The orbit insertion velocity near the Earth’s surface is practically the same as the Earth escape velocity of 11.2 kilometers per second, or 25,805 miles per hour.
Even at an altitude of 200 kilometers, a rocket is still inside the outer reaches of the Earth’s atmosphere. To really leave the atmosphere it probably has to get to a distance of 27,000 miles near the ‘geosynchronous’ limit. There is no sharp boundary to Earth’s atmosphere. It just decreases steadily in density until it eventually matches the density of the interplanetary medium. Here is an image that shows the extent of the geocorona, whose density is so low that it has no effect on spacecraft but qualifies as part of Earth’s atmosphere nonetheless. It was taken by NASA astronaut John Young using an ultraviolet camera on Apollo 16.
About 50,000 years ago, an object about 150-meters across traveling at 45,000 mph created the Barringer Crater in Arizona that is 1,500-meters across. (Credit:Wikipedia-NASA). It was a 10-megaton explosion.
An asteroid for which there is some possibility of a collision with Earth at a future date and which is above a certain size is classified as a potentially hazardous asteroid (PHA). Specifically, all asteroids that come closer to Earth than 0.05 AU or about 8 million km, and diameters of at least 100 meters (330 to 490 ft) are considered PHAs. By December 2024, astronomers at the IAU Minor Planets Center had cataloged about 2,437 PHAs that presented a possible hazard to Earth including impacts. About 153 of these are believed to be larger than one kilometer in diameter.
The graph below shows the cumulative number of PHAs detected since 1999 and you can see that the pace of finding new ones has decreased significantly. This means that we have discovered virtually all of these objects by the present time. Even so, if just one of these rare undiscovered birds were to strike earth it would be catastrophic, so we need 100% to be discovered. This figure is provided by the Vera Rubin Observatory (LSST) website.
The largest known PHA is (53319) 1999 JM8 with a diameter of ~7 km, but it is not currently at risk of any impacts. The asteroid Ida, located in the asteroid belt outside the orbit of Mars is shown in the image below and measures 60km x 25km by 18km gives you some idea of what these huge rocks look like!
The smaller an asteroid, the more numerous they are, is the general rule of thumb for our solar system. According to the best estimates, objects 3 meters across impact the Earth every year and deliver about 2 kilotons of TNT of energy. Objects 100 meters across collide with the Earth every few hundred years and deliver about 2 Megatons of TNT equivalent. A 1 kilometer-sized object impacts the Earth every million years or so and delivers about 100,000 Megatons of TNT.
Now, the good news is that the Earth’s atmosphere shields us from objects that are initially below about 100 meters in size because they break-up and evaporate before reaching the ground. Still, the famous Tunguska Event in 1908 was a 50 meter stony meteor, which evaporated about 20 kilometers above the Earth, yet still flattened trees in a 30 kilometer area. Its yield was about 10 Megatons of TNT, and the frequency tables predict that such strikes should happen every 100 years or so. In 2013 we got lucky again!
On February 15, 2013 a once-a-century, 20-meter asteroid literally ‘came out of nowhere’ and exploded over the town of Chelyabinsk in Russia with 500 kilotons of energy, approximately 30 times the yield of the nuclear bomb over Hiroshima. There were many photos of this event taken but nearly all are copyright-protected and licensed. Here is a set of links to some representative images. Just Google ‘Chelyabinsk Meteor’ and you will find many more.
Over 1,500 people were injured or hospitalized for cuts from flying glass. Had this event happened over New York City, over 100,000 people might have been hospitalized and most of the window glass from sky scrapers would be laying in the streets. Needless to say, the consequences of asteroidal impacts depend on the size of the asteroid. Here is a table of some possible consequences:
Size Yield Crater Effect (Megatons) (km) .......................................................... 75 m 100 1.5 Land impacts destroy area the size of Washington
160 m 500 3.0 Destroys large urban areas
350 m 5000 6.0 Destroys area the size of a small state.
700 m 15,000 12 Land impact destroys a small country
1.7 km 200,000 30 Severe climate effects. Tsunamis destroy coastal communities. 3.0 km 1 million 60 Large nations destroyed, Widespread fires.
