Aero 426, Space System Engineering

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Aero 426, Space System Engineering

• Lecture 4NEA Discoveries (How to Observe NEAs)

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NEAs are dim but stars are bright So lets
begin by considering star light
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Spectral Types, Light Output and Mean Lifetime
Spectral Type (color) Mass (Sun 1) Radius (Sun 1) Temp. (1000 K) Output of visible light (Sun 1) Approximate lifetime (billion years)
O (blue) 16 to 100 15 30 60 4000 to 15,000 0.003 to 0.03
B (blue-white) 2.5 to 16 15 10 30 50 to 4000 0.03 to 0.4
A (white) 1.6 to 2.5 2.5 7.5 10 8 to 50 0.4 to 2
F (yellow-white) 1.1 to 1.6 1.3 6 7.5 1.8 to 8 2 to 8
G (yellow) 0.9 to 1.1 1.1 5 6 0.4 to 1.8 8 to 16
K (yellow-orange) 0.6 to 0.9 0.9 3.5 5 0.02 to 0.4 16 to 80
M (red) 0.08 to 0.6 0.4 lt3.5 10-6 to 0.02 80 to 1000s
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A Hertzsprung-Russell (HR) diagram is a plot of
absolute magnitude (luminosity) against
temperature. The majority of stars lie in a band
across the middle of the plot, known as the Main
Sequence. This is where stars spend most of their
lifetime, during their hydrogen-burning phase.
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The Stellar Pyramid
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Measuring the distance to stars

• If the angle the star moves through is 2
  arcsecond, then the distance to the star 1
  parsec

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Measuring the brightness of stars (and NEAS)

• The observed brightness of a star is given by its
  apparent magnitude. (First devised by Hipparchus
  who made a catalogue of about 850)
• The brightest stars m1. Dimmest stars (visible
  to the naked eye) m6.
• The magnitude scale has been shown to be
  logarithmic, with a difference of 5 orders of
  magnitude corresponding to a factor of 100 in
  actual brightness.
• Brightness measured in terms of radiated flux, F.
  This is the total amount of light energy emitted
  per surface area. Assuming that the star is
  spherical, FL/4pr2, where L is the stars
  luminosity.
• Also defined is the absolute magnitude of a star,
  M. This is the apparent magnitude a star would
  have if it were located ten parsecs away.
  Comparing apparent and absolute magnitudes leads
  to the equation

• where r is the distance to the star, measured in
  parsecs.
• The absolute magnitude of a NEA is its magnitude
  when 1AU distance from the sun, and at zero phase
  angle

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Many Stars Are Brighter than 10th Magnitude
Visible totypicalhuman eye1 Apparentmagnitude Brightnessrelativeto Vega Number of starsbrighter thanapparent magnitude2
Yes -1.0 250 1
Yes 0.0 100 4
Yes 1.0 40 15
Yes 2.0 16 48
Yes 3.0 6.3 171
Yes 4.0 2.5 513
Yes 5.0 1.0 1 602
Yes 6.0 0.40 4 800
No 7.0 0.16 14 000
No 8.0 0.063 42 000
No 9.0 0.025 121 000
No 10.0 0.010 340 000
1 ab Vmaglt 6.5. SIMBAD Astronomical Database
2010-06-25 2 Magnitude. National Solar
Observatory Sacramento Peak. Archived from the
original on 2008-02-06. Retrieved 2006-08-23.
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How many stars brighter than a given magnitude?
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Approximate Star Light Spectrum
A sea of photons is surrounded on all sides by
high temperature plasma and atoms. These
particles randomly absorb or emit photons,
permitting all possible energy transitions
compatible with conservation of overall energy
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Approximate Star Light Spectrum Plancks Law
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Approximate Star Light Spectrum
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COBE (Cosmic Background Explorer) satellite data
precisely verifies Plancks radiation law
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Using Plancks Law Accuracy of intensity
measurement

• As given above Plancks law just gives the rate
  at which energy is emitted. But light is composed
  of discrete packets, called photons, each having
  energy
• Photon arrivals are a Poisson process for which
  all statistics are determined by the average
  number of photons received in a given time
  interval.
• The standard deviation of the fluctuation from
  the mean of the number of photons received is the
  square root of the average number received.
• Then the Signal-to-Noise Ratio (SNR) of an
  intensity measurement during a given time
  interval is
• The key parameter is the average rate of photons
  received per unit area of collecting aperture for
  light in a given wavelength band,

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Summary for Stars

• You have a simple model for the number of stars
  brighter than a given magnitude (see slide
  16)This helps you figure out what type of
  star you should choose to look at.
• You also have a simple model for how many photons
  are received per sec as a function of magnitude
  (see slide 9)This is essential to evaluate
  the goodness of the intensity data. The next
  lecture shows how to compute the SNR from this.

