Touchdown Asteroid

This week’s selection for the Extreme Explorer Hall of Fame is the landmark first asteroid lander–the NEAR Shoemaker spacecraft.

So remarkable was its journey–to soft-land on a peanut shaped asteroid about 176 million kilometers (109 million miles) from Earth. Eros is the Greek name for Cupid, the familiar bringer of love from classical mythology, and somewhat appropriately the lander began its historic dive to the heart of its asteroid companion on Valentine’s Day, 2001.

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

At the time, Andrew Cheng, NEAR Project Scientist wrote: "On Monday, 12 February 2001, the NEAR spacecraft touched down on asteroid Eros, after transmitting 69 close-up images of the surface during its final descent. Watching that event was the most exciting experience of my life."

To relive that remarkable event last month–even after the NEAR Shoemaker spacecraft had exceeded every mission expectation — the project team asked for one more spectacular addition to the mission’s legacy: Talk to us one more time.

But NEAR Shoemaker – the first spacecraft to orbit, land on and send data from the surface of an asteroid – kept mum despite a 12-hour effort to communicate with it.

"The exercise was an experiment to see how robust the spacecraft and its instrumentation and subsystems were given the extremely cold temperature it has been in for nearly two years," says NEAR Mission Director Robert Farquhar. "We didn’t hold out much hope but we had an opportunity to establish an important data point and didn’t want to lose the chance."

The attempt was initiated at 2:40 p.m. EST, Tuesday, Dec. 10, by the NEAR mission operations team at the Johns Hopkins University Applied Physics Laboratory, which managed the mission and built the spacecraft, and the Deep Space Network team, which supported the effort through their 70-meter Goldstone antenna. With asteroid Eros only about 86 million miles (138 million kilometers) from Earth – less than half the distance it was when NEAR Shoemaker landed on it in February 2001 – and NEAR Shoemaker’s solar panels basking in sunlight for the past three months, the timing was ideal.

First, operators listened passively for a carrier signal from the spacecraft. Then they sent commands asking NEAR Shoemaker to transmit data indicating it had survived the last 22 months on the asteroid’s surface, despite temperatures that dipped as low as minus 170 degrees Celsius (-274 degrees Fahrenheit) and long periods of total darkness.

Not knowing which of NEAR Shoemaker’s two computers had access to its transmitter, mission operators tried sending commands to one, then the other. Then they waited — in vain — to receive data. Farquhar says the team will probably never know precisely why NEAR Shoemaker did not respond and they do not expect to try again.

Large, smooth basin. At bottom lie scattered boulders that appear like pebbles by comparison to the crater.
The rocks inside a crater on the Asteroid Eros. Numerous small impacts on the asteroid show brown boulders visible interior to the less exposed (white) lip of the crater. False-color for emphasis. Credit: NEAR Project, JHU APL, NASA

To ensure its picture-perfect crash landing, the scientists had to precisely calculate the rate of the asteroid’s rotation. If they had measured wrong, the spacecraft would have landed 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, had determined that Eros rotates once every 5.27 hours. The Caltech team then 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.

Cheng recollected the human drama of those tense moments of soft-crash: "I was asked immediately afterwards how I felt, and I mumbled something about being tired and happy, but I missed the point. I realized afterward what I should have said: it was like watching Michael Jordan on the basketball court, when the game is on the line and he is in the groove. One miracle after another unfolds, and we are left stunned and speechless. When we learned that the spacecraft had not only landed on the surface, but was still operational, we hardly knew what to think."

The asteroid’s rotation and unusual shape (in places resembling either a heart or giant peanut) can result in a very complex gravitational field, and this further complicated the Eros landing. But the scientists had 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. Gravity did the rest of the work, to a soft crash landing.

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 various measurements conducted by the NEAR Shoemaker spacecraft have helped scientists to understand the composition, mineralogy, and other characteristics of Eros, which, in turn, have allowed them to make assumptions about the history of the asteroid.

"The final weeks of low altitude operations," noted Cheng, "revealed bizarre and surprising aspects of surface structures on Eros, including one type of feature we noticed for the first time in the very last image taken by the spacecraft (the incomplete image taken from a height of 120 meters, 2001 Feb 12F )."

"There are markedly fewer small, fresh craters on Eros than we would expect from our experience at the Moon, and an amazing profusion of boulders, likewise more than we expected. We do not know just what is happening on the surface of Eros to cover and/or obliterate craters while making and/or uncovering boulders. "

What’s Next

Delayed indefinitely this month, ESA’s Rosetta lander, was to be the first man-made object to land on a comet. Hampered by rocketry concerns, the landing phase presented planners with another set of challenges altogether. "Firstly, we don’t know anything about how rough the surface is," said Rosetta Project Scientist Gerhard Schwehm. "It could be covered with fluffy snow like the Alps or it could be hard rocks and craters. We can, however, be sure that it will not be smooth and flat resembling parking lots."

Scientists had designed Rosetta’s landing gear to cope with most nasty surprises as soon as it touches down on Comet Wirtanen in 2011. Two harpoons were to anchor the probe to the surface. The self-adjusting landing gear will ensure that it stays upright, even on a slope. The lander’s feet will drill into the ground. These devices will help counteract the fact that there is no gravity on a comet.

Nevertheless in the next 5 or so years there will be, if everything works out, no fewer than six to seven encounters of spacecraft with comets and asteroids. All the following missions are fully funded, though only not all have already been launched (the others will follow in 2003 to 2004):

2001 Sept. 22 Comet Borrelly Deep Space One (simple flyby)  
2003 Nov. 12 Comet Encke CONTOUR (simple flyby)
2004 Jan. 1 Comet Wild 2 Stardust (coma sample return)
2005 July 3 Comet Tempel 1 Deep Impact (big mass impact)
2005 Sept. Asteroid 1998 SF36 Muses-C (sample return)
2006 June 18 Comet S.-W. 3 CONTOUR (simple flyby)
2006 July 11 Asteroid Otawara Rosetta (simple flyby)

As Cheng concluded after their remarkable encounter with Eros, "The images tell us a tale whose outcome we don’t yet know, but there is more."