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http://www.spaceflightnow.com/news/0003/28eros/
NEAR image of the day for 2000 Mar 9
NEAR image of the day for 2000 Mar 7
NEAR image of the day for 2000 Mar 6
March 3, 2000
At 1 p.m. EST today a successful 22-second engine burn put NEAR into a near-circular orbit around asteroid Eros. The spacecraft is now operating approximately 124 miles (200 kilometers) from the center of the asteroid and will stay in that orbit until April 1.
Space Science News for March 3, 2000
Wanted: A Few Good Solar Flares
A 15-second engine burn on March 3 will
nudge NASA's NEAR spacecraft into a tighter orbit around Eros. With a
little help from the Sun -- in the form of a big solar flare -- the
satellite could get its first readings of the asteroid's composition.
March 2, 2000
Busy Science Orbit Begins
A 15-second engine burn at 1 p.m. EST on March 3 will
nudge NEAR into a 124-mile (200-kilometer) orbit around
Eros, giving the spacecraft its best scientific look at
the asteroid so far. Over the next four weeks, NEAR will
collect images and data for a detailed global surface
map, a topographic model and a more precise estimate of
gravity on Eros.
"We expect to resolve a lot of the features that we've only seen glimpses of," says Louise Prockter, a member of NEAR's imaging team.
NEAR's Multispectral Imager will snap enough photos to create color and monochrome maps of Eros' surface. By measuring the distance between NEAR and Eros, the Laser Rangefinder will begin to shape three-dimensional perspectives of the craters, ridges and various other features in the images. The craft's radio science equipment will use the closer orbit to get a better reading of the asteroid's gravity field.
With a little help from the sun, the satellite could also get its first readings of the asteroid's elements. The X-Ray Spectrometer detects fluorescence from elements that react to solar x-rays. "A lot depends on solar activity," says Ralph McNutt, X-Ray/Gamma Ray Spectrometer instrument scientist. "If there is a strong solar x-ray event, the instrument will get a good measurement."
Moving 3 miles an hour relative to Eros, NEAR will circle the rotating space rock three full times during this orbit. NEAR operates at this range until April 1, when another short engine burn will gradually move it into a 60-mile (100-kilometer) orbit. The asteroid and spacecraft are about 152 million miles (almost 245 million kilometers) from Earth.
The NEAR team will analyze and present its findings from the orbit over the next several months, including a potential first look at the data during a March 13 press briefing at the Lunar and Planetary Science Conference in Houston.
March 2, 2000
On February 29, 2000 NEAR recorded another first: the NEAR Laser Rangefinder (NLR) detected the first laser returns from Eros at a range of 290 km. This is the first time that ranging returns have been detected from an asteroid (see the image-of-the-day for 2000 March 2). NLR was designed to operate at 50 km range, and its successful detection of Eros at 290 km augurs well for the future. The laser rangefinder data will give us a three-dimensional view of the asteroid surface, nicely complementing the information from images. This is because imagers record the distribution of brightness as a function of angles perpendicular to the line-of-sight, whereas the laser rangefinder measures distance to the surface along the line-of-sight. The combination of the two data sets will be powerful, as we hope it will enable us to probe into shadowed regions (because the laser does not depend on solar illumination), and to distinguish between effects of albedo variations and effects of height variations. In an image, a spot may look brighter or darker because of reflectivity differences or because of lighting differences caused by topography (such as shadowing). Laser rangefinder data can be used to separate these effects and to measure topography - e.g., the heights of ridges, the depths of grooves and craters.
The image-of-the-day for 2000 February 25 shows the "eastern and western hemispheres" of Eros. The image shows an amazing diversity of geologic features, which will be the subjects of updates in the coming weeks. For now, I will focus on how we define eastern and western hemispheres and how we locate positions on a celestial body. I feel I should apologize for using the word "hemisphere" to refer to an irregularly shaped body like Eros, but I don't have another word that means "the surface within a 180-degree longitude range". To locate any point on the surface, we use what we call "spherical polar coordinates": the longitude and latitude angles, and the distance from the center of Eros. These three numbers specify the position of any point in three dimensions, but we need to specify which way is "north" and which way is "east". To accomplish this, we first locate the rotation axis of Eros using (for example) image data, and we choose the North pole direction as the reference for latitude, the same as is done for Earth. When viewed from above the north pole, the asteroid rotates counterclockwise; when viewed from the south, it rotates clockwise. The latitude angle is measured from the equator of Eros, which is the plane perpendicular to the rotation axis. Having defined the polar axis, the next step is to define the prime meridian, from which longitudes are measured. On Earth, the prime meridian runs through the poles and through Greenwich, UK. On Eros, a particular crater has been selected to mark the prime meridian. If one walks from the prime meridian in the direction of the rotation, one is going east. In the opposite direction, one is going west. Longitudes can be measured going east from the prime meridian, in which case we speak of east longitude, or they can be measured going west from the prime meridian, giving west longitude. Just to keep us on our toes, geophysicists commonly use east longitudes whereas geologists and cartographers commonly use west longitudes. Unfortunately, there is still another complication, which is that the International Astronomical Union defines "north" in reference to the so-called "invariable plane" which is close to the ecliptic plane defined by Earth's mean motion around the Sun, whereas we have just defined it in reference to the rotation of Eros - but fortunately, the two definitions of "north" coincide for Eros, so we can forget about the invariable plane. I guess scientists have a talent for making even the simplest things seem complicated.
