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Earth has been hit 566 times over 20 years

This map uses data gathered from 1994-2013 and shows where small asteroids hit Earth's atmosphere and produced very bright meteors, technically called 'bolides' and commonly referred to as 'fireballs'. The objects ranged in size from about 1 metre (3 feet) to almost 20 metres (60 feet).

This map uses data gathered from 1994-2013 and shows where small asteroids hit Earth’s atmosphere and produced very bright meteors, technically called ‘bolides’ and commonly referred to as ‘fireballs’. The objects ranged in size from about 1 metre (3 feet) to almost 20 metres (60 feet).

A map released by NASA’s Near Earth Object (NEO) Program reveals that small asteroids frequently enter and disintegrate in Earth’s atmosphere with random spread around the globe.

Released to the scientific community, the map visualises data gathered by U.S. government sensors from 1994 to 2013. (‘Sensors’ probably means, or at least includes, spy satellites. Ed.)

The data indicate that Earth’s atmosphere was hit by small asteroids, resulting in a bolide (or fireball), on 556 separate occasions in a 20-year period.

Almost all asteroids of this size disintegrate in the atmosphere and are usually harmless. The notable exception was the Chelyabinsk event, which was the largest asteroid to hit Earth in this period.

Trail left by the Chelyabinsk meteor.

Trail left by the Chelyabinsk meteor.

The new data could help scientists better refine estimates of the distribution of the sizes of NEOs including larger ones that could pose a danger to Earth.

Finding and characterising hazardous asteroids to protect our home planet is a high priority for NASA, and is one of the reasons the agency has increased by a factor of 10 investments in asteroid detection, characterisation and mitigation activities over the last five years.

In addition, NASA has aggressively developed strategies and plans with its partners in the U.S. and abroad to detect, track and characterise NEOs.

These activities also will help identify NEOs that might pose a risk of Earth impact, and further help inform developing options for planetary defence.

The public can help participate in the hunt for potentially hazardous Near Earth Objects through the Asteroid Grand Challenge, which aims to create a plan to find all asteroid threats to human populations and know what to do about them.

NASA is also pursuing an Asteroid Redirect Mission (ARM) which will identify, redirect and send astronauts to explore an asteroid.

Among its many exploration goals, the mission could demonstrate basic planetary defence techniques for asteroid deflection.

Adapted from information issued by NASA’s Jet Propulsion Laboratory. Chelyabinsk meteor image courtesy Alex Alishevskikh under CC. Map courtesy Planetary Science.

Tracking a dangerous asteroid

Illustration of a spacecraft heating part of Apophis' surface using mirrors

One way to tackle an errant asteroid is to heat part of its surface. Material will be expelled one way, and the asteroid will move slightly in the opposite direction. Done far enough ahead of any potential collision, it will keep a rocky body from coming near Earth.

  • Asteroid Apophis will make a close approach to Earth in 2029
  • Has the potential to strike our planet later this century
  • More observations are needed to clarify its orbit

ASTRONOMERS HAVE TAKEN THE FIRST new images in over three years of the potentially dangerous near-Earth asteroid Apophis as it emerged into view from behind the Sun.

The object became famous in late 2004, when it appeared to have a 1-in-37 chance of colliding with Earth in 2029, but additional data eventually ruled out that possibility.

However, on April 13, 2029, the asteroid, which is 270 metres in diameter, will come closer to Earth than the geosynchronous communications satellites that orbit Earth at an altitude of about 36,000 kilometres. Apophis will then be briefly visible to the naked eye as a fast-moving starlike object.

This close encounter with Earth will significantly change Apophis’s orbit, which could lead to a collision with Earth later this century. For that reason, astronomers have been eager to obtain new data to further refine the details of the 2029 encounter.

Astronomer David Tholen (University of Hawaii (UH) at Manoa), one of the co-discoverers of Apophis, and graduate students Marco Micheli and Garrett Elliott obtained the new images on January 31 using the UH 2.2-metre telescope on Mauna Kea, Hawaii.

Composite image of Apophis

Apophis (circled) in a composite of five exposures taken on January 31 with the University of Hawaii 2.2-metre telescope. (The blemish in the upper left corner is an artefact caused by a dust speck on the camera.)

