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Planetary Defense Experiment

lpetrich

lpetrich

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Double Asteroid Redirection Test (DART) Mission | NASA
 Double Asteroid Redirection Test
DART: Double Asteroid Redirection Test
It will be using this engine:
NEXT-C Advanced Electric Propulsion Engine Cleared to Begin Production | Aerojet Rocketdyne
 NEXT (ion thruster)
It outdoes Dawn's NSTAR engines by having an exhaust velocity of 40 km/s and a thrust of 236 millinewtons. NSTAR's have 30 km/s and 90 mN.

Its target will be the moon of asteroid 65803 Didymos, about 800 m across with a moon about 150 m across.
The DART spacecraft will achieve the kinetic impact by deliberately crashing itself into the moonlet at a speed of approximately 6 km/s, with the aid of an onboard camera and sophisticated autonomous navigation software. The collision will change the speed of the moonlet in its orbit around the main body by a fraction of one percent, enough to be measured using telescopes on Earth.

...

NASA’s DART spacecraft’s launch window range begins in late December 2020 and runs through May 2021. It will intercept Didymos’ moonlet in early October 2022, when the Didymos system is within 11 million kilometers of Earth, enabling observations by ground-based telescopes and planetary radar to measure the change in momentum imparted to the moonlet.
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The spacecraft will be launched along with some commercial or military one.

Fireballs (1988 Apr 15 - 2018 Nov 20) recorded by various US observation satellites. The champion is the 2013 Feb 15 Chelyabinsk impact.

The 1908 Tunguska event would have been devastating if it had been at some major city, and our planet has suffered even bigger hits in the past. If one deflects a near-Earth asteroid enough, one can keep it from hitting.
 
Pushing a rock like this isn't going to be of much use against most threats. We need quick reaction without a long intercept or thrust period--and that means Orion.
 
But if one observes one far enough in advance, a little nudge can make a lot of difference. If it speeds up or slows down the asteroid in its orbit, then this speeding up or slowing down will be cumulative, adding up over the asteroid's orbits. Changing the orbit's eccentricity or orientation will not be cumulative, however.
 
lpetrich said:
But if one observes one far enough in advance, a little nudge can make a lot of difference. If it speeds up or slows down the asteroid in its orbit, then this speeding up or slowing down will be cumulative, adding up over the asteroid's orbits. Changing the orbit's eccentricity or orientation will not be cumulative, however.
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We've done a pretty good job of cataloguing the objects which are close to us. None are a threat at present--we aren't going to need to shove one aside. That means the likely threat will be something coming from the asteroid belt or farther out. We aren't going to have the luxury of a lot of time. To match orbits takes time, considerable time unless you can squander delta-v on the intercept. Look at the one time we have had a detection of a potentially serious threat--Siding Spring. Yes, it was the Martians that were looking down the barrel of the gun but it's an example of what we need to be able to deal with. 20 months from the first indication it might be hazardous to the encounter. The one weakness of Orion is that it can't speed things up very well, you might have to push it to the side even if speeding it up was the better answer.
 
Hera / Space Engineering & Technology / Our Activities / ESA A planned mission to this asteroid and its moon.
Hera is the European contribution to an international double-spacecraft mission. NASA will first perform a kinetic impact on the smaller of the two bodies, then Hera will follow-up with a detailed post-impact survey that will turn this grand-scale experiment into a well-understood and repeatable planetary defence technique.

While doing so, Hera will also demonstrate multiple novel technologies, such as autonomous navigation around the asteroid – like modern driverless cars on Earth, and gather crucial scientific data, to help scientists and future mission planners better understand asteroid compositions and structures.
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It is to be launched in 2023, and it should reach the double asteroid in 2026.

The spacecraft itself:
Powered by solar arrays with a hydrazine propulsion system, Hera will be a small-scale mission in interplanetary terms, roughly the size of a large desk and weighing in at 350 kg, and up to about 800 kg full-fuelled – which is compact compared to the van-sized, 3 tonne Rosetta comet-chaser.
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The spacecraft will have a camera, a lidar device (like radar, but with visible light), a hyperspectral imager for getting surface spectra, and radio-science stuff. It still has 40 kg available, so it might also get a high-frequency radar or a mini-lander.