Andy Cheng
NEAR Project Scientist
NEAR image of the day for 2000 Mar 1
The very oblique illumination in this mosaic is ideal for bringing out small landforms. Many parts of the asteroid have "grooves," linear troughs about 100 meters (330 feet) wide and several kilometers long. Similar features have also been observed on other asteroids such as Gaspra, and they are especially numerous on Mars' moon Phobos. Their origin isn't completely understood, but formation of the grooves probably involves fracturing of the asteroid's subsurface in some way.
NEAR image of the day for 2000 Feb 28
Eros' subtle butterscotch hue at visible wavelengths is nearly uniform across the surface. Two days after these images were taken, mapping by NEAR's infrared spectrometer showed that Eros exhibits a great deal more variety at longer wavelengths. These variations could be due to differences in texture or composition of the surface. Both NEAR's multispectral imager and infrared spectrometer will be used extensively during the month of March to map Eros' color and spectral properties from an altitude of 200 kilometers (120 miles). The images to be returned will show details as small as 20 meters (68 feet) across, providing a new perspective on the asteroid's many fascinating landforms discovered so far by NEAR.
Space Science News for February 25, 2000
NEAR moves closer to Eros
With a short engine burn yesterday, NASA's NEAR spacecraft began a gradual
descent into a tighter orbit around the asteroid Eros. As mission
controllers prepare to activate more science instruments on the space
probe, newly released crater counts show that Eros is a very old space
rock.
http://www.spacescience.com/headlines/y2000/ast25feb_1.htm
NEAR image of the day for 2000 Feb 25
Successful firing of NEAR's thrusters yesterday, February 24, placed the spacecraft on course for insertion into the next lower orbit, at a 200 kilometer (120 mile) altitude. Images from that orbit, commencing in early March, will have nearly twice the spatial resolution of data returned so far.
NEAR Science Update, February 22, 2000
The images we have seen to date provide a tantalizing glimpse of what lies in store for us as NEAR begins its year-long study of Eros. From the images-of-the-day for February 14-17, we have many hints of an underlying global fabric - the nearly parallel markings, the chains of small craters or pits, the grooves - but our exploration has barely begun. Our image resolution will improve by more than ten times over the best we have achieved to date, and we have yet to bring several of our instruments to bear on the asteroid. One interpretation of the global fabric is that it is the surface manifestation of stratigraphy - that is, layered structure - which was produced while Eros was part of a much larger parent body that was disrupted long ago by a monstrous collision. In other words, the global fabric may be a signature of ancient geologic activity (for example, volcanism) on the parent body of Eros. The size of this putative parent body, and the time at which it would have disrupted, are not known, and all we can say is that it must have been much larger than the present maximum diameter of Eros (33 km). It may or may not have been larger than the present-day 500-km asteroid Vesta. Another interpretation is that the global fabric results from large-scale fractures within Eros caused by a giant impact, which may have occurred on the parent body of Eros (so we would not necessarily see the huge crater on present day Eros). We will learn much more from higher resolution images, spatially resolved spectral maps, composition data, and altimetry later in the mission.
Another important early result from the Eros images is reported in the image-of-the-day for February 21, which shows that the surface of Eros is old like the surface of Ida. This graphic summarizes initial results of crater counts, which are made using data like that in the image-of-the-day February 16A. The February 21 plot gives the number of craters of a given size range found per unit area of Eros. Geometric saturation means that craters of a given size basically cover the surface. Of course, much more will be done with crater counting as higher resolution images are obtained. However, it is already clear that Eros does not have a high density of giant craters as found on Mathilde, where the giant craters approach geometric saturation (by giant craters we mean those that have a diameter close to or bigger than the radius of Mathilde itself). It is also clear that Eros does not have a paucity of craters, especially large ones, as does Gaspra. The NEAR data show that Eros has an older surface than Gaspra - Gaspra is less heavily cratered because it has not been exposed to bombardment as long. However, since we are still uncertain about collision rates in the asteroid belt at present and in the distant past, we cannot actually state an age for either asteroid.
