SPACE SCIENCE

NEAR Swings by Earth en Route to Eros

A spacecraft on its way to an asteroid got an up-close look at Earth.

by Noam R. Izenberg and Brian J. Anderson, Johns Hopkins Applied Physics Laboratory, Laurel, Md., USA

The Near Earth Asteroid Rendezvous spacecraft (NEAR) conducted a swingby of Earth on January 23, 1998, en route to its final destination, the asteroid 433 Eros. The NASA spacecraft, launched February 17, 1996, aboard a Delta II-7925-8 rocket from Cape Canaveral, is on a mission to further our understanding of the nature of asteroids and their role in the formation of the solar system. The spacecraft uses four fixed solar panels to power five scientific instruments and transmit data to Earth through a 1.5-meter antenna.

As the first NASA Discovery-class mission, NEAR was designed to return the maximum amount of information on a timely basis with minimal cost. Aside from a transponder that will be used to determine Eros' mass and gravity field, the science instruments are focused on understanding the geology, chemistry, and history of the target asteroid. The Multi-Spectral Imager (MSI) will image the entire surface of the 40 x 14 x 14 kilometer (25 x 9 x 9 mile) asteroid, mapping its morphology and color. The Near Infrared spectrometer (NIS) will map Eros' surface mineralogy using reflected sunlight, while the X ray/Gamma ray Spectrometers (XGRS) will determine surface elemental composition of key rock-forming elements such as Mg, Al, Si, S, Ca, Fe, Ti, and K.The purpose of the NEAR magnetometer is to estimate the magnetic character of EROS, that is, how strong a magnetic field it generates. Finally, the NEAR Laser Rangefinder (NLR) altimeter will measure the asteroid's topography and shape. Soon after launch, NEAR conducted preliminary calibration observations of the Moon with the MSI, but the spacecraft obtained its first new scientific observations when it passed by the asteroid 253 Mathilde in June 1997. NEAR sent back the first close-up images ever of a C-type (low albedo and spectrally neutral) asteroid. The flawlessly executed Mathilde flyby was a "target of opportunity" event, providing both a science bonus early in the mission and a chance to shakedown and calibrate the MSI on an asteroid target. At 1.99 astronomical units (AU) from the Sun, the Mathilde flyby also gave NEAR the distinction of becoming the first solar-powered spacecraft to gather scientific data beyond the orbit of Mars. The purpose of the Earth swingby was three-fold. First, the close pass by Earth altered the spacecraft's course to set it on a final cruise trajectory that will allow NEAR to arrive at Eros in January 1999. The swingby flight path swung NEAR over the South Pole to send the spacecraft to meet Eros' orbit, which is inclined 10° from Earth's ecliptic orbit (see the figure on the cover and (1) on p. _). Second and equally important, the swingby allowed all five instruments onboard to make essential calibration measurements and conduct operational tests on well-known targets (the Earth and Moon). Third, swingby events provided invaluable tests of the ground data system, priming the operations and science teams for the lengthy mission at Eros. As a bonus, the data set accumulated at swingby will provide some unique opportunities for Earth science. During the Earth swingby, each of NEAR's instruments required different conditions and times for calibration measurements.

The Laser Test

In the final days of NEAR's Earth approach, a test of the NLR was conducted to check the alignment of the instrument's 1.06 micron laser. The spacecraft was turned away from Sun-facing orientation to point the laser toward Earth. The instrument attempted to illuminate a 1.2-m optical telescope at the NASA Goddard Geophysical and Astrophysical Observatory in Greenbelt, Maryland. A low-energy, groundbased laser was also fired from Goddard toward NEAR to test the NLR receiver.

Sunglint Maneuver

The night before closest approach to the asteroid, as part of a test of the solar panel alignment and stability of spacecraft pointing axes, NEAR was turned so that light from the Sun bounced off the solar panels to illuminate parts of the Earth. The "sunglint maneuver" took place while the spacecraft was still over 14,000 kilometers above the Earth over the Pacific Ocean. The illuminated 100 x 200 kilometer "spot" was slewed across the continental United States and out to Hawaii, targeting a number of major metropolitan areas as the spacecraft's direction changed. NEAR was observable by the naked eye and sky-watchers with fair weather across the United States and even in continental Europe saw and recorded NEAR's appearance in the early morning hours of January 22. A number of observers recorded the events with still or video cameras, and their images, as well as other swingby information, can be viewed at the NEAR Web site http://near.jhuapl.edu/. Observers on Earth reported that the spacecraft

appeared fainter and was visible more widely than expected. This may indicate that the solar panels are not perfectly aligned, varying from a few tenths of a degree to one degree from each other in orientation. Analysis of solar panel alignment is ongoing, using ground-based observations of the duration, magnitude, and variation of the spacecraft glint.

Magnetic Fields

The NEAR Earth swingby tested the magnetic field instrument and spacecraft contamination correction software by enabling two basic checks. The NEAR magnetic field measurements are reliable to about 1 nanoTesla (nT)*. By surpassing the original 5 nT design goal for the NEAR magnetic field investigation, the NEAR magnetic field measurements are more useful to scientists than ever expected. At Eros, this level of accuracy should allow us not only to measure the direct magnetization field, but also to study perturbations in the solar wind related to the asteroid that should be observed during the initial approach in the higher (about 400 kilometer radius) orbits. This would provide a useful confirmation of the technique used for the Galileo flyby of Gaspra to infer the size of Gaspra's magnetic field. Moreover, the magnetometer calibration data, including the extensive cruise phase observations and the swingby itself, provide unique opportunities for comparison between multiple observations of the interplanetary field and Earth's magnetosphere.

