Landing on an asteroid

THE NEAR-SHOEMAKER MISSION: LANDING ON AN ASTEROID

* Aterrizando en un asteroide

The second largest of the near-Earth asteroids is (433) Eros. By its color, brightness and some radar studies it was possible to know that it is made mainly from silicon, it is irregularly shaped and is about 30-km long, but due to the distance we could not know much more. Basically, there was the discussion of whether meteorites, rocks falling from the sky, are related to asteroids like this or not. To investigate these mysteries, nothing better than to take the instruments used by astronomers and bring them as close as possible to (433) Eros.

Thus, NASA decided to fund a group of scientists to conduct these studies. A magnetometer would be used to measure any magnetic field the asteroid could have; a near-infrared spectrograph to study, from a distance, types of minerals on the surface; a multispectral camera to determine chemical compositions, besides providing pictures of the asteroid's shape, size, surface relief, etc.; also, an X-ray and gamma rays spectrometer, to determine from a distance of what atoms the asteroid consists. Solar X-ray monitors would permit acquiring knowledge of the incidence of the Sun in these readings. As a novelty, a laser altimeter for detailed topographic maps was installed. Finally, the spacecraft radio signal itself could provide information on any change of course or speed of the former, which is used to measure the gravitational field of the asteroid, which depends on its mass. And by joining this datum with photos and laser measurements of its size, its density can be measured and from there its likely internal composition can be inferred. All these equipments were attached to a structure of light-weight aeronautical materials in the shape of an octagonal drum.

Other subsystems include: at one end of the structure, the parabolic "dish" main antenna for communication with Earth, housing a secondary and even a tertiary antenna; around it, four solar panels for electricity; at the opposite end, another tertiary antenna; in the center of the structure, tanks filled with storables hydrazine and nitrogen tetroxide, for the propulsion system; and also auxiliary engines to rotate the spacecraft and for small trajectory adjustments; and multiple control computers. An unusual detail is that the main engine nozzle sticks out from one side, at 90 degrees to the antenna and solar panels. This is because the movement of the spacecraft toward the asteroid should also be, for as long as possible, almost perpendicular to where the Sun and Earth are.

The spacecraft would be built and operated by a private institution, the Applied Physics Laboratory of the Johns Hopkins University.

Communication with Earth would be through huge parabolic dish antennas in Australia, Spain and California. So, even with the Earth's rotation, at least one of them could maintain contact with the spacecraft in space, at any time.

The spacecraft was called NEAR, a pun on the acronym for Near Earth Asteroid Rendezvous, and the word "near". After launch it was decided to alter the name to honor Eugene Shoemaker, a pioneer of Astrogeology.

THE COMPLICATED ROAD TO (433) EROS

The asteroid (433) Eros is the first near-Earth asteroid to be discovered and the second largest in this realm. Therefore, it was the best observed of the "near ones" and its trajectory is very well known.  Its nearness, size and history of observations were decisive factors in electing it for the first space mission targeted exclusively to an asteroid. But its orbit has some complications. First, it is very elliptical, and while at times it comes close at "just" 22 million km from Earth, at other times it goes away beyond the orbit of Mars. But the biggest problem is that this orbit is highly inclined relative to that of Earth's, about 11 degrees.

The secret to put a spacecraft in orbit around another celestial body is that the trajectories have to be matched, so that the spacecraft will be able to accompany it. Initially the spacecraft is moving around the Sun with the Earth, and changing this trajectory to another quite different (equal to that of the asteroid) is the task of a launch vehicle. As in this case the change of trajectory was very large, a very large rocket had to be used, but this is prohibitively expensive. To use a "budget" Boeing Delta II, it was necessary to place the spacecraft around the Sun in a way that it would come back and flyby the Earth in the future, so that the gravity and motion of our planet would "drag along" the spacecraft giving it an extra boost. It is like a race car that in order to overtake another car places itself in the suction that the one ahead of it is creating, so it earns extra speed. Of course, in outer space there is no air so this force of "suction" is the force of gravity; hence the term "gravity-assist" for this kind of astronautical maneuvers. 

