Vikas Nath and Artash Nath The International Academy of Astronautics organized its 6th Planetary Defence Conference from April 29 to May 3rd, 2019 in Washington DC, Area in the USA. The bi-annual conference […]
Vikas Nath and Artash Nath
The International Academy of Astronautics organized its6th Planetary Defence Conferencefrom April 29 to May 3rd, 2019 in Washington DC, Area in the USA. The bi-annual conference brings together world experts to discuss the threat to Earth posed by asteroids and comets and actions that might be taken to deflect a threatening object. There were over 300 participants this year.
Artash (Grade 7 student) submitted a paper to the conference on Using Machine Learning to predict the Risk Index of an Asteroid Colliding with Earth. It was accepted as a poster presentation for the conference and we were happy to attend the event. It was the first time for us to be participating in this conference. We learned a lot from listening to focused presentations from experts from NASA, different Space Agencies, universities, research institutions, and UN Agencies. We met a lot of people and enjoyed participating in the 2019 PDC Hypothetical Asteroid Impact Scenario simulation exercise.
The formal poster presentation was held in the evening of Day 2 of the Conference. There were over 50 posters and it was wonderful to listen to so many research, monitoring and space mission ideas coming out on the topic of NEOs and Planetary Defense.
A large number of people turned up for poster presentation. They were specialists in their respective areas and yet were a very encouraging and welcoming group. They patiently listened to the presentations and asked great questions with the objective of improving the research results and opening up newer opportunities. Artash enjoyed giving his presentation to many of these experts and getting their inputs and feedback to further his research on using machine learning to predict the risk of an asteroid colliding with the Earth.
We have penned below some of our learnings and takeaways from the conference. Being new to this subject area, we enjoyed sessions that were focused on NEO detection, modeling, simulation, and deflection, in addition to poster presentations and the simulation exercise.
The Planetary Defense conference started with the address by the NASA Administrator: Jim Bridenstine. It was interesting to see the high priority NASA gives to planetary defense and the political will to take this issue seriously. Jim succinctly laid out a 4 points road map on how to protect the Earth from known and currently unknown Near Earth Objects. These included:
1. Better identification, classification, tracking of NEOs
2. Increase impact modeling capabilities
3. Robust deflection strategy
4. Emergency preparedness
5. Increased international cooperation
Speakers from other countries and space agencies, including from the European Space Agency, Israel, South Korea, China, and India also talked about sharing of information and international cooperation.
Discovering Near Earth Objects (NEOs)
To protect Earth against a possible collision with the asteroid, we first need to discover these asteroids. Sighting these asteroids is not enough, we need to find more about them starting from their orbital path. Without knowing their orbits we may sight the asteroid once but find it difficult to find them again. We also need to know their other characteristics including size, shape, speed, composition, spin, and more including if they are binary or have satellites or moons of their own.
Over 100,000 asteroids have been discovered, of which over 20,000 are Near Earth Objects. NEOs are asteroids and comets with perihelion distance q less than 1.3 astronomical units (AU). Near-Earth Comets have a perihelion distance less than 1.3 AU and are restricted to include only short-period comets (i.e., orbital period P less than 200 years). The vast majority of NEOs are asteroids, referred to as Near-Earth Asteroids (NEAs). Amongst these, there are around 2,000 potentially Hazardous Asteroids (PHAs) that make threatening close approaches to the Earth. Specifically, all asteroids with an Earth Minimum Orbit Intersection Distance (MOID) of 0.05 AU or less and an absolute magnitude (H) of 22.0 or less are considered PHAs.
So how do we discover and track asteroids? We require a range of equipment to do so including ground and space-based telescopes to carry out observations in visible and infrared. We could also use other techniques such as visual sightings, modeling or infrasound to detect when asteroids enter the Earth’s atmosphere.
Pan-STARRS and the Catalina Sky Survey are currently the most prolific NEO surveys. As the next generation surveys such as the Large Synoptic Survey Telescope (LSST) and the proposed Near-Earth Object Camera (NEOCam) become operational the discovery rate is expected to increase tremendously. Coordination between various survey telescopes will be necessary in order to optimize NEO discoveries and create a unified global NEO discovery network.
