April
2007
Soft landing0
Teaching resources (UK US) designed specifically for this story at Real Science
The story
Scientists at Washington University, with the help of a keen-eyed student, are paving the way for the Phoenix Mission to make a smooth landing on Mars.
The team has been analysing images of the surface of the red planet. Their aim is to make sure the Phoenix, which is due to launch in August, lands in a rock-free spot on the northern plains of Mars.
The craft has to land in a place that won’t have steep slopes or big rocks, said Raymond Arvidson, professor in arts and sciences, and chair of the Washington University earth and planetary sciences department.
“We’ve been looking for locations big enough and homogeneous enough for a high probability of a successful landing. The issue isn’t slopes. The issue is rocks.”
If the lander came down in a place with rocks as big as itself, the whole craft could tilt or tip over. Another problem is the craft’s solar panels. Big rocks would stop these unfurling. Without solar power, which drives seven Phoenix mission instruments, there isn’t much of a mission.
At the heart of the painstaking task of finding a smooth landing is a 21-year-old student at Washington University. Tabatha Heet began working with Arvidson as a work-study student in 2005. She started counting Martian rocks in October 2006.
“Ray asked if I would count some rocks in the original landing area, and I got started, thinking it was going to be a one-time thing,” said Heet. “But it’s turned into a big project. I’ve counted thousands of rocks now.”
Arvidson and his colleagues had settled on a region called Region B for the future landing. But images from an instrument called HIRISE, a feature of the Mars Reconnaissance Orbiter Mission, made them think again.
“The first images for Region B were scary,” Arvidson said. “There are rocks there bigger than the lander – too many big rocks sitting on craters to fit in a landing site.”
With the help of HIRISE images, they looked elsewhere. Heet produced data on the abundance of rocks at different places on the northern plains. This allowed the mission scientists to “zero in on the safe havens”, Arvidson said.
Heet used a software package called ENVI. This shows images and makes measurements.
“All you have to do is draw a line on the image,” she said. “Then ENVI will tell you how long the line is in meters. I go through the image and pick just a small area, because the HIRISE images are too big for one person to count. I’ll make a little subset and then go count every rock in the subset, just by drawing a line where I see the shadow of the rock.
“It’s very slow and makes your eyes go crazy.”
She counted rocks in little areas of the large images. She then made cumulative frequency plots. These showed the number of rocks bigger than any given diameter.
Heet flew out to the Jet Propulsion Laboratory in February. She received a warm round of applause at her introduction to JPL researchers. They questioned her on her technique and stamina. Later, she met a team of automated rock counters who “aggressively” questioned the way she had been counting rocks.
At the meeting, the automated rock counters calibrated their computed numbers to Heet’s hand counts. They are considered ‘ground truth,’ on which all later data are based. She has since corresponded with the group regularly to help make the automated counts more precise.
The automated rock counters map the shape of the shadows, said Arvidson. “From knowing where the sun is, they can compute the rock height and width.
“But they need very intense validation. Tab was the point contact for all of that. We’ve cross-calibrated against the automated counts, because the hand-derived ones are considered anchors.
A human can do a better job with fewer errors “as long as the person is not fatigued”, he added.
Heet’s work has led to the discovery of several possible landing sites. These have at least ten times fewer rocks than the original Region B. They include one desirable location 50 kilometres wide and 250 meters deep. The scientists call it Green Valley.
As data come in from the actual mission, Heet will be at JPL, gathering and interpreting the data, Arvidson said. Just as former Rhodes scholar Bethany Ehlmann, now pursuing a PhD at Brown University, did for the Mars Exploration Rover.
After all her dedicated, painstaking work, Heet often thinks of the mission and the thrill of the Phoenix launch and landing.
“I will certainly be excited when Phoenix launches. I will also probably feel a little bit of pride knowing that I helped make the launch possible. I suspect I’ll be slightly nervous when Phoenix is landing, wondering if I did something wrong and am going to be responsible for making Phoenix crash in a field of huge boulders. Once the lander is on the surface it will be interesting to find out just how accurate all of our predictions were.
“I’m looking forward to it all.”
Topics for group discussion or pupil presentations
1. Research and present current and past thinking about the chances of finding life on Mars.
2. In groups discuss what effect the discovery of life on other planets would have on how people think about science. Would the subject become more popular? Would religions with their view that humans are special disappear? Would the answers to these two questions be different if the extraterrestrial life we discovered were intelligent beings like us or “just” micro-organisms?
3. There are now scientists called astrobiologists who aim to discover if there is life on other planets, by analysing the light that comes from them. Amazingly they are working towards doing this for planets around other stars, as well as in our own solar system. Separating the light of these “exoplanets” and the light of their star has been compared to separating a candle 1000 kilometres away from a lighthouse beside it. Students should read the interview with Giovanna Tinetti then discuss and explain what an astrobiologist actually does.
Links to free activities, resources and lessons
Phoenix mission homepage. Scheduled for launch in August 2007, the Phoenix Mars Mission is designed to study the history of water and habitability potential in the Martian Arctic’s ice-rich soil.
The Phoenix Classroom. Activities and materials to aid understanding of fundamental concepts in science, technology, engineering and mathematics. Includes programs for teacher and student participation.
Ask the Phoenix Mars Mission team a question.
“Spacecraft visiting Mars have returned intriguing images of the surface of the Red Planet for over forty years. Many of these images suggest liquid water once flowed on the surface of Mars. The online video Mars: The Search for Water, the Search for Life looks at some of these images, compares them to similar features on Earth, and looks at the consequences of finding liquid water on Mars.
Phoenix mission fact sheet.
Introduction to what we know and hope to discover about Mars. Learn about the Red Planet by comparing similarities and differences to Earth.
The Phoenix Student Interns Program is an opportunity for high school teachers and students to become part of the science team for the 2007-2008 Phoenix Mars Lander Mission. Selected teachers and students will work with scientists to prepare for surface operations on Mars, analyse data during the mission, and reach out to other students, teachers, and the public through presentations, articles, and web sites. Apply by April 25, 2007
Learn how to organise data in a cumulative frequency table.
Design, construct and test an original model of a bouncing lander. Hold a classroom contest to see which landers work best to keep the cargo from breaking.
Links to more links
Daily tip for science class discussions and groupwork
Only exceptional lecturers are capable of holding students’ attention for an entire lecture period. It is even more difficult to provide adequate opportunity for students to critically think through the arguments being developed. Consequently, lectures simply reinforce students’ feelings that the most important step in mastering the material is memorizing a zoo of apparently unrelated examples.
In order to address these misconceptions about learning, we developed a method, Peer Instruction, which involves students in their own learning during lecture and focuses their attention on underlying concepts. Lectures are interspersed with conceptual questions, called ConcepTests, designed to expose common difficulties in understanding the material. The students are given one to two minutes to think about the question and formulate their own answers; they then spend two to three minutes discussing their answers in groups of three to four, attempting to reach consensus on the correct answer. This process forces the students to think through the arguments being developed, and enables them (as well as the instructor) to assess their understanding of the concepts even before they leave the classroom.
From Peer Instruction
