Near Earth Asteroid Rendezvous Mission Research Articles

Artificial Neural Networks for Multiple NEA Rendezvous Missions with Continuous Thrust

The interest for near-Earth asteroids for scientific studies and, in particular, for potentially hazardous asteroids requires the space community to perform multiple-asteroid missions with close-up observations. To this end, multiple near-Earth asteroid rendezvous missions can help reduce the cost of the mission. Given the enormous number of asteroids, this work proposes a method based on artificial neural networks (ANNs) to quickly estimate the transfer time and cost between asteroids using low-thrust propulsion. The neural network output is used in a sequence-search algorithm based on a tree-search method to identify feasible sequences of asteroids to rendezvous. The rendezvous sequences are optimized by solving an optimal control problem for each leg to verify the feasibility of the transfer. The effectiveness of the presented methodology is assessed through sequences of asteroids of interest optimized using two low-thrust propulsion systems, namely solar electric propulsion and solar sailing. The results show that ANNs are able to estimate the duration and cost of low-thrust transfers with high accuracy in a modest computational time.

An on-line deep learning framework for low-thrust trajectory optimisation

In the preliminary interplanetary mission design stage, a fast low-thrust (LT) transfer cost approximator will improve the mission design efficiency and enable us to design more complex missions. In this study, we propose a Deep Neural Network (DNN) based on-line framework for training approximators for fast low-thrust trajectory optimisation. The online characteristic refers to the ability to continuously adjust the trained DNNs using new LT transfer data from newly found asteroids, which avoids repeated and costly re-training and is particularly useful for mission scenarios where new data are obtained regularly. The framework contains a DNN-classifier and an online-DNN regressor. The Bayesian optimisation (BO) technique is adapted to determine the network structure as well as the feature selection and processing methods. The proposed DNN-classifier significantly improves the LT optimisation convergence rate from 7% to over 98%. The proposed on-line DNN regressor proves to have better generalization ability and scalability comparing to series network structure, giving a mean relative error (MRE) of approximately 0.6% in the test Near-Earth Asteroid (NEA) rendezvous mission scenario. The proposed on-line DNN framework can also be extended to solve other trajectory optimisation problems in other mission scenarios, such as Main-Belt Asteroid (MBA) missions, active debris removal mission (ADR), etc.

Multiple near-earth asteroid rendezvous mission: Solar-sailing options

The scientific interest in near-Earth asteroids (NEAs) and the classification of some of those as potentially hazardous for the Earth stimulated the interest in their exploration. Close-up observations of these objects will drastically increase our knowledge about the overall NEA population. For this reason, a multiple NEA rendezvous mission through solar sailing is investigated, taking advantage of the propellantless nature of this propulsion technology. Considering a spacecraft based on the DLR/ESA Gossamer technology, this work focuses on a method for searching possible sequences of NEA encounters. The effectiveness of the approach is demonstrated through a number of fully-optimised trajectories. The results show that it is possible to visit five NEAs within 10 years with near-term solar-sail technology. Moreover, a study on a reduced NEA database demonstrates the reliability of the approach used, showing that 58% of the sequences found with an approximated trajectory model can be converted into real feasible solar-sail trajectories. Overall, the study shows the effectiveness of the proposed automatic optimisation algorithm, which is able to find solutions for a large number of mission scenarios without any input required from the user.

The global elemental composition of 433 Eros: First results from the NEAR gamma-ray spectrometer orbital dataset

A primary goal of the Near-Earth Asteroid Rendezvous (NEAR) mission was to compare the elemental composition of the S-type asteroid 433 Eros to the chemical compositions of meteorites, with the specific objective of testing the hypothesis that the S-type asteroids are the source of the ordinary chondrite (OC) meteorites. To that end, NEAR carried an X-ray and Gamma-ray Spectrometer (XGRS) to measure the elemental composition of Eros from orbit. To date, no Eros-originating signal had been reported in GRS orbital measurements, a consequence of NEAR's high orbital altitudes about Eros. A reanalysis of the NEAR GRS orbital dataset, particularly data collected during a series of low-altitude flyovers, has finally revealed the first positively identified gamma-ray signals from Eros. This dataset, which amounts to just ~10h of data collection, was used to produce the first GRS-derived global elemental composition values. Results include the first absolute concentrations of Fe and Th, and the first global K concentration. The data confirm prior conclusions that the elemental composition of Eros' surface is inconsistent with achondritic and volatile-rich carbonaceous chondritic compositions. In contrast, ordinary chondrites, volatile-poor carbonaceous chondrites, and enstatite chondrites have compositions that are consistent with Eros' gamma-ray emissions. When placed in the context of other gamma-ray spectrometer investigations, this analysis indicates that successful gamma-ray spectroscopy investigations require extended periods of time (≥10 days) at orbital altitudes less than or equal to the radius of the target body.

