Asteroids Rotation Period

Study the photometric curve

Rotation period

Shape

Is it binary?

Surface compositionspectroscopy

Observations reveal:

Time-dependent brightness variations

Rotation period of the asteroid around its axis

Due to:

Different reflectivity across the asteroid

Non spherical shape of the asteroid

Typical brightness variation : 0.2 mag

Average rotation period: 10 h

Fast rotators period: 2 h

Slow rotators period: few weeks

Most rotators period: 7 h –30 h

Asteroid Rotation Period Determination

Time-dependent brightness variations

for most asteroids are due to:

Non spherical shape-

rugby ball

Cross section changes (as seen by the Earth)

Light reflected towards us varies-

Brightness appears to vary

Asteroid Rotation Period Determination

Shape is 180 degrees symmetrical around an axis perpendicular to its greatest dimension

Rugby-ball shaped asteroid

max min max

maxmin

Other side of the asteroid

same side of the asteroid-full revolution

Asteroid Photometry

Magnitude system

Brighter stars : 1st mag

Dimmest stars : 6st mag

brightness Based on the eye

Real intensity: The intensity becomes 10 times larger every -2.5 magnitudes eg. 6st mag star is 100 times fainter than 1st mag star

Or in mathematics : )log(5.22

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MagnitudeIntensity

The scale that traditionally has being used to denote the brightness of an object

MagnitudeIntensity

)log(5.22

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Asteroid photometry process:

Every certain interval of time take a picture

in each picture measure the signal from the asteroid

Measure the signal from at least one comparison star of similar color in the vicinity of the asteroid

Known brightness

Pick 2 or more comparison stars and use the average intensity

Is the comparison star variable?

)...(log5.2

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Calculate Δm from each measurement (we need to know the brightness variation and not the actual brightness of the asteroid )

Plot Δm vs time (eg. pictures are 3min apart)

Calculate the period by measuring the time between consequitive maxima or minima of the same amplitude

Asteroid photometry process-continued :

)...(log5.2

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Exercise procedure

Pick an asteroid

Plan an observation

Submit a request to the telescope

Analyse the data

Short period : < 4h

Bright enough

Be observable most of the night

List of asteroids

1727 Mette

Cartes du Ciel

Avis