Near Earth Objects
In the first billion years of the history of our Solar System, impacts were incredibly common. One needs only look at the surface of the Moon to get a sense of just how many impacts there were. In fact, collisions of asteroids with the early Earth may have been the key to delivering water to our planet (e.g., Daly & Schultz, 2018), enabling life as we know it. Now, billions of years later, the influx of small bodies in the inner Solar System is much lower. Those that do are called Near Earth Objects (or NEOs).
Formally, an NEO is any comet or asteroid (also referred to, in particular, as Near Earth Asteroids, or NEAs) that passes within 1.3 astronomical units (au) of the Sun — this is slightly farther than the average radius of the Earth’s orbit. Further restrictions may be placed on the orbital period or the orbit’s semimajor axis to avoid including objects with long periods (such as comets with a very close approach to the Sun). NEOs typically only spend a short time orbiting in this region of the Solar System (their median lifetime is only about 10 million years, as most NEOs crash into the Sun). Still, their number is continually replenished and kept in a steady state by dynamical mechanisms that transport asteroids from the main asteroid belt between Mars and Jupiter to the terrestrial planet region.
There are four different classes of NEAs, depending on the object’s current orbit. However, an individual object can move from one class to another as planetary flybys or non-gravitational forces change its orbit. These classes are summarised in Figure 1. Amor and Apollo class NEAs are the most numerous populations; these comprise more than 90% of the known NEAs (JPL, 2023a). Apollo and Aten NEAs have Earth-crossing orbits, though this designation does not necessarily make them hazardous to life on Earth.
To be classified as a “potentially hazardous object” (PHO), an NEA must be on an Apollo or Aten orbit, must be larger than 140m across, and must travel to within 0.05 au of anywhere along Earth’s orbit. Most of the known PHOs are asteroids: there are currently known PHAs (JPL, 2023a). Note that the designation “potentially hazardous object” does not necessarily mean that the object will impact the Earth in the foreseeable future. It simply means that we should pay extra attention to these because (1) these objects are closer to the Earth’s orbit than most NEOs; and (2) in the unlikely event of an impact, the result would have global implications.
Figure 1: The four orbit families of Near Earth Asteroids. Image Credit: Original image courtesy of JPL.
As observed from Earth, NEOs typically move faster with respect to the background sky than more distant objects. When an observer discovers a body moving relative to the background, they will send the information (position on the sky and time of observation) to the IAU Minor Planet Center (MPC), located at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, USA. Most observers will try to obtain at least three sets of observations during the night. MPC combines all data received for this object, calculates, and publicises its orbit in real-time. Since only very preliminary orbits can be determined from a single night of data, more observations must be made — and quickly! — for MPC to precisely calculate the orbit and announce the discovery of the NEO in an MPC Electronic Circular (MPEC). The MPC is not the only institution doing this work: the United States National Aeronautics and Space Administration Jet Propulsion Laboratory (NASA/JPL) also calculates the orbits of newly-discovered, yet-to-be-confirmed NEOs in near time and provides warnings for possible impactors.
Figure 2: The number of NEOs discovered from 1980 to the present. Image Credit: JPL (2023a)
In recent years, several all-sky surveys have come online to spot and track NEOs as they pass by the Earth. For that reason, it might appear that the number of NEOs has dramatically increased (see Figure 2). However, that simply results from being better equipped to observe them. Numerous ground and space-based observation programmes from around the world aim to discover, track, and understand NEOs.
There are many programmes around the world that are currently in production to aid in the effort of scanning the night sky for NEOs. For example, when it is operational, the Vera C. Rubin Observatory plans to undertake the Legacy Survey of Space and Time (LSST). This survey will scan the whole night sky visible from Chile once at least every three days using the world’s largest astronomical camera (at the time of its completion). Another example comes from the European Space Agency, which is building a network of observatories that will be able to observe the entire sky in a single night. Named Flyeye, this project’s optical design got its inspiration from the compound eyes of a fly that enable it to look in multiple directions at once.
There are also major projects that will take on this challenge from space. For example, NASA has initiated a new space mission specifically designed to detect, track, and characterise NEOs: the NEO Surveyor. This will be the first space telescope dedicated to discovering asteroids and comets and has been designed to find more than 90% of all PHAs. NEO Surveyor will observe at the wavelengths where NEOs are the brightest (the infrared), and will launch no later than June 2028 to conduct a 5-year survey of the sky. This spacecraft is expected to discover hundreds of thousands of NEOs, millions of asteroids in the Main Belt, and thousands of comets during its survey, greatly expanding our census of the NEO population. By observing in the infrared, NEO Surveyor will also obtain data needed to measure the size of each asteroid, which is a critical component to constrain the hazard the object would pose to the Earth in the case of an impact.
NEO Risk Determination
There are two scales on which astronomers determine the risk associated with an NEO: the Torino Scale and Palermo Technical Impact Hazard Scale.
