NEAR Spacecraft Research Articles

Independent estimate of asteroid 433 Eros' spin state using surface Doppler from the NEAR-Shoemaker mission

This paper demonstrates the viability of using Doppler collected from an asteroid's surface to precisely estimate the asteroid's rotation state. This technique is applied to archived data from the NEAR-Shoemaker mission. At the end of the NEAR mission, the spacecraft landed on asteroid 433 Eros, and 50 h of two-way Doppler data was subsequently received through the DSN. This data was originally used to determine the landing location of the spacecraft but was never incorporated into estimates of Eros' rotation state. The surface Doppler yields a precise estimate of Eros', with an estimated right ascension of 11.364 ± 0.001° and declination of 17.234 ± 0.001°. No wobble greater than 0.001° has been detected. Additionally, the body-fixed landing location of the NEAR spacecraft is also re-estimated purely from the Doppler data. The estimated landing location shows good agreement with the candidate site determined by the navigation team. When the spacecraft is constrained to lay on the highest resolution shape model, the most precise estimate of the landing location is achieved which is latitude = 41.28 ± 0.40° S, East longitude = 80.34 ± 0.05° W, radius = 6.57 ± 0.04 km.

A flyby anomaly for Juno? Not from standard physics

An empirical formula recently appeared in the literature to explain the observed anomalies of about Δρ̇≈1-10mms−1 in the geocentric range-rates ρ̇ of the Galileo, NEAR and Rosetta spacecraft at some of their past perigee passages along unbound, hyperbolic trajectories.It predicts an anomaly of the order of 6mms−1 for the recent flyby of Juno, occurred on 9 October 2013.Data analyses to confirm or disproof it are currently ongoing.We numerically calculate the impact on the geocentric Juno’s range rate of some classical and general relativistic dynamical effects which are either unmodeled or mismodeled to a certain level in the software used to process the data.They are: (a) the first even zonal harmonic coefficient J2 of the multipolar expansion of the terrestrial gravitational potential causing orbital perturbations both at the (a′) Newtonian (J2) and at the (a″) first post-Newtonian level (J2c-2) (b) the post-Newtonian gravitoelectric (GE) Schwarschild-like component of the Earth’s gravitational field (c) the post-Newtonian gravitomagnetic (GM) Lense–Thirring effect.The magnitudes of their mismodeled and nominal range-rate signatures are:(a′)Δρ̇σJ2≈1μms−1(a″)Δρ̇J2c-2≈0.015μms−1 (b) Δρ̇GE≈25μms−1 (c) Δρ̇GM≈0.05μms−1. If a flyby anomaly as large as a few mms−1 will be finally found also for Juno, it will not be due to any of these standard gravitational effects. It turns out that a Rindler-type radial extra-acceleration of the same magnitude as in the Pioneer anomaly would impact the Juno’s range-rate at a Δρ̇Rin≈1.5μms−1 level. Regardless of the quest for the flyby anomaly, all such effects are undetectable.

Probing the flyby anomaly with the future STE-QUEST mission

In this study, we demonstrate that the flyby anomaly, an unexpected acceleration detected in some of the gravitational assists of the Galileo, NEAR, Cassini and Rosetta spacecraft, could be probed by accurate orbital tracking available in the proposed Space–Time Explorer and Quantum Equivalence Principle Space Test (STE-QUEST); following a recent work, we focus on the similarity between a hyperbolic flyby and the perigee passage in a highly elliptic orbit of the latter, as well as its Global Navigation Satellite System precise orbital determination capabilities.

The shape distribution of boulders on Asteroid 25143 Itokawa: Comparison with fragments from impact experiments

Laboratory impact experiments have found that the shape of fragments over a broad size range is distributed around the mean value of the axial ratio 2:√2:1, which is independent of a wide range of experimental conditions. We report the shape statistics of boulders with size of 0.1–30 m on the surface of Asteroid 25143 Itokawa based on high-resolution images obtained by the Hayabusa spacecraft in order to investigate whether their shape distribution is similar to the distribution obtained for fragments (smaller than 0.1 m) in laboratory impact experiments. We also investigated the shapes of boulders with size of 0.1–150 m on Asteroid 433 Eros using a few arbitrary selected images by the NEAR spacecraft, in order to compare those with the shapes on Asteroid Itokawa. In addition, the shapes of small- and fast-rotating asteroids (diameter <200 m and rotation period <1 h), which are natural fragments from past impact events among asteroids, were inferred from archived light curve data taken by ground-based telescopes. The results show that the shape distributions of laboratory fragments are similar to those of the boulders on Eros and of the small- and fast-rotating asteroids, but are different from those on Itokawa. However, we propose that the apparent difference between the boulders of Itokawa and the laboratory fragments is due to the migration of boulders. Therefore, we suggest that the shape distributions of the boulders ranging from 0.1 to 150 m in size and the small- and fast-rotating asteroids are similar to those obtained for the fragments generated in laboratory impact experiments.