Solar-Sail Trajectory Design for a Multiple Near-Earth-Asteroid Rendezvous Mission

The scientific interest for near-Earth asteroids as well as the interest in potentially hazardous asteroids from the perspective of planetary defense led the space community to focus on near-Earth asteroid mission studies. A multiple near-Earth asteroid rendezvous mission with close-up observations of several objects can help to improve the characterization of these asteroids. This work explores the design of a solar-sail spacecraft for such a mission, focusing on the search of possible sequences of encounters and the trajectory optimization. This is done in two sequential steps: a sequence search by means of a simplified trajectory model and a set of heuristic rules based on astrodynamics, and a subsequent optimization phase. A shape-based approach for solar sailing has been developed and is used for the first phase. The effectiveness of the proposed approach is demonstrated through a fully optimized multiple near-Earth asteroid rendezvous mission. The results show that it is possible to visit five near-Earth asteroids within 10 years with near-term solar-sail technology.

Laboratory simulations of sulfur depletion at Eros

The X-ray spectrometer of the Near-Earth Asteroid Rendezvous (NEAR) mission discovered a low abundance of sulfur on the surface of asteroid Eros, which is seemingly inconsistent with the match of the overall surface composition to that of ordinary chondrites. Since troilite, FeS, is the primary sulfur-bearing mineral in ordinary chondrites, we investigated the hypothesis that sulfur loss from surface FeS could result from ‘space weathering’ by impact of solar wind ions and micrometeorites. We performed laboratory studies on the chemical alteration of FeS by 4 keV ions simulating exposure to the solar wind and by nanosecond laser pulses simulating pulsed heating by micrometeorite impact. We found that the combination of laser irradiation followed by ion impact lowers the S:Fe atomic ratio on the surface by a factor of up to 2.5, which is consistent with the value of at least 1.5 deduced from the NEAR measurements. Thus, our results support the hypothesis that the low abundance of sulfur at the surface of Eros is caused by space weathering.

Implications of the NEAR mission for internal structure of Mathilde and Eros

The Near Earth Asteroid Rendezvous (NEAR) performed a flyby of the C-type main belt asteroid 253 Mathilde and an orbital study of the S-type near-Earth asteroid 433 Eros. These asteroid datasets provide a basis for inferences regarding physical properties and internal structure. The NEAR flyby of Mathilde revealed a heavily cratered surface with at least five giant craters (close to geometric saturation). Mathilde's density was unexpectedly low at 1.3 ± 0.3 g cm −3, indicating a high porosity. Such a high porosity may be consistent with a rubble pile structure and may favor a compressional style of cratering. There are structural features, such as a 20-km long scarp, and polygonal craters indicating that Mathilde is not completely strengthless. At least one of its structural components appears coherent over a few tens of km. NEARs study of Eros found an average density of 2.67 ± 0.03 g cm −3, almost uniform within the asteroid. Several lines of evidence suggest a globally consolidated internal structure: topographic features indicating tectonic deformations, regional scale linear features with related orientations, and structural control of craters in an intermediate size range. Eros is interpreted to be extensively fractured, but it was not disrupted and reaccumulated gravitationally. Some constraints can be placed on its strength. The consolidated interior must support a shear stress at least on the order of a few bars. Crater morphologies can be interpreted as suggesting a `strength' near the surface of a few tens of kPa. Macroscopic fractures within Eros should be filled with fines, so the low average density of Eros relative to ordinary chondrites is not simply explained by macroscopic void space.

Applying successful near mission operations approaches and refining for contour mission operations

On February 17, 1996, the first NASA Discovery Class Mission to launch, the Near Earth Asteroid Rendezvous (NEAR) spacecraft, began its journey to the asteroid Eros. NEAR is the first planetary spacecraft to be designed and operated by the Johns Hopkins University Applied Physics Laboratory (JHU/APL). In July 2002, the comet nucleus tour (CONTOUR) spacecraft, the second planetary spacecraft to be built and operated at JHUAPL and the 6th in the series of NASA Discovery Class Missions, will be launched. Both NEAR and CONTOUR share ambitious “Faster, Better, Cheaper” goals. Furthermore, with both missions being developed and operated at the same institution, a unique opportunity exists to refine CONTOUR designs and operational practices based on 5 years of NEAR operational experience. This paper provides an overview of designs and operational practices implemented by JHU/APL to safely and effectively conduct the NEAR mission. This paper discusses how these will be applied to the CONTOUR mission and what improvements are planned. It also discusses the unique challenges CONTOUR possesses for operating a 4 year mission with widely varying operations activity levels at low cost.

