question: Why doesn't the ISS (or anything in space) cast a huge shadow across land directly beneath?
comment: In order to cast a shadow, it would have to block out the sun for some point on Earth. But it can't, because visually it is much smaller than the sun. [Here]( URL_0 ) is an illustration of what the ISS looks like passing in front of the sun. Obviously it can only block out a tiny, unnoticeable portion of sunlight.
passages: Section::::Reception.
   This was detected only as a slight dimming of the ambient light; the "Viking 1" Lander camera did not image the Sun. The shadow took about 20 seconds to pass over the Lander, moving at about 2 km/s. The shadow was simultaneously imaged from the "Viking 1" Orbiter, which permitted locating the position of the lander in the orbiter pictures.
  
  Section::::Images of the shadow.:26 August 1999 image.
   Viewed from orbit, the penumbral shadow of Phobos can be seen to move rapidly over the Martian surface. This shadow on the Martian surface has been photographed on many occasions by Mars Global Surveyor.
  
  Section::::Images of the shadow.:"Viking 1".
  
  In the 1970s, the "Viking 1" Lander and Orbiter photographed the shadow as well. The Lander detected the penumbral shadow of Phobos passing across it.
   MSTI-3's primary mission was to survey surface and atmospheric features of Earth in SWIR and MWIR to characterize how they vary in appearance across observational angles, times of day, and seasons. This data would be used to build statistical data to determine if it was feasible for space-based surveillance systems to track ballistic missiles in their coast phase against the warm background of Earth. The VIS instrument was used to verify the integrity of the infrared observations, and performed the secondary objective of conducting environmental monitoring at the same spatial resolution of the Landsat 5 and 6 spacecraft but with improved spectral resolution.
   In this case the image start time is 20:13:04.69 UTC, the line integration time is 80.48 milliseconds, and the downtrack summing factor is 27. The shadow is about 8 pixels high, centered at 993 pixels from the bottom of the original 1600-pixel-high image. We add (993 × 0.08048 × 27) = 2157.75 seconds = 35 minutes 57.75 seconds to get a time of 20:49:02.4 UTC for the center of the shadow.
   Jason-1 was launched by a Boeing Delta II rocket in California in 2001 and continued measurements initially collected by TOPEX/Poseidon satellite, which orbited from 1992 up until 2006. NASA and CNES, a French space agency, are joint partners in this mission.
   According to Defense Industry Daily significant limitations were put on the PS1 survey to avoid recording sensitive objects. Streak detection software (known as "Magic") was used to censor pixels containing information about satellites in the image. Early versions of this software were immature, leaving a fill factor of 68% of the full field of view (which figure includes gaps between the detectors), but by March 2010 this had improved to 76%, a small reduction from the approximately 80% available. At the end of 2011, the USAF completely eliminated the masking requirement (for all images, past and future). Thus, with the exception of a few non-functioning OTA cells, the entire field of view can be used.
   BULLET::::- In this 24-hour orbit, the Earth's influence is almost negligible except for the shadowing during eclipses, which can vary in duration from zero at solstice to a maximum of 1.2 hours at equinox. Long eclipses influence the design of both the spacecraft's insulation and heating systems. The seasonal variations in the direction and intensity of the solar input have a great impact on the design, complicating the heat transport by the need to convey most of the dissipated heat to the radiator in shadow, and the heat-rejection systems via the increased radiator area needed. Almost all telecommunications and many meteorological satellites are in this type of orbit.
   Jason-2 was launched on June 20, 2008 by a Delta-2 rocket out of the California site in Vandenberg. Its current mission is to transfer the current data into a second decade of data collection on the ocean height. 
   Far more detailed images of the shadow were taken with the arrival in 1997 of the Mars Global Surveyor and its high-resolution Mars Orbital Camera. One such image was taken on 26 August 1999 and shows the shadow at high resolution. This image was featured in a 1 November 1999 NASA press release.
  
  By examining maps of Mars
  
  we see the shadow is centered at around 10.9°N 49.2°W.
   where formula_15 refers to the brightness temperature and formula_5 is simply the temperature of the surface seawater. Since the viewing angle from nadir is typically set by the remote sensing instrument, measurements of the polarized components of the brightness temperature can be related to the surface seawater's temperature and dielectric coefficient.
   OLI-2 will collect data for nine spectral bands with a ground sample distance (GSD) of 30 m for all bands except the panchromatic band, which has a 15 m GSD. Design changes to the TIR-2 are intended to address the stray light and Scene Select Mechanism (SSM) encoder problems experienced with the TIRS on Landsat 8. Testing and assessment of the TIR-2 demonstrate the stray light magnitude has been reduced to 1%.
  
  Section::::Schedule.
   Section::::History.
  
  ACRIM3 made measurements of the TSI since the start of its mission in April 2000. It extended the TSI measurement database begun by earlier ACRIM instruments on the NASA Solar Maximum Mission (ACRIM1: 1980–1989) and Upper Atmosphere Research Satellite (ACRIM2: 1991–2001).
  