The Effect of General Relativity on Hyperbolic Orbits and Its Application to the Flyby Anomaly

We investigate the impact of the general relativistic gravitoelectromagnetic forces on hyperbolic orbits around a massive spinning body. The gravitomagnetic field, causing the well-known Lense-Thirring precessions of elliptic orbits, is generated by the spin <b>S</b> of the central body. It deflects and displaces the trajectories differently according to the mutual orientation of <b>S</b> and the orbital angular momentum <b>L</b> of the test particle. The gravitoelectric force, which induces the Einstein precession of the perihelion of the orbit of Mercury, always deflects the trajectories inward irrespective of the <b>L</b>&#x02212;<b>S</b> orientation. We numerically compute their effect on the range <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>r</mml:mi></mml:math>, radial and transverse components <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>v</mml:mi><mml:mi>r</mml:mi></mml:msub></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>v</mml:mi><mml:mi>&#x03C4;</mml:mi></mml:msub></mml:math> of the velocity, and speed <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>v</mml:mi></mml:math> of the NEAR spacecraft at its closest approach with the Earth in January 1998 when it experienced an anomalous increase of its asymptotic outgoing velocity <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mrow><mml:mi>&#x221E;</mml:mi><mml:mtext>o</mml:mtext></mml:mrow></mml:msub></mml:mrow></mml:math> of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>13.46</mml:mn><mml:mo>&#x00B1;</mml:mo><mml:mn>0.01</mml:mn></mml:mrow></mml:math>&#x2009;mm <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mrow><mml:mtext>sec</mml:mtext></mml:mrow><mml:mrow><mml:mo>&#x2212;</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>; while the gravitoelectric force was modeled in the software used to process the NEAR data, this was not done for the gravitomagnetic one. The range rate and the speed are affected by general relativistic gravitoelectromagnetism at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mo>&#x2212;</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math> (gravitoelectric) to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mo>&#x2212;</mml:mo><mml:mn>5</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math> (gravitomagnetic) mm <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mrow><mml:mtext>sec</mml:mtext></mml:mrow><mml:mrow><mml:mo>&#x2212;</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math> levels. The changes in the range are of the order of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mo>&#x2212;</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msup></mml:math> (gravitomagnetic) to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mn>1</mml:mn></mml:msup></mml:mrow></mml:math> (gravitoelectric) mm.

Anomalous Orbital-Energy Changes Observed during Spacecraft Flybys of Earth

We report and characterize anomalous orbital-energy changes observed during six Earth flybys by the Galileo, NEAR, Cassini, Rosetta, and MESSENGER spacecraft. These anomalous energy changes are consistent with an empirical prediction formula which is proportional to the total orbital energy per unit mass and which involves the incoming and outgoing geocentric latitudes of the asymptotic spacecraft velocity vectors. We use this formula to predict a potentially detectable flyby velocity increase of less than 1 mm/s for a second Rosetta flyby on November 13, 2007.

The helicial flux rope structure of solar filaments

According to J. B. Taylor's relaxation theory for magnetized plasmas, helical flux ropes should be the fundamental building blocks of magnetism in the solar atmosphere. Although flux ropes are clearly ejected from the Sun, there has been some controversy about whether they exist in the sun's atmosphere before ejection. The issue is important because of the need to understand how eruptions are initiated. A clearer picture of solar flux ropes is emerging with recent high-resolution observations of solar filaments. Their twist and writhe are frequently detectable. This report describes some examples of filaments that suddenly writhe (kink) and do not escape the sun. The observations of November 1, 2001 and May 27, 2002 both appear to show the sudden eruption of a filament with probable transformation of some internal twist into a writhe of approx. + 1. Also described is one event in which a well observed filament did erupt. It is associated with a magnetic flux rope detected with the NEAR spacecraft magnetometer.