The spacecraft will carry two small satellites that it will release at the Didymos system: CubeSats joining Hera mission to asteroid system / Hera / Space Engineering & Technology / Our Activities / ESA The CubeSat unit shape is a cube with 10-cm size. Going on the mission will be two six-unit CubeSats, with a shape of 3*2*1 unit cubes, about the size of a small suitcase.

One of them is the Asteroid Prospection Explorer (APEX). It will made detailed spectral measurements of both asteroids' surfaces, and also magnetic-field measurements. It will eventually land on one of the two asteroids, continuing its measurements there.

The other of them is Juventas, and it will do radio communication with HERA to get a good idea of the gravitational field of Didymoon. It will also use low-frequency radar on that asteroid and eventually land there.

The mission goals will be:
  • Planetary defense
  • Technology demonstration: autonomous operation
  • Bonus science: learning about asteroids
 
I will estimate how well DART will perform with Didymoon, the moon of  65803 Didymos.

I first have to have a density estimate for it, and I will use a likely worst case: the average density of ordinary chondrite stony meteorites (Density & Specific Gravity): 3.4 g/cm^3.

I will also assume a spherical shape with a diameter of 160 m. The resulting volume may be off by a factor of 2 or so, because many asteroids look like lumpy potatoes.

Didymoon's mass is thus 7.3*106 metric tons.

The DART spacecraft's mass is 500 kg and it should impact at 6 km/s. Ignoring escaping backsplatter, that will give Didymoon an impulse of 0.41 mm/s.

Target asteroid / Hera / Space Engineering & Technology / Our Activities / ESA has details on the two asteroids. Their relative orbit velocity is about 175 mm/s. Nearly all of it will be in Didymoon's motion relative to the two asteroids' barycenter, because the size difference suggests that it is a little less than 1% as massive as Didymos. The impact of DART should change its velocity by around 0.24%. But it should be cumulative, and over a year, it should add up to about 1.7 complete orbits. So that impact's effects should be observable.

For planetary defense in general, one must compare that impulse to the asteroid's orbital velocity around the Sun. It is an impulse of about 1.38*10-8, about 13 kilometers ahead or behind in its orbit. It would take about 1000 years to reach 13,000 km and miss the Earth by a good margin.

But if we should ever need to do so, we will be hitting an asteroid with a lot more than 500 kg. For 10 tons, that means reaching 13,000 km in only 50 years. For 100 tons, 5 years.
 
Loren Pechtel said:
lpetrich said:
But if one observes one far enough in advance, a little nudge can make a lot of difference. If it speeds up or slows down the asteroid in its orbit, then this speeding up or slowing down will be cumulative, adding up over the asteroid's orbits. Changing the orbit's eccentricity or orientation will not be cumulative, however.
Click to expand...

We've done a pretty good job of cataloguing the objects which are close to us. None are a threat at present--we aren't going to need to shove one aside. That means the likely threat will be something coming from the asteroid belt or farther out. We aren't going to have the luxury of a lot of time. To match orbits takes time, considerable time unless you can squander delta-v on the intercept. Look at the one time we have had a detection of a potentially serious threat--Siding Spring. Yes, it was the Martians that were looking down the barrel of the gun but it's an example of what we need to be able to deal with. 20 months from the first indication it might be hazardous to the encounter. The one weakness of Orion is that it can't speed things up very well, you might have to push it to the side even if speeding it up was the better answer.
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Chances of being hit by an asteroid which just got deflected are pretty astronomical.
Long period Comets are rare too. And no, they did not get everything catalogued yet.
So I would say next impact will predicted with enough time to act.
 
Both the NASA and the SpaceX broadcasts had a lot of background for the mission during the countdown to launch. The NASA one had a cutesy demonstration of the spacecraft's mission -- an ISS astronaut throwing a bag at another astronaut.

NYT:
Four hours before impact, the DART spacecraft, formally called a kinetic impactor, will autonomously steer itself straight toward Dimorphos for a head-on collision at 15,000 miles per hour. An onboard camera will capture and send back photos to Earth in real time until 20 seconds before impact. A tiny satellite from the Italian Space Agency, deployed 10 days before the impact, will come as close as 34 miles from the asteroid to snap images every six seconds in the moments before and after DART’s impact.
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15,000 mph = 24,000 km/h = 6.7 km/s
34 mi = 54 km
If Dimorphos’s orbit around Didymos is extended by at least 73 seconds, DART will have successfully performed its mission. But mission managers expect the impact to lengthen the asteroid’s orbit even more, by about 10 and 20 minutes.
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BBC:
Dart is carrying a camera called Draco that will provide images of both asteroids and help the spacecraft point itself in the correct direction to collide with Dimorphos.

