NEAR Shoemaker: The Trailblazer that Tamed asteroid Eros
NEAR shoemaker was the first spacecraft to orbit and land on an asteroid, unlocking the secrets of Eros. This budget-friendly mission mapped a batter relic and linked it to meteorites discovered on the Earth, reshaping our view of the chaotic infancy of the Solar System. Eros is the first Near Earth Asteroid to be discovered.
On 17 February, 1996, a Delta II rocket punched through the skies, blasting off from Cape Canaveral. The payload on board was NASA’s Near Earth Asteroid Rendezvous (NEAR) spacecraft that was later named NEAR Shoemaker as soon as the probe entered orbit, in honour of Eugene Shoemaker, a legendary planetary scientist and geologist who died in a car crash in 1997, and left an indelible mark on space exploration. The probe was designed by the Applied Physics Laboratory at the Johns Hopkins University.
The 805 kilogram spacecrafted kicked off NASA’s Discovery Programme, a series of focused missions on shoestring budgets that pushed the limits of frugal engineering. The target of NEAR Shoemaker was the stony asteroid 433 Eros, the second-largest near Earth object, Ganymed. The mission profile involved orbiting the asteroid for a year, and then land in a stunning finale. The route to the asteroid was not a direct one. Budget constraints required a convoluted VEEGA trajectory with one flyby of Venus and two around Earth, a path that was previously taken by the Galileo probe to Jupiter.
A bonus encounter with asteroid Mathilde
The solar panels of NEAR were the first to power a craft venturing beyond Mars, with previous missions such as Galileo using Radioisotope Thermoelectric Generators (RTGs). There were six payloads on board: a gamma-ray spectrometer, a near-infrared spectrograph, a multispectral imager, a laser rangefinder, a magnetometer, and a science experiment that used the telecom system. The aim of the mission was to investigate the bulk properties of Eros, including its composition, mineralogy, morphology, internal mass distribution and magnetic field, all of which were essential for understanding the role of asteroids in the assembly of the Solar System, and their links to meteorites discovered on Earth.
The asteroid Mathilde as captured by the NEAR Shoemaker probe. (Image Credit: NASA).
On 27 June, 1997, NEAR hurtled past 253 Mathilde, a 61 kilometre wide carbonaceous asteroid, at 9.93 kilometres per second, at a distance of 1,200 kilometres. During the flyby, NEAR captured over 500 images that covered 60 per cent of the pockmarked surface of Mathilde, revealing a density half of the expected value. The observations indicated that Mathilde was a pile of rubble loosely held together by tenuous gravity. The data helped redefine models of asteroid interiors. This encounter was before the second of the Earth flybys required to reach Eros.
Uncovering the secrets of Eros
A planned engine burn was aborted on 20 December, 1998 because of a software glitch. The spacecraft hurtled past the asteroid at a distance of 3,830 kilometres without entering into orbit around it. It managed to capture 200 images during the encounter though. The orbit injection was rescheduled for 14 February, 2000, when NEAR successfully managed to enter into an elliptical orbit around Eros, at which point it was renamed NEAR Shoemaker. The craft began its year-long study, reducing the altitude to as little as 35 kilometers at times.
The probe mapped 70 per cent of the surface of Eros, revealing a density similar to the crust of the Earth, with no magnetic field, and a regolith layer encapsulating a fractured interior. Near captured over 160,000 images of Eros, showing large craters named Psyche and Shoemaker. The formation of the latter likely resulted in the ejection of nearly all the boulders on the surface. The deformation of Eros revealed that it had been battered and warped by impacts. The NEAR probe was extremely successful, and returned ten times the planned volume of data.
Images captured over three weeks as the NEAR Shoemaker probe approached the asteroid Eros. (Image Credit: NASA).
On 12 February, 2001, NEAR executed a controlled descent from an altitude of 25 kilometres, continuously snapping images, touching down near a feature known as the Himeros saddle. The spacecraft stunned engineers by surviving the landing. NEAR was not designed to land, and did so on its solar panels and its base. The gamma-ray spectrometer was now ten times more sensitive, and probed the elemental composition of Eros till the freezing temperatures silenced the instrument.
The probe discovered ponds of fine dust on Eros, that were possibly settled via electrostatic levitation. The discovery hinted at dynamic processes. Unlike planets, and some of the larger asteroids and their fragments, Eros does not have a differentiated interior, indicating that it was a pristine shard from the main belt, nudged towards the Earth by the gravitational influence of Jupiter. The probe proved that small, frugal missions could yield big science results, influencing future missions such as Dawn and OSIRIS-REx. NEAR Shoemaker is now resting silently on Eros, a testament to human ingenuity.