Today’s birthdays:

YESTERDAY, about 11 million kilometres from Earth, the US National Aeronautics and Space Administration (Nasa) made the first attempt to divert an asteroid from its orbit. But forget Don’t Look Up, Deep Impact, Armageddon and all the other movies about planetkilling asteroids: this asteroid is only 160 metres wide, and it isn’t coming anywhere near us even if the experiment fails.

On the other hand, there are about 18,000 asteroids of that size or bigger in orbit around the Sun. If Dimorphos (the asteroid in the Nasa experiment) did hit Earth, then the impact would have the energy of a hundredmegaton hydrogen bomb, enough to devastate a city the size of New York or Lagos.

More than that, in fact, because Dimorphos orbits a much bigger asteroid called Didymos which is 780 metres in diameter, and they would arrive together. Now we’re talking about almost nobody surviving in a city the size of Tokyo, and devastation for a hundred kilometres around.

These things don’t happen often, of course, but they do happen. The Lunar and Planetary Laboratory at the University of Arizona estimates that there are more than three million impact craters larger than 1 kilometre in diameter on Earth, although the great majority are buried under subsequent sediment.

The biggest asteroid to hit the planet, Chicxulub on Mexico’s Yucatan peninsula 66 million years ago, was 10 kilometres in diameter. It caused the last great extinction: the worldwide firestorms and the five or 10year ‘‘asteroid winter’’ that followed (due to the ash blocking out the sun) killed off all the nonavian dinosaurs and let the mammals take over.

According to the Planetary Society, the odds on an asteroid the size of Dimorphos hitting Earth are one in a hundred every century. Moreover, we don’t even know where 40% of those asteroids are.

Drop to asteroids of 30140 metres, still big enough to kill a city, and there are about a million of them out there. We have good data on less than 2% of them, but we know that at least one will hit the planet every century. So Nasa and the European Space Agency (ESA) both have ‘‘Planetary Defence’’ offices — and they’re now running the first big experiment.

Nasa’s Double Asteroid Redirection Test, or Dart, was a spacecraft weighing about 500 kilograms fully fuelled, but it weighed a lot less than that when it did a kamikaze dive into Dimorphos yesterday. On the other hand, it was moving at six kilometres a second, so the energy it transferred to the asteroid was not negligible.

The primary aim of the exercise is to see how much that shifts the smaller asteroid’s orbit around its primary, Didymos. It won’t be a lot, because Dimorphos’ mass is an estimated 4.8 billion kilogrammes, but it should be enough to be detectable within weeks by big telescopes.

Then, four years from now, when the ESA’s Hera mission arrives at Dimorphos, we should know how big the crater is, and what shape. That will confirm or refute the growing suspicion that most smaller asteroids, at least, are really not solid boulders but just clumps of rubble weakly held together by microgravity.

If they are, they would be a lot easier to move, because then the collision will not just push the asteroid in the desired direction. It will also spurt out a lot of rubble in the reverse direction, which would magnify the total momentum transferred to the asteroid as much as fivefold.

One step at a time. It will probably be a couple of decades before we can divert even a Dimorphossized asteroid from hitting the Earth and be confident that it will go where we want it to instead.

Larger but much rarer ones, which are more likely to be solid rock, will take a lot longer to get a handle on. Nevertheless, before the end of this century we may be able to protect the planet from all but the very biggest asteroids.

A ‘‘kinetic impact’’ approach to the problem like Dart is currently the favoured technique, but alternative techniques are also being considered. One is to land a small iondrive engine on a threatening asteroid with enough fuel to sustain a very small thrust for a very long time.

Another proposal, particularly useful if we have little warning of the asteroid’s approach, would use interceptor rockets to blow it into a large number of small fragments just hours before impact. Many of the smaller pieces would burn up in the atmosphere, and the damage done by the rest would be far less than that done by a single massive rock.

A good planetary defence system will probably take a century to build, but at least we are moving from theory to practical experiments.

Gwynne Dyer is an independent London journalist.

TODAY is Wednesday, September 28, the 271st day of 2022. There are 94 days left in the year. Highlights in history on this date:

48BC — Pompey the Great, a leading statesman and general of the Roman Republic, is murdered in Egypt.