Andy Cheng
NEAR Project Scientist
NEAR image of the day for 2000 Feb. 18
Space Science News for February 14, 2000
First Light from Eros Orbit: NEAR's first close-up pictures from Eros orbit have arrived at Earth. This story includes a beautiful image of a large crater on the asteroid and highlights from this afternoon's NASA press briefing.
NEAR image of the day for 2000 Feb 14
Features as small as a 100 feet (30 meters) across can be seen. This view shows the 3-mile (5-kilometer) impact crater which the spacecraft has spied for over a week during its approach. The two smaller craters superimposed on its rim are each about 1.2 miles (2 kilometers) across. An enormous boulder a full 170 feet (50 meters) in size sits on the large crater's floor. Other key features of the surface are shallow subsurface layering exposed near the tops of crater walls, and shallow grooves crossing the surface and cutting the crater's rim.
Feb. 14, 2000
Goddard scientists lead two NEAR science teams and believe their research will help the NEAR team in its quest to unravel key mysteries surrounding asteroids, including how they formed and their relationship to meteorites. 433 Eros, named for the Greek god of love, belongs to a class of asteroids that approach Earth's orbit. NEAR researchers therefore also hope to learn how strongly these asteroids are held together, so that the best course of action can be taken if ever one seems likely to impact the Earth.
The NEAR magnetometer, built at Goddard, will let scientists peek inside the asteroid and try to divine its history. The measurements will allow scientists to do this by determining if Eros has a magnetic field and, if so, measuring its strength and shape.
"The magnetometer instrument is one of the very few methods we have for determining what happened on the inside of an asteroid or planet, so we can to understand how it evolved," said Dr. Mario Acuna of Goddard, NEAR Magnetometer Science Team Leader. "If we find a strong magnetic field surrounding the entire asteroid, it implies certain things about the history and origin of 433 Eros. Such magnetic fields can only be created if there was a dynamo in the asteroid's parent body. This implies a core of molten metal, which through convection and rotation, generates a magnetic field. Since Eros is too small to have a molten core, it would have been part of a larger body that did posses a molten core and magnetic field. The magnetic field would have frozen into Eros as the parent body cooled, and then Eros broke off from its parent somehow. This will tell us that asteroids with strong magnetic fields are fragments of larger bodies, rather than aggregations of smaller objects."
Finding a weak magnetic field would make the history of Eros more difficult to fathom. According to Acuna, "Weak magnetic fields can be generated in many ways, and a wider range of phenomena and events will have to be included as possibilities in Eros' history or its interaction with the solar wind." Acuna doubts a magnetic field will be detected immediately, because NEAR's initial orbit will be too remote. He expects to begin taking measurements in late April or mid-May, as NEAR gradually spirals closer to Eros and the orbit is only about 31 miles (50 kilometers) from the surface.
"With the NEAR X-ray Gamma-Ray Spectrometer (XGRS) instrument, we can map the distribution of various elements at or near the surface of Eros, and by comparing their ratios, we can find out if Eros is a primordial relic from the formation of the solar system, or if it has been through thermal change," said Dr. Jacob Trombka of Goddard, XGRS Science Team Leader.
"Eros is a Rosetta stone that can return us to our cosmological roots by determining what happened during the solar system's formation, and the XGRS is a key to translating its language - that of geochemistry," said Dr. Pamela Clark of Goddard. "When a forming planet reaches a certain size, its heat and gravity become sufficient to cause differentiation of its elements - the planet becomes molten, and heavy elements, like nickel and iron, tend to sink to the center, while light elements, like silicon and carbon, rise to the surface. If we find Eros is composed mostly of light elements, this indicates that it is a fragment from near the surface of a larger body. If it is rich in heavy elements, it is likely from the center of a larger object. If instead it is undifferentiated, like the chondrite meteorites found on Earth, it probably never was exposed to much heat. This implies that it is a very primitive, unprocessed object, probably an aggregate of smaller objects from the early solar system nebula."
X-rays from the Sun strike Eros' surface and cause elements to fluoresce (to be stimulated, then glow) in specific X-ray "colors," invisible to the human eye but detectable by XGRS. This is similar to the way a black light uses ultraviolet light to make pop-art posters glow in various garish colors. Each element on Eros glows in unique X-ray colors, allowing XGRS to build a map of the element distribution at and just beneath the surface of Eros. XGRS will also use the cosmic radiation from space to map the elemental composition slightly deeper beneath the surface of Eros (to about four inches) via invisible gamma radiation. Like an atomic billiards game, rapidly moving particles from space crash into atoms within Eros, causing them to emit unique gamma-ray "colors," depending on their elemental classification. In addition, certain elements are naturally radioactive, and will reveal their presence to XGRS with a gamma-ray signature from their own internal decay.
XGRS, together with the other NEAR instruments, will tell us the composition, density and mass of asteroids like Eros, said Trombka.
