Telemetry and geometry capture distant asteroid images
The arrow points to NEAR Shoemaker's saddle-shaped landing site on the asteroid Eros. Image processed by Mark Robinson, Northwestern University/NEAR Imaging Team A high-resolution copy of this photo (3046 x 2196 pixels, 1011K) is available here.
Ithaca, NY -- Will this be 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 University, have 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.
On Feb. 12 the Near Earth Asteroid Rendezvous spacecraft, known as NEAR Shoemaker, will attempt to land on Eros, an Earth-crossing asteroid about 196 million miles from Earth. In mid-descent, an onboard camera will point toward the surface and hopefully send back the best images ever from a small, solar-system body. The navigational prowess of APL and JPL will be complemented by the imaging expertise of the Cornell research team.
"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," says Cornell space sciences researcher Ann Harch. "Other than that, it's easy."
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 is managing the mission) to develop special computer software that, with great precision, displays the shape of Eros and how it will look from the camera's point of view. 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, correlates feature in thousands of images and plots the asteroid's trajectory and orientation. From that information, the program calculates 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 reads input data on the asteroid's location and spin orientation, then shows where the craft and camera will point. The program also displays how the asteroid will look 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 comands take 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.
If all goes as planned, at 10:31 a.m. Eastern time on Feb. 12, the spacecraft will commence firing a series of burns -- firing thrusters away from the asteroid -- to brake the craft for an anticipated 7 mph landing. Control commands to the onboard, multispectral camera will be uploaded to the spacecraft. However, if NEAR goes faster or slower than anticipated, mission controllers at APL will be able to adjust the craft's onboard clock to delay or advance the final photo sequence. "The spacecraft literally has to be in the right place at the right time" for the camera to function as planned, says Harch.
Reflecting on the five-year mission, Harch says: "This final week has been such an emotional one. It was an extraordinary experience working with these people to produce such a fabulous result, and all of us feel that way."
Adds Bell, "Getting this altogether has meant many, many late nights."