroids have come from inner part of the main belt where, over tens of millions of years, their orbits were altered by the gravitational influence of Jupiter and Mars, and some by mutual collisions. Although the vast majority of NEOs that enter Earth’s atmosphere disintegrate before reaching the surface (and more than 100 tons of dust particles disintegrates in Earth’s atmosphere daily), those NEOs that are larger than around 98 to 164 feet (30 to 50 meters) in size may survive the descent and could cause widespread damage in and around their impact sites.
Back to Questions
How often do near-Earth objects come close to Earth?
Small asteroids a few meters in size are detected passing between Earth and the Moon’s orbit several times a month. Meteoroids – very small fragments of asteroids and comets less than 3 feet (1 meter) in size – hit Earth’s atmosphere and explode virtually every day, causing the bright meteor events that people see at night and sometimes leave remnants – meteorites – on the ground. The Jet Propulsion Laboratory’s Center for NEO Studies maintains close approach tables that are updated daily.
Back to Questions
Do we know of any asteroids that pose a threat to Earth?
No known asteroid poses a significant risk of impact with Earth over the next 100 years. The highest risk of impact for a known asteroid is a 1 in 714 chance of impact by an asteroid designated 2009 FD in 2185, meaning that the possibility that it could impact then is less than 0.2 percent. The Sentry Impact Risk Table, which is maintained by the Jet Propulsion Laboratory’s Center for NEO Studies, is updated continuously as new asteroids are discovered and known asteroids are further observed.

One asteroid that NASA is studying up close, called Bennu, has a 1/2700 change of impacting Earth between 2175 and 2195. The OSIRIS-REx spacecraft will complete a 2-year investigation of Bennu before plucking a sample of asteroid material off its surface and sending it back to Earth. Along with collecting a sample, OSIRIS-REx will also be studying how the Sun’s heat affects Bennu’s orbit—and consequently, how that orbit could become more dangerous for Earth. Read more about the OSIRIS-REx mission’s contribution to planetary defense here.
Back to Questions
How are NEOs found and tracked?
Observers find and track NEOs using ground-based telescopes around the world, and, currently, NASA’s space-based NEOWISE infrared telescope. The basic method of finding NEOs is to look for small objects moving across the background of relatively fixed stars. Observers track NEOs by using their predicted orbits, based on initial observations, to look for the objects at the time and in the place where they have been predicted to be visible to telescopes again. It takes a week to a month of observations for scientists to establish a good orbit determination. Observers provide their data to a global database maintained by the Minor Planet Center, which is sanctioned by the International Astronomical Union and funded by NASA’s NEO Observations Program.

Watch these two videos for a deeper look into asteroid-hunting:
How do we spot NEOs?
The international asteroid hunt, how the hunt is coordinated by NASA

Back to Questions
What is a NEO close approach?
A NEO close approach occurs when an object passes by Earth, but it is of particular interest when it passes within the distance from the Earth to the Moon, or a “lunar distance”. NEO close approaches are often measured in lunar distances (1 LD=approximately 240,00 miles, or 384,000 kilometers). The Jet Propulsion Laboratory’s Center for NEO Studies maintains close approach tables that are updated daily.
Back to Questions
How can we prevent an asteroid from hitting Earth?
Currently, an asteroid impact is the only natural disaster we might be able to prevent. There are a few methods that NASA is studying to deflect an asteroid on a course to impact Earth. One of these techniques is called a gravity tractor—it involves a spacecraft that would rendezvous with an asteroid (but not land on its surface) and maintain its relative, optimal position to use the mutual gravity attraction between the satellite and the asteroid to slowly alter the course of the asteroid. A gravity tractor spacecraft could even enhance its own gravitational attraction by first plucking a boulder off the surface of the asteroid to add to its own mass.

A kinetic impactor is currently the simplest and most technologically mature method available to defend against asteroids. In this technique, a spacecraft is launched that simply slams itself into the asteroid at several km per second speed. Scientists will test the kinetic impact technique by the Double-Asteroid Redirect Test mission (DART) on an asteroid system called Didymos in 2022. DART’s target is a binary asteroid system where one football-stadium-sized asteroid (Didymos B) is orbiting a half-mile-wide asteroid (Didymos A). NASA’s goal is to send the car-sized DART spacecraft slamming into Didymos B at 25,000 kilometers per hour (16,000 miles per hour) to determine by how much the impact can shift the orbit of Didymos B around Didymos A. After all, we’d only need to nudge an asteroid’s orbit enough to make it either seven minutes early or seven minutes late in its intersection with Earth’s orbit. It takes seven minutes for the Earth to travel the distance of its diameter, so if an asteroid arrives seven minutes early or late—it’ll miss us completely.