About 10 days before Dart hits its target, the American spacecraft will deploy a small, Italian-built satellite called LiciaCube. The smaller craft will send back images of the impact, the plume of debris kicked up and the resulting crater.

The tiny change in Dimorphos' path around Didymos will be measured by telescopes on Earth. Tom Statler commented: "What we really want to know is: did we really deflect the asteroid and how efficiently did we do it?"

A binary is the perfect natural laboratory for such a test. The impact should change Dimorphos' orbit around Didymos by roughly 1%, a change that can be detected by ground telescopes in weeks or months.
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CNBC: a diagram of the mission's target asteroids to scale with various buildings.
 
This double asteroid orbits outside the Earth's orbit: 1.0133 AU to 2.2760 AU - major axis 1.6446 AU eccentricity 0.3839
Its orbital period is 2.11 years (770 days) and its orbital inclination is 3.4083 deg

Didymos has a mean diameter of 800 meters and a rotation period of 2.26 hours
Dimorphos's orbit has a major axis of 1190 m and an orbit period of 0.4971 d (11.93 h)
Dimorphos has a mean diameter of 170 m

From Dimorphos's orbit, Didymos's mass is 5.40*10^(11) kg, and treating the asteroid as a sphere, its mean density is 2.0 g/cm^3.

Scaling down and assuming the same mean density, Dimorphos's mass is 5.2*10^9 kg.

Dimorphos's orbital velocity is 0.174 m/s

DART's mass is 610 kg, and its impact should change Dimorphos's orbital velocity by 0.45%. Not much, but it should add up as Dimorphos orbits Didymos.
 
DART's only instrument is a camera, the Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO).

The Light Italian CubeSat for Imaging of Asteroids (LICIACube) is a 6*1*1 cubesat that DART will carry.

From Wiki, "LICIACube is equipped with two optical cameras for conducting asteroidal reconnaissance during flyby, dubbed LUKE (LICIACube Unit Key Explorer), a narrow field of view (FoV) camera and LEIA (LICIACube Explorer Imaging for Asteroid), a wide FoV imager with an RGB Bayer pattern infrared filter."

About 10 days before its impact, DART will release LICIACube, and it will observe Dimorphos as it flies by the asteroid system.
 
DART should arrive at Dimorphos in less than a year, around late September.

The first link has some better numbers than Wikipedia and my calculations.
  • Didymos diameter: 780 m
  • Dimorphos diameter: 160 m
  • D-D orbit mean distance: 1.18 km
  • D-D orbit period: 11.92 km -- tidally locked
  • System mass: 5.278*10^(11) kg
  • Didymos mean density: 1.7 +- 0.4 g/cm^3

Meteorites' densities (g/cm^3):
  • Carbonaceous chondrites: 1.60 - 2.80
  • Other stony: 3.22 - 3.47
  • Iron: 7.14 - 7.37
Mars's moons: Phobos: 1.876, Deimos: 1.471
Visited asteroids: 1 Ceres 2.162, 4 Vesta 3.456, 21 Lutetia 3.45, 243 Ida 2.6, 253 Mathilde 1.3, 433 Eros 2.67, 25143 Itokawa 1.9, 101955 Bennu 1.190, 134340 Pluto 1.854, 162173 Ryugu 1.13
Visited comets: 1P/Halley 0.2-0.6-1.5 ?, 67P/Churyumov–Gerasimenko 0.533

I have stuck to the moons, asteroids, and comets with known natural satellites, those that spacecraft have orbited, and those with mass values derived from spacecraft tracking during flybys.

So Didymos does not have an unusually high or low density.
 
This whole thing is a plot by the upper stage to defect from Earth.
 
What if they deflect one object that hits another object that then hits the Earth?

If you want to traslate motion and change the vector you have to hit at the center of mass, otherwise it spins. Sounds low probaility of sucess.

I'd think it would have to be a set of thrusters coordinated to move the object in a predictable manner.

The experiment sounds like an exercise in Newtonian mechanics.
 
steve_bank said:
What if they deflect one object that hits another object that then hits the Earth?

