Our solar system is a dangerous place with asteroids and comets zipping past the Earth on a daily basis. While there are no known extinction-level cosmic rocks heading towards a collision with Earth, history shows that it is a matter of time before one gets close enough to do serious damage, much more so than marauding Aliens.
What is the state of our defense against these age-old cosmic threats?
Asteroids
Asteroids are small, airless rocky worlds leftover from the formation of our solar system about 4.6 billion years ago. Early on, the birth of Jupiter prevented any planetary bodies from forming in the gap between Mars and Jupiter, causing the small objects that were there to collide with each other and fragment into the asteroids seen today.
There are millions of asteroids; the large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter (the Jupiter trojans). However, other orbital families exist with significant populations, including the near-Earth asteroids.
Occasional collisions and gravitational tugs perturb the orbits of asteroids into elliptical ones, some of which cross the orbits of Earth.
As of 6 April 2016, 14,149 near-Earth asteroids are known, ranging in size from 1 meter up to 32 kilometers (1036 Ganymed). The number of near-Earth asteroids over one kilometer in diameter is estimated to be about 981, of which over 90% have been discovered.
Asteroids smaller than about 25 meters generally burn up as they enter the Earth's atmosphere and cause little or no damage
Every 2,000 years or so, a meteoroid the size of a football field hits Earth and causes significant damage to the area.
Only once every few million years, an object large enough to threaten Earth's civilization comes along. Impact craters on Earth, the moon and other planetary bodies are evidence of these occurrences. According to abundant geological evidence, an asteroid roughly 10 km across hit Earth about 65 million years ago. This impact made a huge explosion and a crater about 180 km across. Debris from the explosion was thrown into the atmosphere, severely altering the climate, and leading to the extinction of roughly 3/4 of species that existed at that time, including the dinosaurs.
Comets
A comet is an icy small Solar System body that, when passing close to the Sun, heats up and begins to outgas, displaying a visible atmosphere or coma, and sometimes also a tail. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to a few light years in distance.
As of November 2014 there are 5,253 known comets. The reservoir of comet-like bodies in the Oort cloud is estimated to be one trillion.
NASA Asteroid Grand Challenge
The NASA Asteroid Grand Challenge is a large-scale effort to detect, track, characterize, and create mitigation strategies for potentially hazardous asteroids.
In the United States, NASA has a congressional mandate to catalog all Near Earth Objects (NEOs) that are at least 1 km wide, as the impact of such an object would be catastrophic.
NASA's Near-Earth Object (NEO) Search Program
Since NASA's initiation of the NEO Observations program in 1998, Near-Earth Object (NEO) surveys have been extremely successful finding more than 90% of the Near-Earth Asteroids (NEAs) larger than one kilometer and a good fraction of the NEOs larger than 140 meters. As of 8 August 2016, 873 NEAs larger than 1 km have been discovered, of which 157 are potentially hazardous. The inventory is much less complete for smaller objects, which still have potential for large scale damage.
The vast majority of NEO discoveries have been made by NASA-supported ground-based telescopic surveys. Note that the NEO survey includes comets.
NEO SURVEY PROGRAM |
LOcation |
Status |
LINEAR (Lincoln Near-Earth Asteroid Research) |
Socorro New Mexico |
|
Pan-STARRS (Panoramic Survey Telescope and Rapid Response System) |
Haleakala, Maui, Hawaii |
|
Catalina Sky Survey (CSS) |
Tucson Arizona |
|
Spacewatch |
Tucson Arizona |
Follow-up missions only |
LONEOS |
Flagstaff Arizona |
Discontinued |
NEAT |
NASA/JPL |
Discontinued |
NEOWISE |
Earth polar orbit |
|
Asteroid Impact Avoidance
The following tables (from neo.jpl.nasa.gov/...) summarize various deflection techniques being explored by NASA and other space agencies. PHO stands for Potentially Hazardous Object.
