Potentially Hazardous Asteroids
For additional information go to the Asteroid Watch page
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Potentially Hazardous Asteroids
Potentially Hazardous Asteroids (PHAs). What are they? How many are there? Do they Exist? And, is anybody watching for them?
The number of PHAs increases monthly. Until the last few years little attention was paid to these "space rocks". Perhaps the number of movies concerning errant asteroids has drawn government interest in identifying and classifying asteroids that approach Earth.
The term "space rock" has been used for a number of years. So far as those of us here at the Web Site know, "space rock" is a general term without numerical specifications. The term "Potentially Hazardous Asteroid" has been defined as an object equal to or greater than a 100 meters in diameter that will pass by Earth no farther than 0.05 AU. This definition has been widely accepted as a working definition.
To prove that the U.S. Government is actually "working" on the broad topic of "Potentially Hazardous Asteroids" (PHA), the definition of a PHA continues to be modified.
The definition of a PHA given by NASA (current as of 17 August 2005):
Potentially Hazardous Asteroids (PHAs) are currently defined based on parameters that measure the asteroid's potential to 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. In other words, asteroids that can't get any closer to the Earth (i.e. MOID) than 0.05 AU (roughly 7,480,000 km or 4,650,000 mi) or are smaller than about 150 m (500 ft) in diameter (i.e. H = 22.0 with assumed albedo of 13%) are not considered PHAs. NASA Near Earth Object Program
The definition immediately above is from the NASA's Near Earth Object Program's "Group" page on the World Wide Web (http://neo.jpl.nasa.gov/neo/groups.html. This NASA page was last updated on 27 December 2003.
When a potentially dangerous situation occurs and political bodies need to make important decisions on how to react - could one expect legislative and executive bodies to understand and work with the definition immediately above?
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Classification of PHAs
NASA further classifies Potentially Hazardous Space Objects
| Group | Description | Definition |
|---|---|---|
| NECs | Near-Earth Comets | q<1.3 AU, P<200 years |
| NEAs | Near-Earth Asteroids | q<1.3 AU |
| Atens | Earth-crossing NEAs with semi-major axes smaller than Earth's (named after asteroid 2062 Aten). | a<1.0 AU, Q>0.983 AU |
| Apollos | Earth-crossing NEAs with semi-major axes larger than Earth's (named after asteroid 1862 Apollo). | a>1.0 AU, q<1.017 AU |
| Amors | Earth-approaching NEAs with orbits exterior to Earth's but interior to Mars' (named after asteroid 1221 Amor). | a>1.0 AU, 1.017<q<1.3 AU |
| PHAs | Potentially Hazardous Asteriods: NEAs whose Minimum Orbit Intersection Distance (MOID) with the Earth is 0.05 AU or less and whose absolute magnitude (H) is 22.0 or brighter. | MOID<=0.05 AU, H<=22.0 |
Chart from NASA's http://neo.jpl.nasa.gov/neo/groups.html
PHA Specifications Explained in Simpler Terms
Many reports on PHAs will give the space object's distance from Earth in "LUNAR DISTANCEs" (LD). Lunar Distance being the distance between Earth and the Moon. This distance changes but roughly 1 LD = 384,401 km = 0.00256 AU NASA. The label "MISS-DISTANCE" may be used.
The visual magnitude (MAG) of the PHA is given at the time that the Asteroid is at its shortest distance to Earth.
ASTRONOMICAL UNIT
(AU) = 1.496E+13cm
= 214.94 solar radii.
It is our opinion that changes of detail in the definition complicates the topic. It may be stated that when it comes to PHAs, few know exactly what another is talking about. We feel that a complicated definition compounds the potential hazard if it becomes the "working definition".
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Elsewhere on the Web Site we have commented on the fact that the U.S. Air Force is able to keep track of orbiting space material down to the size of a green pea (roughly)
While pee is monitored, that monitoring is done by NASA. We do not know of anytime that a green pea has been launched to test the Air Force's capability. We are in favor for money to be appropriated by Congress to fund this research.