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NEA Types

• An asteroid is coined a Near Earth Asteroid (NEA)
  when its trajectory brings it within 1.3 AU
  Astronomical Unit from the Sun and hence within
  0.3 AU of the Earth's orbit. The largest known
  NEA is 1036 Ganymede (1924 TD, H 9.45 mag, D
  31.7 km).
• A NEA is said to be a Potentially Hazardous
  Asteroid (PHA) when its orbit comes to within
  0.05 AU ( 19.5 LD Lunar Distance 7.5 million
  km) of the Earth's orbit, the so-called Earth
  Minimum Orbit Intersection Distance (MOID), and
  has an absolute magnitude H lt 22 mag (i.e., its
  diameter D gt 140 m). The largest known PHA is
  4179 Toutatis (1989 AC, H 15.3 mag, D
  4.62.41.9 km).

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Statistics as of December 2012

• 899 NEAs are known with D gt 1000 m (H lt 17.75
  mag), i.e., 93 4 of an estimated population
  of 966 45 NEAs
• 8501 NEAs are known with D lt 1000 m
• The estimated total population of all NEAs with D
  gt 140 m (H lt 22.0 mag) is 15,000 observed
  5456 ( 37 )
• The estimated total population of all NEAs with D
  gt 100 m (H lt 22.75 mag) is 20,000 observed
  6059 ( 30 ).
• The estimated total population of all NEAs with D
  gt 40 m (H lt 24.75 mag) is 300,000 observed
  7715 ( 3) .Estimates lttargetneo.jhuapl.edu/pd
  fs/sessions/TargetNEO-Session2-Harris.pdfgt.Furthe
  r details ltssd.jpl.nasa.gov/sbdb_query.cgigt.

D denotes the asteroid mean diameter H is
the Visible-band magnitude an asteroid would have
at 1 AU distance from the Earth, viewed at
opposition
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NEO Search Programs

• Asiago DLR Asteroid Survey (ADAS), Italy/Germany
• Campo Imperatore Near Earth Object Survey
  (CINEOS), Italy
• Catalina Sky Survey (CSS), USA
• China NEO Survey / NEO Survey Telescope
  (CNEOS/NEOST)
• European NEA Search Observatories (EUNEASO)
• EUROpean Near Earth Asteroid Research (EURONEAR)
• IMPACTON, Brasil
• Japanese Spaceguard Association (JSGA), Japan
• La Sagra Sky Survey (LSSS), Spain
• Lincoln Near-Earth Asteroid Research (LINEAR),
  USA
• Lowell Observatory Near-Earth Object Search
  (LONEOS), USA
• Near-Earth Asteroid Tracking (NEAT), USA
• Panoramic Survey Telescope And Rapid Response
  System (Pan-STARRS), USA
• Spacewatch, USA
• Teide Observatory Tenerife Asteroid Survey
  (TOTAS), Spain
• Wide-field Infrared Survey Explorer (WISE), USA.

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Current Surveys

• Currently the vast majority of NEA discoveries
  are being carried out by the Catalina Sky Survey
  near Tucson (AZ, USA), the LINEAR survey near
  Socorro (NM, USA), the Pan-STARRS survey on Maui
  (HI, USA), and, until recently, the NEO-WISE
  survey of the Wide-field Infrared Survey Explorer
  (WISE).A review of NEO surveys is given by
  Stephen Larson, 2007, in A. Milani, G.B.
  Valsecchi D. Vokrouhlický (eds.), Proceedings
  IAU Symposium No. 236, Near Earth Objects, our
  Celestial Neighbors Opportunity and Risk, Prague
  (Czech Republic) 14-18 August 2006 (Cambridge
  CUP), p. 323, "Current NEO surveys."

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NEA Detection Summary
Diameter(m) gt1000 1000-140 140-40 40-1
Distance (km) for which Fgt100 (?0.5 ?m) gt20 million lt 20 million, gt 400,000 lt400,000 (Lunar orbit) gt32,000 (GEO orbit) lt32,000 gt20
H (mag) 17.75 17.75-22.0 22.0-24.75 gt24.75
N estimated 966 14,000 285,000 ??
N observed 899 4,557 2,259 1,685
O/E 93 33 1 ??
Only 1 detected, and if you wait for sharp
shadows, its probably too late
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