SpaceViews
July 25, 1999
In a pair of papers published in the July 23 issue of the journal Science, members of two science teams associated with the Near Earth Asteroid Rendezvous (NEAR) mission reported on the size, mass, and density of the asteroid 433 Eros, the second-largest of all know near-Earth asteroids.
NASA Headquarters, Washington, DC
Johns Hopkins University Applied Physics Laboratory, Laurel, MD
February 8, 1999
NEAR's science instruments observed about two-thirds of Eros on Dec. 23, 1998, as the spacecraft flew by the asteroid following an unsuccessful firing of its main engine a few days earlier. A subsequent successful firing of the engine put NEAR on course to rendezvous with Eros to begin its planned yearlong orbital mission starting in mid-February 2000.
Scientists and engineers at The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, MD, which manages the mission, and science team members from affiliated institutions quickly planned the valuable flyby observations in the wake of the unsuccessful engine burn on Dec. 20.
During the flyby, 222 photos and supporting spectral observations of Eros were taken from as close as 2,375 miles (3,830 kilometers) from the asteroid by the spacecraft's multispectral imager, infrared spectrometer and radio science experiment. "The flyby of Eros has given us fundamental information that will help us plan a better orbital mission at Eros," said Dr. Andrew F. Cheng, NEAR project scientist at APL. "It has taken some of the risk out of our orbit insertion maneuver and early operations."
First observed from the Earth more than 100 years ago, Eros was known to be an S-type asteroid with high concentrations of silicate minerals and metal. However, few details about its structure or composition are observable from the ground. The NEAR flyby produced evidence of variations in surface color and reflected light (or albedo) that suggest the asteroid has a diverse surface makeup. Closer observations during the comprehensive yearlong orbital study of Eros will be needed to determine its precise composition.
The science team has determined that Eros is slightly smaller than originally estimated from ground-based radar observations, with a size of 21 by 8 by 8 miles (33 by 13 by 13 kilometers), versus an estimate of 25.3 by 9 by 8 miles (40.5 by 14.5 by 14 km). The asteroid rotates once every 5.27 hours and has no discernible moons.
The asteroid's density is approximately 1.55 ounces per cubic inch (2.7 grams per cubic centimeter), close to the average density of Earth's crust. This makes Eros about twice as dense as asteroid 253 Mathilde, a C-type, carbon-rich asteroid that NEAR flew past in June 1997, and about the same density as S-type asteroid 243 Ida, which NASA's Galileo spacecraft flew past in 1993. Eros and Ida are the only S-type asteroids for which mass and density have been determined.
Flyby imaging of the asteroid's surface revealed a prominent elongated ridge that extends along its length for as much as 12 miles (20 km). "This ridge-like feature, combined with the measurements of high density, suggests that Eros is a homogeneous body rather than a collection of rubble" such as Mathilde appears to be, said Dr. Joseph Veverka, of Cornell University, Ithaca, NY, who heads the mission's imaging team. "It might even be a remnant of a larger body that was shattered by an impact."
The surface of Eros is pocked with craters. The two largest craters are four miles and 5.3 miles (8.5 and 6.5 km) in diameter, less than half the size of asteroid Mathilde's largest craters. The existence of fewer, smaller craters could be an indication that Eros has a relatively young surface when compared to Ida.
NEAR and Eros will cross paths again in February 2000. The spacecraft will then be inserted into orbit around the asteroid and begin its yearlong study. Images taken during orbit are expected to have more than 200 times better resolution than those obtained during the flyby and will be taken from as close as nine miles (15 km) from the asteroid's surface.
Flyby images of Eros and a related movie, an asteroid shape model and a chart of spectral observations are available on the NEAR mission Web site.
NEAR image of the day for 1999 Jan 08
A firing of the main engine at 5 PM EST December 20, designed to slow the spacecraft for insertion into orbit around the asteroid, was aborted by the spacecraft. Contact with ground controllers was temporarily lost, but was regained at 8 PM EST December 21 when autonomous spacecraft safety protocols took over and transmitted a signal to the ground. All spacecraft systems were determined to be healthy and operational. Within hours, a flyby observation sequence was developed and uploaded to the spacecraft. 1026 images were acquired by the multispectral imager, to determine the size, shape, morphology, rotational state, and color properties of Eros, and to search for small moons. The infrared spectrometer measured spectral properties of the asteroid to determine what minerals are present, and the magnetometer searched for a natural magnetic field. Analysis of the spacecraft radio signal will yield bounds on the asteroid's mass and density.