Nuclear explosive device methods are considered the last resort when it comes to NEO deflection, though they may be the most effective for preventing a cataclysmic event. When warning time is short or the asteroid is large, deploying a nuclear device is the best option despite the taboo nature of the use of nuclear weapons in space. A standoff detonation is the method with the most controllability and predictability for using a nuclear device to deflect an asteroid. This method works by detonating a nuclear device at a few hundred meters above the surface of the asteroid. The energy from the device is primarily in the form of X-rays, which near instantly strike the surface of the asteroid. The material in the top layers of the asteroid is super-heated and vaporized by this radiation, causing a blow-off of material from the surface. The momentum push from the vaporized and blown off surface material imparts momentum to the rest of the asteroid and pushes it onto a new trajectory. Therefore, it is not the force from the explosion itself that moves the asteroid but rather the force of the radiated energy onto the surface of the asteroid.
Back to Questions
What is Planetary Defense?
Planetary defense is the term used to encompass all the capabilities needed to detect the possibility and warn of potential asteroid or comet impacts with Earth, and then either prevent them or mitigate their possible effects. Planetary defense involves:
  • Finding and tracking near-Earth objects that pose of hazard of impacting Earth;
  • Characterizing those objects to determine their orbit trajectory, size, shape, mass, composition, rotational dynamics and other parameters, so that experts can determine the severity of the potential impact event, warn of its timing and potential effects, and determine the means to mitigate the impact; and
  • Planning and implementation of measures to deflect or disrupt an object on an impact course with Earth, or to mitigate the effects of an impact that cannot be prevented. Mitigation measures that can be taken on Earth to protect lives and property include evacuation of the impact area and movement of critical infrastructure.
  • NASA Planetary Defense Coordination Office feature on International Asteroid Day.
    Back to Questions
    What does the Planetary Defense Coordination Office (PDCO) do?
    A detailed description of what the PDCO does can be found on our "About PDCO" page.
    Back to Questions
    What does the Near Earth Object (NEO) Observations Program do?
    A detailed description of what the NEO Observations Program does and what role it plays in planetary defense can be found on our "About PDCO" page.
    Back to Questions
    What is an asteroid?
    An asteroid is a small, naturally occurring, solar system body that orbits the sun. Asteroids are typically composed of rock-forming minerals, most commonly olivine and pyroxene. However, they often contain metal (iron and nickel), sulfides (chemical mixtures of metals and sulfur), clays, and organic compounds. The structure and composition of asteroids vary from object to object.

    Most asteroids in our solar system reside in the region between Mars and Jupiter known as the Asteroid Belt (sometimes called the “main asteroid belt” or “main belt”). Scientists estimate there are many hundreds of thousands of asteroids 0.6 mile (1 kilometer) or more in size in the main belt, along with millions of smaller objects. Most asteroids are fragments of larger bodies that broke up due to collisions in the early part of solar system history. Only a few of the very largest asteroids have remained intact.