The key to monitoring space junk is that it must ORBIT Earth for regular tracking to take place. Some, but not all, PHAs orbit Earth.
Agencies Responsible For Monitoring Potentially Hazardous Space Objects
Monitoring the "Potentially Hazardous Asteroid" is a responsibility not easily assigned to any one branch of the U.S. Government. Further, quick decision making and plan implementation involving multiple agencies and multi-national organizations would be a difficult task in the face of an emergency and only a very few days to react to the hazard present. Other that the obvious treat posed by a PHA, what criteria can be used to assign their responsibility to one agency or another?
Often times there are very few days between the time of discovery of a PHA and the time of its potential collision with Earth. It is obvious that a responsible Congress can not afford to wait to the last minute to decide. Since other countries are in the same boat as we, how are they handling the potential threat some space rocks pose? PHAs can't be taxed at this time. They can be navigational hazards as well as a sometimes deciding factor in the window of launch for space craft.Can they be regulated or taxed. Are they hazards to navigation?
What are you going to do if you find one? Blow it up? If it is solid rock maybe not.
NASA defines an asteroid as being potentially hazardous " based on parameters that measure the asteroid's potential to make threatening close approaches to the Earth." The specific measurements taken from NASA's Potentially Hazardous Asteroid Site ( http://neo.jpl.nasa.gov/neo/pha.html ) are:
"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. In other words, asteroids that can't get any closer to the Earth (i.e. MOID) than 0.05 AU (roughly 7,480,000 km or 4,650,000 mi) or are smaller than about 150 m (500 ft) in diameter (i.e. H = 22.0 with assumed albedo of 13%) are not considered PHAs."
As of 27 December 2003, NASA reports, on the above web site, that there are "currently 690 known PHAs."
( Note from the Webmaster: 06 April 2005, The web page maintained (?) by NASA showing their "current count" of PHAs has not been updated since 27 December 2003 )
www.spaceweather.com states that 681 PHAs exist as of 06 April 2005
While I was in high school Russia launched a satellite followed closely by the United States. Two things about space held as being as true (at the high school level) were 1). Space was "too large and open" for any large object to just happen by and hit Earth, 2) anything that approached Earth would be burned up in the atmosphere. Apparently that attitude was held true at other levels also, and was held for quite a long time. An early push for the study of PHAs occurred in May of 1998. PHAs were the subject of a Congressional Hearing before the Subcommittee on Space and Aeronautics, Committee on Science and NASA .The time was May 21, 1998. The questions I asked above where the questions being asked during the Hearing.
Taken from the record:
"NASA Memorandum for the Record
By Barbara Cherry, Legislative Affairs Office
Subject: “Asteroids: Perils and Opportunities” hearing before the Subcommittee on Space and Aeronautics, Committee on Science, May 21, 1998."
Members: Rohrabacher , Chairman, (R-CA), Brown (D-CA), Cook (R-UT), Gordon (D-TN),
Present: Bartlett (R-MD), Hall (D-TX), Roemer (D-IN), Weldon (D-FL), Luther (D-MN)
Witnesses: Dr. Clark Chapman, Southwest Research Institute; Dr. William Ailor, The Aerospace Corporation; Dr. Gregory Canavan, Los Alamos National Laboratory; Dr. John Lewis, University of Arizona; Dr. Carl Pilcher, NASA.
OPENING STATEMENTS
"Chairman Rohrabacher opened the hearing by commending Congressman Brown for his leadership and long track record of pushing the Executive Branch to deal with the issue of cataloging and characterizing asteroids. He stated that the potential impact of these hazardous objects is one of national security, economic as well as scientific interest. Congressman Rohrabacher noted that the potential to mine asteroids for metals, minerals and other resources that can be used to build large structures in space was an important aspect of the hearing. Mr. Rohrabacher chided NASA for not “walking the talk” by funding the Near Earth Object (NEO) search program at the levels suggested in the Shoemaker Report. He noted that NASA has no trouble finding $50 million for a program pushed by the Vice President to transmit pictures of Earth into everyone’s living room and cannot find a few million dollars to increase the likelihood of cataloguing all of the potentially hazardous NEOs l km or larger."