The main engine was fired successfully on January 3, 1999, placing NEAR on-course for a February 2000 rendezvous. Eros is NEAR's second asteroid encountered. On June 27, 1997, NEAR flew by the main-belt asteroid Mathilde at a range of 1212 kilometers (750 miles).
Built and managed by The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, NEAR was the first spacecraft launched in NASA's Discovery Program of low-cost, small-scale planetary missions. See the NEAR web page at http://near.jhuapl.edu for more details.
January 3, 1999
NEAR Major Engine Burn Completed
At noon, EST, January 3, the NEAR mission team conducted
a 24-minute, large bipropellant engine burn, to increase
the spacecraft's speed for a rendezvous with Eros in
February 2000. Preliminary indications are that the burn
was successful. NASA's Deep Space Network, which is
tracking the NEAR spacecraft, is expected to confirm the
accuracy of the burn early Monday morning, January 4.
The burn increased NEAR's speed by 2,100 mph (940 meters per second) to catch up to the faster-moving Eros asteroid, which overtook NEAR during the Dec. 23rd flyby. At the time of the burn, the spacecraft was 565,650 miles (910,100 kilometers) from Eros
Once accuracy figures from the burn are received, plans will be finalized for a small hydrazine engine burn to correct any deviation from the spacecraft's intended location. This burn is expected to take place in one to two weeks. Periodic trajectory correction maneuvers will be executed by the Mission Operations Center as deemed necessary to keep the spacecraft on course during its yearlong journey to the asteroid.
For now, NEAR continues on its orbit around the sun, traveling at about 43,000 mph (19 kilometers per second) as it gains on asteroid Eros.
December 24, 1998
Because of the failure of an engine burn needed to begin orbital insertion, the NEAR spacecraft's orbit of asteroid Eros has been postponed for more than a year, from January 1999 to May 2000.
David Morrison
"While the engine burn abort was unfortunate, we still expect to accomplish the rendezvous objectives, but at a later date," says NEAR Mission Manager, Dr. Robert W. Farquhar, of The Johns Hopkins University Applied Physics Laboratory, which manages the NASA mission. "We expect that the later rendezvous date will not diminish the overall science return."
During the Dec. 23, 1998, flyby, scientists will gain a preview look at the asteroid and gain a global perspective that will provide important information in planning an orbit insertion. Mission designers are now expecting the rendezvous will take place by May 2000. Data obtained from the flyby will help determine the shape and size of the asteroid and if it has any moons.
The rescheduling of the NEAR mission was made necessary by the abort of a planned 20-minute engine burn on Dec. 20, 1998. The spacecraft aborted just seconds after initiation of the bipropellant burn, causing communications with the spacecraft to be lost for about 27 hours. Contact was reestablished early Dec. 22, after NASA's Deep Space Network locked onto a radio signal from the NEAR spacecraft at about 8 p.m. EST, on Dec.21.
Responding to a command from the Mission Operations Center, NEAR started downloading stored data early Dec. 22, which the mission team has been analyzing to determine the cause of the abort and why the spacecraft lost attitude control. "We've looked at the data and we believe there has been no damage to the spacecraft or the propulsion system," says spacecraft systems engineer, Andrew G. Santo. "Our fault protection software identified the problem and switched NEAR to a safe mode. Essentially, it worked as designed."
During the Dec. 23 flyby NEAR will take over 1100 images from as close as 4,100 kilometers (2,500 miles). Although the images will be of lower resolution than those taken June 27, 1997, during the flyby of asteroid 253 Mathilde, they will provide scientists with important information about the asteroid.
NEAR will then travel in an orbit around the sun that nearly matches that of Eros. In May 2000, the spacecraft and Eros will meet, making it possible to insert NEAR into orbit around the asteroid.
Three instruments onboard the spacecraft, the Multispectral Imager, the Near-Infrared Spectrometer, and the Magnetometer, are operating as expected. More than 1,100 images of the asteroid are anticipated, which will provide scientists with valuable information on the size, shape, and surface characteristics of Eros and whether it has any moons.
The first photographs will be posted on the NEAR Web site on Dec. 24 between 1:00 - 2:30 p.m.
Despite an aborted engine burn that postponed the January 1999, rendezvous of the NEAR spacecraft with asteroid Eros, mission operations team members were able to quickly upload new commands to the spacecraft, making it possible to obtain valuable information during a December 23, flyby of the asteroid.
Science data, including multicolor images, spectral data, and magnetic field measurements taken during the flyby, are now coming into the Applied Physics Laboratory's NEAR Science Data Center. Doppler navigation data and real-time telemetry were collected that will help determine the mass of the asteroid.