If you want to traslate motion and change the vector you have to hit at the center of mass, otherwise it spins. Sounds low probaility of sucess.

I'd think it would have to be a set of thrusters coordinated to move the object in a predictable manner.

The experiment sounds like an exercise in Newtonian mechanics.
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There are effectively zero other objects to hit. Space is big and empty.

And we already know where all the other objects large enough to matter are.

Accidentally hitting an asteroid in the densest part of the asteroid belt is on a par with accidentally winning the lottery while trying to buy a newspaper.

The solar system is basically empty; as Arthur C Clarke observed, it consists of the Sun, Jupiter, and assorted debris.

Here's a scale map of our Solar System. It's essentially a one dimensional map of a three dimensional volume (although as the objects portrayed are largely close to the ecliptic plane, it's perhaps fairer to say it's a one dimensional map of a two dimensional area), so it significantly underestimates the actual emptiness of space - if you travel outwards from the Sun in a random direction in the ecliptic plane, you likely won't pass anywhere close to any of the planets, and are vanishingly unlikely to pass close to them all.
 
If one was to travel through the asteroid belt, one would not see any asteroids without a telescope unless one went close to one. The asteroid belt isn't thick with asteroids like some planetary ring.
 
lpetrich said:
Changing the orbit's eccentricity or orientation will not be cumulative, however.
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If its original orbit was on the plane of the ecliptic, changing its orientation would greatly reduce the chances of earth impact...
... or so it would seem to my non-astrophysicist brain.
 
Elixir said:
lpetrich said:
Changing the orbit's eccentricity or orientation will not be cumulative, however.
Click to expand...
If its original orbit was on the plane of the ecliptic, changing its orientation would greatly reduce the chances of earth impact...
... or so it would seem to my non-astrophysicist brain.
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True, if it is large enough, but it has to be large enough to make a difference. It's easier to do that with something with cumulative effects, like changing orbit period, because that adds up over the orbits.
 
steve_bank said:
What if they deflect one object that hits another object that then hits the Earth?

If you want to traslate motion and change the vector you have to hit at the center of mass, otherwise it spins. Sounds low probaility of sucess.

I'd think it would have to be a set of thrusters coordinated to move the object in a predictable manner.

The experiment sounds like an exercise in Newtonian mechanics.
Click to expand...

The chance of deflecting one object into another to cause it to hit is minuscule. There are two problems with thrusters: First, you have to match orbits with the target first--unless we have a long lead time that very well might be impossible. Second, they are an inefficient use of your launch mass. You will get more deflection per pound launched by ramming.

Anyway, if there is a seriously threatening object out there the real answer is neither of these. Instead, nuclear-tipped interceptors, fused for standoff detonation. Keep the warheads small enough you don't risk disrupting the target if it turns out to be a pile of gravel. The essence of an Orion drive.
 
lpetrich said:
Elixir said:
lpetrich said:
Changing the orbit's eccentricity or orientation will not be cumulative, however.
Click to expand...
If its original orbit was on the plane of the ecliptic, changing its orientation would greatly reduce the chances of earth impact...
... or so it would seem to my non-astrophysicist brain.
Click to expand...
True, if it is large enough, but it has to be large enough to make a difference. It's easier to do that with something with cumulative effects, like changing orbit period, because that adds up over the orbits.
Click to expand...

You need at most 7,000km of deflection in the optimum direction, or 13,000 of deflection in any direction (the latter might be easier to achieve if minimum deflection was obtained by pushing it towards Earth) to ensure a miss. Note the lack of a time unit, these are purely distance. If you deflect it by a mere meter per second you get 13,000km of deflection in 5 months. 10 centimeters per second generates a miss in just over 4 years in the worst case.

Note that the orbit that it is in is generally irrelevant--if it's in a highly elliptical orbit you want to shove it while it is as close to the sun as possible, but for an object in a highly elliptical orbit you're not likely to get a choice anyway.
 
I don't understand why Armagedon gets bad rap for blowing up asteroid. They argue that few smaller asteroids hitting the Earth are worse than one big one. I don't think it's true.
Rocks below certain size becomes harmless because they don't reach even upper atmosphere. So if is too late, than blowing it up is a good option. Of course even that had to be done relatively far from Earth - months before impact.
 
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