A former computer science student has been sentenced in the US to six months house arrest, two years probation and been banned from using computers for recreational purposes after he hacked Nasa computers last year.

Twenty-nine-year-old Ikenna Iffih, from Boston, Massachusetts, pleaded guilty to charges of defacing a commercial website and wilful malicious interference of communications in June.

Iffih repeatedly hacked a Nasa research project during a period of four months in 1999, but apparently did not access any sensitive information. He also owned up to damaging another US government defence agency website and caused thousands of dollars worth of damage to a marketing company's website.

Nasa has been the target of several hackers recently. During the past six months, law enforcement agencies have made at least four other arrests in conjunction with attacks on Nasa.

In September, Florida teenager Jonathan James was sentenced to six months in a detention centre after he pleaded guilty to intercepting 3300 emails, and stealing passwords and data from 13 Nasa computers.

According to local US newspaper reports, James - who was 15 years old at the time of the attack - downloaded $1.7m in Nasa proprietary software before being caught, causing Nasa to spend $41,000 to check and fix the system after the attack.

In July, law enforcement authorities arrested a 20-year-old New York man, Raymond Torricelli, and an unnamed 15-year-old boy from Long Island, New York, on charges stemming from separate hacking incidents at Nasa facilities.

Torricelli is facing a combination of credit card fraud and password interception charges. He faces up to 10 years in prison and a $250,000 fine on each of the credit card fraud and password possession charges; five years and a $250,000 fine on the password interception charge; and one year plus a $100,000 fine on each hacking charge.

However, 20-year-old Californian Jason Diekman, who pleaded guilty to hacking into several computers at Nasa's Jet Propulsion Laboratory in 1998, faces up to 16 years in jail.