7.0 km 50 million 125 Mass extinction, long term climate change.
16 km 200 million 250 Large mass extinction.
For the ocean impacts of objects about 300 meters across, the tsunami tidal waves produce more damage than an equivalent impact on the land. Had the Tunguska Event happened over a populated city, the damage would have been equal to a major earthquake exceeding 7.0 with perhaps thousands of people killed by the atmospheric concussion wave, which would flatten poorly designed buildings and cause fires just below the impact. Fortunately, humans occupy so little of the surface of Earth that although these impacts happen about once every century or so, in the past no one has been around to see them.
Ocean impacts of bodies in the 700 meter range would produce major tidal waves that would just reach the shores of many continents. In the 1 -2 kilometer range, these tsunamis would be 300 feet high and travel 20 or more kilometers inland putting at risk about 100 million people or 10 percent of the world population. Such an impact would be known several days in advance by direct detection by NORAD so the question is whether enough people could make it to safety. Of course when they return to their coastal homes and cities, most of these dwellings would be severely damaged or washed away by the tremendous return tide!
I am not even going to mention collisions with larger asteroids which could occur every few million years or so. There would be enough devastation caused by the more frequent ‘super Tunguska’ events to keep us busy!
Earth’s magnetic field at the surface has been mapped for decades. This map provided by the British Geological Survey, shows basic polarity difference between the North and South Hemispheres.
The magnetic field of Earth is shaped like the one you see in a toy bar magnet, but there is a very important difference. The toy magnet field is firmly fixed in the solid body of the magnet and does not change with time, unless you decide to melt the magnet with a blow torch! The Earth’s field, however, changes in time. Not only does its strength change, but the direction it is pointing also changes. Here is a computer model of its 3-d shape in space that reveals its complex features even though it is still a ‘dipolar’ field. (Credit:Wikipedia- Dr. Gary A. Glatzmaier – Los Alamos National Laboratory – U.S. Department of Energy.)
Map makers have been aware that the direction of the magnetic field changes since the 1700’s. Every few decades, they had to re-draw their maps of harbors and landmarks to record the new compass bearings for places of interest. Think about it. If you are on a ship navigating a harbor in a fog, a slight change in your compass heading can take you into a reef or a sandbar!
Geologists have also been keeping track of the wandering magnetic poles as well. Instead of using compasses, they can actually detect the minute fossil traces of Earth’s magnetism in rocks. These rocks are dated to determine when they were formed. From this information, geologists can figure out exactly how Earth’s magnetic field has changed during the last two billion years. The results are surprising. Right now, the North point of your compass points towards the magnetic pole in the Northern Hemisphere. That’s why compass creators put the ‘N’ on the tip of the magnetized compass needle. But because opposite’s attract, this means that the magnetic pole in the Northern Hemisphere is actually a south magnetic pole! That’s because scientists named magnetic polarity after the geographic compass direction!
Since the 1800’s, Earth’s magnetic South Pole which lives in the Northern Hemisphere has wandered over 1100 kilometers. By the year 2030, the magnetic pole will actually be almost right on top of our geographic North Pole. Then in the next century, it will be in the northern reaches of Siberia! Scientists are excited, and a bit concerned, by the sudden dramatic change in the magnetic pole’s location. They worry that something may be going on deep within the Earth to cause these changes, and they have seen this kind of thing happen before.
What geologists have discovered is that the magnetic poles of Earth don’t just wander around a little, they actually flip-flop over time. About 800,000 years ago, the Earth’s magnetic poles were opposite to the ones we have today. Back then, your compass in the Northern Hemisphere would point to Antarctica, because in the Northern Hemisphere the polarity had changed to ‘North’ and this would have repelled the North tip of your (magnetized) compass needle. Geologists have discovered in the dating of the rocks that the magnetism of Earth has reversed itself hundreds of times over the last billion years. Careful measurements of rock strata from around the world confirm these reversal events in the same layers, so they really are global events, not just local ones. What is even more interesting is that the time between these magnetic reversals, and how long they last, has changed dramatically. 70 million years ago, when dinosaurs still roamed the landscape, the time between magnetic reversals was about one million years. Each reversal lasted about 500,000 years. 20 million years ago, the time between reversals had shortened to about 330,000 years, and each reversal lasted 220,000 years.
