精东视频

A celestial stocktake: what鈥檚 out there?

The objects in our solar system come in many different shapes and sizes and with lots of different names, so let鈥檚 start by clearing up some terminology.

Planet

A large, round object that orbits a star and has an orbit that鈥檚 mostly clear from other objects. Planets come in different varieties: rocky planets GLOSSARY rocky planetsPlanets that are mostly composed of rocks or metals. like Earth; gas giants GLOSSARY gas giantsPlanets mostly composed of light elements with low boiling points, like hydrogen and helium. like Jupiter; and ice giants GLOSSARY ice giantsPlanets mostly composed of heavier elements with low boiling points, like oxygen, carbon, nitrogen and sulfur. like Uranus.

Moon

A celestial body that orbits another celestial body that isn鈥檛 a star. They don鈥檛 necessarily need to orbit planets鈥攕ometimes they orbit asteroids or dwarf planets.

Dwarf planet

A large object that doesn鈥檛 meet all the criteria to be either a planet or a moon. They include Pluto, Eris and Ceres.

Meteoroid

A small rocky object in space, considerably smaller than an asteroid (typically, no bigger than 10 metres across).

Asteroid

A rocky object that orbits the sun, smaller than a dwarf planet but bigger than a meteoroid. Asteroids usually have an irregular (non-round) shape, because they don鈥檛 have enough gravity to squish them into a ball.

Comet

An object made of ice and dust. They look different from asteroids because of the distinctive tail (or tails) of particles that stream behind them, caused by sublimation GLOSSARY sublimationThe equivalent of melting in space. Heat melts the ice, which boils off into space and then freezes again 鈥� we see those ice crystals twinkling in the sunlight as the tail of the comet. .

Meteor

An object from outer space that enters Earth鈥檚 atmosphere. When they hit the atmosphere at night, they鈥檙e seen as shooting stars (and if they鈥檙e extremely bright, they are also known as fireballs or bolides). Most are no larger than a grain of sand. Comets, asteroids and meteoroids can all become meteors.

Meteorite

A piece of rock or metal that has fallen to the surface of Earth from outer space. If they鈥檙e big enough, meteors become meteorites when they hit the ground. We can by studying meteorites that fall to Earth.

Near-Earth object

A small (non-planet) object in the inner solar system. Despite the name, they can still be quite far away from Earth during most of their orbit.

Perhaps most useful to remember is that when near-Earth objects (including asteroids, comets and meteoroids) enter the atmosphere, they鈥檙e called meteors; and if there鈥檚 anything left when they hit the ground, the resulting object is called a meteorite. We tend to focus on asteroids when talking about potential collisions, because they鈥檙e more likely to hit us than other stuff like comets, but still big enough to pose a threat.

Where do asteroids and comets come from?

Asteroids and comets aren鈥檛 rogue wanderers flying through space鈥攖hey orbit the sun, just like the planets. Here are a few key areas where they鈥檙e found, ordered by their proximity to Earth.

Asteroid belt

Located between Mars and Jupiter. Thought to harbour (and even more smaller ones, too).

Trojan asteroids

Orbit the sun in the same path as Jupiter, trailing in stable positions in front of and behind the giant planet.

Kuiper belt

Located beyond the orbit of Neptune. Is thought to have . It also contains the dwarf planets Pluto, Haumea and Makemake.

Oort cloud

The Oort cloud has not been directly observed, but is thought to be a sphere of icy debris that surrounds our solar system, stretching out so far that it nearly reaches the next-closest star from the sun. If it exists, it probably contains , and is potentially the source of long-period comets GLOSSARY long-period cometsComets that take hundreds or thousands of years to complete their orbits, like Hale-Bopp, last seen in 1997 and next expected to be seen from Earth in the year 4380. .

The vast majority of these asteroids and comets will never come close to our planet. To understand why, it鈥檚 worth taking a brief tour of how the solar system formed.

The building blocks of planets

Before our solar system was the solar system, it was a huge cloud of dust and gas (mostly hydrogen). It started clumping together under the influence of gravity, and little clumps of dust moved and collided with other little clumps, with their overall movement stabilising into a .