The NEAR shoemaker mission to asteroid 433 eros

The Near Earth Asteroid Rendezvous (NEAR) mission inaugurated NASA's Discovery Program. It was the first mission to orbit an asteroid and made the first comprehensive scientific measurements of an asteroid's surface composition, geology, physical properties, and internal structure. NEAR was launched successfully on 17 February 1996 aboard a Delta II-7925. It made the first reconnaissance of a C-type asteroid during its flyby of the main-belt asteroid 253 Mathilde in June 1997. It became the first spacecraft to enter orbit around an asteroid, doing so at the large near-Earth asteroid 433 Eros in February 2000. The spacecraft, renamed NEAR Shoemaker, landed on Eros at 37.2 South by 278.4 West, ending its mission on February 12, 2001 with another spacecraft first. NEAR obtained new information on the nature and evolution of asteroids, improved our understanding of planetary formation processes in the early solar system, and clarified the relationships between asteroids and meteorites. The NEAR Mission Operations Center and Science Data Center were both located at APL. The latter maintained the entire NEAR data set on-line and made data from all instruments accessible over the Internet to every member of the NEAR science team.

Calibration of Radiometric Data for Relativity and Solar Plasma During a Solar Conjunction

The round trip propagation time of a radio signal passing close to the Sun is affected by the charged particle environment of the solar plasma and the increased path length associated with the curvature of space predictedby general relativity. It has been difficult to separate estimates of the individual contributions to the signal delay. In January 1997, the Near Earth Asteroid Rendezvous (NEAR) spacecraft passed through a deep solar conjunction. For a month before and after conjunction, the spacecraft was pointed at the Sun and attitude operations were suspended. Since the NEAR spacecraft is equipped with two way X-band Doppler and range and the spacecraft attitude was favorable, a high precision spacecraft ephemeris was obtained. Data from the solar conjunction period of the NEAR mission is used to solve explicitly for the total electron content of the solar plasma along the signal path by using the signal delay in the range data to calibrate the signal advance in the Doppler data. The result is a precise estimate of the general relativity delay, and of the total electron content along the signal path, which can be used to improve models of the plasma emanating from the Sun. The general relativity parameter y is determined to less than 0.5 percent, which is competitive with the most accurate current verifications of general relativity.

NEAR cruise and mathilde flyby mission operations — report from the front lines of low cost missions

NASA's Near Earth Asteroid Rendezvous (NEAR) mission was launched in February 1996 on a mission to rendezvous with the asteroid 433 Eros in 1999. NEAR, with its five science instruments, is controlled from the NEAR Mission Operations Center at the Johns Hopkins University Applied Physics Lab in Laurel, Maryland by a team of 5–8 sequence planners and flight controllers, with the support of a small engineering group. We examine lessons learned in planning, verifying and executing NEAR cruise and flyby activities. We find that the NEAR prerendezvous phase — so long as science objectives remain tightly focused — has been and can be successfully executed by a small, broadly experienced, highly skilled, closely cooperating team. However, sequence design for the year-long Eros-orbiting phase with multiple, potentially conflicting science activities will require a more robust and tightly integrated structure.

Compositional mapping with the NEAR X ray/gamma ray spectrometer

The X ray/gamma ray spectrometer (XGRS) instrument on board the Near Earth Asteroid Rendezvous (NEAR) spacecraft will map asteroid 433 Eros in the 0.7 keV to 10 MeV energy region. Measurements of the discrete line X ray and gamma ray emissions in this energy domain can be used to obtain both qualitative and quantitative elemental compositions with sufficient accuracy to enable comparison to the major meteorite typies. It is believed that Eros is an S‐type asteroid, the most common of the near‐Earth asteroids. The determination of whether Eros consists of either differentiated or undifferentiated materials is an important objective of this mission. Observations of Eros during the NEAR mission will contribute significantly to our understanding of the structure and composition of this asteroid. The NEAR spacecraft was successfully launched on February 17, 1996. The NEAR XGRS was turned on during the week of April 7, 1996, and all detector systems operated nominally. Background spectra have been obtained.

