Why are MAVEN and Mars Orbiter Mission taking such different paths to Mars?
Posted by Emily Lakdawalla
2013/11/22 12:15 CST
Two spacecraft launched for Mars this month: Mars Orbiter Mission on November 5, andMAVEN on November 18. But MAVEN is already on an interplanetary trajectory, while Mars Orbiter Mission is still in Earth orbit and will not depart for Mars until the end of the month. A lot of people are asking me: why the difference?
I knew it was related to their different launch vehicles, but to get a more informed answer I turned to David Doody, a senior engineer at the Jet Propulsion Laboratory and author of the really awesome JPL online resource called "The Basics of Space Flight." Dave periodically teachescourses on how interplanetary navigation works; registration opens in April for his next one.
Dave confirmed that the different approaches were related to their launch vehicles and to the different spacecraft masses. For MAVEN, the trip to Mars is straightforward. That’s because its Atlas V rocket was able to deliver both MAVEN (with a mass of 2500 kilograms, fully fueled) and an attached, powerful Centaur upper stage into Earth orbit. While already orbiting Earth, Centaur was able to kick MAVEN directly on to Mars. Dave explained:
On Monday the MAVEN spacecraft, stacked atop a Centaur upper stage, enjoyed a flawless launch, rising from Earth on an Atlas 5. The Centaur fired its engine to place itself and the spacecraft into a more or less circular Earth orbit. After twenty-seven minutes, it had coasted around to just the right point to start the trip to Mars: tangent to Earth’s solar orbit on the midnight side. There the Centaur fired again, injecting it into its interplanetary trajectory. The spacecraft then separated from the Centaur and deployed its solar panels for cruise. (This is a familiar sequence, by the way. The Cassini spacecraft started out from Earth using two burns from its Centaur.) From here to Mars, it’s just a long coasting free-fall, with the exception of a few very brief propulsive events to make trajectory corrections as needed.
NASA / Bill Ingalls
MAVEN launches to Mars
MAVEN lifted off at 18:28 UTC on November 18, 2013, atop an Atlas V 401 rocket.
NASA / Ames
Centaur firing in space
This artwork depicts the LCROSS satellite attached to a Centaur upper stage rocket, firing to adjust the spacecraft’s trajectory in space. MAVEN is larger than LCROSS was, but still much smaller than the big Centaur.
India’s four-stage Polar Satellite Launch Vehicle (PSLV) could deliver Mars Orbiter Mission (with a mass of 1340 kilograms, fully fueled) to Earth orbit, but not with a heavy, powerful upper stage like Centaur. Mars Orbiter Mission has to get to Mars using its much smaller Liquid Apogee Motor (LAM) and the relatively small amount of fuel it was able to carry to orbit. Dave explained:
MOM chooses to do things differently. Having less massive a spacecraft than MAVEN, ISRO is depending on its proven Polar Satellite Launch Vehicle, and a reliable propulsion system that is built into the spacecraft: a 440-newton bi-propellant fed rocket engine (Cassini has biprop 440 N engines too!), and instead of one very powerful Centaur burn (about 99,000 newtons), ISRO would use its much smaller engine repeatedly. It has to be repeated burns, rather than one long burn, because in order to raise the spacecraft’s apoapsis – the high point in its orbit – it has to fire when it is speeding past its closest point to Earth, at periapsis. (For more details on this, look for "The Key to Space Flight" here.)
So with repeated burns while going through periapses, the MOM spacecraft is working its way to a high enough energy to get on a trajectory that will be hyperbolic with respect to Earth; it will have the ability to inject onto its interplanetary cruise, orbiting the Sun instead of Earth. MAVEN did this all at once with a powerful rocket; MOM is doing it in steps using its smaller rocket, but the energy required (per unit of spacecraft mass) is about the same.
Mars Orbiter Mission’s troubled fourth rocket burn
It took about five minutes of thrust from the Centaur for MAVEN to depart Earth orbit for its interplanetary transfer. I counted up the rocket burns reported for Mars Orbiter Mission so far, and they amounted to roughly 40 minutes’ total thrust. (They didn’t report times for every burn, so this is just an estimate.) Those minutes were spread out over six maneuvers, all timed to happen right when the spacecraft was near its perigee, to make the most of their precious fuel. One more burn remains, the largest one yet by far, to send Mars Orbiter Mission on to Mars. ISRO chief K. Rhadakrishnan stated the statistics in an interview to The Hindu earlier this week:
In the early hours of December 1, around 00.36 hours [IST, or November 30 at 11:06 PT / 19:06 UT], we have the trans-Mars injection of our Mars spacecraft. On that day, we are going to use the 440 Newton liquid engine again to impart a delta-v, that is, an incremental velocity of nearly 648 metres a second to the spacecraft and the engine will burn for 1,351 seconds…. When this running of the 440 Newton liquid engine takes place on December 1, we also have eight numbers of 22 Newton control thrusters firing.