1066 — William the Conqueror lands at Pevensey, Sussex, and begins the Norman Conquest of England.

1745 — The British national anthem, God Save the King, is first performed, at London’s Drury Lane Theatre.

1781 — American and French troops begin the siege of Yorktown.

1841 — Sir William Martin is appointed New Zealand’s first chief justice.

1850 — Waitera Te Karei rides his horse100km along the coast to the New Plymouth Police Station on Mt Eliot, deposits £35 and becomes the first customer of the newly formed New Plymouth Savings Bank. The bank is now known as the Taranaki Savings Bank (TSB); flogging of sailors in the United States Navy is abolished.

1868 — The Battle of Alcolea causes Queen Isabella II of Spain to flee to France; the Opelousas Massacre at St Landry Parish, Louisiana, in which between 200 and 250 AfricanAmericans were murdered by whites, occurs.

1887 — In one of the deadliest natural disasters on record, the Yellow River (Huang He) floods in China, killing an estimated 900,000 people.

1899 — New Zealand premier Richard Seddon asks Parliament to approve an offer to the British government of a contingent of mounted riflemen to serve in South Africa. Amid emotional scenes, the members overwhelmingly endorsed the motion. Only five voted against it.

1924 — Two US Army planes land in Seattle, having completed the first roundtheworld flight in 175 days.

1939 — The GermanSoviet Frontier Treaty is signed by Nazi Germany’s foreign minister Joachim von Ribbentrop and his Soviet counterpart Vyacheslav Molotov. It redraws German and Soviet spheres of influence in central Europe and transfers most of Lithuania to the Soviet Union.

1941 — The Nazi Germany terror campaign begins in Czechoslovakia.

1942 — The Waikato coalminers’ strike ends and mines come under state control.

1950 — Indonesia is admitted to the United Nations.

1973 — The Sydney Opera House opens after 16 years of building at a cost of $100 million (the original estimate was $7 million).

1975 — The second stage of Dunedin’s oneway street deviation comes into effect, meaning southbound traffic will use Cumberland and Castle sts. The northbound deviation from Cumberland St to Great King St was completed three months earlier.

1978 — Pope John Paul I dies after 33 days in office; he will be succeeded by John Paul II.

1994 — In Europe’s worst peacetime maritime disaster, 852 people drown when the ferry Estonia sinks en route from

Tallinn to Stockholm.

2012 — Mataura’s future is left in the balance when Alliance Group, the township’s biggest employer, announces plans to move its sheepprocessing operation to a sister plant at Lorneville, affecting 325 employees.

2016 — Replacing Sir Jerry Mateparae, Dame Patsy Reddy is sworn in as New Zealand’s 21st governorgeneral, and the third woman to hold the post.

2018 — A 7.5 magnitude earthquake hits just off the island of Sulawesi, Indonesia, setting off a tsunami that hits cities of Donggala and Palu. At least 1649 people are killed.

Confucius, Chinese philosopher (551BC479BC); Sir Robert Stout, twice New Zealand prime minister (18441930); Jane Annie Collier, New Zealand teacher of the blind (18691955); Hubert (Tim) Armstrong, New Zealand politician (18751942); Haane Manahi, distinguished New Zealand serviceman WW2

(191386); Bruce Mason, New Zealand playwright (192182); Bill Hunt, New Zealand alpine skier (19292009); Vincent O'Sullivan, New Zealand poet/short story writer/novelist/playwright (1937);

Graeme James Caughley, New Zealand population ecologist (193794); Leo Madigan, New Zealand author (19392015); Dorothy Buchanan, New Zealand composer (1945); Peter McCardle, New Zealand politician (1955); Wenceslaus Anthony, New Zealand businessman (19572017); Kevin Lawton, New Zealand rower (1960); David

Harland, New Zealand diplomat (1962); John Dempsey, New Zealand cricket umpire (1965); Mira Sorvino, US actress (1967); Naomi Watts, English actress (1968); Hannah Porter, Black Fern player/ manager (1979); Hilary Duff, US actress and singer (1987).