Today, the time between reversals has declined to only about 200,000 years during the last few million years, and each reversal lasts about 100,000 years or so. When did the last reversal happen?
This is a plot of the change in the main field strength of Earth for the last 800,000 years from the research by Yohan Guyodo and Jean-Pierre Valet at the Instuitute de Physique in Paris published in the journal Nature on May 20, 1999 (page 249-252).
The Brunhes-Matuyama Reversal ended 980,000 years ago when the polarity of the field actually did ‘flip’. Since that time, the polarity of Earth’s field has remained the same as what we measure today with the Northern Hemisphere Arctic Region containing a ‘South-Type’ magnetic polarity, and the Antarctic Region containing a ‘North-type’ polarity. You will note that the last reversal ended when the magnetic intensity reached near-zero levels. Since then, there was a near-reversal about 200,000 years ago labeled ‘Jamaica/Pringle Falls’ after the geologic stratum in which these intensity measurements were first identified. Scientists do not know just how low our field has to fall in intensity before a reversal is triggered, but the threshold seems to be below 2.0 units on the scale of the above ‘VADM’ plot. Beginning in the 1920’s, geologists discovered traces of the last few magnetic reversals in rock samples from around the world. Between 730,000 years ago to today, we have had the current magnetic conditions where the South-type magnetic polarity is located in the Northern Hemisphere near the Arctic. Geologists call this the Brunhes Chron. Between 730,000 to 1,670,000 years ago, Earth’s magnetic poles were reversed during what geologists call the Matuyama Chron. This means that the North-type magnetic polarity was found in the Northern Hemisphere. Notice that the time since the last reversal (the end of the Mayuyama Chron) is 730,000 years. This is a LOT longer than the 200,000 years!
Some scientists think that we may be overdue for a magnetic reversal by about 500,000 years!
Is there any evidence that we are headed towards this condition? Scientists think that the sudden, rapid change in our magnetic pole location is one sign of a significant change beginning to occur. Another sign is the actual strength of Earth’s magnetic field.
Scientists are convinced that Earth’s magnetic field is created by currents flowing in the liquid outer core of Earth. Like the current that flows to create an electromagnet, Earth’s currents can change in time causing the field to increase and decrease in intensity. Geological evidence shows that Earth’s field used to be twice as strong 1.5 billion years ago as it is today, but like the weather it has gone through many complicated ups and downs that scientists don’t have a real good explanation for, or ability to predict. But the fossil evidence does tell us something important.
In the 730,000 years since the last magnetic reversal, Earth’s field has at times been as little as 1/6 its current strength. This happened about 200,000 years ago. Also, around 700 AD it was 50% stronger than it is today. There have been many sudden ups and downs in this intensity, but some scientists think that conditions are rapidly becoming very different than the past historical trends have shown.
We’ve only been able to measure the Earth’s magnetic field strength for about two centuries. During this time, there has been a gradual decline in the field strength. In recent years, the rate of decline seems to be accelerating. In the last 150 years, the strength of Earth’s field has decreased by 5% per century. This doesn’t seem like a very fast decrease, but it is one of the fastest ones that has been verified in the 800,000 year magnetic record we now have. At this rate, in 10 centuries we will be 50% below our current field strength, and after 2000 years we could be at zero-strength. The data on past reversals seems to show that, when the field reaches 10% of its current strength, a magnetic reversal can be triggered. It has been 730,000 years since the last reversal ended. We are certainly long overdue for a reversal, by some statistical estimates.