The flyby gave NEAR Mission Operations an opportunity to test tracking and instrument sequences in preparation of a rendezvous event, says Mission Operations Center Manager Mark Holdridge. "The flight recorders, full of Eros science data, are being played back at the present time. The spacecraft is healthy and doing just fine and it has been confirmed that the flyby pointing and instrument command sequence executed flawlessly to completion."
In addition to flyby data processing, team members are also studying data sent by the spacecraft soon after contact with it was reestablished on Dec. 22, following 27 hours of communication blackout to determine the exact nature of the software anomaly that led to the rendezvous burn failure.
NEAR and Eros are now traveling in separate orbits around the sun as plans are being made for a rendezvous sometime between August 1999 and April 2000.
Images of Eros taken during the flyby are being posted on the NEAR Web site as they are processed.
NEAR's First Close-Ups of Eros
This montage of the asteroid Eros was assembled from images acquired by the Near Earth Asteroid Rendezvous (NEAR) spacecraft on Dec. 23, as the spacecraft flew by the asteroid at a distance of 2,500 miles (4,100 kilometers) at 1:43 p.m. EST. This montage shows the first nine of 28 views of Eros that were obtained during the flyby. The images were taken between 10:44 AM and 12:44 PM EST as the spacecraft range closed from 7300 miles (11,100) km to 3300 miles (5300 kilometers). During that time, the asteroid completed nearly half of a rotation. The smallest resolved detail is approximately 1650 feet (500 meters) across.
A firing of the main engine at 5 PM EST December 20, designed to slow the spacecraft for insertion into orbit around the asteroid, was aborted by the spacecraft. Contact with ground controllers was temporarily lost, but was regained at 8 PM EST December 21 when autonomous spacecraft safety protocols took over and transmitted a signal to the ground. All spacecraft systems appear healthy and operational. Within hours, a flyby observation sequence was developed and uploaded to the spacecraft. Over 1100 images were acquired by the multispectral imager, to determine the size, shape, morphology, rotational state, and color properties of Eros, and to search for small moons. The infrared spectrometer measured spectral properties of the asteroid to determine what minerals are present, and the magnetometer searched for a natural magnetic field. Analysis of the spacecraft radio signal will yield bounds on the asteroid's mass and density.
Options for rescheduling firing of the main spacecraft engine are currently being examined, and could lead to Eros rendezvous and orbit insertion as early as mid-1999 or as late as May 2000. Eros is NEAR's second asteroid encountered. On June 27, 1997, NEAR flew by the main-belt asteroid Mathilde at a range of 1212 kilometers (750 miles).
Built and managed by The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, NEAR was the first spacecraft launched in NASA's Discovery Program of low-cost, small-scale planetary missions. See the NEAR web page at http://near.jhuapl.edu for more details.
December 21, 1998
Doppler data leads team members to believe that the spacecraft was transmitting at that time. Around-the-clock strategy sessions are being held to correct the problem. If communications are not established soon it is highly unlikely that the spacecraft will go into orbit on January 10.
December 21, 1998
SPACEMAIL
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The craft is to dip within nine miles of Eros' surface during about a year of orbital operations.
NEAR launched in February 1996 as a joint project between NASA and the Johns Hopkins University Applied Physics Lab, and it is the first space mission aimed at orbiting and probing an asteroid.
Instruments aboard the spacecraft will determine the composition and mass of the 24-mile (38-km) long shoe-shaped object.
After the initial science objectives are completed, mission controllers plan to become more adventurous, orbiting ever closer and perhaps even landing on the asteroid's dusty surface, said mission manager Robert W. Farquhar.
"By taking the risk, we're hoping to get a lot of bonus science," Farquhar said.
Officials hope the mission will provide answers to fundamental questions about the nature and origin of near-Earth objects, such as the numerous asteroids, meteoroids, and comets in the vicinity of Earth's orbit.
Officials said they are interested in learning the composition and density of asteroids as part of fundamental research that would be needed to defend the Earth against any potential collision with a space rock of destructive size.
Farquhar said it would be important to know how to operate on and near such a small body in the event that an asteroid aimed at Earth is ever found. One scenario for protecting the Earth from such a collision is to land nuclear weapons on its surface and shatter the object.
In order to do this, officials said, they have to understand the composition and strength of typical asteroids.
Scientists believe an asteroid collision with Earth was likely responsible for the mass extinction at the end of the Cretaceous period, and another impact in 1908 destroyed thousands of square kilometers of forest near Tunguska, Siberia.
Asteroids are thought to be the building blocks of the solar system -- aggregates of rock and dust that were left over after the formation of the planets. Most of the asteroids are in the so-called astrobelt between the orbits of Mars and Jupiter. Eros, however, is on an orbit that swings it past Mars and close to the orbit of Earth.