Like the NEAR magnetometer, the XGRS has to get close to Eros before taking measurements; they are also scheduled to begin in late April or early May. XGRS research will pay off in other ways as well, from investigating mysterious blasts at the edge of the observable Universe to fighting crime here on Earth.
Gamma-ray bursts (GRBs) are the most powerful explosions known, equal in fury to hundreds of exploding stars, or supernovae. These remote explosions occur randomly and typically last just a few seconds before fading. The trick is to locate them quickly and accurately so that they can be studied by other telescopes sensitive to the different kinds of light emitted by the GRB fireball as it expands and cools.
"The NEAR XGRS instrument has been configured to identify GRBs," said Trombka. "It is electronically linked with GRB detectors on other spacecraft in Earth orbit, and a detector on the Ulysses solar mission, to form a space GRB detection network 130 million miles across. When all spacecraft are transmitting, we can accurately identify GRB locations from the differences in arrival times of the gamma-rays at spacecraft in the network. The data is relayed instantaneously through the Gamma-ray burst Coordinates Network at Goddard to telescopes all over the world. We have already autonomously identified three GRBs with the space network to date."
"The NEAR XGRS team also is working with the National Institute of Justice, applying XGRS technology to identify drive-by shooters from the gunpowder residue on their hands, which fluoresces in X-rays," added Trombka.
Goddard scientists will use the NEAR Laser Rangefinder (NLR) to make three-dimensional maps of the cratered surface of Eros to understand how cosmic collisions influence the shape of low-gravity objects like asteroids. "By measuring the three-dimensional properties of the best preserved and most recent impact events that have occurred on Eros, we can decipher how collisions influence the landscape of asteroids and how low gravity affects the shape of craters," said Dr. James Garvin of Goddard, Participating Scientist on the NLR science team. "Understanding the impact cratering process under low gravity is essential to understanding how planets and other solar system objects formed, because they originated from smaller, low-gravity objects that grew via collision with other solar system material."
The NLR fires extremely short-duration bursts of laser radiation from the NEAR spacecraft to the surface of Eros, which is later received at the spacecraft and timed to billionths of a second. By knowing where the spacecraft is located, where it is pointed, and how long it takes for the laser echo to be received, distance to the surface can be measured precisely, and a three-dimensional profile can be built up as NEAR progresses in its orbit. The NLR team expects to start mapping the asteroid in March, with higher resolution observations during spring and summer.
Space Science News for February 14, 2000
NEAR enters orbit around asteroid Eros!
NEAR Begins Historic Orbit Around Eros
February 14, 2000
At 11:00 a.m. Eastern time, navigation data from the Near Earth Asteroid Rendezvous spacecraft indicates NEAR has achieved orbit around asteroid 433 Eros.
At 10:33 a.m., with Eros about 203 miles (327 kilometers) below, NEAR's small hydrazine thrusters fired for 57 seconds, slowing the spacecraft's approach to walking speed and easing it into the asteroid's weak gravitational pull. The rendezvous took place about 160 million miles (256 million kilometers) from Earth.
"NEAR is now the first spacecraft to successfully lock into orbit around an asteroid," says Mission Director Dr. Robert Farquhar, from the NEAR Mission Operations Center at the Applied Physics Laboratory in Laurel, Md. "We're making history here today."
Over the next 24 hours, instrument data and pictures of the asteroid taken after the orbit insertion burn will provide more details about NEAR's precise position around Eros. The first orbit images from NEAR are expected this afternoon.
Johns Hopkins University
Applied Physics Laboratory
Laurel, Maryland
February 13, 2000
NEAR has snapped about 8,000 photos of its intended since January, and no picture says love like the latest image showing a large heart carved in the asteroid. The image is available on the NEAR Web site.
"It truly is a valentine from Eros," says NEAR Mission Director Robert Farquhar.
NEAR's digital camera captured the feature Feb. 11 from 1,609 miles (2,590 kilometers) away. The image surprised science team members yesterday as they processed the incoming data. The narrow, 3-mile (5-kilometer) heart- shaped depression appears just below a large ridge on the 21-mile (33- kilometer) potato-shaped asteroid. Until the spacecraft sends closer images, however, NEAR team members at the Johns Hopkins University Applied Physics Laboratory (APL) can't say for sure what the shadowy heart really is.
"It's a tantalizing mystery," says Dr. Joseph Veverka, of Cornell University, who leads the NEAR imaging team. "It makes you wonder, what other secrets are lurking in the heart of Eros?"
NEAR will begin unfolding such mysteries when the spacecraft meets up with Eros tomorrow at 10:33 a.m. (EST) and becomes the first spacecraft to orbit an asteroid. APL designed and built NEAR and manages the mission for NASA. For the latest mission news and images, visit the NEAR Web site.
