[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

NEAR Questions & Answers

NEAR Questions & Answers     21 May 98
http://near.jhuapl.edu//Nqa.html

Contents:
   Orbital speed, gravity, carbon dating on Eros?
   Water or other minerals on Eros?
   When is Earth closest and furthest from sun?
   Will Eros and Earth collide?
   Can Earth "capture" asteroids?                          
   Will NEAR see asteroid 1997XF?
   How can I point my telescope toward objects in space?
   How far will the spacecraft go?
   Will the spacecraft land on Eros?
   First "108 years" of solar system?
   When do we see meteor showers?
----------------------------------------------------------------------------
 From: Mike Hartman, USN
 What will be the orbital speed of near around Eros? How much
 would 100 lbs weigh on Eros? What is the theory on Eros's
 orbit being out of plane? Would it have been possible to
 create a miniature carbon dating device on NEAR so that it's
 final mission was to land on Eros and date a sample?

      Speed in orbit of 35 km radius = 5 m/s (approximately,
      depends on asteroid density which is presently
      unknown). 100 lbs would weigh 0.4 ounce on Eros.
      Asteroid orbits are not all in the plane of the
      ecliptic mainly because of collisions. Yes, it would
      have been great to do carbon dating at Eros but it
      would have been beyond available technology; Also, Eros
      as an S asteroid is not expected to have much carbon.
      --Andy Cheng
      (see also next response).

 -------------------------------
 From: Gene
 I know that NEAR is designed to do spectrographic analysis
 of EROS. In light of the Lunar Prospector findings of water
 on the moon, is there expectation that EROS would also have
 a water signature? If it does, what would be the
 significance of that? What other types of elements are
 expected to be found on Eros?

      Eros belongs to a class of asteroids (the so-called "S"
      class) expected to be dominated by iron-magnesium
      silicates and iron-nickel metal. S asteroids are
      believed to contain no measurable water, although if
      Eros did contain water, its spectral signature could be
      measured by the infrared spectrometer and the hydrogen
      in the water could be detected by the gamma-ray
      spectrometer. The gamma-ray instrument would "see"
      water occurring only over broad regions tens of
      kilometers in size, but the infrared spectrometer could
      detect water occurring in much smaller, localized
      occurrences. An important difference between the moon
      and Eros is the form in which water could occur. On the
      moon, water is though to occur as ice in permanantly
      shadowed polar regions. Because Eros's rotional axis is
      nearly in its oribtal plane, the asteroid lacks
      permanantly shadowed regions in which water could be
      preserved as ice. However, water could still be locked
      in minerals on the asteroid, chemically comined with
      the rock. This form of water is common on Earth and
      Mars, for example in clays where water forms an
      intrinsic part of the mineral crystals.

 -------------------------------
 From: Stephanie
 When will the earth will be closest to and furthest from the
 sun in 1998? I have enjoyed sharing your site with my 3rd
 grader and he is interested in these dates for a report he
 is preparing for science class.

      Perihelion is the closest point to the Sun and aphelion
      is the farthest point. Earth's perihelion occurred on
      Jan. 4 at 4 pm EST, distance to Sun 91.4 million miles.
      Aphelion will occur on July 3 at 8 pm EDT, distance to
      Sun 94.5 million miles.

      These occur each year within one day of those dates. I
      obtained them from the Sky Publishing Corporation's Web
      site

      At skypub.com there is a good menu of selections on
      many astronomical topics. You will find their list of
      planetariums, and astronomy clubs useful to find local
      sources of help.                                       
      -- David Dunham                                           
 -------------------------------
 From: Robby
 How close of an orbit is Eros coming to earth and what
 continents will it pass over?
 Is there a sight where I can find out more about Eros ?

      The closest distance in recent times (1975) between
      Earth and Eros was in the 13-14 million mile range. In
      addition to the Education pages on APL's NEAR website,
      here are a few other sites containing 433 Eros info:
      1     2     3     4     5

 -------------------------------
 From: Joe Santi
 Could the earth ever capture an asteroid into close orbit?
 Hypothetically, what are the conditions (size, speed,
 composition of asteroid) that could make this possible?

      This would be very difficult, since in general, an
      asteroid leaves the Earth with the same relative
      velocity (and hence geocentric orbital energy) that it
      arrives at the Earth. If the asteroid's orbit is close
      to the Earth so that the geocentric orbital energy is
      relatively small, a leading- edge flyby of the Moon
      could decrease the energy enough to capture the
      asteroid into Earth orbit, but the apogee distance
      would always be greater than the distance to the Moon.
      The relative velocity near the Earth must be less than
      2.7 km/sec for this to work, and no known Near-Earth
      Objects have relative velocities that small.