Impulsive Technique
|
Description
|
Conventional Explosive (surface)
|
Detonate on impact
|
Conventional Explosive (subsurface)
|
Drive explosive device into PHO, detonate
|
Nuclear Explosive (standoff)
|
Detonate on flyby via proximity fuse
|
Nuclear Explosive (surface)
|
Impact, detonate via contact fuse
|
Nuclear Explosive (delayed)
|
Land on surface, detonate at optimal time
|
Nuclear Explosive (subsurface)
|
Drive explosive device into PHO, detonate
|
Kinetic Impact
|
High velocity impact
|
Slow Push Technique
|
Description
|
Focused Solar
|
Use large mirror to focus solar energy on a spot, heat surface, “boil off” material
|
Pulsed Laser
|
Rendezvous, position spacecraft near PHO and focus laser on surface, material “boiled off” surface provides small force
|
Mass Driver
|
Rendezvous, land, attach, mine material and eject material from PHO at high velocity
|
Gravity Tractor
|
Rendezvous with PHO and fly in close proximity for extended period, gravitational attraction provides small force
|
Asteroid Tug
|
Rendezvous with PHO, attach to PHO, push
|
Enhanced Yarkovsky Effect
|
Change albedo of a rotating PHO; radiation from sun-heated material will provide small force as body rotates
|
Key Findings for Diverting a Potentially Hazardous Object (PHO)
A study report from 2007 provides the following assessment of the various PHO deflection techniques —
- Nuclear standoff explosions are assessed to be 10-100 times more effective than the non-nuclear alternatives. Because of international treaties, use of a nuclear device would likely require prior international coordination. Conventional explosives were found to be ineffective against most threats.
- Non-nuclear kinetic impactors are the most mature approach and could be used in some deflection/mitigation scenarios, especially for NEOs that consist of a single small, solid body.
- "Slow push" mitigation techniques are the most expensive, have the lowest level of technical readiness, and their ability to both travel to and divert a threatening NEO would be limited unless mission durations of many years to decades are possible.
Next we look at a few interesting past and future missions for Asteroid Deflection.
Deep Impact
Deep Impact was a NASA space probe launched in January 2005. It was designed to study the interior composition of the comet Tempel 1, by releasing an impactor into the comet. At 05:52 UTC on July 4, 2005, the impactor successfully collided with the comet's nucleus.
The 370 kg impactor included a 100 kg copper cratering mass. Since copper was not expected to be found on a comet, scientists could ignore copper's signature in any spectrometer readings. At its closing velocity of 10.2 km/s, the impactor's kinetic energy was equivalent to 4.8 metric tons of TNT.
Although the primary goal of the mission was to study the comet and its composition, it provided some insights into kinetic impact based deflection. The impactor generated a predicted small 0.0001 mm/s velocity change in the comet's orbital motion and decreased its perihelion distance a tad by 10 meters. Note that Tempel 1 is a relatively large comet.
Asteroid Impact and Deflection Assessment (AIDA)
The Asteroid Impact and Deflection Assessment (AIDA) mission will demonstrate the kinetic impact technique to change the motion of an asteroid in space. AIDA is a dual-mission concept, involving two independent spacecraft – NASA’s Double Asteroid Redirection Test (DART), and ESA’s Asteroid Impact Mission (AIM).
Its target is the binary near-Earth asteroid 65803 Didymos, which consists of a primary body approximately 800 meters across, and a secondary 150m “moonlet”.
The DART spacecraft will achieve the kinetic impact by deliberately crashing itself into the moonlet at a speed of approximately 21,600 km/hour. The collision will change the speed of the moonlet in its orbit around the main body by a fraction of one percent, enough to be measured using telescopes on Earth. By targeting the small moonlet in a binary system, the AIDA mission plan makes these precise measurements possible and ensures that there is no chance the impact could inadvertently create a hazard to Earth. The DART will be launched in Dec 2020 and intercept Didymos’ moonlet in early October 2022 when the Didymos system is within 11 million kilometers of Earth, enabling observations by ground-based telescopes and planetary radar.