    Based on studies of meteorites that have fallen to Earth and telescopic studies of asteroids, scientists have learned that most meteorites are fragments of asteroids that were broken off during collisions. Chemical and age-dating studies of meteorites have shown that asteroids formed about 4.5 billion years ago. Studying asteroids provides a way of looking back into processes and conditions that existed during the formation of the solar system.
    Back to Questions
    What is a comet?
    A comet is a small body composed mostly of dusty material embedded with icy volatiles, such as water and carbon dioxide, that formed in the cold outer solar system.
    Back to Questions
    Is it likely that an asteroid will impact Earth in the near future?
    While no known asteroid larger than 140 meters in size has a significant chance to hit Earth for the next 100 years, only about 40 percent of those asteroids have been found to date.
    Back to Questions
    How many near-Earth asteroids have been discovered so far?
    As of 2016, the number of discovered near-Earth asteroids totaled more than 15,000. An average of 30 new discoveries are added each week. The 15,000 milestone, reached on October 13, 2016, marks a 50 percent increase in the number of known near-Earth asteroids since 2013, when discoveries reached 10,000 in August of that year. More than 95 percent of these objects were discovered by NASA-funded surveys (primarily using ground-based telescopes) since 1998, when NASA began tracking and cataloguing them. The Jet Propulsion Laboratory’s Center for NEO Studies provides up-to-date asteroid discovery statistics.
    Back to Questions
    How good are we at finding and tracking NEOs?
    Over 2015-2016, observers discovered more than 1500 previously unknown NEOs each year. Roughly half of the known catalogue of NEOs are objects larger than about 460 feet (140 meters) in size. The estimated population of NEOs of this size is about 25,000. Current surveys are finding NEOs of this size at a rate of about 500 per year.

    The 460-foot cutoff point was established by a NASA NEO survey science definition team (SDT) in 2003. The SDT determined that impacts from objects of that size would only produce regional effects, while larger objects would have corresponding wider effects such as large sub-global effects from impacts of a 984-foot (300-meter) object and global effects from 0.6 mile (1-kilometer) object impacts. In 2016, NASA appointed a new NEO survey SDT to reevaluate this cutoff point in light of research conducted and events occurring since 2003. The new SDT’s recommendations should be available in 2017.

    Ground-based telescopes alone have limitations - for instance, they can only survey the skies at night and in clear skies. Based on statistical population estimates, about 74 percent of NEOs larger than 460 feet still remain to be discovered.
    Back to Questions
    What can be done to improve the NEO detection rate?
    Larger ground-based telescopes and a dedicated space-based infrared asteroid survey telescope would substantially increase the discovery rate and meet the goal in the NASA Authorization Act of 2005 to detect, track, catalogue, and characterize the physical characteristics of 90 percent of the NEO population down to 140 meters in size. NASA’s currently operating NEOWISE space-based survey was not designed for this purpose. NEOWISE is a repurposed astrophysics spacecraft, and while it has made significant contributions to NEO discovery and characterization, its capabilities are limited.
    Back to Questions
    How are NEOs characterized?
    NEOs are characterized by using optical and radio telescopes to determine their size, shape, rotation, and physical composition. Some of the most detailed characterization data is obtained by planetary radar, performed by radio telescopes at NASA’s Deep Space Network and the National Science Foundation’s Arecibo Observatory in Puerto Rico. Other methods employed to characterize NEOs are ground-based and space-based spectroscopic and infrared measurements, light-curve measurements, and long-arc high-precision astrometry.
    Back to Questions
    How do NEO scientists assess asteroid impact risks?
    The Jet Propulsion Laboratory’s Center for NEO Studies maintains an impact risk assessment Web page that describes this process, explaining how an asteroid’s orbit is calculated, how soon after discovery a search for potential collisions is initiated, and how uncertainties in initial calculations of an orbit are reduced over time.
    Back to Questions
    Does Bennu, the asteroid that will be studied by the OSIRIS-ReX spacecraft, pose an impact risk to Earth?
    Asteroid Bennu orbits the sun every 1.2 years and makes a relatively close approach to Earth about every six years. Bennu has been measured by planetary radar and is roughly spherical with an equatorial bulge. Its average diameter is about 1,614 feet (492 meters).

    By current definition, Bennu is a potentially hazardous asteroid (PHA). A PHA is an asteroid whose orbit is predicted to bring it within 0.05 Astronomical Units (just under 5 million miles, or 8 million kilometers) of Earth’s orbit; and of a size large enough to reach Earth’s surface – that is, greater than around 30 to 50 meters. (Smaller objects entering Earth’s atmosphere will likely disintegrate.) The potential for an asteroid to make a close approach to Earth does not mean that it will impact Earth. By monitoring PHAs and updating their orbits as new observations are made, observers can improve their predictions of the risk of impacting Earth. Sometimes the term potentially hazardous object, or PHO, is used to describe an asteroid, or comet, that meets these criteria.