  ACRIMSAT/ACRIM3 tracked TSI during a 2004 transit of Venus and measured the 0.1% reduction in the solar intensity caused by the planet's shadow. It also recorded data for the 2012 Transit of Venus.
   Section::::Pollution.
   A typical basic depth measuring device is the US AN/UQN-4A. Both the water surface and bottom are reflecting and scattering boundaries. For many purposes, but not all naval tactical applications, the sea-air surface can be thought of as a perfect reflector. In reality, there are complex interactions of water surface activity, seafloor characteristics, water temperature and salinity, and other factors that make "...range predictions difficult. Multi-path degradation affects overall figure of merit and active classification. As a result, false target identifications are frequent."
   BULLET::::- A satellite could be illuminated over 99% of the time, and be in Earth's shadow a maximum of only 72 minutes per night at the spring and fall equinoxes at local midnight. Orbiting satellites can be exposed to a consistently high degree of solar radiation, generally for 24 hours per day, whereas earth surface solar panels currently collect power for an average of 29% of the day.
   Satellite surface salinity
  
  Satellite surface salinity refers to measurements of surface salinity made by remote sensing satellites. The radiative properties of the ocean surface are exploited in order to estimate the salinity of the water's surface layer.
  
  The depth of the water column that a satellite surface salinity measurement is sensitive to depends on the frequency (or wavelength) of the radiance that is being measured. For instance, the optical depth for seawater at the 1.413 GHz microwave frequency, used for the Aquarius mission, is about 1–2 cm.
  
  Section::::Background.
   BULLET::::- Subsurface Radar Sounder (SRS) would image faults, stratigraphy and weathering in the upper ~100 m of the areas mapped by VenSAR, to identify structural relationships and geological history. The Principal Investigator of the Subsurface Radar Sounder is Lorenzo Bruzzone, Università di Trento, Italy.
  
  Section::::Gravity Science.
   Section::::Observation and exploration.:Transits.
  
  On 26 July 2006, the Hubble Space Telescope captured a rare transit made by Ariel on Uranus, which cast a shadow that could be seen on the Uranian cloud tops. Such events are rare and only occur around equinoxes, as the moon's orbital plane about Uranus is tilted 98° to Uranus's orbital plane about the Sun. Another transit, in 2008, was recorded by the European Southern Observatory.
  
  Section::::See also.
  
  BULLET::::- Planetary Science Decadal Survey
  
  Section::::External links.
  
  BULLET::::- Ariel profile at NASA's Solar System Exploration site
  
  BULLET::::- AN, 33 (1852) 257/258
  
  BULLET::::- Ariel basemap derived from Voyager images
   Whether an object is visible varies significantly on the height above sea level from where it is observed. The Kármán line, at , is accepted by the World Air Sports Federation, an international standard setting and record-keeping body for aeronautics and astronautics, as the boundary between the Earth's atmosphere and outer space. However, astronauts typically orbit the Earth at several hundreds of kilometres; the ISS, for example, orbits at 394 km (245 miles) above the earth and the Moon orbits at about away.
  
  Section::::Examples.
   Only a small fraction of the light incident on the ocean will be reflected and received by the satellite. The probability for a photon to reflect and exit the ocean decreases exponentially with length of its path through the water because the ocean is an absorbing medium. The more ocean a photon must travel through, the greater its chances of being absorbed by something. After absorption, it will eventually become part of the ocean's heat reservoir. The absorption and scattering characteristics of a water body determine the rate of vertical light attenuation and set a limit to the depths contributing to a satellite signal. A reasonable rule of thumb is that 90 percent of the signal coming from the water that is seen by the satellite is from the first attenuation length. How deep this is depends on the absorption and scattering properties of both the water itself and other constituents in the water. For wavelengths in the near infrared and longer, the penetration depth varies from a metre to a few micrometres. For band 1, the penetration depth will usually be between 1 and 10 metres. If the water has a large turbidity spike below 10 metres, the spike is unlikely to be seen by a satellite.
   It launched on 5 December 2018 on board a Falcon 9 and it is part of the SpaceX CRS-16 mission. It will be mounted on the Exposed Facility of the Japanese "Kibo" module for a two-year mission. The mission is being led by Professor Ralph Dubayah of the University of Maryland.
  
  Section::::Instrument.
   Its shape was a 12 sided prism (to fit the shuttle orbiter payload bay). There were 5 or 6 experiments on each of the 12 long sides and a few more on the ends. It was designed to fly with one end facing earth and the other away from earth. 
   In the image below you can see the measurement system using by the satellite Jason-1.
  
  Section::::Measurement.:Satellites.
  
  Currently there are two different satellites calculating the earth ocean topography, Jason-1, and Jason-2. These two satellites are currently both in space orbiting Earth in a tandem rotation. They are approximately 330 kilometers apart.
   BULLET::::- Glaciologists have used photoclinometry to characterize the surfaces of Antarctica and Greenland. Repeat photoclinometry is a method of building surfaces from a series of satellite image to investigate changes in ice sheet topography over time.
  
  BULLET::::- Photoclinometry has been used to measure volcanic plume top topography on Earth.
  
  Section::::Problems.
  
  Light direction is very important to the quality of a photoclinometric image. Light that comes from directly over the surface (behind the camera) makes it hard to distinguish the shadows. Multiple light sources are also a problem, since they destroy important shadows required for the algorithms to work properly.
  