[NEAR Trajectory]

NEAR Mission Profile

Launch Phase - 17 February 1996  

     NEAR was launched on 17 February 1996, at 20:43 UT 
(3:43 p.m. EST) from Pad 17-B at Cape Canaveral Air Station, 
FL. using a three-stage Delta II-7925 expendable launch 
vehicle.

	The Delta II parking orbit had an altitude of 100 miles 
(183 kilometers) and an inclination of 28.74 degrees. The 
launch azimuth was fixed at 95 degrees. The coast period in 
the parking orbit was relatively short (13 minutes), allowing 
solar power to be used starting one hour after launch. The 
injection burn, accomplished mainly by a four-minute burn of 
the third stage solid motor, was entirely inside Earth's shadow.

     Approximately 22 minutes after launch, the spacecraft 
separated from the third stage. A yo-yo despin mechanism 
simultaneously released the solar panels from their stowed 
launch position and despun the spacecraft from a maximum of 
69 rpm to a nominal rate of 0 rpm. Once the solar panels were 
released, spring-loaded hinges deployed them to the on-orbit 
configuration. The spacecraft exited the Earth's shadow 37 
minutes after launch.  From launch until this time, the 
spacecraft was battery-powered. Because weight constraints 
limited the size of the battery, only those components 
considered mission critical during this phase were powered. 
At third stage separation, responsibility for attitude control 
passed from the Delta to the spacecraft's guidance and control 
subsystem.


Cruise Phase

     NEAR followed a so-called "Delta VEGA" trajectory to 
provide the extra energy needed to accomplish the rendezvous 
with Eros, which orbits the Sun at an angle of 10.8 degrees 
to the ecliptic. "Delta V" stands for change in velocity; 
"EGA" is an abbreviation for Earth Gravity Assist.

     During the first few weeks of cruise, a series of 
component functional tests checked the health of the 
spacecraft. Also during this time, low-level burns were 
performed to calibrate the propulsion system and to correct 
for any trajectory errors.

     After this checkout period the spacecraft maintained 
minimal activity. Due both to limited electrical power 
beyond 2 AU (186 million miles/300 million kilometers) and a 
desire not to thermally stress the solar panels during 
cruise, spacecraft operations Sunward of 1.5 AU (140 million 
miles/225 million kilometers) and outward of 2 AU were 
intentionally limited. All of the instruments were off. The 
telemetry subsystem periodically sampled low-level 
housekeeping and navigation data and stores the information 
on the solid state recorder. Heaters were used to maintain 
the temperature of the inactive systems.

     The spacecraft maintained this hibernation mode except 
during ground station contacts. Conducted during four-hour 
passes, three times per week, the ground contacts permit the 
mission operations team to analyze current spacecraft 
health, upload the next week's series of command sequences, 
and dump recorded telemetry.

     NEAR maintained this routine until preparations 
began for two critical mission events -- the Mathilde flyby 
and the Deep-Space Maneuver -- in the June/July 1997 
timeframe. In support of the increased activity, the DSN 
provided continuous coverage from its 34-meter network 
from June 20 to July 10, 1997. NEAR required 21 eight-
hour passes per week from these antennas instead of the 
normal cruise coverage. A third major event during cruise -- 
the Earth swingby -- occurred in January 1998.

     As part of preparations for the flybys, the 
multispectral imager periodically pointed at the target and 
an image was transmitted to Earth. This optical navigation 
(OpNav) data was combined with ground tracking information to 
calculate a flyby trajectory. The trajectory calculation was 
refined throughout the flyby approach as more ground data 
was taken and more images returned. Prior to the flyby, a 
time-tagged command sequence was uploaded to the spacecraft 
defining a time-ordered sequence of command to be executed 
during the flyby. These commands include an open-loop 
pointing trajectory and instrument data capture sequences.


Mathilde Flyby - 27 June 1997

     NEAR's flyby of the 50 x 53 x 57 kilometer C-type 
main belt asteroid 253 Mathilde took place on June 27, 1997, 
at a distance from Earth of 2.2 AU (205 million miles/330 
million kilometers). Closest approach distance was 1,200 
kilometers and occurred at 12:56 UT. Although the approach 
phase angle was almost 140 degrees, NEAR's imaging system 
was able to obtain useful optical navigation images 
beginning about three days before the encounter. OpNav 
sequences were scheduled at four-hour intervals with each 
sequence consisting of four pictures. Flyby speed was 9.93 
kilometers/sec.