A Global Solution for the Gravity Field, Rotation, Landmarks, and Ephemeris of Eros

As part of the NEAR Radio Science investigation, a global solution that includes both spherical and ellipsoidal harmonic gravity fields of Eros, Eros pole and rotation rate, Eros ephemeris, and landmark positions from the optical data was generated. This solution uses the entire one-year in orbit collection of X-band radiometric tracking (Doppler and range) from the Deep Space Network and landmark tracking observations generated from the NEAR spacecraft images of Eros. When compared to a constant density shape model, the gravity field shows a nearly homogeneous Eros. The Eros landmark solutions are in good agreement with the Eros shape model, and they reduce the center-of-mass and center-of-figure offset in the z direction to 13 m. Most of the NEAR spacecraft orbits are determined in all directions to an accuracy of several meters. The solution for the ephemeris of Eros constrains the mass of Vesta to 18.2±0.4 km 3/s 2 and reduces the uncertainty in the Earth–Moon mass ratio.

Modeling of the Eros gravity field as an ellipsoidal harmonic expansion from the NEAR Doppler tracking data

The gravity field for asteroid 433 Eros has been determined in terms of ellipsoidal harmonic functions by processing the Doppler tracking data of the NEAR spacecraft while it was in orbit about the asteroid. Using the same set of NEAR spacecraft Doppler tracking data, comparative descriptions of the Eros gravity field are provided for both the ellipsoidal and the traditional spherical harmonic models. It is shown that for elongated bodies, like the asteroid Eros, the ellipsoidal harmonics model permits a better representation of the gravity signature than does the spherical harmonics model. Eros has a nearly uniform density but there are negative gravity anomalies near the ends of Eros and positive gravity anomalies near the Psyche crater and the Himeros depression.

Web Watch

http://near.jhuapl.edu/iod/20010731 Section:ChooseTop of pagehttp://near.jhuapl.edu/io... <<http://www.inquinamentolu...http://hoaxbusters.ciac.o...On 12 February, the NEAR spacecraft broke out of its orbit 5 km above the asteroid Eros to head on a collision course for the asteroid’s surface. Throughout NEAR’s descent, the spacecraft’s Multi-Spectral Imager (MSI) took pictures of the approaching surface. The online movie MSI Final Descent captures this final phase of the NEAR mission. http://www.inquinamentoluminoso.it/dmsp Section:ChooseTop of pagehttp://near.jhuapl.edu/io...http://www.inquinamentolu... <<http://hoaxbusters.ciac.o...Because of street lights and other forms of artificial light, about one fifth of Earth’s population can’t see the Milky Way unaided. The University of Padua’s Pierantonio Cinzano and his collaborators reached this sobering conclusion by calculating, for any spot on the globe, the overhead brightness of the night sky due to artificial light. Their calculations, which are based on remotely sensed data, are available in the form of the online atlas The Night Sky in the World. http://hoaxbusters.ciac.org Section:ChooseTop of pagehttp://near.jhuapl.edu/io...http://www.inquinamentolu...http://hoaxbusters.ciac.o... <<From the US Department of Energy’s Computer Incident Advisory Center comes Hoaxbusters, a Web site devoted to combating the pernicious problem of hoax e-mail. Among the actual e-mails documented on the site is the chilling but false “Flesh Eating Bananas Hoax,” which you can find under the Urban Myths category. To suggest topics or sites for Web Watch, please e-mail us at [email protected].© 2001 American Institute of Physics.

Anatomy of a Partially Differentiated Asteroid: A “NEAR”-Sighted View of Acapulcoites and Lodranites

With the insertion of the NEAR spacecraft into orbit around the asteroid 433 Eros, a new stage of asteroid exploration has begun, with the return of spatially resolved mineralogical, geological, and chemical information from specific asteroids. In this paper, we outline the features of a partially differentiated asteroid using the acapulcoite and lodranite meteorites as a guide. Such a body would likely contain a variety of lithologies, including chondritic rocks largely unaltered by melting, residues of partial melts, and basaltic and Fe,Ni–FeS rocks crystallized from the partial melts. The presence or nature of the regolith remains largely unconstrained. Mineralogical and isotopic heterogeneities preserved in these rocks suggest spatial separation of these rock types by distances in excess of 200 m. Spacecraft-based remote sensing instrumentation should detect substantial spectral heterogeneity (most prominently manifested in the band area ratio) and chemical heterogeneity. Observation of these various lithologies in situ would provide important new constraints on the processes involved during the earliest differentiation of asteroids.

