This was the gang that could shoot straight.
For the past year, engineers and computer programmers from Johns Hopkins University's Applied Physics Laboratory (APL), assisted by NASA's Jet Propulsion Laboratory (JPL) and the imaging team at Cornell, had been figuring out how to slew a spacecraft precisely and aim its camera perfectly for the final act of its mission: alighting on an asteroid.
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| This is the last image of asteroid 433 Eros received from NEAR Shoemaker, taken from a range of 394 feet. What can be seen of the rock at the top of the image measures 12 feet across. The streaky lines at the bottom indicate loss of signal as the spacecraft touched down on the asteroid during transmission of the image. APL/NEAR imaging team |
On Feb. 12 the Near Earth Asteroid Rendezvous spacecraft, known as NEAR Shoemaker, touched down on 433 Eros, an Earth-crossing asteroid about 196 million miles from Earth. In middescent, an onboard camera pointed toward the surface sent back the best images ever from a small, solar-system body. The navigational prowess of APL and JPL was complemented by the imaging expertise of the Cornell research team.
"This final week has been such an emotional one," said Ann Harch, Cornell space sciences researcher. "It was an extraordinary experience working with these people to produce such a fabulous result, and all of us feel that way."
Use of the navigation team's telemetry, geometry and other calculations -- for this never-before-attempted maneuver -- required unique software to point the camera, and it took more than a year to perfect. Harch and her Cornell research colleagues Maureen Bell and Colin Peterson and programmer Brian Carcich worked with APL (which built the spacecraft and managed the mission) to develop special computer software that, with great precision, displayed the shape of Eros and how it looked from the camera's point of view.
Prior to the landing, Harch quipped about the mission's difficulty: "It's not like this craft is landing on a sphere. It's descending on a potato-shaped rock that is 22 miles long, and the rock has a large, saddle-shaped hole on one side. The rock continuously spins end-over-end. Geometry is forcing us to land there -- where there is more motion than at the poles -- so that NEAR's solar panels face the sun, its antenna points to Earth and its camera faces the asteroid. Other than that, it's easy."
First an exact model of the asteroid's shape had to be determined. This shape-model software, called POINTS, developed by Cornell's Jonathan Joseph, programmer analyst, and Peter Thomas, senior research associate, correlated features in thousands of images and plotted the asteroid's trajectory and orientation. From that information, the program calculated a detailed, three-dimensional asteroid model.
Harch, Bell and Peterson then used Orbit, a computer program developed at Cornell by Carcich, to design pointing commands for the multispectral imaging camera. Orbit read input data on the asteroid's location and spin orientation, then showed where the craft and camera pointed. The program also displayed how the asteroid looked to the camera at each instant.
This information allowed Harch, Bell and Peterson to cobble together command sequences that were uploaded to NEAR Shoemaker throughout the mission. The commands took about 17 minutes for the information to be received by the distant spacecraft and the same amount of time for the craft to send back confirmation that the data was received.
All operations went as planned. At 10:32 a.m. Eastern time on Feb. 12, the spacecraft commenced firing a 20-second series of burns -- firing its thrusters away from the asteroid 22 miles below -- to begin braking the craft for the 4 mph landing.
Reflecting on the end of the five-year mission, Bell remarked: "Getting this all together meant many, many late nights."
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