A Visit to Cupid for Valentine’s Day

Artist's conception: NEAR Shoemaker spacecraft just above the surface of Erosspace
Animation: NEAR Shoemaker spacecraft descending to the surface of Eros. Credit: Johns Hopkins Univ. APL

Around Valentine’s Day this year, the hearts of many scientists will beat a little faster. That’s because on February 12, a spacecraft will land on an asteroid for the very first time. For the past year, the Near Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft has been orbiting 433 Eros, a large asteroid located about 176 million kilometers (109 million miles) from Earth. Eros is the Greek name for Cupid, the familiar bringer of love from classical mythology. The spacecraft has relayed thousands of photographs of the asteroid back to Earth, giving scientists the opportunity to pick the perfect landing spot. But why land on an asteroid at all? One reason is because near-Earth objects like Eros may contain clues about the origin of our solar system. They can even tell us something about our own planet, since Eros’s pristine surface reflects the conditions in space 4.5 billion years ago when the Earth was formed. Objects such as Eros may also help us understand the formation of other planets, and even tell us something about the evolution of the entire universe.

Long, slightly
Video: Eros rotating.Credit: Johns Hopkins Univ. APL

Despite the depictions of asteroid landings in movies like Armageddon and Deep Impact, landing on an asteroid is not quite like landing on Earth, or even like landing on the Moon. The NEAR Shoemaker spacecraft is scheduled to make a “soft crash” rather than a touchdown type of landing. The spacecraft will slowly move inward toward the asteroid, approaching closer and closer until it finally makes contact. A crash landing is necessary because of the low gravitation of Eros. When “crash-down” occurs on February 12, the spacecraft will most likely be destroyed upon impact. Scientists are hoping, however, that the on-board camera will photograph the asteroid’s surface right up until the last moments.

Full view of oblong asteroid Eros, oriented horizontally. Land site at left, large craters at center and right.
Landing site on Eros, shown by Dot at left. Credit: Johns Hopkins Univ. APL

To ensure a picture-perfect crash landing, the scientists have to precisely calculate the rate of the asteroid’s rotation. If they measure wrong, the spacecraft may land on the asteroid at the wrong angle spoiling the opportunity for picture taking. Scientists at Caltech in Pasadena, California, and the National Space Studies Centre in Toulouse, France, have determined that Eros rotates once every 5.27 hours. The Caltech team measured Eros’s rotation by tracking the movement of landmarks on the series of photographs beamed back from the NEAR Shoemaker spacecraft’s on-board camera.

Large, smooth basin. At bottom lie scattered boulders that appear like pebbles by comparison to the crater.
False-color close-up of Eros.Credit: Johns Hopkins Univ. APL

An asteroid’s rotation and unusual shape can result in a very complex gravitational field, and this further complicates the Eros landing. But the scientists have plotted the contours of Eros to an accuracy of 1 kilometer (0.6 mile), allowing them to estimate the gravitation of Eros at various points on the asteroid. The scientists also plan to pinpoint the spacecraft’s position relative to the surface by using the on-board laser altimeter. Never before has an asteroid been studied as intensely as Eros. The various measurements conducted by the NEAR Shoemaker spacecraft have helped us understand the composition, mineralogy, and other characteristics of Eros, which, in turn, have allowed scientists to make assumptions about the history of the asteroid.

close-up of Eros as it rotates.
Video: NEAR Shoemaker flyover of Eros.Credit: Johns Hopkins Univ. APL

For instance, these measurements have shown that Eros is related to the primitive ordinary chondrites, the most common type of meteorite. From this finding, along with other measurements, scientists have inferred that Eros did not condense from a swirling disk of gas and dust part of the “clumping” theory of planet formation but instead broke off a much larger object during the birth of the solar system. To have a gravitational pull strong enough to attract other large bodies, an object in space has to be at least 100 kilometers (60 miles) wide. Eros is approximately 30 kilometers (20 miles long), so its parent body could very well have been big enough to form one of the planets in our solar system.


The shape of Eros. Credit: Johns Hopkins Univ. APL

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