The Torino Scale (Morrison et al., 2009) paints a broad picture of the risk associated with a NEO. It is based on impact probability (how likely an object is to hit the Earth or its atmosphere) and kinetic energy (this depends on how fast the object moves and how massive it is). The Scale ranges from 0 to 10, where 0 indicates no hazard, and 10 indicates a certain impact with likely catastrophic global effects. While several NEAs are initially categorised as a 1 (normal) on the Torino Scale every year, as more data is taken of these objects, they are usually downgraded to a level 0 in due course. As of this writing, there is 1 NEO that rates higher than level 0 on the Torino Scale (JPL, 2023b).
Credit: After Morrison, et al. (2004), Figure 16.2
Compared to the Torino Scale, the Palermo Scale (Chelsey et al., 2002) provides more precise information about the risk associated with a NEO. The Palermo Technical Impact Hazard Scale compares information about the probability of an impact and its kinetic energy to the background frequency of similarly sized objects colliding with the Earth. This background level can be thought of as a kind of status quo of NEO impacts: it allows us to understand how entire populations of objects of a particular size will behave over long periods of time.
Because there is a wide range of sizes in the overall NEO population, with a wide range of velocities, this comparison can yield a very wide range of numbers. For this reason, the Palermo Scale is most often discussed on a logarithmic scale. This scaling helps scientists notice patterns in datasets with a large range of numbers. A NEO with a Palermo Scale of -2 is 1% (or 10-2) as likely to occur as a random background event, and an NEO with a Palermo Scale of 0 is just as likely to occur as a random background event. Most often, NEOs have a Palermo Scale between -2 and 0. Positive Palermo Scale scores indicate that astronomers should proceed with some concern. As of this writing, no NEOs rank above 0 on the Palermo Scale (JPL, 2023b).
The International Astronomical Union (IAU) has its own NEO Working Group, composed of international experts who are highly involved in dedicated projects and institutions. Since the early 1990s, the UN Office of Space Affairs (UNOOSA) and the UN Committee on the Peaceful Uses of Outer Space (COPUOS) have convened international partners “to ensure international information sharing in discovering, monitoring and physically characterising potentially hazardous NEOs with a view to making all countries aware of potential impact threats, particularly developing countries with limited capacity in predicting and mitigating a NEO impact” (UN, 2018). From their recommendations and task forces, the International Asteroid Warning Network and the Space Mission Planning Advisory Group were formed in 2013.
Every two years, the international community gets together at the Planetary Defense Conference (hosted by the UN Office for Outer Space Affairs) to present recent advances in the various activities dedicated to this topic. During this conference, an exercise is conducted during which the experts must mitigate a simulated hypothetical threat, allowing them to measure their level of readiness.
International Asteroid Warning Network
The International Asteroid Warning Network (IAWN) aims to coordinate the international effort to detect, observe, and characterise Near Earth Objects. An essential part of its mission is to devise comprehensive communication plans to ensure all its members have access to the most up-to-date information in the case of an emergency. They also work with governments to make policy recommendations and plan impact mitigation responses. To date, IAWN has representatives from North and South America, Europe, and Asia.
Space Mission Planning Advisory Group
While the IAWN focuses on observations and communications on the surface of the Earth, the Space Mission Planning Advisory Group (SMPAG) makes recommendations for research and missions in space. Part of their scope is to coordinate international efforts to produce new research and technologies on planetary defence. The IAWN and SMPAG work together to produce policy recommendations and defence strategies for governing bodies worldwide.
The world’s space agencies are looking for direct ways to preemptively protect the Earth from astronomical threats. Organisations such as NASA, the European Space Agency (ESA), and the China National Space Administration (CNSA) have plans to ramp up their space-based planetary defence. Below are only a few examples of ongoing work.
DART and Hera
In November 2021, the US National Aeronautics and Space Administration (NASA) launched its Double Asteroid Redirection Test (DART) to test the asteroid deflection method called the kinetic impactor technique (Rivkin et al., 2021). Less than a year later, the spacecraft, with a mass of about 570 kg, intentionally collided at about 6 km/s with the minor planet moon, Dimorphos of the near-Earth asteroid, Didymos. An international observational campaign from the ground and space was organised to observe the event on 26 September 2022 at 23:14 UTC. Though the event occurred about 11 million kilometres from Earth, astronomers were able to measure the change in the orbital period of the small moon around its main body, Didymos. Furthermore, a small cubesat called LICIACube, provided by the Italian Space Agency (ASI), observed the impact event from a distance of a few tens of kilometres during the first few minutes after impact, providing images of the early ejecta. Using radar and optical telescopes, observers from around the world determined that DART had shortened the orbital period of Dimorphos by about 32 minutes (Handal & Surowiec, 2022), confirming that the DART impact had a consequent effect on the small moon’s trajectory.
