Asteroid 5261Eureka (Mars Trojan)Frames taken 10 minutes apart, tracking asteroid motion

Discovery of Asteroids

● Asteroid 1 Ceres discovered 1801 January 1 by Giuseppe Piazzi in Palermo, Sicily, and observed for 40 nights.

Discovery of Asteroids

● Asteroid 1 Ceres discovered 1801 January 1 by Giuseppe Piazzi in Palermo, Sicily, and observed for 40 nights.

● Carl Friedrich Gauss invents “Gauss’ Method” of orbit determination to allow recovery in 1802.

Discovery of Asteroids

● Asteroid 1 Ceres discovered 1801 January 1 by Giuseppe Piazzi in Palermo, Sicily, and observed for 40 nights.

● Carl Friedrich Gauss invents “Gauss’ Method” of orbit determination to allow recovery in 1802.

● Three more asteroids discovered over the next few years, no more until 1847.

Discovery of Asteroids● 3000 numbered by 1985.● 20957 numbered as of 2000 Jan 19.● 26073 numbered as of 2001 June 7● 43721 numbered as of 2002 June 24.● 65634 numbered as of 2003 June 19.● 85117 numbered as of 2004 June 14● 99947 numbered as of June 2005, then a 5 month

hiatus.● 129437 numbered as of 2006 June 19● 189005 numbered as of 2008 June 20● 241562 numbered as of 2010 May 27● 279722 numbered as of 2011 May 17

Near-Earth Asteroid Discoveries

Why Study Asteroids?

● Solar System Formation– Many asteroids are believed to be relatively

unprocessed remnants of Solar-System formation.

Why Study Asteroids?

● Solar System Formation– Many asteroids are believed to be relatively

unprocessed remnants of Solar-System formation.● Space Resources

– Substantial colonization of space requires bulk materials that are too expensive to haul from Earth. Near-Earth asteroids can provide a very low delta-V source of materials.

Meteorites

Meteorites provide samples of extraterrestrial bodies, but where did they come from?

Los Angeles Shergottite (Martian Meteorite)

Korra Korrabes H3 ChondriteOrigin Unknown

Bilanga Meteorite

5 mm across

Why Study Asteroids?

● Solar System Formation– Many asteroids are believed to be relatively

unprocessed remnants of Solar-System formation● Space Resources

– Substantial colonization of space requires bulk materials that are too expensive to haul from Earth. Near-Earth asteroids can provide a very low delta-V source of materials.

● Impact Hazard

Fig. 8-28, p. 179

Impact Hazard

Where are the Asteroids?

When Worlds Collide

Meteorites are fragments of asteroids created in a collision

Collisional Evolution

Asteroids and Meteorites

● Meteorites can be studied in great detail. ● Almost all of our understanding of Solar System

formation and evolution comes from analysis of meteorites.

● We typically have little information as to their parent bodies.

● What do we learn from the asteroids themselves?

Eight-Color Asteroid Survey(1980s)

Modern Asteroid Spectroscopy

Mineral Spectra

Modern Asteroid Spectra

Overlapping overtones of H2O/OH at 3 microns, from 2.6–2.85 lost to atmosphere

951 Gaspra

Dactyl

433 Eros

253 Mathilde

1999 JM8

radar astronomyThe basics:

A radar transmitter transmits radio waves at a known frequency for a certain time interval.

The waves hit the object, bounce off of it, and return to the telescope. The receiver, now moved into the focus of the telescope, detects the weak echo.

Transmitted wave

Echo from distant object

Planetary Radar● Absolute calibration to speed of light.● Extremely high fractional precision.● Astronomical Unit.

– And thus all parallax-based distance measurements.● Rotation rate of Mercury.

– Not sun-synchronous.● Images of Venus.● Tests of general relativity.● Ice at the poles of Mercury.

Ice at the Poles of Mercury

Ice at the Poles of Mercury

asteroid experiments

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• Two main types of radar experiments can be performed on asteroids

1. Continuous Wave Experiments• Uses continuous 2380 MHz wave• Produces one-dimensional spectra• Provides information on rotational velocity,

composition, and orbit of asteroid

2. Ranging Experiments• Uses encoded 2380 MHz wave• Produces 2D Delay-Doppler images• Provides information on size, shape, and

spin state of asteroid

continuous wave

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• For continuous wave (CW) experiments, we send a constant, umodulated 2380 MHz signal

• Upon reflection, the echo consists of many waves of slightly different frequencies• It is Doppler shifted as a result of the rotation of the asteroid

time

V

Fourier transform the echo to get the

spectrum

asteroid spectra

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• A spectrum is obtained by taking the Fourier transform of the echo• Gives the strength of each

reflected frequency• Determine resolution after data

has been taken!