    The Jet Propulsion Laboratory’s Center for NEO Studies (CNEOS) has predicted the future orbital movements of Bennu based on 29 radar observations and 478 optical observations of the asteroid, conducted by trackers around the world between September 1999 and January 2011. CNEOS predicts that the next time Bennu will pass Earth within the Moon’s orbit will be in 2135. This particularly close approach will change Bennu’s orbit by a small amount, which is uncertain at this time and which may lead to a potential impact on Earth sometime between 2175 and 2199. CNEOS has calculated that the cumulative risk of impact by Bennu during this 24-year period is 0.037 percent or a 1 in 2,700 chance. That means there is a 99.963 percent probability that Bennu will not impact Earth during this quarter-century period.

    Although there is some uncertainty in precisely predicting its orbital motion more than a century from now, future observations of Bennu – including those made by the OSIRIS-REx mission – will enable experts to update orbit predictions and revise the future impact probabilities.
    Back to Questions
    Is it likely that an asteroid will impact Earth in the near future?
    Asteroid impacts are a continuously occurring natural process. Every day, 80 to 100 tons of material falls upon Earth from space in the form of dust and small meteorites (fragments of asteroids that disintegrate in Earth’s atmosphere). Over the past 20 years, U.S. government sensors have detected nearly 600 very small asteroids a few meters in size that have entered Earth’s atmosphere and created spectacular bolides (fireballs). Experts estimate that an impact of an object the size of the one that exploded over Chelyabinsk, Russia, in 2013 – approximately 55 feet (17 meters) in size – takes place once or twice a century. Impacts of larger objects are expected to be far less frequent (on the scale of centuries to millennia). However, given the current incompleteness of the NEO catalogue, an unpredicted impact – such as the Chelyabinsk event – could occur at any time.
    Back to Questions
    Would it be possible to shoot down an asteroid that is about to impact Earth?
    An asteroid on a trajectory to impact Earth could not be shot down in the last few minutes or even hours before impact. No known weapon system could stop the mass because of the velocity at which it travels – an average of 12 miles per second.
    Back to Questions
    Why is it necessary to characterize a NEO that might impact Earth in order to develop a plan for an impact mitigation mission?
    Research indicates that the best technique to use to divert an asteroid from its impact course with Earth is scenario-dependent. That is, the choice of method for impact mitigation depends on the orbit of the object and its composition, bulk properties, and relative velocity, as well as the probability of impact and the predicted impact location. Some NEOs could be in orbits that are especially hard to reach unless we find them many years to decades in advance of impact. Some asteroids are essentially rubble piles, making them difficult to “push on” without breaking them up, while others could be coherent monolithic bodies. Some are too small or fragile to reach the surface of Earth (for example, the asteroid that disintegrated over Chelyabinsk, Russia in 2013) and would not warrant a mitigation mission but rather emergency response planning. Prior to mounting a mitigation mission, it is especially important to adequately characterize the asteroid.
    Back to Questions
    What is the role of planetary radar in NEO observations?
    Planetary radar observations contribute to the characterization of near Earth objects by more precisely determining their orbits and by measuring their size, shape, body dynamics, and surface features if they approach close enough to the Earth. NASA’s Near Earth Object Observations Program supports planetary radar observations of NEOs at the Arecibo Observatory in Puerto Rico and the Goldstone station of NASA’s Deep Space Network in California.
    Back to Questions
    Can planetary radar be used to search for near-Earth objects?
    No current radar is powerful enough to sweep vast areas of space even relatively near Earth and return enough signal to detect unknown NEOs. It is much easier to detect light from the Sun that is reflected by an object, using optical telescopes. However, ground-based radar can be used to more precisely track and determine orbits for NEOs discovered by optical telescopes and to determine their physical characteristics and body dynamics, when they approach within millions of miles of Earth.

    Optical or infrared observations cannot be used to directly determine distance to an asteroid ("range") or the speed at which it is moving, data that radar is uniquely useful in obtaining. Using radar tracking data, NEO observers can more precisely determine the orbital path of an asteroid and predict that path out years into the future. Observing an asteroid for less than an hour with radar will provide a more precise determination of the orbit than months of observations from an optical telescope.

