NEAR Shoemaker

• 1. Introduction

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  NEAR Shoemaker touched down on an asteroid on the 12th of February 2001, and nobody expected it to survive. The probe had been designed to orbit, not land. Controllers had planned a controlled descent that would end in impact, but when the spacecraft finally came to rest on the surface of 433 Eros, it was still in one piece, still transmitting, and still operational. At an estimated speed of 1.5 to 1.8 meters per second, it had become the first spacecraft to soft-land on an asteroid.

  The mission had been built around a single question that touches on the deepest history of the solar system: what are asteroids made of, and how do they connect to the meteorites that fall to Earth? Answering that question required getting closer to an asteroid than any spacecraft had gone before. What followed was a five-year journey marked by a near-catastrophic failure, a nail-biting recovery, and a landing nobody had planned.

• 2. First of Its Kind

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  NEAR was the first robotic space probe ever built by Johns Hopkins University's Applied Physics Laboratory. Before NEAR, APL spacecraft relied on magnetic tape recorders or magnetic cores for data storage. NEAR was the first APL spacecraft to use solid-state data recorders, and also the first to use significant numbers of plastic encapsulated microcircuits.

  The mission carried a broader significance for NASA as well. NEAR was the first launch of NASA's Discovery Program, a series of small-scale spacecraft designed to move from development to flight in under three years for less than $150 million. Construction, launch, and the first 30 days of operation were estimated at $122 million. The final total mission cost reached $224 million, broken down as $124.9 million for spacecraft development, $44.6 million for launch support and tracking, and $54.6 million for mission operations and data analysis.

  The original mission concept had been considerably more ambitious. An early plan called for visiting asteroid 4660 Nereus, with a flyby of 2019 van Albada along the way. An even grander scheme, called the Small-Body Grand Tour, proposed visiting two asteroids and two comets over a decade. Among the destinations discussed were 2P/Encke, 1036 Ganymed, 4 Vesta, and 4015 Wilson-Harrington. The mission that ultimately flew was narrower in scope but no less demanding in execution.

• 3. The Road Through Mathilde

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  NEAR launched on the 17th of February 1996 aboard a Delta 7925-8 launch vehicle, fitted with nine strap-on solid-rocket boosters and a Star 48 third stage. After leaving Earth orbit, the spacecraft entered the first leg of its cruise phase, spending most of that time in a low-activity hibernation state.

  On the 27th of June 1997, NEAR flew by asteroid 253 Mathilde, passing within 1,200 km at a speed of 9.93 km per second at 12:56 UT. Mathilde is 61 km in diameter. The flyby returned more than 500 images, covering 60% of the asteroid's surface, alongside gravitational data that allowed scientists to calculate Mathilde's dimensions and mass.

  Just days later, on the 3rd of July 1997, NEAR executed the first major deep-space maneuver: a two-part burn of the main 450-newton thruster that reduced velocity by 279 meters per second and lowered the perihelion of the spacecraft's orbit from 0.99 AU to 0.95 AU. The Earth gravity assist swingby followed on the 23rd of January 1998, at 7:23 UT. At a closest approach of 540 km, the maneuver shifted the orbital inclination from 0.5 to 10.2 degrees and brought the aphelion distance down from 2.17 to 1.77 AU, nearly matching the orbit of Eros.

• 4. The Near-Loss

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  On the 20th of December 1998, at 22:00 UT, mission controllers initiated the first of four scheduled rendezvous burns to set up orbital insertion around Eros. The burn sequence started and then immediately aborted. The spacecraft entered safe mode and began tumbling.

  What followed was one of the most tense stretches in the mission's history. The thrusters fired thousands of times during the anomaly, burning through 29 kg of propellant and reducing the program's propellant margin to zero. The spacecraft lost solar orientation and began draining its batteries. For over 24 hours, mission control could not establish contact. The probe came close to being lost entirely. The root cause was never definitively determined, though software and operational errors were identified as factors that made the anomaly worse.