Near‐Earth Asteroid Rendezvous: Mission overview

The Near‐Earth Asteroid Rendezvous (NEAR) mission, the first launch of NASA's Discovery Program, will be the first mission to orbit an asteroid. NEAR will make the first comprehensive scientific measurements of an asteroid's surface composition, geology, physical properties, and internal structure. NEAR launched successfully on February 17, 1996, aboard a Delta 11–7925. It will orbit the 20‐km‐diameter near‐Earth asteroid 433 Eros for about 1 year, at a minimum orbit radius of about 35 km from the center of the asteroid. The NEAR is a solar‐powered, three‐axis stabilized spacecraft with a launch mass including propellant of 805 kg. NEAR uses X band telemetry to the NASA Deep Space Network, with the data rates at Eros up to 8.8 kbits/s using a 34‐m High EFficiency (HEF) dish, and up to 26.5 kbits/s using a 70‐m dish. A solid‐state recorder is accommodated with a memory capacity of 1.8 Gbytes. Attitude control is to 1.7 mrad, line‐of‐sight pointing stability is within 50 μrad over 1 s, and post processing attitude knowledge is within 50 μrad. NEAR accommodates 56 kg of instruments and provides them with 84 W. The instruments are a multispectral imager (MSI), a near‐infrared spectrograph (NIS), an X ray/gamma ray spectrometer (XRS/GRS), a magnetometer (MAG), and a laser rangefinder (NLR), while a radio science (RS) investigation uses the coherent X band transponder. NEAR will make a flyby of the C‐type asteroid 253 Mathilde in June 1997 and will rendezvous with 433 Eros in February 1999. It will execute an initial slow flyby of Eros, with a flyby speed of 5 m/s and a closest approach distance of 500 km. Subsequently, its orbit will be lowered to 35 km. The NEAR Mission Operations Center and the Science Data Center are at the Johns Hopkins Applied Physics Laboratory. The Science Data Center will maintain the entire NEAR data set on‐line, and data from all instruments can be accessed by every member of the NEAR Science Team. Data, including images, are released over the Internet as soon as they are validated.

Postlaunch Contingency Trajectories for the Near-Earth Asteroid Rendezvous Mission

The August 1995 Koreasat 1 Delta II launch vehicle failure pointed to the prudence of developing a postlaunch contingency strategy for varying levels of Delta II launch vehicle failure. The objective of this investigation is to deŽ ne contingency strategies that maximize accomplishment of the Near-Earth Asteroid Rendezvous mission objectives. The baseline mission incorporates a trajectory correction maneuver at one week after launch,  yby of main belt asteroid (253) Mathilde in June 1997, and rendezvous with near-Earth asteroid (433) Eros in February 1999.After a partial launch vehicle failure for the Near-Earth Asteroid Rendezvous spacecraft, the need to reduce D V penalties associated with a delayed initial trajectory correction maneuver would require mission planners to quickly formulate a contingency trajectory design. Therefore, postlaunch contingency trajectories were designed prior to launch to avoid selection of a less-than-optimal alternative mission option resulting from emergency contingency planning after launch. Strategies were deŽ ned for how to perform the initial trajectory correction for launch vehicle failure approaching twice the velocity shortfall experienced by Koreasat 1. These strategies enable completionofmostmissionobjectives with 1) rendezvouswith Eros after an injection velocity shortfall up to 69m/s and 2) rendezvouswith near-Earth asteroid (4660)Nereus in 2003 after an injection velocity shortfall up to 300m/s.

Remote X‐ray spectrometry for NEAR and future missions: Modeling and analyzing X‐ray production from source to surface

Remote X‐ray spectrometry will play a key role in the geochemical exploration of solar system bodies, provided the methodology for data analysis efficiently detects and removes solar source and flight trajectory‐induced geometric variations. In this paper, we discuss the nature of such variations expected for missions to an asteroid, the Moon, and Mercury. An effective means of removing the effects of solar variability from surface measurements, as indicated by the agreement between theoretical models presented here and Apollo X‐ray observations, is also discussed. We calculate X‐ray spectra anticipated for these targets using probable surface compositions, solar outputs, and flight trajectories. Generally, the spectra show three distinctive regions where line intensities are clearly correlated with surface abundances: a high‐energy Fe region, a moderate‐energy Ca region, and a low‐energy region which contains Mg, Al, and Si lines. In addition, we calculate anticipated integration times required for acceptable levels of certainty and estimate spatial resolutions achievable for those integration times for elements Mg, Al, Si, S, Ca, Ti, and Fe. Required integration times are lower (on the order of minutes or even seconds) and achievable spatial resolutions improved (on the order of kilometers) for the lower energy lines and for periods of higher solar activity. Using the Near Earth Asteroid Rendezvous (NEAR) mission to asteroid 433 Eros as an example, we describe a recommended approach for analysis of X‐ray measurements based on our findings. Most importantly, we clearly demonstrate that major scientific goals for future exploration of asteroids, Mercury, and the Moon can be met by obtaining remote orbital X‐ray measurements of these bodies.

The NEAR Mission Operations System

Successful completion of the Near Earth Asteroid Rendezvous (NEAR) mission demands not only cost effective spacecraft design and development, but also cost effective mission operations system development and implementation. To control mission operations costs, several steps have been taken. Engineering of the mission operations system is concurrent with engineering of the spacecraft to trade off spacecraft capability with ground system capability. A single integrated system is being developed to support both spacecraft integration and test (I&T) operations and mission operations. Development of this system, which emphasizes the use of Commercial-Off-The-Shelf (COTS) components, is the responsibility of one development team. The mission operations team will be trained by supporting spacecraft I&T operations and will be cross-trained to fill any team position on-orbit. In this paper, status and plans in these areas will be discussed, as well as aspects of the concept of operations that afford cost efficient operations.