      There is one other way, if the asteroid passes through
      the Earth's atmosphere on a grazing trajectory,
      friction can decrease the orbital energy enough to
      capture it into an elliptical orbit about the Earth.
      But such an orbit would probably be short-lived, since
      lunisolar perturbations would lower the perigee so that
      it would collide with the Earth, possibly after just
      one or two orbits, but in some situations after a few
      years. For this to work, the asteroid would need to be
      small enough and cohesive enough (a metallic-type
      asteroid would be best) to survive the strong heating
      and deceleration forces in the Earth's atmosphere.

      Although no asteroids are known to ever do this, a
      large meteoroid did pass through the Earth's atmosphere
      over the Rocky Mountains in August 1976 and skipped
      back out into space, but it was still in a heliocentric
      orbit, not captured by the Earth. If it had struck the
      Earth, it would have done so with the force of a large
      atomic bomb. In the early part of this century, a swarm
      of meteors called Cyrillids (don't know how it got that
      name) was seen by many observers from the Midwest to
      Brazil. In order for those meteors to be seen over such
      a long great-circle arc, they must have been in Earth
      orbit before they entered the atmosphere that night.

 -------------------------------
 From: bjluby@avalon.nf.ca
 Will the NEAR see the approaching asteroid 1997XF ?

      No, NEAR's (and Eros') orbit is not close to that of
      1997XF11. The closest approach of Eros' orbit to that
      of Earth is in late January (remember that NEAR's Earth
      flyby was on Jan. 23) while 1997XF11's orbit crosses    
      the Earth's orbit in late October.                        
 -------------------------------
 From: hezrikh@pc.jaring.my
 How to point telescope from earth so I can see objects from
 outer space?

      For this, you generally need an ephemeris (list of
      right ascensions and declinations at intervals of one
      or two days) for the object you want to find, and a
      good set of star charts on which you can plot the
      ephemeris positions, and then locate the object in the
      sky. Ephemerides of the brighter comets and asteroids,
      and sometimes even star charts with their paths already
      plotted, are often given on Sky and Telescope's Web
      site at skypub.com. Look for the items about comets and
      events for the week or month. Star charts, such as the
      Uranometria 2000, are advertised in magazines such as
      Sky and Telescope.

      I am copying your question to V. Ramalinggam of the
      Malaysian Astronomical Society. He might be able to
      give you some local help; perhaps you already know him.

      --David Dunham

 -------------------------------
 From: Edward
 Where does it [spacecraft] go now? Will it pass by earth
 again?

      The spacecraft is scheduled to begin slow-down
      maneuvers on December 20, 1998 in order to go into
      orbit around asteroid 433 Eros on January 10, 1999.
      NEAR will travel 376 million miles in the next 10
      months. It recently flew by Earth for a gravity assist
      that adjusted the orbit size and tilt (or orbital
      inclination) to be very close to that of asteroid 433
      Eros. NEAR's only Earth "swingby" helps make this
      mission possible by allowing the spacecraft to carry
      all the propellant needed to complete its mission. The
      spacecraft will orbit the asteroid for over a year, and
      finally land on Eros in February 2000.

 -------------------------------
 From: Will
 What do you mean by "land on Eros" at the end of the
 mission? How soft would it have to be for the instruments to
 survive? What ground measurements would you do? What if it
 lands upside down?

      Spacecraft "survival" is not an issue for this mission,
      which was designed to explore the asteroid via low
      orbit. By end of mission around Feb. 2000, the science
      instruments will have fulfilled their purpose, which is
      to survey the asteroid's electromagnetic properties and
      send the science data back to us. After having
      succeeded doing that, they are indeed expendable, and
      will have the same fate as most other science probes we
      have sent into space for the past 30+ years.

 -------------------------------
 From: Bob
 In Dr. Cheng's Science Overview presentation, he says:
 "Most asteroids are thought to be fragments of planetesimals
 that underwent most of their evolution in the first few 108
 yrs of the solar system." Does he mean "the first few years"
 or is the "108" a number that was incorrectly posted?

      Thanks for catching the error! In fact that number
      should be 108. The "8" should have been an exponent,
      which as you know, is an expression that means "10 x
      10" 8 times, or 100,000,000. This number represents the
      difference in age between the oldest meteorites -- 4.6
      x109 years -- and the oldest planetary measurements --
      4.5 x 109 years. We want to understand the natural
      forces and processes at work during those formative
      "childhood" years of our solar system.

 -------------------------------
 From: F Jolicoeur
 When is the next Meteor shower? Hope it is soon! I like to
 watch them! Do you?

      Cynthia Phillips, on kidsastronomy.guide@miningco.com
      writes: "You can usually see one or two meteors on just
      about any night that you're out looking at the stars. A
      few times a year, though, there's... a "meteor shower".
      The most famous of these is called the Perseids, and
      it's visible every year around August 11th and 12th.
      [This occurs each year] when the Earth passes through
      an area in its orbit around the sun that contains a lot
      of dust and small rocks" [which burn up as they pass
      through Earth's atmosphere]. For more information, go
      to your favorite web search engine site and search for
      "perseids" or "meteor shower" or "meteor"!             
                                                                
----------------------------------------------------------------------------