The US Political Imperative
In 2005, Congress again directed NASA to find at least 90 percent of potentially hazardous NEOs sized 140 meters or larger by the end of 2020. NASA has found over 20,000 of them. Following up of Near Earth Asteroids (NEAs) is essential for refining the orbit of asteroids and to prevent them from getting lost, particularly for virtual impactors (VIs) or asteroids with non-zero impact solutions.
What happens when an Asteroid is detected?
NEO detection and tracking is a 24/7 job. As soon as an object is discovered, its details are entered into the Scout: NEOCP (NEO Confirmation Page) Hazard Assessment database. These objects could be real asteroids, but they cannot be officially designated until they are confirmed by additional observations. Their status is unconfirmed; their designations are user-assigned and unofficial. Scout continually monitors the objects on the NEOCP and for each provides the orbital, ephemeris, and hazard assessment information. Once an object is entered into the NEOCP database, NEOCP candidate can become a NEO discovery, NEO attribution, non-NEO attribution, a comet, unconfirmed, artificial satellite, retracted by observers. Some small fraction of objects remain unconfirmed or currently not traceable.
Scout: NEOCP database is managed by the Minor Planet Center (MPC) which is the single worldwide location for receipt and distribution of positional measurements of minor planets, comets and outer irregular natural satellites of the major planets. The MPC is responsible for the identification, designation and orbit computation for all of these newly discovered objects. The MPC operates at the Smithsonian Astrophysical Observatory, under the auspices of the International Astronomical Union (IAU).
Monitoring of Asteroids Impact (Meteorites) using Infrasound
Infrasound is low-frequency sound with a range of less than 10 Hz. Around the world, the CTBTO has 45 infrasound monitoring stations that are constantly listening for sound waves far too low for the human ear to hear. Based on the sounds the CTBTO recorded, scientists are able to study the meteor’s direction, energy release, and duration. Asteroid Impacts are not a fixed explosion as the position is constantly changing as the meteorite moves towards the earth. It’s not a single explosion, it’s burning, traveling faster than the speed of sound. That’s how these impacts are distinguished from mining blasts or volcanic eruptions.
Identifying Short-term Impactors with Large Synoptic Survey Telescope (LSST)
Large Synoptic Survey Telescope (LSST) is an under-construction 8.4-meter, special three-mirror design telescope, which creates an exceptionally wide field of view and has the ability to survey the entire sky in only three nights. Each image will be the size of 40 full moons. The goal of the LSST project is to conduct a 10-year survey of the sky designed to address four science areas:
• Understanding the Mysterious Dark Matter and Dark Energy • Hazardous Asteroids and the Remote Solar System
• The Transient Optical Sky
• The Formation and Structure of the Milky Way
But how effective is LSST at identifying Asteroids during their final approach to the Earth? The analysis is being carried out using a simulated LSST baseline and a synthetic impactor population. LSST is designed in a way that a given field would typically be visited twice each night in a 25 minutes interval and inter-night visits would happen with an interval of about 2 days.
As LSST revisits each field of view twice every night, one would get a pair of observation for each object. It would take pairs of detection on 3 nights to discover and catalog a previously unknown asteroid. But if an asteroid is sufficiently close to Earth, it would appear moving very fast. Thus it would present a trail in the 30-second exposure given to each field of view. And measuring these trail ends would provide the 4 astrometric data points which could be sent to MPC for follow-up on impact probabilities.
For brighter impactors (H < 28) it is easier to get observation pairs for 3 nights. But for fainter ones, it is difficult to get such observations. Such impactors are more likely to be discovered using trailing pairs but even this method starts becoming ineffective for H greater than 31.
Characterization Of Near-Earth Objects By The Neowise
Funded by the Planetary Defence Coordination Office (PDCO) and led by Amy Mainzer (JPL) to find NEOs and characterize them (ie getting their physical properties). The telescope cannot point at a particular object but surveys the entire sky looking for things that are moving. It is currently on its 11 sky survey to measure diameters and albedos of asteroids when combined with visual data.