    Radar observations can reduce the uncertainly in position of an asteroid from the several thousands of kilometers provided by optical observations to just a few meters. The impact risk posed by a potentially hazardous asteroid can be relatively quickly resolved with radar observations, while it might otherwise remain uncertain for years if only optical observations are available. Such was the case with the asteroid Apophis, discovered in 2004 and initially thought to pose a risk of Earth impact in April 2029. Radar observations made by Arecibo Observatory in 2005 eliminated that possibility of impact.
    Back to Questions
    What is NASA’s Near-Earth Object Observations Program funding the Arecibo Observatory to do?
    The Near-Earth Object Observations Program provides $3.5 million a year in funding to the Arecibo Observatory in Puerto Rico – about one third of the observatory’s yearly budget – to fund its planetary radar capability. The National Science Foundation is currently assessing whether to continue supporting the Arecibo Observatory. NASA has agreed to continue to fund planetary radar capability at Arecibo while NSF continues to operate the observatory. If Arecibo is closed, NASA will continue to use planetary radar capability at its own Deep Space Network Goldstone station. However, Goldstone’s radar is not as powerful as Arecibo’s, so it does not have the same range into space, meaning that fewer NEOs could be characterized by radar. Planetary radar observed about 100 asteroids per year in 2014 and 2015.
    Back to Questions
    What is NASA’s role in the Large Synoptic Survey Telescope (LSST) project?
    The LSST is a project of the National Science Foundation and Department of Energy, in partnership with the government of Chile (where the telescope will be located) and other international public-private partners. NASA is not directly involved in the construction of LSST. However, several individual NASA-supported scientists are members of the LSST Science Collaborations, which are contributing to plans for the operation of the telescope, and the LSST Science Advisory Committee.
    Back to Questions
    How will LSST contribute to finding, tracking, and characterizing near-Earth asteroids?
    The current congressionally directed objective of the NEO Observations Program is to find, track, and catalogue at least 90 percent of the estimated population of NEOs that are equal to or greater than 460 feet (140 meters) in size by 2020 and to characterize a subset of those objects that is representative of the entire population. Roughly half of the known catalogue of NEOs are objects larger than 140 meters in size. The predicted population of NEOs of this size is about 25,000. Current surveys are finding NEOs of this size at a rate of about 500 per year.

    LSST will survey the entire observable sky for 10 years, taking hundreds of images of individual fields every night. Under its current plan, LSST will return to each field, acquiring two images, approximately every third night. Combining the images will generate an unprecedentedly wide and deep picture of the universe. Comparing them against each other will reveal objects that change in brightness or color, or that move — including near-Earth objects (NEOs). Because LSST will be significantly larger than the telescopes currently used for NEO searches, scientists say LSST will be able to discover substantially more NEOs than are currently being found. They also have proposed that their project could help NASA meet its congressionally mandated detection goal but by well after 2020.

    However, LSST is not designed specifically for NEO observations. The project’s scientific goals range from the study of the content of the solar system to understanding the evolution of the universe. In 2016 NASA’s Planetary Defense Coordination Office commissioned the Jet Propulsion Laboratory’s Center for NEO Studies to conduct an assessment study, in collaboration with the LSST Solar System Science Team, of LSST’s proposed performance in detecting and cataloging NEOs. The study results, published in 2017, show that, assuming 10 years of operations, currently scheduled to begin in 2022, and also assuming that LSST’s proposed multi-night observation linkage algorithms are successful, LSST could detect about 60 percent of NEOs 140 meters or larger in size. The study indicates that by the end of LSST’s 10-year survey period (ca. 2032), adding together LSST’s projected NEO detections and the projected results of all other ongoing NEO surveys, the catalog of 140 meter and larger NEOs could be 80 percent complete (plus or minus 5 percent).
    Back to Questions
    How are human-accessible asteroids identified?
    NASA initiated a Near-Earth Object Human Space Flight Accessible Targets Study (NHATS) in September 2010. Its purpose was to identify any known NEOs that might be accessible by future human space flight missions. This study led NASA researchers to develop a process that automatically downloads orbital information on newly discovered NEAs from the Center for NEO Studies’ Small Bodies Database (SBDB) on a daily basis. Multiple trajectory calculations are made to determine which objects may meet NHATS accessibility parameters, such as minimum delta-V (the energy required to go from Earth to the object and return) and mission duration (typically less than 400 days). For objects found NHATS-compliant, future observation opportunities are identified, so that researchers can know when they may further characterize the objects.
    Back to Questions