At the time, the asteroid was less than 44 degrees from the Sun and about a million times fainter than the faintest star that the average human eye can see without optical aid.

“The superb observing conditions that are possible on Mauna Kea made the observations relatively easy,” said Tholen.

Out of the glare

Astronomers measure the position of an asteroid by comparing it with the known positions of stars that appear in the same image as the asteroid. As a result, any tiny error in the catalogue of star positions—due, for example, to the very slow motions of the stars around the centre of our Milky Way galaxy—can affect the measurement of the position of the asteroid.

“We will need to repeat the observation on several different nights using different stars to average out this source of imprecision before we will be able to significantly improve the orbit of Apophis and therefore the details of the 2029 close approach and future impact possibilities,” noted Tholen.

Apophis’s elliptical orbit around the Sun will take it back into the Sun’s glare in the middle of 2011, inhibiting the acquisition of additional position measurements.

However, in 2012, Apophis will again become observable for approximately nine months. In 2013, the asteroid will pass close enough to Earth for ultra-precise radar signals to be bounced off its surface.

Adapted from information issued by the University of Hawaii at Manoa. Apophis image courtesy D. Tholen, M. Micheli, G. Elliott, UH Institute for Astronomy. Apophis illustration courtesy SpaceWorks Engineering, Inc.

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Near misses for Earth today

Asteroid Itokawa

This is asteroid Itokawa, which was visited in recent years by a Japanese spacecraft. The two asteroids due to pass by Earth today are much smaller than Itokawa.

Two small asteroids are due to pass close to Earth within the next 24 hours. So close, in fact, that they’ll be close to us than the Moon is.

Discovered on Sunday by astronomer Andrea Boatini at the Mount Lemmon Observatory in Arizona, they are known only by their catalogue names: 2010 RX30 and 2010 RF12.

2010 RX30 is thought to be 32 to 65 feet (10 to 20 metres) in width. It’ll pass within 0.6 of the distance between the Moon and the Earth (about 154,000 miles, or 248,000 kilometres) at 5:51am US EDT on Wednesday…that’s 7:51pm Sydney time on Wednesday).

The second object, 2010 RF12, estimated to be 20-46 feet (6 to 14 metres) in size, will pass within 0.2 of the Moon-Earth distance (about 49,088 miles or 79,000 kilometres) at 5:12pm US EDT on Wednesday…that’s 7:12am Sydney time on Thursday.

Asteroid orbit diagrams

Orbits of the inner planets (white circles) and the asteroids (light blue lines). Arrows show where the orbit of Earth and the asteroids' trajectories almost intersect.

Experienced amateur astronomers will have a chance of seeing them, depending upon where the observer is located on the Earth (eg. whether it is daylight or night-time at the observing location). Details are available on the Fawkes Telescope web site.

But the two small space rocks won’t be visible to the unaided eye—they’ll be hundreds of times fainter than the naked eye can see.

Neither of the objects have any chance of hitting us during these close approaches, nor indeed at any reasonable time into the future.

So why weren’t they discovered earlier?

Asteroids don’t give off any light of their own…all they do is reflect sunlight. And being made of rock, they aren’t very reflective. Add this dullness to the fact that small asteroids don’t reflect as much light as big ones do (because of their smaller surface area), and you can see that small ones are dim. And this makes them harder to spot.

They have to be almost upon us before they can be seen. Indeed, some have been spotted only once they’ve gone past!

Small asteroids such as these don’t pose much of a risk to anyone on Earth. If they hit, they would explode in the upper atmosphere—due to the tremendous deceleration forces experienced during atmospheric entry—with some small fragments probably surviving to fall to the ground as meteorites.

This is what happened in October 2008, when a small asteroid that had been spotted only a few days before, entered the atmosphere over Sudan in Africa and exploded. Searchers went out and found some fragments a few days later.

And we’re probably being buzzed by them all the time. NASA says that a “10-metre-sized near-Earth asteroid from the undiscovered population of about 50 million would be expected to pass almost daily within a lunar distance, and one might strike Earth’s atmosphere about every 10 years on average.”

Adapted from information issued by NASA / JAXA.

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