But the caveat is that magnetic changes come in a variety of timescales from the major reversal events every few hundred thousand years to micro changes called ‘excursions’ that come and go withing a few thousand years. Two detailed “studies of the geomagnetic field in the last 1 million years have found 14 excursions, large changes in direction lasting 5-10 thousand years each, six of which are established as global phenomena by correlation between different sites. Excursions appear to be a frequent and intrinsic part of the (paleomagnetic) secular variation”.(Gubbins, David. 1999. The distinction between geomagnetic excursions and reversals. Geophysical Journal International, Vol. 137, pp. F1-F3.). The figure below shows on the left-side the magnetic intensity measurements since 500,000 years ago during the current Brunhes magnetic chron. You can easily see the ‘spiky’ fast excursions, but the overall magnetic intensity is decreasing in time to the present day. We may be living inside one of these fast excursions which will be replaced by a growing field in a few thousand years, but it seems that the big picture is still that the overall largescale field is declining slowly over 100,000 year timescles. It isn’t the excursions we need to worry about for ‘reversals’ but this larger trend downwards that seems to be going on.
So, what will happen when the field reverses? The fossil record, and other geological records, seem to say ‘Not much!’
Scientists have recovered deep-sea sediment cores from the bottom of the ocean. These sediments record the abundance of oxygen atoms and their most common isotope: Oxygen-18. The increases and decreases in this oxygen isotope track the ebb and flow of periods of global glaciation. What we see is that, during the time when the last reversal happened, there was no obvious change in the glacial conditions or in the way that the conditions came and went. So, at least for the last reversal, there was no obvious change in Earth’s temperature other than what geologists see from the ‘normal’ pattern of glaciation. By the way, because glaciation depends on the tilt of Earth’s spin axis, this also means that a magnetic reversal doesn’t change the spinning Earth in any measurable way.
Loess deposits in China have recently given climatologists a nearly unbroken, continuous record of climate changes during the last 1,200,000 years. What they found was that the sedimentation record shows the summer monsoons and how severe they are. The only significant variation in the data could be attributed to the coming and going of glacial and inter-glacial periods. So, summer monsoons in China were not affected by the reversal in any way that can be obviously seen in the climate-related data from this period. The fossil record, at least for large animals and plants, is even less spectacular when it comes to seeing changes that can be tied to the magnetic reversal.
The Brunhes-Matuyama reversal happened 730,000 years ago during what paleontologists call the Middle Pleistocene Era (100,000 to 1 million years ago). There were no major changes in plant and animal life during this time, so the magnetic reversal did not lead to planet-wide extinctions, or other calamities that would have impacted existing life. It seems that the biggest stresses to plant and animal life were the comings and goings of the many Pleistocene Ice Ages. This led very rapidly to the evolution of cold-tolerant life forms like Woolly Mammoths, for example.
So, it seems that we may be headed for another magnetic reversal event in perhaps the next few thousand years. This event, based on past fossil and geological history, will not cause planet-wide catastrophies. The biosphere will not become extinct. Radiation from space will not cause horrible mutations everywhere. Ocean tides will not devastate coastal regions, and there will certainly not be volcanic activity that leads to global warming.
Of course, scientists cannot predict which minor effects may take place. A magnetic reversal could be a big nuisance to many organisms that will not lead to their extinction, but it just might lead to temporary changes in the way they would normally conduct themselves. The fossil record doesn’t record how a species reacted to minor nuisances! Some animals use Earth’s field to magnetically navigate, but we know that these same animals have back-up navigation systems too. Pigeons use Earth’s magnetism to navigate, as do dolphins, whales and some insects. They also use their eyes as a backup, and a knowledge of land forms and geography, or the location of the Sun and Moon to get about. Humans have used compasses to navigate for thousands of years, but now we rely almost entirely on satellites to steer by. In the future, only those few anachronistic people using the ancient technology of compasses to get around, would have any problems!
The magnetic field of Earth shields us from cosmic rays, so losing this shield may seem like a big deal, but it really isn’t. Cosmic rays are not the same kind of radiation as light, instead it consists of fast-moving particles of matter such as electrons, protons and the nuclei of some atoms. Our atmosphere is actually a far better shield of cosmic radiation than Earth’s magnetic field. Losing the magnetic field during a reversal would only increase our natural radiation background exposure on the ground by a small amount – perhaps not more than 10%. The long term result might be a few thousand additional cases of cancer every year, but certainly not the extinction of the human race.