     The primary science instrument was the camera, but 
measurements of magnetic fields and mass were also made. 
The whole illuminated portion of the asteroid was imaged 
in color at about 0.6-mile (1-kilometer) resolution, with 
the best monochrome views at some 660 to 980 feet (200 to 
300 meters) resolution. As the spacecraft recedes from 
Mathilde, a thorough search for satellites was 
conducted but none were observed.


Deep Space Maneuver - 3 July 1997

     The Deep Space Maneuver (DSM) was executed about 
one week after the Mathilde flyby, on July 3, 1997. The DSM 
represents the first of two major burns during the NEAR 
mission of the 100-pound (450-Newton) bi-propellant 
(hydrazine / nitrogen tetroxide) thruster. The spacecraft 
was slowed by 625 miles/hour (279 meters/second).  This 
maneuver is necessary to lower the perihelion distance of 
NEAR's trajectory, from 0.99 AU to 0.95 AU (92 million 
miles/148 million kilometers to 88 million miles/142 
million kilometers).

     The DSM was conducted in two segments to minimize 
the possibility of an overburn situation. The first segment, 
DSM-1, provided 90 percent of the required delta-V of 279 
m/sec and was performed with the high gain antenna in 
operation to monitor critical engineering data. 
Accelerometer measurements of DSM-1 were then be used to 
update DSM-2, which will supply the remaining 10 percent of 
the delta-V.


Earth Swingby - 22 January 1998

     The next critical phase of NEAR's flight profile 
occurred on January 22, 1998, when the spacecraft passed 
by the Earth at an altitude of 540 kilometers at 07:23 UT.  
This maneuver altered NEAR's heliocentric trajectory, 
changing the inclination from 0.5 to 10.2 degrees and 
reducing the aphelion distance from 2.17 to 1.77 AU (200 
million miles to 165 million miles/325 million kilometers 
to 265 million kilometers). Consequently, NEAR's 
post-swingby trajectory virtually matched the inclination 
and aphelion distance of Eros' orbit, which significantly 
reduces the magnitude of the rendezvous maneuver.

      An interesting aspect of the Earth flyby is that the 
post-swingby trajectory remained over the Earth's south polar 
region for a considerable time. This provided an 
opportunity for NEAR to obtain some unique images of the 
Antarctic continent. Also, because of its extreme southerly 
declination, the spacecraft could be viewed continuously from 
the DSN Canberra station for 71 days following the Earth 
flyby. The first visibility from the Goldstone and Madrid 
stations could not occur until 110 and 120 days, 
respectively, after the flyby.


Asteroid 433 Eros Approach - 9 January 1999

     First detection of Eros by the multispectral imager 
occurred on 5 November 1998, approximately 200 days prior to 
closest approach. Following this early observation, clusters 
of images were obtained weekly for optical navigation and 
for initial shape and rotation determination.

     Beginning on Jan. 9, 1999, a series of four rendezvous 
maneuvers with the main thruster were scheduled -- spaced 
seven days apart -- to slow NEAR by 2,123 miles/hour 
(949 meters/sec) to achieve a relative velocity between the 
spacecraft and Eros of 11 miles/hour (5 meters/sec).

     However, the first of the scheduled rendezvous burns on 
20 December 1998 at 22:00 UT aborted due to a software problem. 
Contact was lost immediately after this and was not 
re-established for over 24 hours. The original mission plan 
called for the burns to be followed by an orbit insertion burn 
on 10 January 1999, but the abort of the first burn and loss of 
communication made this impossible. A new plan was put into 
effect in which NEAR flew by Eros on 23 December 1998 at 
18:41:23 UT at a speed of 0.965 km/s and a distance of 3827 km 
from the center of mass of Eros. Images of Eros were taken by 
the camera, data was collected by the near IR spectrograph, and 
radio tracking was performed during the flyby.  Orbit insertion 
around Eros is now planned for 14 February 2000 after a 13 
month heliocentric orbit which closely matches the orbit of 
Eros.