NEAR Lightcurves of Asteroid 433 Eros

The NEAR spacecraft flew by Asteroid 433 Eros on December 23, 1998, and obtained images with a best angular resolution of about 400 m/pixel. These images were used to derive a shape model (Thomas et al. 2000, Icarus). In addition, images obtained at phase angles of 88° to 115° during the flyby were used to modify our photometric model of Eros. NEAR has continued to observe Eros in flight, taking a series of lightcurves. The multispectral imager (MSI) lightcurve data consist of six observations of one full rotation each and range in phase angle from 62° to 98°. With the post flyby shape and photometric models, we have predicted the lightcurve of Eros. We used viewing angles which corresponded to actual viewing angles of lightcurve data obtained in flight by the MSI. Comparisons between observed and modeled lightcurves reveal relative discrepancies of up to 0.3 magnitudes. Our analysis shows that these differences indicate significant albedo features located between longitudes 220° and 300°. These longitudes were imaged during the NEAR flyby of December 1998, and indeed they show that a bright facet, well constrained in the shape model, appears to have an anomalously high albedo.

The Shape of Eros from NEAR Imaging Data

Images obtained during the NEAR spacecraft flyby of 433 Eros in December 1998 reveal a curved object, of a mean size and shape close to those inferred from lightcurve and radar data. The resolution of the images was sufficient to identify landmarks and establish a surface-based coordinate system.

Interplanetary magnetic clouds: Statistical patterns and radial variations

Abstract Pioneer Venus IMF data have been used to study the correlation between the structure of magnetic clouds in the inner heliosphere and the evolution of solar magnetic fields in the course of the solar cycle. We find that the leading polarity of these magnetic clouds follows the global polarity of the sun's magnetic field. In addition if we interpret the polarity structure of the clouds in terms of twisted magnetic fields, we find that the axes of these twisted ropes must vary greatly over the course of the solar cycle. The helicity of the ropes show no obvious pattern of behavior. Case studies of flux ropes seen at both Earth and the NEAR spacecraft are used to test a new technique for determining the radial variation of flux rope speeds.

Intercomparison of NEAR and Wind interplanetary coronal mass ejection observations

Nearly 4 months of continuous interplanetary magnetic field measurements September 1997 through December 1997 have allowed us to compare four interplanetary coronal mass ejection (ICME) events seen by the NEAR and Wind spacecraft. When the spacecraft are in close proximity (separated by 1° in azimuth relative to the sun) the ICMEs seen by Wind and NEAR have similar signatures as expected for structures with dimensions along the solar wind flow of ∼0.2 AU. When the NEAR spacecraft is separated by ∼5.4° in azimuth from the Earth the vector signature of ICMEs seen at NEAR begins to differ from those seen at Wind even though the magnitude of the field in the events and the background solar wind show similarities at the two spacecraft. When the spacecraft are separated by 11.3° the magnetic signatures are quite different and sometimes ICMEs are seen only at one of the two locations. Nevertheless, in all cases the magnetic helicity of the cloud structures seen at NEAR is the same as at Wind. The radial speeds of the shock and ICME leading edge as they cross Wind and the time delays of those events, for which we have some assurance that they also arrived at NEAR, indicate that the ICMEs decelerate measurably as they travel near 1 AU.

NEAR Encounter with Asteroid 253 Mathilde: Overview

On June 27, 1997, the NEAR spacecraft carried out the first-ever encounter with a C-type asteroid, flying by 253 Mathilde at a distance of 1212 km. We summarize findings derived from 330 images obtained by NEAR's MSI camera which cover about 60% of the surface of the asteroid. The highest resolution achieved was about 160 m/pixel.Mathilde is a low-reflectance object (geometric albedo=0.047) with principal diameters of 66×48×44 km. The mean radius of 26.4±1.3 km is somewhat smaller than the value of 30 km suggested by previous telescopic data. Mathilde's surface morphology is dominated by large craters, at least four of which have diameters comparable to the radius of Mathilde. The two largest, Ishikari and Karoo, have diameters of 29.3 and 33.4 km, respectively. No evidence of layering is exposed in the crater walls, but suggestions of downslope movement are present. The surface density of craters in the diameter range from 0.5 to 5 km is close to equilibrium saturation, a situation in which as many craters are being destroyed as are being produced. Observed depth-to-diameter ratios for craters in this size range are close to those observed on the lunar surface. A disruption lifetime of about 4 billion years has been estimated for Mathilde.Based on the mass determination obtained from Doppler tracking (D. K. Yeomans et al., 1997, Science 278, 2106–2109) and the volume derived from MSI images, the average density of Mathilde is remarkably low: 1.3±0.3 g/cm3, a value consistent with a rubble pile structure for the interior. Assuming that Mathilde's rock type is similar to that found in CM meteorites, the porosity of the interior must be some 50%. Shock and seismic disturbances associated with major impacts are expected to be transmitted very poorly by Mathilde's underdense interior, a fact which may explain the remarkable degree to which surface morphology and topography have been preserved in spite of later major collisional events.Except for the lower geometric albedo (0.047±0.005), the photometric properties of Mathilde are closely similar to those of Phobos. The surface is extremely homogeneous in terms of both color or albedo: specifically, no color or albedo variations associated with craters have been identified.