Space Science News for February 13, 2000
Guess Who's Coming to Breakfast?: The NEAR spacecraft is now less than 1000 km from asteroid 433 Eros. It is scheduled to enter orbit around the space rock at 10:33 EST on Monday morning. Critical science observations are slated to begin 11 hours earlier when the spacecraft passes directly between the Sun and Eros.
NEAR Approaching Eros on Schedule
February 13, 2000
NEAR is cruising toward 433 Eros for tomorrow's historic rendezvous with the asteroid. The NEAR team at the Johns Hopkins University Applied Physics Laboratory indicates all spacecraft systems are operating as planned.
Around 2 a.m. (EST) today, NEAR began a rendezvous sequence that includes a unique low-phase flyby of Eros. Scheduled to happen around midnight tonight, the flyby will put NEAR directly between the sun and Eros and allow it to map the minerals on the asteroid's northern half under optimal lighting.
NEAR team members will download data from this sequence early tomorrow, about four hours before the spacecraft moves into position for orbit insertion at 10:33 a.m. Eastern time.
NEAR Science Update February 13, 2000
On February 11, 2000 at 15:20 UT, the NEAR imager recorded a most curious observation of Eros. It was late afternoon of that day when I was pulled aside and asked to 'have a look at something'. That something turned out to be the amazing heart-shaped feature that can be seen in today's image-of-the-day, which was taken at a distance of 1609 miles (2590 km). This image has undergone only our standard processing and has not been retouched (except for the arrow drawn on it). The 'heart of Eros' is actually a 5 km long depression in the surface, and it appears as a heart because of an accidental confluence of shadows. The same feature can be discerned in the Eros images from December 1998 but was not as well resolved at that time. It can be seen in the image-of-the-day from December 23, 1998. In that nine-frame montage, the first frame on the left in the bottom row shows the feature clearly, about half way to the upper end of Eros starting from the bright saddle-shaped depression. The three earlier frames and the next frame also show the feature. The saddle-shaped depression may be the largest crater on Eros, but the heart-shaped depression is basically mysterious at this time. By April we should have a much clearer idea as to what it is.
Andy Cheng
NEAR Project Scientist
NEAR image of the day for 2000 Feb 9
Over the last few days new details of Eros's surface have become resolved. Craters as small as a couple of kilometers across are now becoming visible. By the time of orbit insertion on February 14, features as small as 330 feet (100 meters) in size will be distinguishable.
Space Science News for February 8, 2000
Eros or Bust: As any dinosaur can tell you, it's important to keep an eye on Near-Earth Asteroids. On February 14, 2000, NASA's NEAR spacecraft will go into orbit around 433 Eros for a year-long closeup look at a 21 mile long space rock. Data collected during the mission could revolutionize our understanding of the solar system's "minor planets."
NASA Headquarters, Washington, DC
Johns Hopkins University Applied Physics Laboratory, Laurel, MD
February 8, 2000
The NEAR (Near Earth Asteroid Rendezvous) mission, a NASA Discovery Program being conducted by the Johns Hopkins University Applied Physics Laboratory, Laurel, MD, is the first mission to orbit an asteroid. For a year the spacecraft will use its instruments to scrutinize the potato-shaped space rock to learn about its chemical and physical features and evolutionary history. The asteroid is known to be 21 by 8 by 8 miles (33 by 13 by 13 kilometers) -- about twice the size of Manhattan Island.
NEAR is less than 2,900 miles (4,700 kilometers) from Eros and is slowly closing in at about 18 mph relative to the asteroid. The spacecraft is alive with preparations for its rendezvous. Its multispectral imager has been taking daily images for the past few weeks to confirm that the spacecraft is on track, to look for any moons orbiting the asteroid, and to measure its brightness variations for clues to its rotation.
The last scheduled rendezvous burn prior to orbit insertion will take place Feb. 8 at 5 p.m. EST. On Feb. 13 at about 11:33 p.m. EST, the spacecraft will fly directly between the sun and the asteroid, enabling NEAR's near-infrared spectrometer to take critical observations of Eros' northern hemisphere under near-perfect lighting conditions, which will allow it to distinguish the asteroid's mineral composition. In October a similar sweep will be made over its southern hemisphere.
On Feb. 14, at 10:33 a.m. EST, when NEAR is 207 miles (333 kilometers) from the center of Eros, it will fire its hydrazine engines to slow it enough to be captured by the asteroid's weak gravitational pull. Confirmation of orbit is expected to come at about 11:30 a.m. EST to waiting team members in the Mission Operations Center on the Applied Physics Laboratory campus.
During the first few weeks after achieving orbit the spacecraft will slowly descend toward the asteroid. Because the asteroid is irregularly shaped and rotating (it rotates once every 5.27 hours), this early stage of the mission can be very tricky, says Dr. Robert Farquhar, NEAR mission director. "No one has ever orbited a small body in space," Farquhar says. "The orbital stability is rather tenuous, and as we travel around Eros our navigation maneuvers must be perfect to keep us from crashing into it."