Some objects clumped together more quickly, giving them more gravity and surface area than other bits of matter. This made those objects more likely to accrete GLOSSARY accreteCome together under the influence of gravity. more matter, and so they became even bigger as they moved through space. The centrepiece became the sun. Around it, the planets formed鈥攍arge bodies that cleared their orbits by collecting or pushing apart all the stuff in their way.

Asteroids can be thought of like the building blocks of planets (they鈥檙e sometimes called planetoids or minor planets). The jury鈥檚 still out on exactly , though the gravity of Jupiter is thought to have played a key role in agitating their orbits. Regardless of their size, these objects tend to stay in their orbits, far from Earth.

But occasionally, asteroids and comets are nudged by the gravity of nearby planets into orbits closer to us, becoming near-Earth objects. These 鈥榥udged鈥� asteroids constitute a tiny fraction of all those in the solar system, but there are still a lot of them. And they can鈥攁nd do鈥攈it us. 

Earth鈥檚 pockmarked past

Collisions with asteroids and other bodies in space aren鈥檛 rare for Earth, even on the timescales of our own lives. Small meteoroids about a metre wide hit the atmosphere about , becoming little more than a shooting star. But past collisions with bigger offenders have caused dramatic changes to our planet.

The (proposed) story of our moon

The biggest collision Earth has faced is the one that (probably) led to the formation of the moon. Under the leading鈥攂ut still debated鈥攈ypothesis of the moon鈥檚 formation, around 4.5 billion years ago a protoplanet GLOSSARY protoplanetLarge bodies that are thought to still be accreting matter and developing. If their orbits stabilise, they end up as dwarf planets or planets. known as Theia . Theia was almost the size of what Mars is today, and the collision between these two huge bodies changed the course of our planet forever: it , changed the length of our days, changed the makeup of the Earth鈥檚 core, and created a band of rocky debris that eventually formed into the moon. 

Not long after Earth collided with Theia, the solar system started to settle down. The planets had formed, and their orbits were de-cluttered. But that was not the end of the story.

The late heavy bombardment

Based on the ages of rocks that Apollo astronauts brought back from the moon, we know that something major happened around 4 billion years ago: the Late Heavy Bombardment. We don鈥檛 know exactly what caused this event鈥攐ne idea is that the orbit of Jupiter flung Neptune into the outer solar system, and another is that the bombardment was caused by the late formation of Uranus and Neptune. Whatever happened, it created a gravitational shift that sent a barrage of asteroids from their orbits. Some hit Earth, some hit the moon, and most of the other planets of the inner solar system received hits too.

More recent collisions

Perhaps the most well-known collision is the one that formed the Chicxulub crater in Mexico, coinciding with the extinction of most of the dinosaurs. It threw a huge amount of dust and debris into the atmosphere, causing an impact winter GLOSSARY impact winterA long period of darkness and cold weather caused by dust and debris thrown into the atmosphere by an impact with an asteroid or comet. . This contributed to the extinction of around 60 to 80% of all living species. The energy released鈥攅quivalent to over a billion Hiroshima bombs鈥攖riggered a near-global firestorm that burned huge quantities of the ancient forests. This had flow-on effects that further contributed to environmental change.

Although this happened around 66 million years ago, it鈥檚 very recent in Earth鈥檚 4.5 billion year history. More recent collisions haven鈥檛 been as extreme, but explosive impacts have still happened within the window of humanity鈥攆rom the Tunguska event in 1908 to the Chelyabinsk meteor in 2013, we鈥檝e come to learn that threatening asteroids are part of our cosmic environment.

So even though the rocky mass of Earth itself was resilient enough to survive a collision with a Mars-sized rock billions of years ago, our relatively recent life and climate is much more susceptible to the dangers of asteroids鈥攁nd that means we鈥檙e susceptible, too.

• The end of the dinosaurs?

  You may have learned that dinosaurs became extinct due to an asteroid collision, but this isn鈥檛 the full story. First, not all dinosaurs became extinct. , and passed their legacy on to descendants who roam the planet today: birds!

  Second, it鈥檚 true that the Chicxulub asteroid caused huge devastation to the global climate of our planet 鈥� we know this due to geological evidence. This cataclysmic event would have led to the extinction of many species who could no longer survive in the new conditions.