While the telescope has lost its cryogen, its 4.6-micron channel is sensitive to infrared radiation emitted by the asteroids. Infrared measurements have been calculated of 193,000 small bodies in the solar system and 1500 NEOs.
Asteroid Deflection Models
This seems to be one of the most interesting segments of the conference. We have found the asteroid and determined that it is on a collision path with Earth. How do we deflect it?
The popular imagination may say blowing up the asteroids. But this is not the preferred scenario as it would break the asteroid into pieces- all of which are not hurtling towards the Earth complicating the scenario further.
Nuclear vs. Non-Nuclear Asteroid Deflecting Option
The Los Alamos National Laboratory is a part of the NASA Interagency Agreement on Planetary Defense. The Laboratory models asteroid and comet impacts on Earth, and how to prevent these impacts. This could include deflecting asteroid by using a nuclear explosive. The nuclear option could be most appropriate for surprise asteroids or comet that have not been observed – they have come out of nowhere, and there are only a few months to respond.
Nonnuclear options could prevent impact by deflecting the incoming asteroid through
the use of gravitational tractors (spacecraft that travel alongside the asteroid for a decade or two and have enough mass to pull the object off its collision course with Earth) and impactors (rockets that make direct hits on the asteroid and throw it off course). But these options would need a decade of planning and development before they could be deployed. In addition, they would need to be deployed many years in advance of the impending collision.
Kinetic Impactors: Momentum Transfer
To change the momentum (or velocity) of the asteroid, it could be impacted by a spacecraft acting as a kinetic impactor. When adding momentum to asteroids via kinetic impactors, it is important to assess how much momentum would be transferred (Beta) and how much would be enhanced with by impact ejecta. The quantity Beta is defined as momentum transferred divided by input momentum. Ejecta release enhanced momentum transfer, making Beta value greater than 1. Estimation of Beta becomes critical for deflection performance.
For the kinetic impactor method to be successful, extensive 2D and 3D modeling have to be done as there are known and unknown factors. While the velocity and mass of the impactor are known the size, composition, and mass of the asteroid is unknown. Furthermore different angles of impact would produce different Beta values. It could even drop to less than 1 in some cases, for example from very asymmetric ejecta curtains produced by oblique impacts (which pushes ejecta in the wrong direction).
International Law and Considerations
International cooperation is pivotal to a successful planetary defense. If an asteroid was to be detected on on a possible collision course with Earth, it would require making decisions that rest on international relations, international law, and considerations. Who gets to decide what strategy to adopt? Would countries that are directly on the collision corridor have more say than those countries that are not? If the asteroid deflection strategy could change the orbit of the asteroid bringing other countries into the risk corridor: would those countries approve of it?
The fact that a possible option to deflect an asteroid could include using nuclear weapons complicates the situation even further. Do all countries need to agree on the nuclear option to pursue to deflect the asteroid, can a country act unilaterally when it is under threat, can non-state actors or even the private sector act on its own to protect its assets?
Asteroid Apophis: 2029 Close Encounter
On Friday the 13th in 2029, the asteroid Apophis (340 meters) will make a close flyby to Earth. It will pass within 19,000 miles (31,000 kilometers) of Earth’s surface and is well within the distance of Earth’s geosynchronous satellites. During the 2029 approach, Apophis’s brightness will peak at magnitude 3.1 and will be visible to the naked eye from Europe, Africa, and western Asia.
Upon its discovery in 2004, Apophis was briefly estimated to have a 2.7% chance of impacting the Earth in 2029. Additional measurements later showed there was no impact risk at that time from Apophis which has been identified spectroscopically as an Sq type similar to LL chondritic meteorites.
Nevertheless, it will be a historically close approach to the Earth, estimated to be a 1 in the 800-year event (on average, for an object of that size). It will be a tremendous science and outreach opportunity. Scientists will be able to study such asteroids near Earth in detail and educate the public to the importance of space programmes and planetary defense.