"Congressman Brown echoed his long-standing interest in this subject and the importance of addressing the issue of cataloging and characterizing NEOs even though the risk of impact is small because of the enormous potential catastrophic consequences."
Dr. Chapman discussed the “possibility that an asteroid or comet might strike Earth in our lifetime, perhaps destroying civilization as we know it.” He presented a chart which illustrated the chances of dying from an asteroid impact against selected other causes (USA). Congressman Cook noted that the chances of dying from an airplane crash and from an asteroid impact were both l in 20,000.
Dr. Chapman noted that if a mile-wide asteroid hit earth, it would create a hole larger than Washington DC, it would be deeper than 20 Washington monuments stacked on top of each other, ruin agriculture production, and hundreds of millions to billions of people would die. He noted that the consequences were devastating and, therefore, it was prudent to implement the recommendations contained in the Shoemaker Report of cataloging 90% of all of the NEOs with diameters of 1 km or larger within a decade. This would reduce by a factor ten the uncertainty of knowing if an asteroid were headed toward Earth and would likely provide sufficient time to try and deal with the situation.
...testimony not included : may be read in full at http://impact.arc.nasa.gov/
QUESTIONS:
Chairman Rohrabacher stated that NASA has not been spending adequate funding to search for NEOs (Near Earth Objects) as recommended in the Shoemaker Report. Dr. Pilcher noted that, the Office of Space Science has issued their Strategic Plan which includes a goal of cataloging 90% of the 1 km asteroids, that NASA funding wasn’t adequate to accomplish this goal, and NASA had doubled the funding of the NEO program.
Mr. Rohrabacher asked Dr. Pilcher how many of the asteroid missions he discussed were actually in the budget. Dr. Pilcher replied that DS-1, DS-4, Contour, STARDUST, Comet Nucleus Sample Return, and Pluto Kuiper Express were all assumed in NASA’s budget.
Congressman Gordon asked if NASA was the only Agency working on the problem. Dr. Pilcher responded that NASA supports researchers at Universities to address this issue and works closely with the Air Force. He stated that NASA is developing collaborations with the international community as well. Mr. Gordon asked if the Federal Government was coordinating adequately. Dr. Chapman responded that FEMA has little appreciation for the hazard of such an event. Dr. Canavan stated that interagency cooperation between NASA and the Air Force hasn’t percolated down to the troops beyond the Administrator and General Estes.
Congressman Hall asked if we have to have a calamity before anyone takes something seriously and noted that it is an international problem and should have international participation. He asked how much NASA is spending on search activities. Dr. Pilcher responded that NASA is spending $3 million per year and approximately $1 billion over the next decade in asteroid/comet missions. Mr. Hall questioned if NASA is actually spending all of the money allocated for the purposes which the Congress appropriated the funds, indicating that Life and Microgravity funding was being spent for hardware and not research.
Mr. Roemer noted that Dr. Chapman had provided the sound bite for the evening news-- that “a mile wide asteroid could hit the Earth tomorrow and we wouldn’t know anything about it.” He asked if the Federal Agencies had held discussions among themselves on what you would need to do to coordinate a response if an impact were imminent. The witnesses indicated that not much had been done.
....additional testimony omitted. Testimony can be read in full at http://impact.arc.nasa.gov/
The above testimony was given and heard not 50 years ago, or even 10 years ago. The above took place less than 5 years ago.
The pace of study has picked up and the number of countries "interested" in the topic has increased by a few ( "interest" is defined in at least forming some type of committee or greater effort ). In the next issue we will start to provide information on who is looking at the topic of Potentially Hazardous Asteroids and what they are doing. Things have changed in 5 years.
If you question the merit of studying PHAs, you can get a brief look at the Asteroid of 18 August 2002... the one no one really talked about.
http://neo.jpl.nasa.gov/neo/pha.html
http://neo.jpl.nasa.gov/neo/groups.html
This page was last updated : 10 Aug 2008 01:29 AM CST U.S.A.