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Fwd = NEAR Science Update - March 2, 2000

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  • Frits Westra
    Forwarded by: fwestra@hetnet.nl Originally from: baalke@jpl.nasa.gov Original Subject: NEAR Science Update - March 2, 2000 Original Date: Thu, 2 Mar
    Message 1 of 1 , Mar 3, 2000
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      Forwarded by: fwestra@...
      Originally from: baalke@...
      Original Subject: NEAR Science Update - March 2, 2000
      Original Date: Thu, 2 Mar 2000 17:26:00 -0800 (PST)

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      NEAR Science Update
      March 2, 2000

      On February 29, 2000 NEAR recorded another first: the
      NEAR Laser Rangefinder (NLR) detected the first laser
      returns from Eros at a range of 290 km. This is the
      first time that ranging returns have been detected from
      an asteroid (see the image-of-the-day for 2000 March 2
      NLR was designed to operate at 50 km range, and its
      successful detection of Eros at 290 km augurs well for
      the future. The laser rangefinder data will give us a
      three-dimensional view of the asteroid surface, nicely
      complementing the information from images. This is
      because imagers record the distribution of brightness as
      a function of angles perpendicular to the line-of-sight,
      whereas the laser rangefinder measures distance to the
      surface along the line-of-sight. The combination of the
      two data sets will be powerful, as we hope it will
      enable us to probe into shadowed regions (because the
      laser does not depend on solar illumination), and to
      distinguish between effects of albedo variations and
      effects of height variations. In an image, a spot may
      look brighter or darker because of reflectivity
      differences or because of lighting differences caused by
      topography (such as shadowing). Laser rangefinder data
      can be used to separate these effects and to measure
      topography - e.g., the heights of ridges, the depths of
      grooves and craters.

      The image-of-the-day for 2000 February 25 shows the
      "eastern and western hemispheres" of Eros. The image
      shows an amazing diversity of geologic features, which
      will be the subjects of updates in the coming weeks. For
      now, I will focus on how we define eastern and western
      hemispheres and how we locate positions on a celestial
      body. I feel I should apologize for using the word
      "hemisphere" to refer to an irregularly shaped body like
      Eros, but I don't have another word that means "the
      surface within a 180-degree longitude range". To locate
      any point on the surface, we use what we call "spherical
      polar coordinates": the longitude and latitude angles,
      and the distance from the center of Eros. These three
      numbers specify the position of any point in three
      dimensions, but we need to specify which way is "north"
      and which way is "east". To accomplish this, we first
      locate the rotation axis of Eros using (for example)
      image data, and we choose the North pole direction as
      the reference for latitude, the same as is done for
      Earth. When viewed from above the north pole, the
      asteroid rotates counterclockwise; when viewed from the
      south, it rotates clockwise. The latitude angle is
      measured from the equator of Eros, which is the plane
      perpendicular to the rotation axis. Having defined the
      polar axis, the next step is to define the prime
      meridian, from which longitudes are measured. On Earth,
      the prime meridian runs through the poles and through
      Greenwich, UK. On Eros, a particular crater has been
      selected to mark the prime meridian. If one walks from
      the prime meridian in the direction of the rotation, one
      is going east. In the opposite direction, one is going
      west. Longitudes can be measured going east from the
      prime meridian, in which case we speak of east
      longitude, or they can be measured going west from the
      prime meridian, giving west longitude. Just to keep us
      on our toes, geophysicists commonly use east longitudes
      whereas geologists and cartographers commonly use west
      longitudes. Unfortunately, there is still another
      complication, which is that the International
      Astronomical Union defines "north" in reference to the
      so-called "invariable plane" which is close to the
      ecliptic plane defined by Earth's mean motion around the
      Sun, whereas we have just defined it in reference to the
      rotation of Eros - but fortunately, the two definitions
      of "north" coincide for Eros, so we can forget about the
      invariable plane. I guess scientists have a talent for
      making even the simplest things seem complicated.

      Andy Cheng
      NEAR Project Scientist

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