On a Possible Rotation State of (433) Eros

Due to its extremely prolate figure, Asteroid (433) Eros may exhibit an unusual nonprincipal axis rotation state. As a result of a relatively small difference between the maximum and intermediate moments of inertia, a small perturbation such as the gravitational torques experienced during a close planetary encounter could have induced a rather large amplitude libration about the longest axis. Observation of such a state will permit measurement of the moment of inertia ratios; this task may be possible once the NEAR spacecraft enters orbit around Eros in early 2000 to begin an extensive study of its composition and structure.

1620 Geographos and 433 Eros: Shaped by Planetary Tides?

Until recently, most asteroids were thought to be solid bodies whose shapes were determined largely by collisions with other asteroids. It now seems that many asteroids are little more than rubble piles, held together by self-gravity; this means that their shapes may be strongly distorted by tides during close encounters with planets. Here we report on numerical simulations of encounters between a ellipsoid-shaped rubble-pile asteroid and the Earth. After an encounter, many of the simulated asteroids develop the same rotation rate and distinctive shape (i.e., highly elongated with a single convex side, tapered ends, and small protuberances swept back against the rotation direction) as 1620 Geographos. Since our numerical studies show that these events occur with some frequency, we suggest that Geographos may be a tidally distorted object. In addition, our work shows that 433 Eros, which will be visited by the NEAR spacecraft in 1999, is much like Geographos, which suggests that it too may have been molded by tides in the past.

Satellite Dynamics about Small Bodies: Averaged Solar Radiation Pressure Effects

The effect of the solar radiation pressure force on a spacecraft orbiting a small body (asteroid or comet) is analyzed for the general case when the small body is in an elliptic orbit about the Sun and the spacecraft is in a relatively close orbit to the small body. The effect of this force is parameterized in terms of the spacecraft mass, spacecraft surface area exposed to the Sun, the small body mass, and the small body orbit about the Sun. Constraints and general results on orbit dynamics are presented and discussed for the averaged solar radiation pressure effect. These solutions are seen to be integrable and a function only of the small body true anomaly. Four families of stable, frozen, Sun-synchronous orbits close to the small body are found, all for the case of elliptic motion of the small body about the Sun. Such orbits have applications for extended station-keeping at a small body or for mission operations when the solar radiation pressure effect is large. Specific results for the NEAR spacecraft at asteroid 433 Eros and the Rosetta spacecraft at comet Wirtanen are given.

The NEAR magnetic field investigation: Science objectives at asteroid Eros 433 and experimental approach

The NEAR magnetic field investigation is an integrated technical, scientific, and management undertaking that was selected by NASA to achieve the following goals: to coordinate and support the development, calibration, and integration aboard the NEAR spacecraft of the magnetometer facility instrument (MAG), to provide technical assistance to the NEAR project in the area of magnetic cleanliness, and to ensure maximum science data quality. The prime objective of the MAG investigation is to establish the nature of the magnetic field of 433 Eros and any detectable effects of its interaction with the solar wind. The detection of an intrinsic magnetic field would place a strong constraint on the composition and structure of the asteroid's interior and thermal evolution. NEAR magnetometer data will be primarily available through the Johns Hopkins University Applied Physics Laboratory (APL) Science Data Center, http://sd‐www.jhuapl.edu/NEAR. The basic building block of the MAG investigation is the facility‐class magnetometer instrument flown aboard the NEAR spacecraft [Lohr et al., 1997]. The NEAR MAG was built as a cooperative undertaking between the Laboratory for Extraterrestrial Physics of the Goddard Space Flight Center (GSFC) and APL. GSFC provided the sensors and analog systems, and APL provided the digital processing unit and power converter as well as oversight for test and integration activities on the spacecraft. No booms or appendages were used to place the MAG sensor away from the spacecraft; instead and in analogous fashion to ESA's Giotto spacecraft, the sensor assembly was mounted on the high‐gain antenna feed structure. This arrangement results in a minimum detectable magnetic signature of 2–5 nT.