Categories: AstronomyMeteors

There Could be Meteors Traveling at a Fraction of the Speed of Light When They Hit the Atmosphere

It’s no secret that planet Earth is occasionally greeted by rocks from space that either explode in our atmosphere or impact on the surface. In addition, our planet regularly experiences meteor showers whenever its orbit causes it to pass through clouds of debris in the Solar System. However, it has also been determined that Earth is regularly bombarded by objects that are small enough to go unnoticed – about 1 mm or so in size.

According to a new study by Harvard astronomers Amir Siraj and Prof. Abraham Loeb, it is possible that Earth’s atmosphere is bombarded by larger meteors – 1 mm to 10 cm (0.04 to 4 inches) – that are extremely fast. These meteors, they argue, could be the result of nearby supernovae that cause particles to be accelerated to sub-relativistic or even relativistic speeds – several thousand times the speed of sound to a fraction of the speed of light.

Their study, titled “Observational Signatures of Sub-Relativistic Meteors“, recently appeared online and is being considered for publication in the Astrophysical Journal. Their work addresses an ongoing mystery in astrophysics, which is whether or not the ejecta created by a supernova can be accelerated to relativistic speeds and travel through the interstellar medium to reach Earth’s atmosphere.

Artist’s concept of the meteorite entering Earth’s atmosphere. Credit: University of Oxford

The existence of these kinds of meteors, which would measure about 1 mm in diameter (0.04 inches), has been proposed by several astronomers in the past (like Lyman Spitzer and Satio Hayakawa). The question of whether or not they could survive the trip through interstellar space has also been studied at length. As Siraj explained to Universe Today via email:

“Empirical evidence indicates that at least one supernova has rained heavy elements on Earth in the past. Supernovae are known to release significant quantities of dust at sub-relativistic speeds. We also see evidence of clumpiness or ‘bullets’ in supernova ejecta. The fraction of mass contained in small clumps is unknown, but if just 0.01% of the dust ejecta is contained in objects of millimeter size or larger, we would expect one to appear in the Earth’s atmosphere as a sub-relativistic meteor every month (based on the rate of supernovae in the Milky Way galaxy).”

Despite having a sound theoretical basis, the question remains as to whether or not meteors larger than a grain of dust enter Earth’s atmosphere at sub-relativistic or relativistic speeds. These would be meteors that measure 1 mm (0.04 in), 1 cm (0.4 in), or 10 cm (4 in) in diameter. Much of the problem has to do with our current search methodology, which is simply not set up to look for these kinds of objects.

“Meteors typically travel near 0.01% of the speed of light,” said Siraj. “Therefore current searches are tuned to find signals from objects moving at that speed. Meteors from supernovae would travel a hundred times faster (around 1% of the speed light), and so their signals would be significantly different from typical meteors, making them easily missed by current surveys.”

Meteors are pieces of comet and asteroid debris that strike the atmosphere and burn up in a flash. Credit: Jimmy Westlake

For the sake of their study, Siraj and Loeb developed a hydrodynamic and radiative model to track the evolution of hot plasma cylinders that result from sub-relativistic meteors passing through our atmosphere. From this, they were able to calculate what kind of signals would be produced, thereby providing an indication of what astronomers should be on the lookout for. As Siraj explained:

“We find that a sub-relativistic meteor would give rise to a shock wave that could be picked up by a microphone, and also a bright flash of radiation visible in optical wavelengths – both lasting for about a tenth of a millisecond. For meteors as small as 1mm, a small optical detector (1 square centimeter) could easily detect the flash of light out to the horizon.”

With this in mind, Siraj and Loeb went on to outline the kind of infrastructure that would allow astronomers to confirm the existence of these objects and study them. For instance, new surveys could incorporate infrasound microphones and optical-infrared instruments that would be able to detect the acoustic signature and optical flashes created by these objects entering our atmosphere and the resulting explosions.

Based on their calculations, they recommend that a global network of about 600 detectors with all-sky coverage could detect a few of these types of meteors per year. There is also the option of searching through existing data for signs of sub-relativistic and relativistic meteors. Last, but not least, there is the possibility of using existing infrastructure to look for signs of these objects.

Map displaying location and energy of meteor explosions detected by CNEOS. Credit: NASA/CNEOS

A good example of this, Siraj explained, is to be found in NASA’s Center for Near-Earth Object Studies (CNEOS) network and database:

In addition, we note that the U.S. government’s global classified network of sensors (including microphones and optical detectors) that provides the CNEOS Fireball and Bolide Database likely comprises a capable existing infrastructure. We urge the U.S. government to declassify broader swaths of the CNEOS data so scientists can search for sub-relativistic meteors without spending more taxpayer money to develop a new global network – with one already in operation!

The payoff for this would be nothing less than the ability to study an entirely new set of objects that regularly interact with Earth’s atmosphere. It would also provide a new perspective to the study of supernovae by allowing astronomers to place important constraints on the ejecta they produce. With this in mind, a low-cost, global network of all-sky cameras seems well worth the investment!

Further Reading: arXiv

Matt Williams

Matt Williams is the Curator of Universe Today's Guide to Space. He is also a freelance writer, a science fiction author and a Taekwon-Do instructor. He lives with his family on Vancouver Island in beautiful British Columbia.

View Comments

  • Brings a question to mind: How much energy would be released by a 1 cm meteor traveling at near light speed?
    I'm guessing quite a lot, and we wouldn't even see it coming until a fraction of a second before impact. (If at all)

    • kinetic energy is equal to half of an object's mass (1/2*m) in Kg multiplied by the velocity squared.
      Let's say 0.8 light speed 239,833,600 metres/sec
      and assume a 1cm cubic meteor that would weigh about 3.3 grams, 0.0033Kg
      that's 0.00165 * 5.75 x 10^16
      94,908,257,000,000 J
      A watt is one Joule/sec so if it transfers that energy to the atmosphere in 1 second that's 94 Terawatts

      • Wow! Extrapolating from there, it wouldn't take a huge rock to cause some serious damage.
        I can picture an alien ship bombarding the Earth, not with nukes, but with high speed pebbles. A few tons of sand, launched toward a planet at near light speed could toast a hemisphere.

  • if you use 1% the speed of light and a 1cm diameter asteroid (like suggested in the article)
    speed of light = 2.998x10^8 m/s
    typical asteroid density is 5.3 g/cm^3
    KE= 1/2mv^2
    V = 4/3 pi*r^3
    m = 4/3*pi*(1/2)^3 * 5.3 = 2.78g
    KE = 1/2*2.78*(.01*2.998*10^8) = 1.2e*10^10 Joules
    or anther unit 2.98 Tons TNT

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