Asteroid grazes past Earth in 'critical' rehearsal

October 12, 2017
A composite image of the Western hemisphere of the Earth. Credit: NASA

A house-sized asteroid grazed past Earth Thursday, passing harmlessly inside the Moon's orbit, as predicted, to give experts a rare opportunity to rehearse for a real strike threat in future.

Dubbed 2012 TC4, the object's passing allowed scientists to practice spotting incoming objects, predicting their size and trajectory, and tracking their passage with a of telescopes and radars.

"We pretended that this was a critical and exercised our communication," said Detlef Koschny of the European Space Agency's Near-Earth Object programme.

The trial run was "a big success," he said, despite some instruments not working as planned.

A radar system in Puerto Rico, for example, was out of service due to damage from the recent hurricane there.

"This is exactly why we do this exercise, to not be surprised by these things," Koschny told AFP.

The asteroid flitted past around 0541 GMT at less than 44,000 kilometres (27.300 miles) from Earth's surface—just above the 36,000 km plane at which hundreds of geosynchronous satellites orbit our planet.

This was about an eighth of the distance between the Earth and the Moon.

Scientists had predicted that TC4 was between 10 and 30 metres (33-99 feet) wide. In the end, it measured some 10-12 metres—the smaller end of the range.

"This means it must be very bright," to make it appear bigger, said Koschny. Observations also revealed that TC4 spins around its axis in about 12 minutes, "which is quite fast."

When it comes back

The asteroid was about half the size of the meteoroid that exploded in the atmosphere over Chelyabinsk in central Russia in 2013 with the kinetic energy of 30 Hiroshima atom bombs.

The resulting shockwave blew out the windows of nearly 5,000 buildings and injured more than 1,200 people.

While the Chelyabinsk event caught everyone unawares, TC4 is one of thousands of space rocks whose whereabouts are known.

Millions are not.

On a 609-day loop around the Sun, TC4 will return to Earth in 2050 and 2079, Koschny's colleague Ruediger Jehn has told AFP.

It will not hit Earth on its next approach but could come close in 2079—another reason for studying its route through the Solar System.

With a one-in-750 chance of hitting in 2079, TC4 is listed at number 13 on the "risk list" of objects posing even the remotest impact threat.

As its full name suggests, the asteroid was first spotted five years ago when it called on Earth at about double the distance, before disappearing from view.

Astronomers will comb through the data they have now gathered, to learn more about the asteroid's composition.

Flybys like these are quite common—about three objects in TC4's size range graze past at a similar distance every year. What made this one special is that it was chosen to test the global warning system.

Scientists believe Earth will be hit again by a space rock of the size that wiped out the dinosaurs, though nobody knows when.

And even if we become better at predicting a strike, there is very little we can do about it.

Futuristic projects mooted to deflect or destroy incoming have come to nought so far, and the only strategy would be to evacuate people in zones at risk.

Explore further: Asteroid to shave past Earth on Oct 12: ESA (Update)

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5 / 5 (1) Oct 12, 2017
Close flybys of these smallest of asteroids occur fairly often. It costs hundreds of millions of dollars (or even a billion or so) to send multi-year probes to very distant asteroids. How much would it cost to send a "sprint" probe on short notice to one of these Earth-grazers? How cost-efficient could Elon Musk do it?
not rated yet Oct 12, 2017
Close flybys of these smallest of asteroids occur fairly often. It costs hundreds of millions of dollars (or even a billion or so) to send multi-year probes to very distant asteroids. How much would it cost to send a "sprint" probe on short notice to one of these Earth-grazers? How cost-efficient could Elon Musk do it?

Good questions. Perhaps there should be a special space probe mission set up in place, well before we know when the next Earth-grazer will be, for whenever we get the next Earth-grazer to take full economic advantage of the next cost-saving opportunity.
5 / 5 (2) Oct 12, 2017
We could have sent a probe for only slightly more than the launch of a geosynchronous satellite, **but** the relative motion of the two objects mean that they would pass each other in a fraction of a second. Any attempt at a "landing" would simply be a high velocity impact that nothing we've ever developed could withstand.

One of the reasons that we spend multiple years to send a probe to an asteroid is to allow it to get up to speed. Ion engines have a high specific impulse but the acceleration imparted is minuscule, it takes years to catch up to an asteroid.
3.7 / 5 (3) Oct 12, 2017
ow much would it cost to send a "sprint" probe

i don't think you appreciate the relative velocities involved (to which a probe would have to be accelerated to do anything meaningful with such an object)

'Sprint' is not something you do in space.
not rated yet Oct 12, 2017
You could easily launch up a bunch of solar wind propelled cube sats to investigate our surrounding neighborhood. Just have them in a constant figure eight between Earth and the Moon to build up speed by gravity assists.
not rated yet Oct 12, 2017
'Sprint' is not something you do in space
I dunno this sounds like a sprint of sorts

"The notional mission trajectory, shown in Figure 2(b), departs Earth on August 2, 2019 with an Earth departure C3 energy of 11.99 km2/s2 and intercepts the NEA 121.41 days later with a relative velocity at intercept of 11.5 km/s and an approach phase angle of 3.04◦. The small approach phase angle places the Sun nearly behind the spacecraft during terminal approach, providing excellent natural illumination of the target. The trajectory design for the spacecraft requires only 58 m/s of ∆V for deterministic and statistical mid-course correction maneuvers, calibration burns, ACS maneuvers, and terminal guidance phase maneuvers, which include maneuver execution errors and relative navigation uncertainties." -NASA
a "landing" would simply be a high velocity impact
-A lot you can do on approach.
5 / 5 (2) Oct 13, 2017
You could easily launch up a bunch of solar wind propelled cube sats to investigate our surrounding neighborhood.

Flybys to get a gravity assist aren't done willy nilly. You have to plan these ahead (and you have to be sorta in the right direction. Asteroids can come from all over the place)

The thing is: you don't need speed, you need velocity (difference between a scalar and a vector). Just going fast isn't enough. You have to go fast in the right direction. Gravity assist only works in the direction of the body you're using as the assist. Ifthe Moon isn't going inthe direction of the asteroid then you can't use it.

Also: as soon as you go too fast you tend to leave the Earth's vicinity. Asteroids come in at - on average - 25km/s (and up to something like 70km/s) relative velocity to Earth.
But at 11km/s you already have escape velocity from Earth. So 'preparing' by speeding sats up on the off chance they'll be needed doesn't really work.
not rated yet Oct 13, 2017
That's why you have a bunch of cubesats in constant rotation, so you have one ready to break off formation at a moments notice in any direction.
5 / 5 (3) Oct 13, 2017 doesn't work that way. Look at the numbers
Cubesats also don't have the type of fuel on board that says "Oh...I need to make a flyby of the Moon at a moment's prob"

Space is big. Really big. A LOT bigger than you seem to think.
not rated yet Oct 16, 2017
at a moment's notice

"A photonic laser thruster (PLT) is a concept for space propulsion that works on the principle of a photon-pushed sail, generating thrust directly from the momentum of a photon from a laser reflected from a mirror... Photonic laser thrusters have a very high specific impulse, and can permit spacecraft to reach much higher speeds that approach a fraction of the light speed... In addition, a small 1U CubeSat satellite was propelled and stopped in simulated zero-gravity. The laser power of the trapped photon beam exceeded 500 kW, which was powered by a 500 W laser. The concept is proposed for beaming thrust from a conventional heavy "tanker" vehicle to a more expensive, lightweight mission vehicle..."

-cubesats are ideal for sprints.

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