  But it鈥檚 also thought that some dinosaurs were on the way out anyway, regardless of the asteroid. This happens all the time鈥攁s the environment gradually changes over time, some animals die out while others thrive. Even if the asteroid hadn鈥檛 hit, we probably wouldn鈥檛 see dinosaurs around today鈥攁nd if we did, they鈥檇 probably look very different to the ones from the fossil records.

Future collisions: how big? How likely?

Before you start planning your asteroid survival strategy, it鈥檚 worth noting that cataclysmic asteroid collisions are incredibly unlikely, due to the relative stability of our solar system and the vastness of space. But when you factor in millions or billions of years of time, we expect them to happen again.

The stable orbits of today

鈥楽tability鈥� is a tricky concept in the context of the solar system. There are still some collisions between smaller objects, and the orbits of planets will shift over time鈥攕o it鈥檚 not a pristine, unchanging system. But so much time has passed since the formation of the planets that most of the dramatic stuff seems to be out of the way. The vast majority of bits of matter bound for collision have already collided, and the orbits of bigger bodies have settled down.

Collisions are even rare within the asteroid belt鈥攄espite the rocky obstacle course that it tends to be portrayed as, it鈥檚 mostly empty space. The is around 1 to 3 million kilometres鈥攕o basically, it鈥檚 hard to hit anything in space these days, no matter where you are.

But the orbits in the solar system are part of a dynamic system, and small gravitational effects can add up in unpredictable ways. Plus, there are many moving pieces, including those 1,700 potentially hazardous asteroids鈥攕o it鈥檚 just a matter of time before the next one hits. The question is: how much time?

Likelihood statistics

In accepting the reality of potential asteroid and comet collisions, we have two main options in determining the likelihood of collision: first, to look at how often these events happen based on what we know about our planet鈥檚 past, and extrapolating that out to get a rough guide of how often impacts of various sizes occur; and second, to find and track the trajectories of asteroids that are out there today, and figure out which will hit us.

Interactive

Collision size and frequency

Based on the past frequency of impacts with asteroids/comets and Earth, we can get a very rough guide of how often, on average, impacts occur. Slide the bar to see the collision frequencies of different sized objects.

Size of asteroid/comet

[ANSWER] (diameter before hitting the atmosphere)

Approximate collision frequency

[ANSWER] (average interval between past collisions)

Energy released

Equivalent to [ANSWER]

• View this data as a table

  Approximate projectile diameter	Mean interval between impacts	Comparable event (in terms of energy released)
  2 metres	3 months	Minimum damaging earthquake (magnitude 5)
  6 metres	2 years	Atomic bomb explosion, Hiroshima (Japan, 1945)
  23 metres	32 years	鈥楾ypical鈥� hydrogen-bomb explosion
  55 metres	1,900 years	Tunguska explosion (Siberia, Russia, 1908)
  90 metres	4,400 years	San Fransisco earthquake (magnitude 8.4; USA, 1906)
  155 metres	12,000 years	Total energy of Mt. St. Helens eruption, including thermal energy (USA, 1981)
  350 metres	51,000 years	Largest recorded earthquake (magnitude 9.6; Chile, 1960)
  610 metres	142,000 years	Total energy of Tambora volcano eruption, including thermal energy (Indonesia, 1815)
  1 kilometre	350,000 years	The impact that caused the Haughton Dome, Canada
  1.5 kilometres	720,000 years	Total annual energy release from Earth
  2.5 kilometres	4.5 million years	The impact that caused the crater in Montagnais, Canada
  5 kilometres	26 million years	The impact that caused the crater in Popigai, Russia
  10 kilometres	150 million years	The impact that caused the creater in Chicxulub crater, Mexico

Source: Adapted from ; some figures updated with more recent data from .

Like most probabilities, these numbers are only useful as a guide鈥攊mpact events don鈥檛 run like clockwork. And don鈥檛 forget that asteroids above a certain size have the potential for global catastrophe鈥攖hey affect more than the place where they land.

But we don鈥檛 have to accept inevitable catastrophe, whether it鈥檚 now or far in the future. We may be able to prevent disasters. First, we need to locate the incoming projectiles.