• From the bandwidth, one can determine the rotational velocity of the asteroid

cv2

length FFTrate sample resolution moving towardmoving away

BW

devi

atio

n from

noise

1998 FH12

Hz from ephemeris

asteroid images• Delay Doppler images map a 3D object into a 2D image

• Circles: Represent lines of constant range• Lines: Represent lines of constant Doppler shift

range

Doppler shift

Direction of radar illumination

Direction of radar illumination

13

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• Delay Doppler images map a 3D object into a 2D image• Circles: Represent lines of constant range• Lines: Represent lines of constant Doppler shift

• Like cutting a potato up into many individual pieces (or pixels)

range

Doppler shift

asteroid images

13

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#1 #2 #3

pole

• One important piece of information we obtain from the Delay-Doppler images is the size of the asteroid• The extent of the asteroid in range indicates its radius

asteroid images

rKnowing the size

and rot. velocity of the asteroid gives

its period

• The frequency resolution is selected after the data has been collected by adjusting the FFT length

• The range resolution is fixed by the sampling rate• Common sampling rate for image = 100ns (or 15 m)

Changing Frequency Resolution● Can rescale frequency to increase SNR of fast

rotator.● Eye is pretty good at picking out linear structure

anyway.

MODEL OF 216 KLEOPATRA FROM ARECIBO RADAR DELAY-DOPPLER IMAGES COLOR

CODED FOR GRAVITATIONAL SLOPES

2003 YT1 May 2 May 3

2001 SN263

12 13 14 18 21 23 24 26

2001 SN263

1999 KW4 viewed in orbit plane

1996 HW1

Internal Structure● We measure Shape and Spin, which provide information on

internal structure.– Sphere strengthless

– Slowly-rotating bar tidally stretched (strengthless)?

– Fast-rotating bar at least some tensile strength

– Very irregular collisional fragment?

● The gross physical structure of NEAs is critical to all of the conventional reasons for studying them: – Meteorite delivery Space resources

– Hazard mitigation

● We see all kinds, implying multiple formation mechanisms

Summary: Variety● In spite of our attempt to classify NEAs, the

clearest observation is their great Variety.● Spacecraft images can be more detailed than radar

images, and can be more complete, but the vastly larger number of objects observable by radar allows us to detect and explore this variety.

● Radar imaging can be used to aid in spacecraft target selection, and to identify the most interesting targets for further study.

asteroid armageddon?

Asteroid impact* Airplane crash

Likelihood of death 1 in 20,000 1 in 20,000

Frequency of event

1 in 10,000,000 years ~40 a year

Number of casualties All life on Earth 1-3000

people

Scientists also study asteroids because of the potential hazards they pose to the Earth:

*Data based on a 7.0 km asteroid.

Hazard per Year

Normalized Hazard

Comparison to Other Causes(Deaths per Year)

● Cancer 473 000● Suicide 30 000● Homicide 19 000● Car Crash 46 000● AIDS 15 600

● Falls 12 000

● Drowning 5 000● Fires/Burns 5

000● Falling objects1 000● Impact 200From Weissman (1994), “The Comet and Asteroid Impact Hazard in

Perspective,” in Hazards of Near-Earth Space, p1191, and other sources

Preventable DeathsWorldwide average, normalized to US population.

● Vaccine Preventable Diseases 129 000● Antibiotic Preventable Diseases 225 000● Diarrhea 185 000● Globally Destructive Impact 200

What's the Comparison?● Impact can cause global, civilization-destroying

catastrophe. The whole is greater than the sum of its parts.

● Drama: Air travel is much safer than automobile travel, but people drive because they are afraid to fly.

● AIDS: Since the impact hazard came under study, AIDS became a clear worldwide hazard, and receives much more funding.

● Terrorism: Since the impact hazard came under study, terrorism became a clear worldwide concern (but perhaps not a hazard), and also receives much more funding.

Minimizing the Hazard

● Radar distance measurements improve our knowledge of the position of a “potentially hazardous” asteroid by orders of magnitude.

● Radar imaging of asteroids provides a “faster, cheaper” way to image these objects.