The AIM spacecraft will be launched in Oct 2020, arrive at Didymos in May 2022, before DART’s impact, and perform close-up studies of the binary asteroid, providing high-resolution imagery of the surfaces of the binary system as well as measurements of the masses, densities, and shapes of its two bodies. AIM will maneuver to a safe distance to observe DART’s impact and observe the effects of the impact and make precise determinations of the momentum transferred to the moonlet. AIM will deploy a surface package, MASCOT-2 [Mobile Asteroid Surface Scout] to characterize the moonlet before, during and after the DART impact. Also, AIM will be the first spacecraft to demonstrate interplanetary optical communications.
The DART spacecraft will utilize the NASA Evolutionary Xenon Thruster – Commercial (NEXT-C) solar electric propulsion system as its primary in-space propulsion system. Ion thrusters create thrust by accelerating ions to high velocities using electrical energy and ejecting them. The thrust is small compared to conventional chemical rockets, but it requires significantly less amount of propellant. Ion thrusters are practical only in the vacuum of space and cannot take vehicles through the atmosphere.
Here is a video describing the AIDA mission -
Asteroid Redirect Mission (ARM)
ARM is an ambitious NASA robotic mission to visit a large near-Earth asteroid, collect a multi-ton boulder from its surface, deflect the asteroid and transport the boulder into a stable orbit around the moon.
The ARM consists of two mission segments: 1) the ARRM, which will be the first robotic mission to visit a large (greater than ~100 m diameter) near-Earth asteroid (NEA), collect a multi-ton boulder and regolith from its surface, use the boulder to perform an enhanced gravity tractor asteroid deflection demonstration, and then transport the asteroidal material to a stable orbit around the Moon; and 2) the Asteroid Redirect Crewed Mission (ARCM), in which astronauts will explore the boulder and return samples to Earth. The ARRM is planned to launch at the end of 2020 and the ARCM is planned for late 2025.
The gravity tractor method uses the gravitational force of a large spacecraft to deflect the asteroid slowly over a time. The boulder effectively increases the mass of the spacecraft, thereby increasing the gravitational force and reducing the total time needed to achieve deflection to a few months. The spacecraft and the asteroid mutually attract one another; if the spacecraft counters the force towards the asteroid by, e.g., an ion thruster, the net effect is that the asteroid is accelerated towards the spacecraft and thus slightly deflected from its orbit. While slow, this method has the advantage of working irrespective of the asteroid composition, structure or spin rate.
The crew segment of the mission ARCM will include spacewalk activities for sample selection, extraction, containment and return; and mission operations of integrated robotic and crewed vehicle stack - all key components of future in-space operations for human missions to the Mars system (read “How NASA’s Asteroid Redirect Mission Will Help Humans Reach Mars.”)
The ARM robotic spacecraft will be propelled by advanced solar electric propulsion (SEP), very likely a Hall effect thruster (which is slightly different than the NEXT-C ion thruster which will be used in the AIDA mission). Electricity will be provided by high efficiency UltraFlex-style solar panels (50 kW). The advanced ion engine uses 10 times less propellant than equivalent chemical rockets. The Hall-effect provides low acceleration but can fire continuously for many years to thrust a large mass to high speed. Hall effect thrusters trap electrons in a magnetic field and use them to ionize the onboard xenon gas propellant. The magnetic field also generates an electric field that accelerates the charged ions creating an exhaust plume of plasma that pushes the spacecraft forward. Hall thrusters are now routinely flown on commercial GEO communications satellites where they are used for orbital insertion and minor in-orbit adjustments called stationkeeping. The ARM spacecraft concept would have a dry mass of 5.5 tons, and could store up to 13 tons of xenon propellant. Each thruster will have a 30- to 50-kilowatt power level, and several thrusters will be combined to increase the power of the ARM spacecraft.
