The late July emergency repositioning of the NASA Mars Odyssey orbiter after a serious onboard malfunction should enable the spacecraft to relay critical data to Earth from the Mars Science Laboratory (MSL) spacecraft throughout all aspects of the rover’s daring Entry, Descent and Landing (EDL) on Mars.
The transmission will be a real time confirmation of events, minus the 13.5 min of light time delay caused by the 154 million mi. distance from Mars to Earth.
Here is how the relay satellite arrangement was set up, the nature of the Odyssey emergency , its dramatic fix and the spacecraft maneuvering that went into reestablishing a real time relay for EDL.
Mars Odyssey, the primary relay spacecraft for Curiosity arrived in Martian orbit in 2001, long before the MSL mission was even defined in detail.
It was followed in 2006 by the high resolution Mars Reconnaissance Orbiter (MRO) which has played the primary role of imaging the best landing site candidates leading to Gale Crater’s selection. Odyssey has also provided critical data on the presence of past and possible current subsurface water in the landing site area to helping MSL define whether Gale Crater ever had, or possibly still has, habitable environments.
Odyssey, a spacecraft with older but still very capable technology can receive data at either 128K or 256K bits per second. At the low end, this is only twice as fast as an internet dialup connection, but useful amounts of data can be exchanged at this rate.
MRO’s data rate is far greater and variable up to 2048 kilobits per second owing to more powerful radios, advanced compression schemes and a new generation of error correction software.
To return its data to Earth, Curiosity will transmit signals at high data rates to Odyssey and MRO, which in turn will use their more powerful radio and directional antenna to send the data to Earth.
As defined by their science objectives, the orbital paths of the Odyssey and MRO are nearly circular sun-synchronous polar orbits traveling over the same locations on Mars at nearly the same time twice each day.
Unlike the communications satellites that orbit Earth over fixed locations, MRO and Odyssey are in low orbits, ranging between 160 and 250 miles above the Martian surface.
At these altitudes, the relays are only above Curiosity’s horizon for about 8 minutes. Passing over the Gale crater area twice per day, once in the pre-dawn hours and once in the late afternoon, Curiosity will transmit data on both of these passes. Here lies the difficulty in setting up the relays for the most eventful part of the mission–EDL, The Seven Minutes of Terror.
Mission planners decided on a landing time months before Curiosity’s November 2011 launch to begin the coordination process between MRO, Odyssey and Curiosity. A fixed date and time for landing are necessary due to the fact Odyssey and MRO both have orbital paths that pass over Gale, but the timing of their paths in orbit are not synchronized.
The hardest trick for mission planners was to get all three spacecraft together at the same general area at the same time, with maneuvers starting over 7 months before MSL lands.
Once MSL settled into its cruise mode, controllers slightly altered the inclination of Odyssey and MRO’s orbits and allowed them to drift towards their positions to cover the landing. In their new orbits, both would be able to receive signals from the beginning of entry before passing over the Martian horizon just after landing. With so many months to complete this task, only a small adjustment was necessary, saving fuel and creating only a minimal disruption to the science plan.
Despite the precision of the maneuvers, enough variations could occur, however, that the orbiters might still be out of range and unable to monitor Curiosity’s landing. Just a small error in timing would place the orbiter early or late by just a few minutes, missing the chance to capture any useful data from the descent. With both spacecraft on track to be in position, all appeared well.
Then on June 8, a serious problem struck Odyssey ultimately causing it to change its orbit to one that would miss the final two minutes of landing. A reaction wheel used to maintain Odyssey attitude control malfunctioned, and the spacecraft put itself into a protective “safe mode”.
In this state, its science instruments turned off and the spacecraft listened for communications from Earth. During seven days engineers worked to start up a spare reaction wheel then had to carry our several spacecraft maneuvers to reorient the spacecraft. That resulted in a small change to the orbit, but enough to prevent Odyssey from being in the right place at the right time to relay the critical last two minutes of the landing.
MSL’s entry sequence will be completely automatic, and does not require the relays for a safe landing. With a round-trip communications delay of nearly 30 min. there is nothing that controllers can do even if a problem does occur. During the last few minutes of the landing, Earth will have set at Gale crater making it impossible for NASA’s Deep Space Network antennas to receive a signal directly from MSL if Odyssey is out of position, MRO becomes the only means to record the EDL events.