  With the original plan for a the 10th of January 1999 orbit insertion now impossible, controllers devised a new approach. NEAR flew by Eros on the 23rd of December 1998 at 18:41:23 UT, passing at 965 meters per second and a distance of 3,827 km from the asteroid's center of mass. The camera captured images, the near-infrared spectrograph collected data, and radio tracking was performed during the flyby. A rendezvous maneuver on the 3rd of January 1999 matched NEAR's orbital speed to Eros; a hydrazine thruster burn on January 20 fine-tuned the trajectory; and by the 12th of August, a two-minute burn had slowed the spacecraft's velocity relative to Eros to 300 km per hour.

• 5. In Orbit Around Eros

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  Orbital insertion around Eros was achieved on the 14th of February 2000, at 15:33 UT. Eros itself is an S-type asteroid measuring approximately 13 by 13 by 33 km, making it the second largest near-Earth asteroid. NEAR entered an initial elliptical orbit of 321 by 366 km.

  Over the following months, the orbit was progressively tightened. The radius dropped to 50 by 50 km on the 30th of April 2000. By the 14th of July, the spacecraft had settled into a 35 by 35 km circular polar orbit, where it remained for ten days before being pulled back out in stages to a 100 km circular orbit by the 5th of September. In mid-October, a low flyby took NEAR within 5.3 km of Eros's surface at 07:00 UT on October 26.

  Searches for natural satellites of Eros were conducted on the 28th of January and on the 4th and the 9th of February; none were found. The scans served both scientific purposes and as a precaution against collision. Between December 1999 and February 2001, the gamma-ray spectrometer was also used as part of a broader network, detecting gamma-ray bursts for the InterPlanetary Network.

  By the 13th of December 2000, the orbit had been returned to a circular 35 km low orbit. Starting on the 24th of January 2001, NEAR began a series of close passes, coming within 5 to 6 km of the surface. On January 28, it passed within 2 to 3 km.

• 6. The Unplanned Landing

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  On the 12th of February 2001, at approximately 20:01 UT (3:01 p.m. EST), NEAR Shoemaker touched down just south of Himeros, the saddle-shaped feature on Eros's surface. Controllers had not planned a soft landing; the descent was intended as a controlled impact. When the spacecraft survived at an estimated speed of 1.5 to 1.8 meters per second, it was an outcome nobody had designed for.

  After receiving an extension of antenna time on the Deep Space Network, engineers reprogrammed the gamma-ray spectrometer to collect compositional data on Eros from the surface. Positioned about 4 inches from the ground, the instrument became ten times more sensitive than it had been in orbit. The improvement had two causes: the signal from Eros was far stronger at that range relative to noise from the probe itself, and the impact of cosmic rays on the sensor was reduced by about 50%.

  The last data signals from NEAR Shoemaker were received at 7:00 p.m. EST on the 28th of February 2001, when the spacecraft was shut down. A final attempt to contact the probe was made on the 10th of December 2002. It failed. The extreme cold on the asteroid surface, measured at approximately -173 degrees Celsius, had almost certainly ended the spacecraft's ability to function.

• 7. What Science Sought

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  The primary scientific objective of NEAR was to characterize Eros in detail: its bulk properties, composition, mineralogy, morphology, internal mass distribution, and magnetic field. The instrument suite reflected those goals directly. It included an X-ray/gamma-ray spectrometer, a near-infrared imaging spectrograph, a multi-spectral camera with a CCD imaging detector, a laser rangefinder, and a magnetometer. Together, the instruments had a total mass of 56 kg and required 80 watts of power to operate.

  The magnetometer was supplied by NASA's Goddard Space Flight Center and could measure fields from DC to 10 Hz across three axes. The near-infrared spectrograph covered a spectral range of 0.8 to 2.6 micrometers in 62 bins. A radio science experiment used NEAR's tracking system to estimate the gravity field of Eros. The gamma-ray component of the spectrometer used a sodium iodide scintillator with an active bismuth germanate shield, the same material used to shield the instrument from spacecraft-generated noise.

  The deeper scientific ambition was to establish whether Eros could be linked to meteorites recovered on Earth. If chemical composition data were sufficient, a causal connection might be drawn between Eros, other S-type asteroids, and the meteorites believed to have originated from S-type bodies. Establishing that link would allow scientists to study asteroid material using Earth-based equipment and extrapolate those findings to bodies still in space. NEAR did not resolve that question to scientific satisfaction, but the data it returned continued to inform how researchers understand the early conditions of the solar system.