     On 3 January 1999 a large bipropellant thruster burn was 
executed to close the gap between NEAR's orbital speed and that 
of Eros.  On 20 January a hydrazine thruster burn was completed 
to fine-tune the spacecraft's trajectory and speed. The last 
major trajectory correction was completed on 12 August with a 
2-minute burn of the hydrazine engine, slowing the spacecraft to 
188 mph relative to Eros


Asteroid 433 Eros Orbit Insertion occurred on 14 February 2000 
at 15:33 UT (10:33 EST).  The thruster burn put NEAR into a 
327 x 450 km near-polar orbit with a period of 27.6 days. 

The schedule now calls for the following:
	 
Feb. 14 to April 30, 2000 
      High-Orbit Phase. NEAR orbits Eros at distances decreasing 
	  from 311 to 31 miles (500 to 50 kilometers) from the center 
	  of the asteroid. 
March 10, 2000 
      NEAR spacecraft descends to 125-mile (200-kilometer) orbit. 
April 10, 2000 
      NEAR reaches orbit of 62 miles (100 kilometers). 
April 30, 2000 
      NEAR arrives at a polar orbit of 31 miles (50 kilometers), 
	  where the spacecraft spends 100 days. 
April 30  Aug. 27, 2000 
      Low-Orbit Phase. NEAR travels in nearly circular orbits at 
	  about 31 miles (50 kilometers) from Eros. The
      X-Ray/Gamma-Ray Spectrometer measures element abundances, 
	  which will help to determine the relationship between 
	  meteorites and asteroids. 
July 6, 2000 
      For the first time since NEAR arrived at Eros, all of Eros, 
	  excluding deep polar craters, is illuminated by the sun.  
	  Sunlight shines directly over Eros' equator as the sub-solar 
	  point moves south. 
Aug. 27  Dec. 20, 200 
      High-Orbit Phase. NEAR travels in orbits of 31 to 311 miles 
	  (50 to 500 kilometers) from Eros. During this period, the 
	  retrograde orbit shifts from nearly polar to nearly 
	  equatorial, where NEAR travels opposite the direction of 
	  Eros' spin. 
Oct. 15, 2000 
      Zero-phase measurements occur using the Near-Infrared 
	  Spectrometer as the spacecraft flies between Eros'
      southern hemisphere and the sun. 
Dec. 20, 2000 
      Low-altitude operations begin as the spacecraft passes 
	  within 21 miles (35 kilometers) or closer during each
      orbit. 
Feb. 14, 2001 
      Mission ends. 


 
     The irregular shape of Eros requires that NEAR remain 
in retrograde orbit relative to the asteroid spin. Prograde 
orbits tend to be unstable in the sense that the spacecraft 
would typically be ejected from orbit or caused to impact 
the surface. An orbital plane flip maneuver at approximately 
mid-mission is required to maintain a retrograde orbit.

     When data are to be downlinked, the spacecraft will be 
slewed if necessary to point the high-gain antenna at Earth. 
The instruments face 90 deg. from the direction of the antenna, 
so they can point at Eros as the spacecraft rolls in its 
orbit. All or any combination of the instruments can operate 
simultaneously, taking data and storing data on the solid 
state recorders. The spacecraft also can take data and 
downlink data simultaneously, although the instruments can 
be pointed at the asteroid for only a small portion of the 
downlink periods.

     Navigation constraints at Eros are designed to permit 
the spacecraft to orbit as low as possible for as long as 
possible to accomplish scientific objectives. They include:

     - Spacecraft orbit should be safe and stable for a 
timespan of weeks.

     - Normally there should be no less than seven days 
between propulsive maneuvers.

     - Total rendezvous delta-V expenditure should be less than 
224 miles/hour (100 meters/sec).

     - Sun pointing angle must be limited to less than 30 degrees 
because of power and payload pointing constraints resulting 
from the fixed mounting of the solar arrays and instruments.


This information has been adapted from the NEAR press kit and 
the mission timeline.


* NEAR home page
* Asteroid Fact Sheet
* Planetary Science home page

Author/Curator:
Dr. David R. Williams, dave.williams@nasa.gov
NSSDC, Mail Code 690.1
NASA Goddard Space Flight Center
Greenbelt, MD 20771
+1-301-286-1258
[NSSDC Logo]
NASA Official: Ed Grayzeck, edwin.j.grayzeck@nasa.gov
Last Updated: 08 February 2000, DRW