Eros poses no threat. Its last pass went by the planet in 1975, missing by 45 million miles.
In June 1997, the NEAR spacecraft passed within 750 miles of an asteroid known as Mathilde and sent back photos of features as small as a football field. The photos of Eros should be far more detailed, scientists said.
The NEAR mission is the first launch in the Discovery Program, a NASA initiative for small planetary missions with a maximum three-year development cycle and a cost capped at $150 million for construction, launch, and 30 days of operation.
Space Science News
December 16th
NASA's NEAR spacecraft may find that asteroids need dusting - NASA scientists think that asteroids scoop up dust from space over the eons, possibly blanketing themselves in layers up to a meter thick. Asteroids are too small for gravity to capture dust, but static electricity might do the job instead. If this idea is correct, then asteroids could provide storehouses of primordial matter from the early Solar System for future exploration.
The University of Arizona
Dec. 14, 1998
"This is the first time ever a spacecraft will orbit an asteroid," said Professor William V. Boynton of the Lunar and Planetary Laboratory at The University of Arizona in Tucson. "There have been flybys and snapshots, but not much in the way of quantitative scientific data."
Boynton is on one of six science teams that will study 433 Eros on the year-long NEAR mission that begins on Jan. 10 and is scheduled to end Feb. 6, 2000. The asteroid, which measures 24 miles in length and 10 miles in diameter (40 kilometers x 17 kilometers) was the first near-Earth asteroid (those whose orbits come close to or cross the orbit of Earth) spotted by astronomers.
Boynton is a scientist on the X-Ray/Gamma Ray Spectrometer, or XGRS, experiment. It is the primary experiment for determining the elemental composition of the surface and layers just beneath the surface. The instrument will begin taking data next spring while NEAR is orbiting above the asteroid's surface, coming as close as 9 miles (15 kilometers).
The XGRS was designed and built by the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., which also built the NEAR spacecraft and manages the NEAR mission. Boynton and his group at the UA Lunar and Planetary Laboratory will process XGRS data and manage the data base for all XGRS data, as well as work on the scientific interpretation of results. Boynton's UA colleagues on the NEAR mission include Samuel (Hop) Bailey, project manager, and software specialists Jazbir (Jesse) Bhangoo and Irina Mikheeva.
XGRS results are basic to solving such mysteries as the source of meteorites and their relationship to asteroids.
"Eros is a very important asteroid because it is a member of a class called ‘S' asteroids, which appear to be similar to a rare type of meteorite on Earth called ‘stony-irons,'which have 50 percent metal and 50 percent silicate. Though the S-asteroids are very common in space, they do not seem to match many of the meteorites that fall to Earth," Boynton said.
"The other half of this problem is that the most common meteorites found on Earth, called ordinary chondrites, are very common on Earth but appear to be rare in space. Some people think that ordinary chondrites might come from S-asteroids and that S-asteroids actually might have a lower metal content than ground-based astronomical data suggest. This mission should really answer this question."
It's possible that the composition of Eros might turn out to be different from any of the known meteorites, Boynton added. It's also possible that Eros, a highly irregularly shaped object, is "possibly a chip off some larger, pre-existing asteroid that was smashed up," richer in silicates on one side and richer in metal on another, Boynton said. "This might allow us to learn something about the processes that go on in asteroids."
NEAR is the first in NASA's Discovery Program for "faster, better, cheaper" planetary missions. It was launched Feb. 17, 1996, from Cape Canaveral, Fl., 9 months sooner than its 36-month schedule and $41.6 million under the $150 million budget. The other NEAR instruments include a multispectral imager, a laser rangefinder, a near-infrared spectrometer, a magnetometer and a radio science package.
NEAR is the first spacecraft powered by solar cells to operate beyond the orbit of Mars. It returned 500 images of asteroid 253 Mathilde when it flew within 750 miles of that object on June 27, 1997. Last January NEAR returned to the Earth's vicinity for a "slingshot" gravity assist toward Eros' orbital plane.
Next month, Boynton will travel from Cape Canaveral, site of the Jan. 3 launch of the Mars Polar Lander, to the Johns Hopkins lab to witness NEAR's Jan. 10 arrival at Eros. On the Mars mission, Boynton heads an experiment in the Mars Volatiles and Climate Surveyor integrated payload. That experiment, TEGA, or the Thermal and Evolved Gas Analyzer, is to discover how much water and carbon dioxide is in soil at the south pole landing site on Mars, and what minerals make up that soil.
