ION BEAM DEFLECTION(AKA PUSH-ME/PULL-YOU)

October 1, 2013

John BrophyJet Propulsion Laboratory

California Institute of TechnologyPasadena, CA

Concept Overview

Kitamura, S., “Large Space Debris Reorbiter Using Ion Beam Irradiation,” IAC-10-A6.4.8, 61st International Astronautical Congress, September 2010.

C. Bombardelli and J. Pelaez, J. of Guidance, Control, and Dynamics, Vol. 34, No.4, July-August 2011.

spacecraft

Ion Beam / Asteroid Interactions The electric thruster exhaust impinging on the

asteroid will impart a force approximately equal to the thrust (within a couple of percent).o This has been verified experimentally where a

“plasma momentum flux sensor” that intercepts the entire exhaust plume of a Hall thruster feels a force equal to the measured thrust produced by the thruster*.

Other Effects:o Some of the thruster exhaust may miss the

asteroid Minimized by selecting the right separation distance

between the thruster and asteroid surface based on the size of the asteroid and the divergence angle of the thruster exhaust.

o Sputtering of the asteroid surface Sputtered products are known to leave the surface

with low velocities relative to that of the incoming ions

o Elastic collisions of the ions Experiments indicate this to be a minor effect

o Asteroid charging Not expected to be a significant effect since the

thruster exhaust is quasi-neutral

*Longmier, B.W., et al., “Validating a Plasma Momentum Flux Sensor to an Inverted Pendulum Thrust Stand,” J. of Propulsion and Power, Vol. 25, No. 3, May-June 2009

High-power, Low-beam-divergence Ion Thruster(NEXIS)

Thruster Input Power: 20 kW Ion Velocity: ~70 km/s Thrust: 0.45 N Flat Carbon-carbon Grids Ion Beam Divergence Half-angle: < 5o

Wear Tested for 1,200 hours

J. Polk, et al., “Performance and Wear Test Results for a 20 kW-Class Ion Engine with Carbon-Carbon Grids,” AIAA 2005-4393, 10 - 13 July 2005, Tucson, Arizona

Other Asteroid Deflection Concepts

Gravity Tractor

o F = 0.10 N for a

100-m asteroid

Kinetic Impactor

o Representative Values V∞ between 5 km/s and 25 km/s

m ≤ 10,000 kg

𝐹=𝐺𝑀𝑚𝑟2

∆𝑉=𝛽 (𝑚𝑀 )𝑉 ∞

Gravity Tractor

0.01

0.10

1.00

10 100 1000

Forc

e Ap

plie

d to

the

Aste

roid

(N)

Asteroid Diameter (m)

Spacecraft mass, m = 10,000 kgAsteroid Density, r = 1.9 g/cm3

Max thruster cant angle = 60o

Ion Beam Deflection (IBD)

Assume the approximate capabilities of the Asteroid Redirect Vehicle (ARV) 10,000 kg delivered to the

asteroid 6,000 kg of propellant Ion Exhaust Velocity 30 km/s

Since the IBD must thrust in both directions, only approximately half the propellant is available to impact the asteroid 3,000 kg at 30 km/s This is comparable to the

momentum in a kinetic impactor, and can be directed along the velocity vector

0.01

0.10

1.00

10 100 1000

Forc

e Ap

plie

d to

the

Aste

roid

(N)

Asteroid Diameter (m)

40-kW (ARV) Ion Beam Deflector

IBD Compared to Gravity TractorFor ARV-sized SEP vehicles

0.01

0.10

1.00

10 100 1000

Forc

e Ap

plie

d to

the

Aste

roid

(N)

Asteroid Diameter (m)

40-kW (ARV) Ion Beam Deflector

IBD Compared to Gravity Tractor (cont.)For ARV-sized SEP vehicles

1

10

100

1,000

10,000

10 100 1000

Hei

ght A

bove

the

Surf

ace

(m)

Asteroid Diameter (m)

Ion Beam Deflector(5-deg beam div.)

Gravity Tractor(30-deg. beam div.)

Itoka

wa

Height Above the Asteroid Surface

IBD enables greater separation distances from the asteroid surface

These distances minimize back-sputter contamination of the spacecraft

NEXIS ion thruster has an ion beam divergence < 5o

IBD Potential Capability

Assume 5,000 kg of propellant delivered to the asteroid

Assume half of this impinges on the asteroid

Total impulse delivered is independent of: The power level of the

electric propulsion system

The asteroid size and spin state 0.0E+00

2.0E+07

4.0E+07

6.0E+07

8.0E+07

1.0E+08

1.2E+08

1.4E+08

1.6E+08

1.8E+08

2.0E+08

0 1000 2000 3000 4000 5000

Tota

l Im

puls

e D

eliv

ered

(N-s

)

Mass of Kinetic Impactor (kg)

2000 s

3000 s

4000 s

5000 s

6000 s

7000 s

Ion Beam Deflector with 5000 kg of propellant (1/2 of this impinges on the asteroid)

Kinetic Impactor with b = 1

More Detailed Comparison to Kinetic Impactor

Compare using asteroid 2011 AG5:* Determine latest arrival time for a

required deflection

For a 10-Re deflection, an 80-kW IBD provides comparable performance to a 3000-kg, 12.5 km/s impactor Would use about 4,000 kg of xenon

But 10-Re may not be necessary for IBD

For a 2-Re deflection, a 40-kW IBD can arrive at the asteroid just 1.5 years before the keyhole and use about than 1,300 kg of xenon

0

2

4

6

8

10

12

14

16

18

20

0.00.51.01.52.02.53.03.54.0

defle

ction

(Ear

th ra

dii)

years before keyhole passage February 3, 2023

40kW, 3000s40kW, 2000s80kW, 3000s80kW, 2000s

Kinetic Impactor

0

2

4

6

8

10

12

14

16

18

20

0.00.51.01.52.02.53.03.54.0

Xeno

n m

ass

(ton

)

years before keyhole passage February 3, 2023

40kW, 3000s40kW, 2000s80kW, 3000s80kW, 2000s

*Grebow, D., et al., “Deflection Missions for Asteroid 2011 AG5,” 23rd International Symposium on Space Flight Dynamics, Pasadena, California, October 29 – November 2, 2012.

De-spinning with IBD

An offset between the center-of-pressure of the ion beam and the center-of-mass of the asteroid will result in a torque on the asteroid changing its rotation state

Andy Thomas (JSC) suggested that IBD could be used to slow down the rotation rate of a target asteroid for the Asteroid Redirect Mission, simplifying the capture process

Propellant required would be manageable (~100 kg of xenon) due to the high Isp of the electric propulsion system

Easy

Harder

Conclusions

The Ion Beam Deflection provides the following potential advantages over other asteroid deflection systems Like the gravity tractor, it doesn’t require despinning of the

asteroid. Unlike the gravity tractor, it provides a significantly higher

coupling force that is independent of the asteroid size The concept could be tested as part of the baseline Asteroid

Redirect Robotic Mission. The thrust and total impulse are entirely within the design of the

SEP vehicle. The total impulse is potentially competitive with kinetic impactors

and eliminates the need for a second rendezvous spacecraft Gridded ion thrusters provide beam divergence angles of a few

degrees enabling long stand-off distances from the asteroid.o Mitigating control issues o Minimizing back-sputter contamination risks