The VIS-IR Swingby

Data collection during the swingby in the visible and near infrared wavelengths consisted of four overall phases organized by target and calibration objectives: Earth I (Asia, Africa) and II (South Africa, Antarctica), Moon (NIS and MSI observations), and coalignment. NEAR's MSI and NIS instruments conducted a variety of calibration measurements in each of these phases. For Earth I, during closest approach to Earth, the NEAR spacecraft was oriented so that MSI and NIS faced the surface as the spacecraft passed over Siberia, the Aral Sea, western Iran, Kuwait, Saudi Arabia, and Ethiopia. The instant of closest approach was 07:22:56.60 UTC, at an altitude of 539.8 kilometers (334.8 mile) over Qal'eh Safid, Iran. The spacecraft passed over a variety of interesting geologic targets, including forested land, open desert, and ocean, all of which serve as control observations for spectral calibration.

For Earth II, as NEAR's ground track continued south and west, MSI panned across the west coast of South Africa, while the NIS scanned out a spectral mosaic of the whole of South Africa. The images and spectra from Asia and Africa are still being analyzed. MSI data are being used to understand the imager's reaction to light scattered in from outside the field of view. NIS data over South Africa provide its first test for creation of a spectral "image cube," with two spatial dimensions and one spectral dimension. NIS data will also be compared to other terrestrial remote sensing and weather data for calibration purposes. It is interesting to note, however, that NIS in particular may provide unique observations for parts of the Middle East and Africa since to date no spaceborne spectrometer covering the same spectral range has observed in that part of the world.

After looking at Africa, NEAR reoriented itself to point MSI and NIS toward Antarctica. These observations provided an essential set of calibration data for the two optical instruments, as well as some of the most spectacular images of the swingby. With MSI and NIS coaligned, and the field of view of the instruments buried in the nearly featureless "eastern" portion of Antarctica (latitude 80°S, longitude 90°E), a series of MSI images were taken concurrently with a large number of NIS calibration spectra. The figure (2) on p. _ shows an MSI frame of Antarctica with an example NIS footprint indicated. A "footprint" is the area on the ground that the instrument can see when the spacecraft is at a given distance and orientation.

Following the eastern Antarctica observations, pointing was centered on the South Pole for a series of images as NEAR receded from Earth. These images, averaging one set taken every 12 minutes for a day and a half, have been combined to create an "Earth spin movie." Though extended time sequences of the Earth's rotation have been made in the past, the South-Pole-centered perspective of NEAR's movie is unique. The cover figure shows 12 equally spaced frames from the movie. The fourth frame in the figure is an image of the Moon taken when the spacecraft panned away from the Earth for lunar radiometry measurements before recentering on Antarctica. Antarctica observations interspersed with the spin movie were used to test an auto-exposure process for MSI, which will allow the science and operations teams to program MSI data gathering more easily. The receding Earth provided good data for characterizing MSI scattered light and other artifacts seen during the Mathilde encounter.

Observations of the Moon with both MSI and NIS were primarily for radiometric calibration of both instruments to a planetary body and to each other. MSI had observed the Moon soon after NEAR's launch, but only through a protective cover that was opened before the Mathilde encounter. MSI observations targeted both the Moon and space around the Moon to gather detailed data on scattered light. MSI's observations were consistent with expected lunar brightness and color, and are being used to better understand the instrument's behavior throughout the swingby and during the Mathilde encounter.

The distance of the Moon from NEAR during the swingby meant that the Moon would barely fill the NIS field of view if centered perfectly. Since the instrument's pointing was not known to the required level of accuracy, NIS observations consisted of slewing back and forth across the Moon to view as much of it as possible. The Moon observations provided NIS with its first planetary target unobscured by an atmosphere. As the cover figure shows, NEAR observed a crescent, southern hemisphere Moon. An appropriate spectrum for comparison with NIS observations is being synthesized from laboratory spectra of known lunar materials and current knowledge of the Moon's southern regions.

Onward

Having completed the swingby, NEAR is now on course for rendezvous with Eros, where it will arrive January 10, 1999, for its year-long survey. During the months remaining before arrival, the NEAR science teams are analyzing the remaining swingby data, developing rendezvous data analysis procedures, and working with mission operations and navigation in planning, preparing, and testing command sequences and working modes for operations at Eros. The MSI will be able to glimpse Eros as early as mid-August, and the XGRS and magnetometer will be operated for several months before rendezvous to further refine calibration and correction procedures. Final calibrations and instrument checkouts are to be conducted during the approach to ensure that the entire suite of instruments will be ready. Upon arrival at Eros, NEAR will be placed in a 400-kilometer (about 250 mile) orbit, training all five instruments on the asteroid in a full-scale mapping mission. Over the course of the primary mission, the orbit will be tightened to radii of 200, 100, 50, and 25 kilometers successively. End of mission operations may be carried out even closer to the asteroid.

GLOSSARY

ground track--the imaginary line across the Earth's surface that is exactly below the spacecraft's flight path; magnetometer--an instrument that measures magnetic field and its changes; morphology--the external structure, form, and surface relief; nanoTesla--unit of magnetic field strength; spectrometer--a device for measuring intensity of radiation as a function of wavelength; topography--configuration of land surfaces, including its relief and the position of its natural features Source: Eos, June 23, 1998.

(1) The NEAR Earth swingby ground track. Times cover the day and a half around closest approach.

(2) NIS view of Antarctica from the Earth spin movie taken about 2.5 hours after the NIS data were acquired. The image is an MSI image and the box is the NIS footprint.