With less than 10 years to go, the missions to Apophis should start now to get science data about the asteroid, photographs, and for planetary defense outreach. Experiments could include deploying a seismometer (insert, release, depart maneuver similar to Near Earth Asteroid Rendezvous – Shoemaker (NEAR), Hayabusa and OSIRIS-REx missions). This would require matching the rotational speed of the asteroid and brief contact while allowing for non-deployment and abort.
If a spacecraft could be positioned to get the picture of the asteroid with Earth in the background, it would be an iconic picture that would galvanize public interest. The naked eye asteroid viewing event could be something similar to the 2017 and 2025 Total Solar Eclipse and there could be citizen events and citizen experiments centered around it.
Other Close Flybys
In addition to Apophis, there would be 5 more very close Potentially Hazardous Asteroid flybys in the late 2020s. So the period between 2027 and 2029 would be very interesting in terms of near earth asteroids. 1990 MU (with flyby 6 Jun 2027) is almost 3 kms wide. The resolution of images of these asteroids which we will get from groundbased telescopes (Goldstone and Arecibo) would be equivalent to those we get from spacecraft flybys. This would be an excellent opportunity to get science data such as the non-gravitational Yarkovsky effect, allowing us to measure their mass and density, and for science outreach.
Science Questions for Different Asteroid Missions
What is the evolutionary history of target asteroids?
How can remote observations of asteroids be linked to their interiors?
Has recent resurfacing occurred?
What are the monolithic fragments remaining from the collisional cascade?
What are the impacts of tidal interactions with a large planet?
Asteroids related Space and Ground-based Missions (Ongoing / in Pipeline / Suggested)
DISCUS – The Deep Interior Scanning CubeSat mission to a rubble pile near-Earth asteroid
DISCUS will be operated either as an independent mission or accompanying a larger one. It is designed to determine the internal macroporosity of a 260-600 m diameter Near Earth Asteroid (NEA) from a few kilometers distance. The main goal will be to achieve a global penetration with a low-frequency signal as well as to analyze the scattering strength for various different penetration depths and measurement positions.
Will be the first mission to investigate a metallic world rather than that of rock and ice. It is expected to be launched in 2022 and will provide insights to the history of collisions and accretions that created terrestrial planets.
It would be the first-close view of all three major types of bodies in the Main Belt asteroid and the six trojans. All of them would be fly-by missions. The highlight of the mission would be the exploration of Jupiter Trojans.
Double Asteroid Redirection Test (DART)
The DART mission aims to demonstrate kinetic impact to asteroid deflection technique for NASA’s Planetary Defense Coordination Office. Its current target is Asteroid Didymos which will have a distant approach to Earth in October 2022.
The DART impact will join Deep Impact and LCROSS as planetary-scale impact experiments. While the initial impactor (spacecraft) parameters are well known, the physical properties of the target are not well constrained.
Pan-STARRS NEO Survey
The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) operations are mostly funded by the NASA Near Earth Observation Program. Each night, PS1 observes about 1,000 square degrees of the night sky, using a sequence of four exposures that span a period of about an hour. The images are compared to each other, and objects that move during the one hour period are identified. Pan-STARRS1 has amassed over 1 million science images comprising over 2.5 Petabytes of data.
The NEO Characterization Assets include:
Interplanetary Radars (Goldstone and Arecibo) – have resolutions up to 4 meters
NASA Infrared Telescope Facility (IRTF) – allows spectroscopy of asteroids and detecting their thermal signatures.
Spitzer Infrared Space Telescope – in its extended warm phase mission it is used to detect thermal signatures, albedo/sizes of NEOs.
The Near-Earth Object Camera (NEOCam) is a new space mission that is designed to discover, track, and characterize at least two-thirds of potentially hazardous asteroids (PHAs) that are larger than 140 m and therefore capable of causing significant regional damage. NEOCam consists of an infrared telescope and a wide-field camera operating at thermal infrared wavelengths. NEOCam has been funded for an extended Phase A study by NASA in the Planetary Defense Coordination Office. NEOCam’s primary science objectives are threefold:
To assess the present-day risk of near-Earth object (NEO) impact.
To study the origin and ultimate fate of our solar system’s asteroids.
To find the most suitable NEO targets for future exploration by robots and humans.