Return to Dr. Odenwald’s FAQ page at the Astronomy Cafe Blog. References: Guo, Zhengtang, et al., 2000, “Summer Monsoon Variations Over the Last 1.2 Million Years from the Weathering of Loess-soil Sequences in China”, Geophysical Research Letters, June 15, pp. 1751-1754. Guyodo, Yohan and Valet, Jean-Pierre 2003, “Global Changes in Intensity of the Earth’s Magnetic Field During the Past 800 kyr”, Nature, May 20, 2003, p. 249. Jacobs, J. A., “Reversals of the Earth’s Magnetic Field, (pp. 48-50) Jacobs, J. A. “Geomagnetism” Academic Press (pp. 186-89, 215-220, 236-42) Merrill, Ronald, McElhinny, M. and McFadden, P., “The Magnetic Field of the Earth”, Academic Press, (pp.120-125) Raymo, M., Oppo, D. W., and Curry, W. 1997, “The Mid-Pleistocene Climate Transition: A deep Sea Carbon Isotopic Perspective”, Paleoceanography, August 1997, pp. 546-559. Rikitake, Tsuneji and Honkura, Yoshimori, “Solid Earth Geomagnetism”, D. Reidel Publishing Co. (pp. 42-45) Ruddiman, W., et al. 1989, “Pleistocene Evolution: Northern Hemisphere Ice Sheets and North Atlantic Ocean”, Paleoceanography, August, pp. 353-412. Wollin, G., Ericson, D., Ryan, W. and Foster, J. 1971, “Magnetism of the Earth and Climate Changes”, Earth and Planetary Science Letters, vol. 12, pp. 175-183.
In an article in the journal Nature, March 28, 1996 vol. 380, page 323, Dr’s A.D. Taylor, W. J. Baggaley and D. I. Street at the University of Adelaide in Australia discuss the results of their one- year radar monitoring of incoming meteors. When meteorites slam into the atmosphere, they produce ionization in the atmosphere. Radar echoes from this momentary ionization allow the velocity, altitude and distance to be determined if you have two or more such installations for triangulation. The AMOR radar in New Zealand was used for a year in this fashion to detect 350,000 faint echoes from very small meteorites with sizes between 10 – 100 microns. This works out to nearly 1000 every day, just from this site alone! Over 1508 of these meteorites ( 0.9 percent) were found to be traveling at speeds up to several hundred kilometers per second!
On any given day, the estimates are than the Earth intercepts about 19,000 meteorites weighing over 3.5 ounces, every year of which fewer than 10 are ever recovered. About 2800 meteorites are in museums from previous ‘falls’ and are chemically found to represent about 20 or so distinct parent-bodies. The Earth acquires about 100 tons per day of dust-sized micro- meteoroids.
Every night, a network of NASA all-sky cameras scans the skies above the United States for meteoritic fireballs. Most meteors you see in the sky are causes by rice-grain-sized rocks that burn up rapidly through friction. But bolides are especially large. These are objects that can be as large as marbles or basketballs! Automated software maintained by NASA’s Meteoroid Environment Office calculates their orbits, velocity, penetration depth in Earth’s atmosphere and many other characteristics. Daily results are presented at Spaceweather.com. Here is a map for a typical day’s worth of bolides. On February 27, 2017 there were 9 bolides detected by the network. Note all orbits intersect at the location of Earth…of course!
Here is a particularly busy day around the time of the 2016 Perseid Meteor Shower when 262 fireballs were recorded on August 12!
Since 1967, some astronomers have intensively studied the evolution of stars similar in mass and age to our Sun. For example, Prof. Iko Iben at MIT published a ground breaking paper in 1967 ( The Astrophysical Journal, vol. 147, page 624) in which he calculated the changes in temperature, size and luminosity of stars with masses similar to our Sun. What he found out is that at the present time, 4.5 billion years after its birth, the Sun will change its luminosity by a factor of two in the next 5 billion years. In the next billion years, the amount of solar radiation reaching the Earth will increase by 8 percent. Here is a typical timeline for the increase of solar luminosity as it evolves, courtesy of David Taylor at Northwestern University.