Tracking what鈥檚 out there

Thanks to their streaking tails and reflective surfaces, comets are a visual treat for backyard astronomers (even without a telescope). Asteroids are much trickier to spot, being that they are dark objects in the darkness of space. So astronomers use that track the heat that the asteroids absorb from the sun. We don鈥檛 pick up everything鈥攑articularly the asteroids that approach us from the direction of the sun, as the sun鈥檚 light blinds us and the light reflected by the asteroids is sent in the opposite direction.

Overall, though, while a few asteroids here and there slip under the radar, the ones that are missed tend to be the smaller ones. And based on what we know so far, there鈥檚 nothing out there raising alarm bells.

NASA鈥檚 NEOWISE (Near Earth Object Wide-field Infrared Survey Explorer) project freely publishes its , including orbit diagrams, impact probabilities, and past and upcoming close approaches. They have a huge database of near-Earth objects, but most of those that have been found have tiny, almost insignificant chances of hitting Earth. For instance, 鈥攁n asteroid that receives occasional media attention, and shares its name with an ancient Egyptian god of chaos鈥攊s estimated to have only a 0.00089 per cent chance of making a direct hit in the coming years. But we still need to keep watch, as it might break into smaller chunks when it passes by in 2029.

Planetary defence

While the likelihood of any one large asteroid striking our planet is incredibly low, the immense damage they have the potential to cause makes them difficult to ignore. If we spotted an asteroid or comet that had us worried, what could we do?

Sending explosives to detonate it into smaller pieces is probably a bad idea, . Breaking one object into several could make it an even bigger threat, and certainly more difficult to track. So, more subtle options tend to stand ahead of the rest. Here are some proposed options.

Tow it away

A spacecraft could rendezvous with the asteroid or comet, then throw a bag or rope around it to . This will only work if the asteroid (or comet) is nice and solid, so it won鈥檛 break apart. It could also be tricky to do this for very large objects鈥攊magine trying to build a robot that could throw a rope or bag over a 1 kilometre-wide asteroid while in space!

Nudge it with gravity

Another kind of tow truck, called a , could use the gravity of the spacecraft itself to slowly divert the asteroid. When the spacecraft gets near the asteroid, the gravity of the more massive object (the asteroid) will pull the less massive object (the spacecraft) towards it. But the asteroid will also move towards the spacecraft a tiny bit, too. You could fire the spacecraft鈥檚 rockets to move away and then repeat this process many times until the asteroid is no longer on a collision course with Earth.

Paint it

white would alter the amount of heat it absorbs on that side. This could make a tiny shift in the direction it travels, which would add up over time.

Attach a solar sail

Wrapping an object with a , a bit like aluminium foil around a potato, could change the degree it reflects light, shifting its course.

Shoot lasers at it

This isn鈥檛 quite the solution you may think it is 鈥� rather than using lasers to blow up an asteroid, they鈥檇 be used to . Earth-bound lasers may not be powerful enough for this task (because it鈥檚 much easier to affect an object鈥檚 trajectory long before it reaches us), but a swarm of tiny craft sent close to the asteroid might do the trick.

These and other deflection strategies require a long notice period鈥攖he earlier we spot the threat and start planning, the less dramatic our efforts will need to be to direct it away. But while successful exploratory missions like Rosetta/Philae, and are essential stepping stones, we鈥檙e not yet able to claim we have a comprehensive planetary defence system.

More than science and technology

Unfortunately, tracking asteroids and finding a great technology to divert them are not our only challenges. We are divided by country borders and have our own economies, laws and priorities. Who would fund and manage our planetary defences? What would we do if we were faced with a decision that could prevent a collision with one country, but put another at higher risk? What if we made a mistake?

Although Hollywood has created some colorful methods for stopping an object that is on a collision path with Earth, no government agency, national or international, has been tasked or accepted the responsibility to stop such an asteroid, should one be discovered.

So while we shouldn鈥檛 be consumed with fear about asteroids today or tomorrow, it鈥檚 worth more than a passing thought of what we could鈥攁nd should鈥攄o when faced with a threat in the future. Celestial collisions have the potential to change more than the face of our planet, so it seems wise to keep watching, thinking, and preparing.