Using a multispectral imager, laser rangefinder, and onboard radio science experiment, mission scientists and engineers will acquire enough information on Eros' shape, mass and gravity field to allow the spacecraft to come closer. "Soon after we go into orbit we should know the asteroid's mass and therefore its density to within 5 percent," says Dr. Andrew Cheng, mission scientist.
The onboard magnetometer will determine the strength of the asteroid's magnetic field -- if there is one. "This will give the scientific community the first definitive measurement of an asteroid's magnetism, which contains clues to its thermal and geologic history," Dr. Cheng says. "The results of these measurements and others that we will take over the next year will help us to determine the origin of the asteroid and give us an unprecedented understanding of asteroids in general."
For the first two months NEAR will slowly descend to within 31 miles (50 kilometers) from Eros. During this low- orbit phase the x-ray/gamma-ray spectrometer will measure elemental abundances -- important information to help determine the relationship between meteorites and asteroids.
In late August the spacecraft will begin to climb from 31 to 311 miles (50 to 500 kilometers) above the center of Eros. During this ascent the multispectral imager will continue to take images of the asteroid's surface that will be compiled into a complete map of the asteroid. In December the spacecraft will descend, possibly to less than a mile, from the surface of the asteroid. At that vantage point the near- infrared spectrometer can collect extremely high resolution data of the asteroid's surface, making it possible to distinguish the composition of rocks as small as a grapefruit. Final events of the mission, which will end in February 2001, will be determined sometime this summer.
NEAR was launched Feb. 17, 1996, from Cape Canaveral Air Station, FL. Its original rendezvous date of Jan. 10, 1999, was postponed when a firing of the spacecraft's bipropellant engine, designed to put the spacecraft on target for the rendezvous, exceeded preset acceleration limits and caused the spacecraft to retreat into safe mode. But valuable information about the asteroid was collected by a hastily programmed flyby of Eros on Dec. 23, 1998. Early images can be found on the Internet at: http://near.jhuapl.edu
NEAR Gets Final Flight Adjustment
February 8, 2000
A slight press on the pedal today adjusted NEAR's flight path to Eros one more time before the spacecraft's historic encounter with the asteroid on Valentine's Day.
The 23-second engine burn bumped NEAR from 18 mph to 22 mph relative to Eros, and shifted the target point for its closet approach to the asteroid by 0.6 degrees. The maneuver was the second of two speed-and-trajectory adjustments the NEAR team designed just last week, after the craft went into "safe" hold while preparing for a larger rendezvous burn on Feb. 2.
The first maneuver - which NEAR executed flawlessly on Feb. 3 - slowed the spacecraft's approach speed from 43 mph to 18 mph. These were the last scheduled rendezvous burns before Monday's orbit insertion, during which NEAR becomes the first spacecraft to orbit an asteroid.
NEAR is 2,879 miles (4,633 kilometers) from Eros - about the same distance as a straight path between the Florida Keys and the northwest corner of Washington state.
ITHACA, N.Y. -- In deep space, there are very few second chances. But one year later and one year wiser, a team of Cornell University astronomers and researchers is preparing for the first spacecraft to orbit an asteroid, named 433 Eros, on Valentine's Day.
In December 1998 the NEAR (for Near Earth Asteroid Rendezvous) spacecraft almost had been given up for dead after a problem with an attempted rocket firing left the $224.1 million mission unable to complete its goal of going into orbit around asteroid Eros. But thanks to fast thinking at Cornell, NASA's Jet Propulsion Laboratory (JPL) and Johns Hopkins University's Applied Physics Laboratory (APL), which is managing the mission, signals were sent enabling the spacecraft instead to fly by 21-mile-long Eros, capturing images as it went.
Since then the spacecraft has been crawling back toward Eros (last Thursday, Feb. 3, its speed was slowed to 18 mph from 45 mph), once again preparing to enter into orbit and make the first detailed exploration of an asteroid with a full complement of instruments. By Saturday, Feb. 12 the spacecraft finally will be closer to Eros -- about 1,310 kilometers (814 miles) away -- than it was on Dec. 20, 1998, when the rocket problem occurred. At that time, the Cornell team expects to receive the highest resolution pictures of Eros yet. Cornell senior researcher Peter Thomas likens this slow yet finely detailed approach to "having to sneeze to get into orbit -- but sneezing in the right direction."
The year spent in preparing for this historical event has been well used by the mission team. Indeed, some researchers look on the unexpected months of preparation as serendipity.
"The unplanned extra year provided time to gain a lot more experience so that everyone is much more ready now than before," says Joseph Veverka, the Cornell astronomy professor who is leading the mission's science team in charge of the visible light camera and near-infrared spectrometer, two of the five science instruments carried by NEAR. His operations team also designed the spacecraft command sequences that point and operate the instruments.