Odyssey is especially useful because it functions like a simple “bent pipe” retransmitting the signal it receives, allowing engineers and the public to listen to the landing in real-time. MRO uses a different technique, storing the signals first, and retransmitting up to several hours later.
On July 24 engineers commanded Odyssey to fire its thrusters for six seconds, adjusting its orbit by about six minutes. Now repositioned, the spacecraft will be able to retransmit data from Curiosity during the entry, and for about two minutes after the landing.
Even with all the redundancies, a complete picture of the landing sequence might not emerge for a day or two after the landing. If the relay spacecraft fail to receive any signals, several passes over Gale Crater may be necessary before a complete understanding of the rover and its health–or a lack thereof– are available to engineers on Earth.
Only after status information is sent to Earth will Curiosity send the first pictures of the landing site. Once the rover deemed healthy, sequences of commands will be sent to deploy the camera mast and prepare other systems for exploration. Now able to settle into a regular sequence of data relay passes, Curiosity should first be able to uplink several weeks of checkout data and especially images of its surroundings.
Relay operations will be augmented by the European Mars Express orbiter, and towards the end of MSL’s nominal mission, the NASA MAVEN orbiter, scheduled for launch in 2013.
The vast distance between the Earth and Mars–154 million mi. at landing then growing to more than 200 million miles as the flight proceeds– makes communications between the controllers at JPL and the Mars Science Laboratory one of the greatest challenges of the mission. In performing its science mission, MSL will generate an unprecedented amount of data, up to 2 GB of data per day in the form of photography, science results and engineering data, and is expected to return 250 megabytes per day to Earth.
Curiosity has two radios for communication. The first is the high gain system using a hexagonal, highly directional antenna nearly a foot in diameter mounted on the rovers deck.
JPL Deep Space Network antennas like this 70 ft. dish at Canberra, Australia will play a critical role in receiving MSL data transmitted by the Odyssey and MRO spacecraft and eventually from Curiosity itself.
The high gain antenna can send data at rates from 500 to 32,000 bits per second and is intended as a direct link to Earth. This is not sufficient for the bulk of the science return, but is entirely adequate for sending basic engineering data and receiving commands from Earth. A second radio is named Electra, and transmits in the 400MHz band using an omni antenna to communicate with the relays.
To transmit 250 megabytes of data up to 200 million miles is far beyond the ability of the radios on Curiosity, requiring the assistance of the relay spacecraft in Mars orbit to transmit such a torrent of information back to Earth.
The biggest obstacles in communication are the laws of physics. Data transmissions, whether cell phone conversation or science results between Earth and a spacecraft on Mars require balancing two conflicting requirements signal strength and the amount of data that can be transmitted.
So it is the Mars orbiter relay satellites that have made the 2004 Spirit and Opportunity rovers and the 2008 Phoenix Mars polar lander missions a success. MSL will be on its own during EDL, and if successful the relays will remain a pivotal part of Curiosity’s mission just like they still are for Opportunity which continues to rove.
About the author
Frank O’Brien has lent his spaceflight history expertise to NASA for nearly 20 years as a contributing editor for the Apollo Lunar Surface Journal. He also helps co-edit the Apollo Flight Journal. Through the use of mission transcripts, interviews with the flight crews and a vast collection of technical resources, the Apollo Journals are the canonical resource for those interested in mankind’s greatest voyage of exploration.
From this work, Frank was invited to the Cradle of Aviation Museum on Long Island to assist in their May, 2002 reopening. He prepared a rare Lunar Module Mission Simulator for exhibition, wrote software for their Lunar Module cockpit trainer, and prepared an Apollo space suit for the museum’s centerpiece Apollo 11 diorama.
His background on the lunar missions and computing led him to write a well-received book on the Apollo Guidance Computer, and is now working on a new book on Apollo spacecraft engineering. This year, Frank became a Solar System Ambassador for NASA’s Jet Propulsion Laboratory and lectures on a wide range of space topics. Frank has always been passionate about aerospace, and was a pilot and aircraft owner for 25 years.
He is a 1979 graduate of Rutgers University (computer science), and later returned to Rutgers to earn his MBA. His day job as a SAP and database administrator is far less interesting than traveling to the Moon.
AmericaSpace and The Mars Society have partnered to provide in-depth coverage of the arrival of the Mars Science Laboratory rover “Curiosity” to Mars. Stay tuned for regular updates as AmericaSpace correspondents Craig Covault and Frank O’Brien travel to NASA’s Jet Propulsion Laboratory in California for live coverage.