This 8 percent increase doesn’t sound like much, but if you look at a recent 1994 report by the National Academy of Science “Solar Influence on Global Climate”, you will discover that a 0.1 percent increase in solar radiation causes a ‘climate forcing of 0.24 watts per square meter, which leads to an increase in the mean global temperature of 0.2 degrees Celsius. From this, we can estimate that our 8 percent increase in solar radiation will cause a 16 degree increase in the mean solar temperature over the next billion years. Or 5 degrees in the next 300 million years.
The map above, courtesy of Robert A. Rhohde and the Global Warming Art project, shows the average annual temperature of Earth based on satellite data. An 8 percent increase in solar energy would cause all of the annual temperatures to increase by 5 Celsius, which means a significant expansion of the brown temperate zone into the sub-Arctic latitudes of Canada, and an expansion of the equatorial high temperature zone into the mid-latitudes.
If a billion years seems too long, just realize that our atmosphere will probably respond to this increase by becoming cloudier as its water vapor content climbs, and will also become richer in carbon dioxide as plant growth is stimulated, and the various terrestrial reservoirs ( oceans) begin to give-up some of their dissolved carbon dioxide. The increased greenhouse heating could make the global temperature increase somewhat higher that the simple 16 degree centigrade change due to the Sun alone.
Lets say that the mean winter temperature, now, is about 20 degrees Celsius. A 5 degree increase in 300 million years means we are now talking about a 25 degree average winter temperature, and very few places where we can expect to see snow and ice. The average summer temperatures would be closer to 30 degrees Celsius. If we add enhanced greenhouse heating, these estimates might easily be much higher, with the average global temperature in 300 million years looking more like 35 – 40 degrees Celsius.
The biggest problem facing life on Earth is Continental Drift. 200 million years ago, the continents came together to form a Supercontinent called Pangea. There have been many re-creations of the continent such as the one shown here. (Credit:Wikipedia-Fama Clamosa).
In about 250 million years a new supercontinent will have formed as the current continents continue their movements. Called Pangea Ultima, the interior of the continent will be utterly uninhabitable by life with daytime temperatures, by some forecasts, exceeding 160 Fahrenheit! This may cause global warming to the degree that a new Hothouse Earth is established due to water vapor and CO2 buildup in the atmosphere. Although a temporary condition that will subside as the continent breaks apart, the timescale for this change is long enough that any extant humans or land-based life will be under enormous environmental stresses, with inevitable population reductions.
Apart from this temporary change due to Pangea Ultima, a review of long term climate variations among the inner planets by Michael Rampino and Ken Caldeira appearing in volume 32, of the Annual Reviews of Astronomy and Astrophysics ( 1994 page 83) suggests that an even bleaker outlook may be in store for Earth when you take into account the carbon dioxide gas in the atmosphere.
The various sources and sinks are sensitive to temperature, and in the next 1.5 billion years, the global mean temperature could well exceed 80 degrees Celsius. The evaporation of the Earth’s oceans would be well underway by 1 billion years from now. We can assume that millions of years before this, Earth will have become uninhabitable. Life more complex than a bacterium has only been around for 600 million years, so it looks like we are about half way through the ‘Golden Years’. To me, this is rather uncomfortably short, because it suggests that in perhaps as short as a few hundred million years, life could get very uncomfortable here!!
The bottom line is that in less time than it has taken higher life forms to evolve into land creatures, the Earth’s biosphere may be changed by the inevitable course of the evolution of our Sun. In 300 million years or less, it may become very inhospitable for life to continue to exist on the land, and if we leave it alone, evolution may encourage life to return to the sea where the climate will be a bit more moderate.
As for humans, we may adapt to living on the land, or we may decide to leave the planet. Estimates suggest that it only takes about 5 – 10 million years to colonize the ENTIRE Milky Way galaxy, so I think we will have plenty of opportunity to survive as a species, even though Earth has become a second cousin to what Venus now looks like.
An astronomer's point-of-view on matters of space, space travel, general science and consciousness