SEP effectively uses solar energy for propulsion, of which there is plenty in long duration missions within the solar system. It requires ~10 times less propellant than equivalent chemical rockets which translates to a ~90 times less fuel in the launch rocket needed to launch the propellant.
The ARM mission has a constant budget cap of $1.25 billion. NASA’s total 2017 budget is estimated at $19.5 billion.
The concept animation video below opens with a rendering of the mission's spacecraft trajectory, rendezvous, and approach to asteroid 2008 EV5. The animation concludes with the notional crew operations that will take place after the asteroid boulder is placed in lunar orbit.
Target Asteroid
NASA has identified the NEA 2008 EV5 as the reference target for ARM mission planning. Final target selection will be made approximately a year before launch. 2008 EV5 is a carbonaceous (C-type) asteroid that has been remotely characterized (radar, visual, and infrared wavelengths) and is believed to be hydrated.
2008 EV5 was discovered in 2008, by the Catalina Sky Survey in Arizona. The asteroid has a slightly eccentric orbit close to Earth’s orbit with an orbital period of 0.94 years. EV5’s overall shape is a 400m spheroid with an equator‐aligned ridgeline broken by a single ~150 m concavity.
EV5 has been extensively observed using infrared telescopes and planetary radars, such as those at NASA's Goldstone facility in California and the Arecibo Observatory in Puerto Rico. Observations made in December 2008 suggest that 2008 EV5 should have plenty of boulders to choose from.
In general, boulders or blocks of a wide range of sizes, from ~100 m to sub-meter cobbles, are present in large numbers on the surfaces of all three NEAs so far visited by spacecraft (Eros, Itokawa, and Toutatis). Boulders are evident in radar images of a large number of other NEAs.
Going to an asteroid such as 2008 EV5 is particularly appealing to the scientific, exploration and industrial communities because it is a primitive, C-type (carbonaceous) asteroid, believed to be rich in volatiles, water and organic compounds. The ability to extract core samples from the captured boulder will allow us to evaluate how its composition varies with depth and could significantly advance our understanding of the origins of our solar system. Astronaut sampling and potential commercial activities could help establish the value of C-type asteroids for commercial mining purposes, which in turn could have significant impacts on how deep space missions are designed and implemented in the future.
Mission Status
NASA's Jet Propulsion Laboratory in Pasadena, California, has issued a request for proposal (RFP) seeking design, development and build of the ARM robotic spacecraft. The RFP is open to the four industry partners that previously completed conceptual designs of the spacecraft — Lockheed Martin Space Systems, Space Systems/Loral, Boeing Satellite Systems and Orbital ATK. Proposal submissions are due to JPL by Oct. 24, 2016, with contract award expected in 2017.
The House Appropriations Committee has decided to block funding for NASA’s Asteroid Redirect Mission (ARM) in the 2017 budget. The sub-committee explained that decision by saying “The Committee believes that neither a robotic nor a crewed mission to an asteroid appreciably contribute to the overarching mission to Mars.”
Mars over civilization-destroying asteroids?
References
- NASA NEO Program — neo.jpl.nasa.gov
- Near-Earth Object Survey and Deflection Analysis of Alternatives — Report to Congress neo.jpl.nasa.gov/…
- Asteroid impact avoidance — en.wikipedia.org/...
- AIDA mission at ESA — www.esa.int/...
- NASA ARM Mission — www.nasa.gov/…
- Asteroid Redirect Mission (ARM) Formulation Assessment and Support Team (FAST) Final Report, Feb 2016 — www.nasa.gov/…
- Enhanced Gravity Tractor Technique for Planetary Defense — ntrs.nasa.gov/…
- Technology Development for NASA’s Asteroid Redirect Mission — www.nasa.gov/...
- OSIRIS-REx Mission — www.dailykos.com/...