Once Mars Orbiter Mission has left Earth orbit, its trajectory toward Mars will be very similar to MAVEN’s, Dave explained:
After their Earth departures, MAVEN on November 18 and MOM planned for November 30, their interplanetary trajectory will be the well-known, minimum propellant, Hohmann Transfer, in which they coast "up" away from the Sun, slowing all the way to Mars. Upon arrival, each spacecraft will again fire its rocket engine to be captured by Mars’s gravity and enter orbit around the planet.
Hohmann transfer orbit diagram
A Hohmann transfer orbit can take a spacecraft from Earth to Mars. The orbit is an elliptical one, where the periapsis is at Earth’s distance from the Sun and the apoapsis is at Mars’ distance from the Sun. The transfer orbit has to be timed so that when the spacecraft departs Earth, it will arrive at its orbit apoapsis when Mars is at the same position in its orbit. Earth and Mars align properly for a Hohmann transfer once every 26 months.
Both spacecraft get only one chance to enter Mars orbit. Mars Orbiter Mission will again be using its single 440-Newton rocket to perform the orbit insertion maneuver. MAVEN has six rocket motors, each of which can achieve 200 Newtons of thrust. But MAVEN needs that much more power, because it is a much heavier spacecraft. When MAVEN arrives at Mars, it will still possess nearly all the fuel it launched with, and will weigh in at around 2400 kilograms. Mars Orbiter Mission will have burned a great deal of its onboard fuel simply to depart Earth orbit, so will weigh under 1000 kilograms.
|Mars Orbiter Mission||MAVEN|
|Launch mass||1340 kg||2454 kg|
|Propellant||852 kg||1645 kg|
|Dry mass||488 kg||809 kg|
I highly recommend the Basics of Space Flight as a well-explained resource on how we get spacecraft from one place to another!
Other related posts:
Chris Jones: 11/22/2013 01:41 CST
The second sentence in the caption under the diagram headed "Hohmann transfer orbit diagram" flips "apoapsis" and "periapsis" (the later use of apoapsis is correct).
Emily Lakdawalla: 11/22/2013 02:09 CST
Oops, you’re right, I’ve fixed the error.
Pankaj: 11/23/2013 03:12 CST
One positive fallout of the MOM is that a lot more people in India are suddenly taking a lot more interest in things scientific – specially in trying to understand the concepts behind space flight. So I guess this scores one for "pros" in the pros-vs-cons-of-a-developing-country-putting-scarce-resources-in-space-flight" debate.
Your well-written explanation above is a valuable contribution to this. So is JPL’s "Basic of Space Flight" site which you have referred.
Shreerang Kaulgi: 11/23/2013 01:06 CST
Apart from development of the vehicle for launching, it is better to use as less fuel as possible. Space Missions are very well planned and usually, there is no time restraint to reach their targets. A little energy taken from natural space bodies will save great amount of fuel and weight to be launched. Most deep space missions have done this, then why not also for near space missions.
Your well written articles should reach more common people so that the misconceptions are removed and science is better understood, appreciated and supported.
Ethan Walker: 11/24/2013 01:33 CST
Sheerang Kaulgi: It is true that gravity slingshot maneuvers are not used to get to the closest planets. However I think this makes sense when you consider the requirements for this maneuver. You cannot get a gravity boost from the planet you launch from, at the point in the orbit where you initially depart it. Most gravity boosts are done on the neighboring planets, and by the time you get to those you have already gotten to them. I believe that the earth is sometimes used for the first gravity boost, but only after departing on a significantly different orbit, and returning the following year. It is likely that the amount of fuel required for this maneuver, plus the added time, mean that the direct Hohmann transfer is probably the best option for going to the nearest planets. I suppose that departing missions can always get a boost from the moon, but its low mass and low velocity relative to earth probably makes this not worth the effort.
Prasad Mandava: 11/24/2013 11:39 CST
Very nice explanation. Such explanations make every one to understand the complexity of interplanetary missions and the basic science behind it. It definitely ignite the younger brains. Hopefully MOM and MAVEN could succeed in that.
Shreerang Kaulgi: 11/25/2013 12:36 CST
Thank you for the lucid explanation. Do you think that Chandrayaan was an economic way to get to the Moon? And the Mars Orbiter Mission? Perhaps they may serve as alternate ways to get to space bodies if one doesn’t have the large launchers, despite an intense wish to explore space.
I hope that this blog reaches many eager readers like me and attracts them to this society.