NASA Headquarters, Washington, DC
The Johns Hopkins University Applied Physics Laboratory, Laurel, MD
January 20, 1998
Launched Feb. 17, 1996, NEAR completed a flyby of the asteroid Mathilde in June 1997 and is now on its way back to Earth. Late Thursday, Jan. 22, the spacecraft will approach Earth over the Pacific Ocean traveling at about 20,000 mph. Because the United States will be in darkness as NEAR approaches, if there is no cloud cover, several geographic areas will be able to see the Sun reflecting off the spacecraft's solar panels, which will act as large mirrors. These sunglints will be visible on the East Coast, Friday, Jan. 23, at about 1:30 a.m. EST and the West Coast at about 1:45 a.m. EST (Thursday, 10:45 p.m. PST).
The spacecraft then swings around the Aleutian Islands and over Siberia before reaching its closest point to Earth, about 336 miles above Ahvaz in southwest Iran, Friday, Jan. 23, at 11:23 a.m. local time (2:23 a.m. EST), traveling at about 29,000 mph, its fastest speed for the swingby. Although NEAR will be close to Earth at this time, daylight may obscure its image.
The spacecraft then swings over Africa and on to Antarctica before pulling away from the Earth at a speed of about 15,000 mph. The swingby will have changed NEAR's trajectory to approximately 11 degrees south of the Earth's ecliptic plane, the orbital path the Earth takes as it circles the Sun, and put the spacecraft on target for its Jan. 10, 1999, rendezvous with Eros.
NEAR scientists and engineers are using the swingby as an opportunity to test performance and calibration of the spacecraft's six instruments and to practice coordinated multi- instrument observations of the type that will be used at Eros.
The Multispectral Imager, a visible light camera that will help determine the physical characteristics of Eros, and the NEAR- Infrared Spectrograph, used to study surface minerals, will be calibrated by comparing their readings of geological features with proven measurements of the same areas. These instruments will also be used to take images of the Earth along the spacecraft's path. NEAR's Magnetometer will be calibrated by comparing swingby data with known measurements of the Earth's magnetic field.
Other activities during the swingby will include using the X-Ray/Gamma-Ray Spectrometer to observe celestial gamma ray bursts and to collect data on gamma ray and X-ray backgrounds. These data are needed so scientists can better remove background impurities from the measurements to be made at Eros.
NEAR is expected to capture its first images of Eros, a 25- mile-long near-Earth asteroid, a few months prior to the 100th anniversary of the asteroid's discovery on Aug. 13, 1898. After reaching Eros, the spacecraft will start its orbit about 600 miles above the asteroid's surface, descending to 200 miles by February and coming as close as 10 miles during its yearlong study. Scientists will thoroughly map Eros and will examine its surface composition and physical properties. On Feb. 6, 2000, the mission is expected to end with a controlled descent onto the asteroid, sending dozens of high-resolution pictures as the spacecraft closes in on Eros.
The NEAR mission will be the first close-up study of an asteroid. APL, the first non-NASA center to conduct a NASA planetary mission, is managing the mission for NASA's Office of Space Science, Washington, DC.
Johns Hopkins University
Applied Physics Laboratory
JHU/APL Office of Public Affairs
NASA Headquarters Office of Space Science
July 3, 1997
"Everything went beautifully and we are now on target for a rendezvous with Eros in January 1999," says NEAR Mission Director Dr. Robert W. Farquhar of The Johns Hopkins University Applied Physics Laboratory (APL), Laurel, Md., where NEAR was designed and built. Spacecraft Systems Engineer Andrew G. Santo, also of the Applied Physics Laboratory, says, "The burn was performed so accurately that fuel that had been allocated for further correction can now be used during the orbital phase."
Today's maneuver was the first time the large thruster engine was fired. The next scheduled firing of the engine, Dec. 20, 1998, will mark the beginning of the Eros rendezvous sequence.
The flawless deep space maneuver continued the success story of the NEAR mission that began with its Feb. 17, 1996 launch from Cape Canaveral Air Station in Florida. On June 27, 1997, the spacecraft completed a flyby of asteroid 253 Mathilde, sending back spectacular images of a dark, battered carbon-rich rock believed to date from the beginning of the solar system. The flyby was the closest look at any asteroid to date and the first encounter with a C-type (carbon-based) asteroid. The Mathilde images were the first science return of NASA's Discovery Program. A radio science experiment measured for the first time the mass of an asteroid.
The NEAR mission, which will end Feb. 6, 2000, after a year-long study of Eros, will provide the first comprehensive study of a near-Earth asteroid. The mission has been managed by APL, where the Mission Operation and the Science Data centers are located.
Mathilde flyby updates can be obtained on the Mathilde homepage.
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