In October 2024, ESA will launch the Hera mission (Michel et al., 2022) that will rendezvous with Didymos in early 2027 for a 6-month investigation to measure in detail the outcome of the DART impact. In particular, they hope to measure the momentum transferred by the impact (the deflection technique’s efficiency), the effect of the impact on Dimorphos (e.g., the size of the crater or the possible global deformation), and the physical and compositional properties of the moonlet (e.g., determine its internal properties using radar sounding). In the framework of the Asteroid Impact & Deflection Assessment (AIDA) international cooperation, DART and Hera thus offer us the first fully documented asteroid deflection test using the kinetic impactor technique, with the initial conditions and early stage results provided by DART and LICIACube images and ground-based measurements, followed by the detailed impact outcome and target properties provided by Hera.
Hayabusa and Hayabusa2
The Japanese Space Agency, JAXA, has performed two successful missions, namely Hayabusa and Hayabusa2, to retrieve samples from Near Earth Asteroids. The analysis of samples has given us unprecedented insights into the link between asteroid types and meteorite groups, the early Solar System history, with potential implications on the origin of life and its evolution on Earth. Their second mission, Hayabusa2, collected a surface sample and tested the surface material's response to an explosive event. After returning its sample to Earth, Hayabusa2 has been extended to perform the flyby of an 800-meter size asteroid in 2026 and a rendezvous with a 60 m-size, quickly spinning NEA whose size is close to the estimated one for the object that exploded over Siberia in 1908. This rendezvous will allow us to know the properties of such an object, and these visits, which require precise navigation and control, will facilitate improvements to the technology and procedures in support of future missions to deflect potentially hazardous asteroids (Hirabayashi et al., 2021).
After returning the samples of the near-Earth asteroid Bennu to Earth on 24 September 2023, the NASA OSIRIS-REx mission will perform a new mission under the name of OSIRIS-APEX. This new mission’s aim is to perform a rendezvous of the famous near-Earth asteroid Apophis a few days after its close approach to Earth, at less than 36,000 kilometres, on 13 April 2029. During this close approach, humans will be able to see the light of an asteroid. It is possible that tidal forces from the Earth can trigger some surface or internal motion on the asteroid, offering us a natural laboratory to observe the effects of a tidal encounter on an NEO. Because OSIRIS-APEX will arrive after the encounter, other projects are under study at the ESA and the French space agency (CNES), in collaboration with NASA/JPL, to send a spacecraft to visit the asteroid before and during the encounter itself, allowing a complete study of the asteroid and the effect of tidal forces. Characterising NEOs in detail with rendezvous and flyby space missions is an important aspect of the planetary defence roadmap.
It is not difficult to find articles on the Internet that claim the Earth will soon be bombarded with an Earth-shattering asteroid. However, these claims are almost always misguided and often deliberately misleading. More often than not, these articles appear when predictions for the future motion of the NEA are based on a poor determination of the initial orbit. Here are some key questions to keep in mind when figuring out whether the article you have found is legitimate or is just preying on the natural fear of its readers:
- What is a credible source of information? Does the source you found reference or cite an international space agency or IAU website that also has an article on the hazard of the event? Simply citing data from the MPC or CNEOS is not enough — does the source discuss the data using expert text or quotes from named scientists? When you look up whom the source says the experts are, do they seem reliable?
- Have I stumbled upon a simulation event? Scientists like to be prepared for any situation. Astronomers regularly team up with policymakers to better understand what might happen in the event of an astronomical catastrophe like an asteroid impact and how they can most effectively work together to solve it. Read the article you have found carefully. Does it say that the event described is “fictional”, “not real”, or an “exercise”? If so, you have found an article about a fictitious event and have no reason to fret.
- Is there international coverage of this doomsday event? If you have stumbled upon an article that says the world is in imminent danger, check to see if there is international consensus. In the event of a real emergency, there would be widespread global coverage. If you are struggling to find more than a few “clickbait” articles, chances are you do not have anything to worry about.
- Where can I find more information about NEAs? Suppose you have found an article you think is credible (after going through all the previous questions on your own), and you would like to know more about the object in question. In that case, you can search for an asteroid by name using the IAU Minor Planet Centre website search function.
The UN designated June 30th as Asteroid Day to raise awareness about asteroids. This day was initiated by the Asteroid Foundation, and co-founders Brian May, Grig Richter and Danica Remy, to inform the public about asteroid science and planetary defence. Every year, on that day, a great number of events are organised around the world. In particular, scientists, engineers, and decision-makers convene a virtual live show in which they discuss asteroids and NEAs and inform the public about the most recent advances in asteroid science.
We thank the following people for their time and effort to greatly improve this document: Chair of Inter-Division A-F Working Group on NEOs and Organizing Committee Member of Commission F4 Asteroids, Comets & Transneptunian Objects, Patrick Michel, Organizing Committee Member of Commission F4 Asteroids, Comets & Transneptunian Objects, Joe Masiero, Members of Commission F4 Asteroids, Comets & Transneptunian Objects, Ellie Sansom and Quanzhi Ye, and Member of Division F Planetary Systems and Astrobiology, Robert Weryk. This article is based on text written by the past-President of Division III, Iwan Williams.
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