Landing on Eros

Nearly 11 years ago — in February, 2001 — the NEAR spacecraft had finished exploring the asteroid Eros from the vantage point of an orbit around the asteroid.

With its primary mission accomplished, the mission team decided to try a riskier maneuver — landing on the asteroid. Mark Robinson from Northwestern University in Evanston, Illinois, was in Mission Control.

Mark Robinson: I think there was probably only one or two people who really believed the spacecraft would still operate once it landed — again, it wasn’t desgned to land. . . And we started getting pictures over the last 15 or 20 minutes, and they were the highest-resolution pictures taken from orbit of any body. . . As soon as we touched down, I heard this big uproar in the back of the room — I was up at the front of the room looking at images — and that was indicating to us that we were still getting data from the spacecraft after it had set down. . . Apparently there was absolutely no damage whatsoever at all. So it had set down on the surface at about a walking pace.

NEAR seems to have landed in soft dust on Eros, collected in what space scientists are calling “ponds.” Robinson said it was like landing in a sandbox, rather than on solid rock.

The craft continued to send back useful data about Eros for another two weeks.

More resources:

  • M. S. Robinson, P. C. Thomas, J. Veverka, S. Murchie, and B. Carcich. “The Nature of the Ponded Deposits on Eros,” Nature, Vol. 413, September 27, 2001, pp. 396-400.
  • J. Veverka, et al. “Imaging of Small-Scale Features on 433 Eros from NEAR: Evidence for a Complex Regolith,” Science, Vol. 292, April 20, 2001, pp. 484-488.

Mark Robinson: It’s a very dynamic and exciting geologic surface that we see on asteroids. It’s not just a big hunk of rock up there in space, doing nothing.

Mark Robinson of Northwestern University in Evanston, Illinois — who helped explore an asteroid up close. Eros is the second-largest known Near-Earth Asteroid . . .

It’s about the size of Manhattan island. The NEAR spacecraft visited Eros in 2000 and 2001. It gazed toward this little world from orbit for one year. NEAR images showed that Eros is covered by boulders and craters. Some craters contain small, mysterious “ponds” made of fine dust . . .

Robinson’s team believes that, as the sun rises over a given part of Eros, sunlight causes dust particles to become electrically charged — which in turn causes them to levitate above the asteroid’s surface. A short time later, the particles lose their electric charge and settle in the craters. Other scientists have offered less exotic explanations for the “ponds” — for example, dust slides and shaking caused by “Eros-quakes.”

Since there are no more missions to Eros in the works, we might never know what causes the dust ponds. Robinson is now back at work on a new mission — this time to the planet Mercury.

Eros – the second-largest Near-Earth Asteroid (NEA)

Eros is the second-largest Near-Earth Asteroid (NEA), and the first discovered. Eros measures 33 kilometers long by 11 kilometers wide by 11 kilometers high. The asteroid’s shape has been likened to that of a shoe or a telephone handset. Major Erosian landmarks are the large craters Shoemaker, Psyche, and Himeros. Shoemaker, the youngest crater, is 7.6 kilometers across.

The asteroid comes as close to the Sun as 1.13 Astronomical Units (AU) (one AU is equivalent to Earth’s distance from the Sun, or about 93 million miles) and moves as far away as 1.73 AU. Eros does not cross Earth’s orbit, but over time the gravity of the Earth, Mars, Jupiter, and the Sun might shape its orbit so it becomes an Earth-crosser. Millions of years from now, Eros might crash into the Earth.

Eros turns once in 5.27 hours. Eros’ axis of rotation is parallel to the plane of its orbit around the Sun. This means that the poles take turns pointing directly at the Sun. At the height of northern hemisphere summer, the north pole points at the Sun while the south pole is permanently shadowed. Halfway around its orbit, the south pole points directly at the Sun and north pole hides in shadow. In between, neither pole points at the Sun. Sometimes during the Erosian year, points near a given pole can go without a sunrise or sunset for months. Points near the equator, on the other hand, frequently experience sunrise and sunset. (Incidentally, the planet Uranus has a similar axial tilt, with similar effect.)

Gravity is weak all over Eros. What is more, it varies from point to point. On average, gravity pulls with only 1/1000th the force of Earth gravity. At the asteroid’s ends, however, farthest from the center of rotation, gravity’s pull is halved. Near the asteroid’s middle, close to the center of rotation, gravity is about twice average.

Interestingly, the pond deposits on Eros occur in areas that experience both the most day-night transitions and the weakest gravity. This would be expected if electrostatic levitation of surface dust is the cause of Eros’ pond deposits. Sunlight would electrically charge the dust, causing it to repel everything around it. This would push the dust off the surface. The dust would rise above the surface at dawn and sink back down soon after. The more sunrises a given location experienced, the more the dust could move. At the same time, weak gravity would make it easier for electrostatic levitation to raise dust.

Science, however, is about testing hypotheses. Electrostatic levitation fits many of the facts we know about Eros, but there might also be other processes at work on that odd little world. Perhaps asteroid impacts and “Eros-quakes” shake Eros, causing dust to avalanche into craters, then shaking it flat like flour in a shaken bowl. This theory fails, however, to explain why the ponds congregate in places with many day-night transitions and low gravity. Undiscovered processes might also be at work on Eros. Electrostatic levitation must, for now, remain a theory awaiting additional data to support (or undermine) it.

Scientists think that dust on the Moon does this same trick of electrostatic levitation that is theorized to explain the Eros dust ponds.

Surveyor missions (pre-Apollo moon landers) took pictures of the [moon’s] horizon shorty after sunset and saw horizon glow. Since the Moon has no appreciable atmospher,e no light should have been visible. The same phenomena was observed by astronauts in lunar orbit (in fact they made sketches). The Lunar Ejecta and Meteorites experiment (LEAM) delivered to the Moon by Apollo 17 detected increased particle contacts during passage of the terminator. The theory is that these disparate observations were the result of electrostatic levitation of dust. This idea was supported by conductivity measurements of lunar soil. So the answer is – something was observed by instruments (photography and particle detectors) and human observation. The explanation is a theory – there has been no definitive 100% certain absolute iron-clad nail-in-the-coffin measurement.

For a quick summary, check out “Lunar SourceBook” page 532, though it leaves out the Surveyor results bizarrely enough. So also see page 459 of “Surveyor Project Final Report”, JPL Tech Doc 32-1265, 1968, (particularly the picture on page 460).

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