For example, the unexpected images obtained in the December 1998 flyby greatly assisted in the design of commands sent to the spacecraft in recent weeks because, says Cornell researcher Colin Peterson, "we know more about the asteroid and what it looks like." Also, the flyby images helped Thomas make computer models of the asteroid's peanut shape, which the latest approach images show to be highly accurate and have helped confirm the location of Eros in the sky as seen from the spacecraft. "One of the first tests was not how good the shape model was but which way is the asteroid pointing and is it in the rotational phase we thought many months before," says Thomas. "The answer is yes, and we are much relieved to see that."
The mission team is being very cautious the second time around. In recent weeks there have been two "rehearsals" in which instructions have been sent to the spacecraft, tested and verified. On Feb. 13, 30 hours before entering orbit 200 kilometers (124 miles) above Eros, the spacecraft will perform a low-phase flyby of the asteroid. Partly this is to make sure the commands to the spacecraft are keeping it "healthy and safe," in Peterson's words, before the engine burn that will insert the spacecraft into orbit.
About 11 hours before the orbit maneuver, the spacecraft will pass directly between the asteroid and the sun at an angle that will erase shadows from Eros' surface -- as seen from NEAR. This will permit the NEAR infrared spectrometer aboard the spacecraft to measure the brightness of the surface in infrared wavelengths of light. Cornell researcher Beth Clark explains that the infrared spectroscopic observations of the asteroid's surface will tell researchers much about the minerology of Eros, helping to answer such questions as: Is Eros a rubble pile or a shard of strong material? What kinds of rock is the asteroid made of and have the rocks been changed by long exposure to space? In particular, the spectrometer will focus on the asteroid's northern hemisphere, which is currently in continual summer sunlight. This will be the only time during the mission that the spacecraft will be focused on Eros' north pole.
The spectrometer will be observing Eros at different wavelengths than can be perceived by a camera lens (or a human eye). Indeed, the imaging camera will not be able to see the surface of the asteroid during this period because the solar panels must remain in sunlight to receive power, and turning the spacecraft so that the camera could image the asteroid would result in turning the solar panels away from the sun.
At approximately 11:33 a.m. Eastern time Feb. 14, NEAR's main engine will burn to slow down the spacecraft, thrusting it into orbit around the asteroid, which is some 240 million miles from Earth. Veverka says that for the first three weeks NEAR will stay in a "loose, adjustable orbit" -- the modest mass of the asteroid means that the spacecraft will be barely bound by gravity. Eventually, he says, the orbit will settle at 200 kilometers. By April the orbit will be lowered to 100 kilometers, and by the end of May it will go into a very low 50-kilometer orbit, where it will remain for the rest of the year.
"What most of us hope," says Thomas, "is that after most of the mission's goals have been accomplished the spacecraft will go even closer, perhaps 5 kilometers (3 miles) above the surface, to obtain much higher resolution pictures." There is even a possibility, says Veverka, that next January the decision will be made to land the spacecraft on the surface of Eros.
With the orbit of Eros just a week away, the Cornell operations team remains entirely confident. "Our real apprehension came last January when the spacecraft was moving so fast, about a kilometer a second, that we had to make a big engine burn to slow it down," says Veverka. "What we have to do now is relatively easy, but we still have to make some very fine adjustments, which is a big challenge."
Latest images from Near Earth Asteroid Rendezvous http://www.news.cornell.edu/releases/Feb00/NEAR.CUteam.deb.html">The web version of this release, including accompanying photos.
February 3, 2000
NEAR Rendezvous Burn a Success
Preliminary indications show the NEAR spacecraft is on a
steady path to Eros, after a braking maneuver today
adjusted its approach speed and trajectory toward the
large asteroid.
At noon EST, NEAR's medium-sized thrusters fired for 90 seconds and eased the spacecraft from 43 mph (relative to Eros) to 18 mph. The maneuver also moves NEAR's trajectory about six-tenths of a mile (100 kilometers) closer to its target.
The operation was a slightly modified version of the rendezvous burn scheduled for Feb. 2, which was canceled after NEAR went into "safe" hold early yesterday morning. Mission operators at the Applied Physics Laboratory sent new commands to NEAR late last night, dividing the original Feb. 2 maneuver into two parts. A second burn on Feb. 8 will bump NEAR's approach speed to 22 mph and put it back on its original track to the asteroid.
"We were able to come back right away and devise a turnaround burn," says NEAR Mission Director Bob Farquhar. "It really shows the resiliency of the mission plans."
All critical science operations - including a low-phase flyby on Feb. 13 - remain on schedule. The flyby will put NEAR directly between the sun and Eros, affording a unique opportunity to map the asteroid's minerals under optimal lighting.
