Today, June 30, is Asteroid Day. On this UN designated day, events are held globally to educate the public and to raise awareness about asteroids, and about the science of defending our planet from future asteroid impacts. June 30 marks the date of Earth’s largest asteroid impact in recorded history, the Siberia Tunguska event, in 1908.
Asteroid Day was co-founded by astrophysicist and famed musician Dr. Brian May, Apollo 9 astronaut Rusty Schweickart, filmmaker Grig Richters, and B612 Foundation President Danica Remy. Over 200 astronauts, scientists, technologists and artists co-signed the Asteroid Day Declaration. Asteroid Day was officially launched on December 3, 2014.
The Asteroid Day website has lots of online events today -
NASA has online events today as well -
What are asteroids and why should we worry about them?
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 January 8, 2019 and according to statistics maintained by CNEOS, 19,470 near-Earth asteroids (NEOs) have been discovered. ranging in size from 1 meter up to 32 kilometers (1036 Ganymed). The number of near-Earth asteroids over one km in diameter is estimated to be about 900, of which over 90% have been discovered. 1,955 of the NEOs are classified as potentially hazardous asteroids (PHAs).
Asteroids smaller than about 25 meters generally burn up as they enter the Earth's atmosphere and cause little or no damage.
The Tunguska event was caused by a stony asteroid estimated to be 50–60 metres wide. The airburst at 6–10 km above the Earth's surface flattened an estimated 80 million trees over an area of 2,150 square km. No impact crater was found.
Every 2,000 years or so, a meteoroid the size of a football field hits Earth and causes significant damage to the impacted area.
Only once every few million years, an object large enough to threaten life on Earth 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.
A reminder from the Nobel Prize committee about the most famous asteroid that collided with earth -
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 April 2021 there are 4,595 known comets. The reservoir of comet-like bodies in the Oort cloud is estimated to be one trillion.
Astronomers recently discovered a giant comet (100 - 200 km in diameter) that will come visiting us in January 2033.
Its highly eccentric orbit reaches out into the Oort cloud, at a distance of 40,000-50,000 AU from the Sun (1 AU = mean Earth-Sun distance). We won’t see it again for another 3-4 million years.
C/2014 UN271 is currently as far as Neptune is from the Sun (20 AU). As its closest approach, it will be just outside Saturn’s orbit, so nothing for us to fear … yet.
Searching for and cataloging Asteroids and Comets
The Center for Near-Earth Object Studies (CNEOS) is the Jet Propulsion Laboratory (JPL) center for computing asteroid and comet orbits and their probability of Earth impact.
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.
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 |
|
ATLAS |
Haleakala and Mauna Loa, Hawaii |
|
A new space based NEO Surveyor telescope is planned for launch in 2026 -
The following animation represents a map of the increased count of all known asteroids in the solar system between Jan. 1, 1999, and Jan. 31, 2018. Blue represents near-Earth asteroids. Orange represents main-belt asteroids between the orbits of Mars and Jupiter.
Asteroid Impact Avoidance
The following tables (from cneos.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.
Double Asteroid Redirection Test (DART)
Double Asteroid Redirection Test (DART) is a planned space probe that will visit the binary asteroid Didymos and demonstrate the kinetic effects of crashing an impactor spacecraft into an asteroid moon for planetary defense purposes.
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 DART 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 was expected to be launched in Dec 2020 and intercept Didymos’ moonlet in early October 2022 when the Didymos system is within 11 million km of Earth, enabling observations by ground-based telescopes and planetary radar. The 2021 launch date has been moved to 2022. However, DART will still arrive at the Didymos binary asteroid system within a few days of the originally scheduled impact date of September 30, 2022 and will carry out its kinetic impact test as planned.
Here are a couple of videos on the DART mission —
Originally, DART was part of a joint mission with the European Space Agency (ESA) called AIDA. ESA would launch the AIM spacecraft, which would have orbited the larger asteroid to study its composition and that of its moon, as well as measure the effect on the asteroid moon's orbit around the larger asteroid. The AIM mission was cancelled.
Instead, DART will be accompanied by LICIACube (Light Italian CubeSat for Imaging of Asteroids), a small 6-unit CubeSat being developed by the Italian Space Agency (ASI). LICIACube will separate shortly before impact to acquire images of the impact and ejecta as it drifts past the asteroid.
In addition, ESA will launch the Hera spacecraft to Didymos in 2024, which will arrive in 2027 — 5 years after DART's impact — to do a detailed reconnaissance and assessment.
Asteroid Redirect Mission (ARM)
ARM was 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 project was cancelled in 2017. Here is a description of the exciting mission nevertheless; hopefully, something like this might get funded in the future.
The ARM consisted of two mission segments: 1) the ARRM, which would 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 would explore the boulder and return samples to Earth. The ARRM was planned to launch at the end of 2020 and the ARCM was 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 would have included 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.
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 would have taken place after the asteroid boulder was placed in lunar orbit.
In 2016-17, the House Appropriations Committee decided to block funding for NASA’s Asteroid Redirect Mission (ARM). 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?
Epilogue
From en.wikipedia.org/… — While the chances of a major collision are low in the near term, it is a near-certainty that one will happen eventually unless defensive measures are taken. In April 2018, the B612 Foundation reported "It's 100 percent certain we'll be hit by a devastating asteroid, but we're not 100 percent sure when." Also in 2018, physicist Stephen Hawking, in his final book, Brief Answers to the Big Questions, considered an asteroid collision to be the biggest threat to the planet.
That’s not much the average person can do for planetary defense against marauding asteroids, but it helps to be aware and knowledgeable about asteroids, comets and the science of our solar system. Let’s support our scientists and various R&D efforts in this area and let’s impress upon our lawmakers that a few billion dollars spent on planetary defense will do lot more for humanity than the trillions we spend on the military and high-tech weapon systems.
References
- Center for NEO Studies (CNEOS) — cneos.jpl.nasa.gov
- Near-Earth Object Survey and Deflection Analysis of Alternatives — Report to Congress (2007) — cneos.jpl.nasa.gov/…
- National Near-Earth Object Preparedness Strategy and Action Plan (2018) — www.nasa.gov/...
- Asteroid impact avoidance — en.wikipedia.org/...
- Double Asteroid Redirection Test (DART) Mission — www.nasa.gov/...
- Double Asteroid Redirection Test (DART) — en.wikipedia.org/…
- DART at APL — dart.jhuapl.edu
- Enhanced Gravity Tractor Technique for Planetary Defense — ntrs.nasa.gov/…
- NEO Earth Close Approaches — cneos.jpl.nasa.gov/… (database of NEOs, default view shows upcoming NEOs)
- 5 Planetary Defense Systems That Could Keep Us Safe From Asteroids — interestingengineering.com/…
- Asteroids and Planetary Defense — www.dailykos.com/…