Ethan Walker: 11/25/2013 11:32 CST
The proof, as they say, is in the pudding. If MOM successfully gets to Mars, then for a price of $69M, it will be very economical. However, I think it would be a mistake to attribute too much of the cost difference between these missions to their different approaches to getting to Mars. The biggest factor determining which path they take is the launch vehicle. And while the Atlas is significantly more expensive than the PSLV, it is a fraction of MAVEN’s total cost, so clearly there are other things going on here also.
Kevin N: 11/25/2013 04:51 CST
Thanks for the post. If I can summarize what I understand: The last stage has low thrust (why?) and has to do a very long burn to get into trans-Mars orbit. Because of the Oberth Effect, this has to be done at perigee, and time at perigee is too short for one burn, so many perigee burns are required, generating ever-larger ellipses. I think Ethan is correct in that there is NO "slingshot/gravity assist" effect, which you can’t get from the planet from which you launch.
It’s hard to get clarity on this stuff because there is so much misinformation on the web, such as from the commenter above who promulgates misinformation and then ironically expresses the desire that "misconceptions are removed".
Several questions are left unanswered, such as:
Why so long between burns? Even the last ellipse has a period of about 26 hours (by my calculation) so why do many days go by between the six burns?
What is the advantage of a low-thrust engine? Is it lighter or does it have better heat dissipation or what?
Nik D: 11/25/2013 09:32 CST
The burns are done when the orbiter is at it’s highest speed in it’s elliptical orbit, which is when it’s at the perigee. Due to the nature of this Mars Orbiter’s flight path, the apogee is above the Sun facing side and perigee at the night facing side of the Earth.
The burns are controlled from the ISRO center in Southern India. So, they are done when night time in India coincides with the Orbiter’s perigee. The orbital period has been increasing with every "orbit raising" maneuver, and at present, it is almost 4 days.
Yes, every orbit raising maneuver takes advantage of the gravity boost, and the final burn this weekend takes advantage of the earth’s gravity to "slingshot" itself into the Heliocentric Hohmann Transfer Orbit.
The difference between the two, as explained in this blog, is due to NASA’s Centaur final stage, which could push the much heavier Maven out to the Hohmann Orbit in a much shorter and single burn. Hence Maven can not only carry more equipment but also more fuel on it’s way to Mars, whereas, since ISRO’s PSLV does not have nearly as much lift capacity, MOM is smaller (lighter, with less fuel and equipment) and has used up a good part of it’s fuel just to get out to the transfer orbit.
The only advantage of a low-thrust engine is, it is what ISRO has, and they are making do with what they have (ingenuity).
Nik D: 11/25/2013 09:36 CST
Sorry, I said the burns are done when the Orbiter is at its perigee.
They are actually done at periapsis, when it is speeding past its closest point to Earth, shortly before the perigee.
Emily Lakdawalla: 11/26/2013 09:57 CST
Kevin: India doesn’t have anything like a Centaur upper stage at present — they must make do with the small PSLV (which can lift less mass to orbit) and the relatively small LAM. LAM is only small when compared to Centaur; as Dave Doody points out, it is a standard size for a deep-space restartable rocket motor.
Nik, there is no difference between periapsis and perigee. Periapsis is the general term, the closest approach that an orbiting body makes to the thing it orbits. Perigee is the same, except it only applies to an orbit at Earth. Perihelion is the term for that position on an orbit around the Sun, periselene or sometimes perilune for the Moon, pericrone for Saturn, etc. But perigee and perihelion are the only two that I see in common use; most of the rest of the time people use "periapsis." Also, Earth’s gravity is not involved in any kind of slingshot here. The spacecraft is starting out with Earth’s orbital velocity, and must add velocity to that in order to climb "up" out of the Sun’s gravity well toward Mars.
Easwar Sankar: 11/27/2013 02:56 CST
Thanks for the great article. I struggled to understand why a burn at the perigee gives more kick than elsewhere in the orbit until I read about the Oberth effect. The Wikipedia page is pretty good
skumaran: 11/27/2013 08:28 CST
Thanks to Mr.Dave ,
He clearly pointed out the differences between MOM and MAVEN . Cost of the both launch vehicle also differ.
Paras: 11/27/2013 03:04 CST
Maven is already on its way to mars and MOM will exist earth orbit on early hours of 1 dec. Maven will reach mars on 22 sept and MOM on 24 sept 2014. Is it means MOM’s speed will be little bit faster than Maven’s?
Shubham: 11/30/2013 10:03 CST
Really good explanation. To add to information. PSLV C25 was not the first choice to place MOM in orbit, it was GSLV Mk III which is more powerful than PSVL but, due to failures of GSLV’s upper stage and high success rate of PSLV, PSLV was chosen for this ambitious mission.