NEAR is now 5,047 miles (8,123 kilometers) away from Eros, which it will reach and begin orbiting on Valentine's Day.
NEAR image of the day for 2000 Feb 2
At this resolution of 0.7 miles (1.2 km) per pixel, a "saddle" is apparent at the upper left of the asteroid, near the terminator separating the day and night sides. This saddle, discovered in NEAR flyby images taken in December 1998, is the location of unusual bright rock deposits that scientists are eagerly waiting to see in high resolution images from Eros orbit later this month.
February 2, 2000
NEAR Rendezvous Burn Delayed The NEAR spacecraft went into a "safe" hold this morning while preparing for today's scheduled rendezvous burn. The NEAR team will attempt the maneuver tomorrow and the mission remains on schedule for the Valentine's Day encounter with asteroid Eros.
The rendezvous burn is a braking maneuver designed to slow NEAR's speed relative to Eros (from 45 mph to 21 mph) and refine its trajectory toward the asteroid. The spacecraft's onboard computer initiated the safe mode at 5 a.m. EST during a routine procedure that precedes a change in NEAR's velocity. Mission operators stayed in constant touch with NEAR and the spacecraft appears healthy, though the NEAR team is looking into potential causes of the problem.
NEAR image of the day for 2000 Jan 31
At this resolution of 1.1 miles (1.7 km) per pixel, the peanut-like shape of Eros is apparent. A detailed shape model of Eros was constructed from NEAR flyby images taken in December 1998. At that time the Sun illuminated Eros's southern hemisphere. In the views shown here the northern hemisphere is illuminated, showing the face of Eros that was not imaged during the 1998 flyby.
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 for more details.
Applied Physics Laboratory
Johns Hopkins University
Laurel, Maryland
NEAR Mission Status
2000 Jan 17
Snapped from 27,200 miles (45,350 kilometers) away, Eros appears only as a white speck on the black background of deep space. However, mission navigators use these early images to confirm the asteroid's location and keep the spacecraft on the right course. The NEAR team also uses them to measure variations in the light reflected off Eros, a key to determining the asteroid's exact rotation.
NEAR is now about 22,500 miles (or 35,300 kilometers) from its target -- closer than the distance at which most weather and communications satellites orbit the Earth.
IMAGE CAPTION:
Applied Physics Laboratory
Johns Hopkins University
Laurel, Maryland
This week, science operations moved into high gear as NEAR approaches to within 41000 km of Eros. Images of Eros were obtained today, yesterday, and the day before. These distant image sequences had three objectives: to confirm the location of Eros in the sky as seen from the spacecraft, to search for satellites of Eros, and to measure its brightness variations. We look to see whether Eros is in the expected part of the sky in order to confirm that NEAR is headed in the proper trajectory -- this process is called 'optical navigation'. We are also looking to see if Eros has any natural moons of its own. We do not know of any moons of Eros, but we do know of a few asteroids (such as Ida and Eugenia) that have their own moons. If Eros has a moon, we surely wish to know about it before we go into orbit. Of course, once NEAR goes into orbit around Eros, it will become the first artificial moon of an asteroid. Finally, we are measuring the brightness variations of Eros -- we call these 'lightcurve' measurements. Eros is an elongated, kidney-bean shaped object. As Eros rotates, sometimes greater areas are lit by the sun and Eros appears brighter, but sometimes smaller areas are lit and Eros appears dimmer. Especially large lightcurve variations are observable when Eros rotates between broadside to the sun and narrow end to the sun -- Eros is not in that geometry now, but will be later this year. We will use our Eros lightcurve observations to refine our knowledge of Eros's rotation. The other instrument teams on NEAR have also been busy. The laser rangefinder test mentioned last week was completely successful. There was a successful calibration of the near infrared spectrometer -- more about that instrument next time!
NEAR Watching the 'Rock' Around the Clock
January 10, 2000
Get the coffee ready. Starting late this evening - as NEAR begins regular observations of asteroid Eros - the NEAR Mission Operations Center at the Applied Physics Laboratory stays open around-the-clock to monitor the spacecraft.
The 24-hour operations mark a milestone in the mission to Eros, which intensifies with rendezvous maneuvers on Feb. 2 and the actual orbit insertion on Feb. 14. NEAR is 29,800 miles (48,000 kilometers) from Eros, but its instruments are already collecting valuable navigation data and detailed scientific information on the large space rock. Starting Jan. 11, the NEAR team will begin downloading this information daily through NASA's Deep Space Network. The raw data will be available on the NEAR Science Data Center Web site a week after it's collected.
"The spacecraft is doing a lot of work, and we need to make sure things are happening the way we expect," says Mark Holdridge, NEAR mission operations manager. "This is the stretch drive as we prepare for the orbit insertion."
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