The launch took place on 17 February 1996. In its first lap around the Sun, NEAR-Shoemaker penetrated briefly in the Main Asteroid Belt, beyond Mars (and [433] Eros). Technicians knew that a peculiarity of launching precisely at, or at most within days from that date was that the spacecraft would pass very close to asteroid (253) Mathilde, on 27 June 1997. An encounter sequence was programmed in the computers, starting with 24 images 5,2 minutes before closest approach, followed by 144 high-resolution images from 2,8 minutes before until 3 minutes after closest approach. The spacecraft raced passing at just 1200 km of (253) Mathilde. Between 3 minutes and 10 minutes later 188 global images followed, and then between 10 minutes and 20 minutes after closest approach 178 images in search of tiny satellites. These images, plus data from other sensors, allowed us to know that (253) Mathilde is a bunch of loose rubble 66 km x 48 km x 44 km, with a high carbon content.

Month after a significant course correction, NEAR-Shoemaker passed by Earth, which helped tilt the plane of its orbit to match that of (433) Eros. Finally, when in December 1998 the day of the last major trajectory correction maneuver to match its speed to that of (433) Eros arrived, the large main engine did not work, due to a computer failure. Technicians spent Christmas and New Year's Day desperately trying to fix the problem, while the asteroid was slipping away and their mission of many years was being ruined. Finally they managed to ignite the engine, but the asteroid was already too far away. However, they realized that the spacecraft would be able to follow it, until after a year it could reach it again. This "Plan B" was therefore implemented, and the result was that on 14 February 2000, NEAR-Shoemaker became the first human-made object to orbit around such a small body.

There, the gravity is so low that an object at the surface may take up to 29 seconds to fall 1 m. As the asteroid furthermore rotates every 5 hours and is irregular, all the spacecraft maneuvers had to be very careful and precise. Slowly, the initial orbit, of more than one month, was being reduced to reach 200 km, and then to 100 km, and when technicians were more confident, just 50 km and even 35 km from the center of this 34-km long object.

SCIENTIFIC RESULTS

The asteroid (433) Eros is shaped like a peanut, and like other asteroids, is full of craters of different sizes. So many that in most of its surface saturation was reached, that is, it is not possible to open a new impact crater without destroying previous ones. It has mountains and valleys, and some regions are brighter than others. The main bowl crater is 5 km in diameter and hundreds of meters deep. There are many loose stones, some up to 100 m, which are certainly remnants of collisions. The asteroid has its rotation axis passing through its narrowest part, and due to the angle to the plane of its orbit, it has seasons where one pole and then the other are left in total darkness.

Following tradition, the different terrain features were receiving names related to the theme of the asteroid's name: then there are craters on (433) Eros now called Cupid, Lolita, Valentine, Don Juan, Dulcinea, etc.

The color images (actually taken in black and white, for higher quality, but through color filters that then a computer interprets) confirm that the asteroid is redder than ordinary silicon stony meteorites, as it was already known by Earth telescopes. But this and other sensors show that the composition is remarkably similar to that of the most common meteorites found on Earth, the chondrites. The color might be some surface staining due to solar irradiation and/or micrometeorite weathering.

So, it would not be far from the truth to say that this asteroid is a giant chondrite, confirming decades of suspicion that stones falling from the sky and asteroids are the same thing but in different sizes. The importance of this discovery is that it means that what we have in our museums, planetariums and laboratories are actually samples of asteroids.

Over time technicians became increasingly daring and ordered the spacecraft to perform low passes over the surface of this dangerous rotating object. The photos were worth the risk. They were able to study dusty craters and debris, product of millions of years of erosion by collisions; fissures caused by collisions with other large asteroids; big blocks of stones that have rolled down to the bottom of craters; and even craters erased by movements of loose soil, possibly shaken by earthquakes following impacts.

After a year orbiting the asteroid, and with fuel running low by the constant changes in trajectory, a Grand Finale that was not in the script was proposed: trying to land on the surface. On 12 February 2001, computers carefully made the NEAR-Shoemaker spacecraft descend, with its sensors capturing the moment, until touching the surface of this strange celestial body.

Today, unable to point its main antenna toward Earth or its photovoltaic panels toward the Sun, the NEAR-Shoemaker has become a monument to Humankind's pioneering efforts in space exploration.

Aldo Loup.

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Based on a lecture given at USP, originally on 3 October 1998. Originally published in ABC Color, on 6 November 2006. Photograph: Global mosaic of (433) Eros based on information from various instruments aboard the spacecraft NEAR-Shoemaker. Credit: Near-Earth Asteroid Rendezvous Project, NEAR Laser Rangefinder Science Team, The Johns Hopkins University Applied Physics Laboratory, Goddard Space Flight Center Scientific Visualization Studio, NASA.